Clamping systems. Clamping elements of devices. Installation elements of devices
CONTENT
Page
INTRODUCTION………………….…………………………………… ……..…….....2
GENERAL INFORMATION ABOUT DEVICES…………………………... …3MAIN ELEMENTS OF DEVICES……………….………………...6
Clamping elements of devices……………………………….……. …..61 Purpose of clamping elements………………………………………...6
2 Types of clamping elements……………………………………….…..…. .7
REFERENCES………………………………… ……………………..17
INTRODUCTION
The main group of technological equipment consists of devices for mechanical assembly production. In mechanical engineering, devices are auxiliary devices for technological equipment used when performing processing, assembly and control operations.
The use of devices allows you to: eliminate the marking of workpieces before processing, increase its accuracy, increase labor productivity in operations, reduce production costs, facilitate working conditions and ensure its safety, expand the technological capabilities of equipment, organize multi-machine maintenance, apply technically sound time standards, reduce the number of workers necessary for the production of products.
The frequent change of production facilities, associated with the increasing pace of technological progress in the era of scientific and technological revolution, requires technological science and practice to create structures and systems of devices, methods for their calculation, design and manufacture, ensuring a reduction in production preparation time. In mass production, it is necessary to use specialized, quickly adjustable and reversible fixture systems. In small-scale and individual production, the system of universal prefabricated (USP) devices is increasingly being used.
New requirements for devices are determined by the expansion of the fleet of CNC machines, the readjustment of which for processing a new workpiece comes down to replacing the program (which takes very little time) and replacing or readjusting the device for basing and securing the workpiece (which should also take little time) .
Studying the patterns of the influence of devices on the accuracy and productivity of operations performed will make it possible to design devices that intensify production and increase its accuracy. Work on the unification and standardization of fixture elements creates the basis for automated design of fixtures using electronic computers and automatic machines for graphic display. This speeds up the technological preparation of production.
GENERAL INFORMATION ABOUT DEVICES.
TYPES OF DEVICES
In mechanical engineering, a variety of technological equipment is widely used, which includes fixtures, auxiliary, cutting and measuring tools.
Devices are additional devices used for machining, assembly and control of parts, assembly units and products. According to their purpose, devices are divided into the following types:
1. Machine tools used for installing and securing processed workpieces on machines. Depending on the type of machining, these devices, in turn, are divided into devices for drilling, milling, boring, turning, grinding machines, etc. Machine tools make up 80...90% of the total fleet of technological equipment.
The use of devices ensures:
a) increasing labor productivity by reducing the time for installing and securing workpieces with partial or complete overlap of auxiliary time with machine time and reducing the latter through multi-place processing, combining technological transitions and increasing cutting conditions;
b) increasing processing accuracy due to the elimination of alignment during installation and associated errors;
c) facilitating the working conditions of machine operators;
d) expanding the technological capabilities of equipment;
e) increasing work safety.
2. Devices for installing and securing a working tool, communicating between the tool and the machine, while the first type communicates the workpiece with the machine. Using devices of the first and second types, the technological system is adjusted.
3. Assembly devices for connecting mating parts into assembly units and products. They are used to fasten base parts or assembly units of an assembled product, ensure correct installation of the connected elements of the product, pre-assemble elastic elements (springs, split rings, etc.), as well as to make tension connections.
4. Inspection devices for intermediate and final inspection of parts, as well as for inspection of assembled machine parts.
5. Devices for capturing, moving and turning over workpieces and assembly units used in the processing and assembly of heavy parts and products.
According to their operational characteristics, machine tools are divided into universal ones, designed for processing a variety of workpieces (machine vices, chucks, dividing heads, rotary tables, etc.); specialized, intended for processing workpieces of a certain type and representing replaceable devices (special jaws for a vice, shaped jaws for chucks, etc.), and special, intended for performing certain operations of machining a given part. Universal devices are used in conditions of single or small-scale production, and specialized and special devices are used in conditions of large-scale and mass production.
Using a unified system of technological preparation of production, machine tools are classified according to certain criteria (Fig. 1).
Universal prefabricated devices (USF) are assembled from prefabricated standard elements, parts and high-precision assembly units. They are used as special short-term devices for a specific operation, after which they are disassembled, and the delivery elements are subsequently reused in new arrangements and combinations. Further development of the USP is associated with the creation of units, blocks, individual special parts and assembly units that ensure the layout of not only special, but also specialized and universal adjustment devices for short-term operation,
Collapsible fixtures (CDF) are also assembled from standard elements, but less precise, allowing local modification according to the seats. These devices are used as special long-term devices. After disassembling the elements, you can create new layouts.
Rice. 1 – Classification of machine tools
Non-separable special devices (NSD) are assembled from standard parts and general-purpose assembly units, as irreversible long-term devices. The structural elements of the layouts included in the system, as a rule, are used until they are completely worn out and are not reused. The layout can also be carried out by constructing a device from two main parts: a unified base part (UB) and a replaceable setup (SN). This design of the NSP makes it resistant to changes in the design of the workpieces being processed and to adjustments in technological processes. In these cases, only the replaceable adjustment is replaced in the fixture.
Universal non-adjustment devices (UPD) for general purpose are most common in mass production conditions. They are used for securing workpieces from rolled profiles and piece workpieces. UBPs are universal adjustable housings with permanent (non-removable) basic elements (chucks, vices, etc.), included with the machine upon delivery.
Specialized adjustment devices (SAD) are used to equip operations for processing parts grouped according to design characteristics and basing schemes; the arrangement according to the assembly diagram is the basic design of the housing with interchangeable settings for groups of parts.
Universal adjustment devices (UND), like SNP, have permanent (body) and replaceable parts. However, the replacement part is suitable for performing only one operation to process only one part. When switching from one operation to another, the devices of the UNP system are equipped with new replaceable parts (adjustments).
Aggregate means of mechanized clamping (ASMZ) are a set of universal power devices, made in the form of separate units, which, in combination with devices, make it possible to mechanize and automate the process of clamping workpieces.
The choice of device design largely depends on the nature of production. Thus, in mass production, relatively simple devices are used, designed mainly to achieve the specified accuracy of processing the workpiece. In mass production, high demands are placed on fixtures in terms of performance as well. Therefore, such devices, equipped with quick-release clamps, are more complex designs. However, the use of even the most expensive devices is economically justified.
MAIN ELEMENTS OF DEVICES
The following equipment elements exist:
installation - to determine the position of the workpiece surface being processed relative to the cutting tool;
clamping - for securing the workpiece being processed;
guides - to give the required direction to the movement of the cutting tool relative to the surface being processed;
device housings - the main part on which all elements of the devices are located;
fastening - for connecting individual elements to each other;
dividing or rotary, - to accurately change the position of the workpiece surface being processed relative to the cutting tool;
mechanized drives - to create clamping force. In some devices, installation and clamping of the workpiece is performed by one mechanism, called installation-clamping.
Clamping elements of fixtures
1 Purpose of clamping elements
The main purpose of clamping devices is to ensure reliable contact of the workpiece with the mounting elements and prevent its displacement relative to them and vibration during processing. By introducing additional clamping devices, the rigidity of the technological system is increased and this results in increased processing accuracy and productivity, and a reduction in surface roughness. In Fig. Figure 2 shows a diagram of the installation of workpiece 1, which, in addition to the two main clamps Q1, is secured with an additional device Q2, which imparts greater rigidity to the system. Support 2 is self-aligning.
Rice. 2 - Workpiece installation diagram
Clamping devices are used in some cases to ensure correct installation and centering of the workpiece. In this case, they perform the function of installation and clamping devices. These include self-centering chucks, collet clamps, etc.
Clamping devices are not used when processing heavy, stable workpieces, compared to the mass of which the forces arising during the cutting process are relatively small and are applied in such a way that they cannot disturb the installation of the workpiece.
Clamping devices of devices must be reliable in operation, simple in design and easy to maintain; they should not cause deformation of the workpiece being fastened and damage to its surface, and should not move the workpiece during the process of its fastening. The machine operator should spend a minimum of time and effort on securing and detaching workpieces. To simplify repairs, it is advisable to make the most wearing parts of clamping devices replaceable. When securing workpieces in multiple fixtures, they are clamped evenly; with limited movement of the clamping element (wedge, eccentric), its stroke must be greater than the tolerance for the size of the workpiece from the installation base to the place where the clamping force is applied.
Clamping devices are designed taking into account safety requirements.
The location where the clamping force is applied is selected according to the conditions of greatest rigidity and stability of the fastening and minimal deformation of the workpiece. When increasing the processing accuracy, it is necessary to comply with the conditions of a constant value of the clamping force, the direction of which must be consistent with the location of the supports.
2 Types of clamping elements
Clamping elements are mechanisms directly used to secure workpieces, or intermediate links in more complex clamping systems.
The simplest type of universal clamps are clamping screws, which are activated by keys, handles or handwheels mounted on them.
To prevent the movement of the clamped workpiece and the formation of dents on it from the screw, and also to reduce the bending of the screw when pressing on a surface not perpendicular to its axis, swinging shoes are placed on the ends of the screws (Fig. 3, a).
Combinations of screw devices with levers or wedges are called combined clamps, a type of which are screw clamps (Fig. 3, b). The clamping device allows you to move or rotate them so that you can more conveniently install the workpiece in the fixture.
Rice. 3 – Schemes of screw clamps
In Fig. Figure 4 shows some designs of quick-release clamps. For small clamping forces, a bayonet device is used (Fig. 4, a), and for significant forces, a plunger device is used (Fig. 4, b). These devices allow the clamping element to be moved a long distance from the workpiece; fastening occurs as a result of turning the rod through a certain angle. An example of a clamp with a folding stop is shown in Fig. 4, c. Having loosened the handle nut 2, remove the stop 3, rotating it around its axis. After this, the clamping rod 1 is retracted to the right at a distance h. In Fig. 4, d shows a diagram of a high-speed lever-type device. When turning the handle 4, the pin 5 slides along the bar 6 with an oblique cut, and the pin 2 slides along the workpiece 1, pressing it against the stops located below. Spherical washer 3 serves as a hinge.
Rice. 4 - Quick release clamp designs
The large amount of time and significant forces required to secure the workpieces limit the scope of application of screw clamps and, in most cases, make high-speed eccentric clamps preferable. In Fig. Figure 5 shows disk (a), cylindrical with L-shaped clamp (b) and conical floating (c) clamps.
Rice. 5 – Various clamp designs
Eccentrics are round, involute and spiral (along the Archimedes spiral). Two types of eccentrics are used in clamping devices: round and curved.
Round eccentrics (Fig. 6) are a disk or roller with an axis of rotation shifted by the eccentricity size e; the self-braking condition is ensured at the ratio D/e ? 4.
Rice. 6 – Diagram of a round eccentric
The advantage of round eccentrics is the ease of their manufacture; the main disadvantage is the inconsistency of the elevation angle a and the clamping forces Q. Curvilinear eccentrics, the working profile of which is carried out according to an involute or Archimedes spiral, have a constant elevation angle a, and, therefore, ensure constancy of the force Q when clamping any point in the profile.
The wedge mechanism is used as an intermediate link in complex clamping systems. It is simple to manufacture, easily placed in the device, and allows you to increase and change the direction of the transmitted force. At certain angles, the wedge mechanism has self-braking properties. For a single-bevel wedge (Fig. 7, a) when transmitting forces at right angles, the following dependence can be accepted (with j1=j2=j3=j, where j1...j3 are the friction angles):
P=Qtg(a±2j),
Where P is the axial force;
Q - clamping force.
Self-braking will take place at a
P = Q sin (a + 2j/cos (90°+a-b+2j).
Lever clamps are used in combination with other elementary clamps to form more complex clamping systems. Using the lever, you can change the magnitude and direction of the transmitted force, as well as simultaneously and uniformly secure the workpiece in two places.
Fig. 7 – Diagrams of a single-bevel wedge (a) and a double-bevel wedge (b)
Figure 8 shows diagrams of the action of forces in single-arm and double-arm straight and curved clamps. The equilibrium equations for these lever mechanisms are as follows:
for single-arm clamp (Fig. 8, a)
,
for direct double-arm clamp (Fig. 8, b)
,
for double-arm curved clamp (for l1
where r is the friction angle;
f is the friction coefficient.
Rice. 8 - Schemes of the action of forces in single-arm and double-arm straight and curved clamps
Centering clamping elements are used as installation elements for the external or internal surfaces of rotating bodies: collets, expanding mandrels, clamping bushings with hydroplastic, as well as membrane cartridges.
The collets are split spring sleeves, the design varieties of which are shown in Fig. 9 (a - with a tension tube; b - with a spacer tube; c - vertical type). They are made from high-carbon steels, for example U10A, and are heat treated to a hardness of HRC 58...62 in the clamping part and to a hardness of HRC 40...44 in the tail parts. Collet cone angle a=30. . .40°. At smaller angles, the collet may jam. The cone angle of the compression sleeve is made 1° less or greater than the collet cone angle. The collets ensure installation eccentricity (runout) of no more than 0.02...0.05 mm. The base surface of the workpiece should be processed according to the 9th...7th accuracy grade.
Expanding mandrels of various designs (including designs using hydroplastic) are classified as mounting and clamping devices.
Diaphragm cartridges are used for precise centering of workpieces along the outer or inner cylindrical surface. The cartridge (Fig. 10) consists of a round membrane 1 screwed to the faceplate of the machine in the form of a plate with symmetrically located protrusions-cams 2, the number of which is selected in the range of 6...12. A pneumatic cylinder rod 4 passes inside the spindle. When the pneumatics are turned on, the membrane bends, pushing the cams apart. When the rod moves back, the membrane, trying to return to its original position, compresses the workpiece 3 with its cams.
Rice. 10 – Membrane cartridge diagram
A rack and pinion clamp (Fig. 11) consists of a rack 3, a gear 5 sitting on a shaft 4, and a handle lever 6. By rotating the handle counterclockwise, lower the rack and clamp 2 to secure the workpiece 1. The clamping force Q depends on the value force P applied to the handle. The device is equipped with a lock, which, jamming the system, prevents the reverse rotation of the wheel. The most common types of locks are:
Rice. 11 - Rack and pinion clamp
The roller lock (Fig. 12, a) consists of a drive ring 3 with a cutout for roller 1, which is in contact with the cut plane of the gear shaft 2. Drive ring 3 is attached to the handle of the clamping device. By rotating the handle in the direction of the arrow, rotation is transmitted to the gear shaft through roller 1. The roller is wedged between the bore surface of the housing 4 and the cut plane of the roller 2 and prevents reverse rotation.
Rice. 12 – Schemes of various lock designs
A roller lock with direct transmission of torque from the driver to the roller is shown in Fig. 12, b. Rotation from the handle through the leash is transmitted directly to the 6th wheel shaft. Roller 3 is pressed through pin 4 by a weak spring 5. Since the gaps in the places where the roller touches ring 1 and shaft 6 are selected, the system instantly jams when the force is removed from handle 2. By turning the handle in the opposite direction, the roller wedges and rotates the shaft clockwise .
The conical lock (Fig. 12, c) has a conical sleeve 1 and a shaft 2 with a cone 3 and a handle 4. The spiral teeth on the middle neck of the shaft are engaged with the rack 5. The latter is connected to the actuator clamping mechanism. At a tooth angle of 45°, the axial force on shaft 2 is equal (without taking into account friction) to the clamping force.
An eccentric lock (Fig. 12, d) consists of a wheel shaft 2 on which an eccentric 3 is wedged. The shaft is driven into rotation by a ring 1 attached to the lock handle; the ring rotates in the housing bore 4, the axis of which is displaced from the shaft axis by a distance e. When the handle rotates in reverse, transmission to the shaft occurs through pin 5. During the fastening process, ring 1 is wedged between the eccentric and the housing.
Combined clamping devices are a combination of elementary clamps of various types. They are used to increase the clamping force and reduce the dimensions of the device, as well as to create greater ease of control. Combination clamping devices can also provide simultaneous clamping of a workpiece in several places. Types of combined clamps are shown in Fig. 13.
The combination of a curved lever and a screw (Fig. 13, a) allows you to simultaneously secure the workpiece in two places, uniformly increasing the clamping forces to a given value. A conventional rotary clamp (Fig. 13, b) is a combination of lever and screw clamps. The swing axis of lever 2 is aligned with the center of the spherical surface of washer 1, which relieves pin 3 from bending forces. Shown in Fig. 13, in an eccentric clamp, is an example of a high-speed combination clamp. At a certain lever arm ratio, the clamping force or stroke of the clamping end of the lever can be increased.
Rice. 13 - Types of combined clamps
In Fig. 13, d shows a device for securing a cylindrical workpiece in a prism using a hinge lever, and in Fig. 13, d - diagram of a high-speed combined clamp (lever and eccentric), providing lateral and vertical pressing of the workpiece to the supports of the device, since the clamping force is applied at an angle. A similar condition is provided by the device shown in Fig. 13, e.
Hinge-lever clamps (Fig. 13, g, h, i) are examples of high-speed clamping devices actuated by turning the handle. To prevent self-release, the handle is moved through the dead position to stop 2. The clamping force depends on the deformation of the system and its rigidity. The desired deformation of the system is set by adjusting pressure screw 1. However, the presence of a tolerance for size H (Fig. 13, g) does not ensure a constant clamping force for all workpieces of a given batch.
Combined clamping devices are operated manually or by power units.
Clamping mechanisms for multiple fixtures must provide equal clamping force at all positions. The simplest multi-place device is a mandrel on which a package of blanks (rings, disks) is installed, secured along the end planes with one nut (sequential clamping force transmission scheme). In Fig. 14a shows an example of a clamping device operating on the principle of parallel distribution of clamping force.
If it is necessary to ensure the concentricity of the base and workpiece surfaces and to prevent deformation of the workpiece, elastic clamping devices are used, where the clamping force is uniformly transmitted by means of a filler or other intermediate body to the clamping element of the device (within the limits of elastic deformations).
Rice. 14 - Clamping mechanisms for multiple devices
Conventional springs, rubber or hydroplastic are used as an intermediate body. A parallel clamping device using hydroplastic is shown in Fig. 14, b. In Fig. 14, c shows a device of mixed (parallel-series) action.
On continuous machines (drum-milling, special multi-spindle drilling), workpieces are installed and removed without interrupting the feed movement. If the auxiliary time overlaps with the machine time, then various types of clamping devices can be used to secure the workpieces.
In order to mechanize production processes, it is advisable to use automated clamping devices (continuous action), driven by the feed mechanism of the machine. In Fig. 15, a shows a diagram of a device with a flexible closed element 1 (cable, chain) for securing cylindrical workpieces 2 on a drum milling machine when processing end surfaces, and in Fig. 15, b - diagram of a device for securing piston blanks on a multi-spindle horizontal drilling machine. In both devices, operators only install and remove the workpiece, and the workpiece is secured automatically.
Rice. 15 - Automatic clamping devices
An effective clamping device for holding workpieces made of thin sheet material during finishing or finishing is a vacuum clamp. The clamping force is determined by the formula
Q=Ap,
where A is the active area of the device cavity limited by the seal;
p=10 5 Pa - the difference between atmospheric pressure and the pressure in the cavity of the device from which air is removed.
Electromagnetic clamping devices are used to secure workpieces made of steel and cast iron with a flat base surface. Clamping devices are usually made in the form of plates and chucks, the design of which takes as initial data the dimensions and configuration of the workpiece in plan, its thickness, material and the necessary holding force. The holding force of the electromagnetic device largely depends on the thickness of the workpiece; at small thicknesses, not all the magnetic flux passes through the cross section of the part, and some of the magnetic flux lines are scattered into the surrounding space. Parts processed on electromagnetic plates or chucks acquire residual magnetic properties - they are demagnetized by passing them through a solenoid powered by alternating current.
In magnetic clamping devices, the main elements are permanent magnets, isolated from one another by non-magnetic gaskets and fastened into a common block, and the workpiece is an armature through which the magnetic power flow is closed. To detach the finished part, the block is shifted using an eccentric or crank mechanism, while the magnetic power flow is closed on the device body, bypassing the part.
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LECTURE 3
3.1. Purpose of clamping devices
The main purpose of fixture clamping devices is to ensure reliable contact (continuity) of the workpiece or assembled part with the installation elements, preventing its displacement during processing or assembly.
The clamping mechanism creates a force to secure the workpiece, determined from the condition of equilibrium of all forces applied to it
During machining the workpiece is subject to:
1) forces and cutting moments
2) volumetric forces - workpiece gravity, centrifugal and inertial forces.
3) forces acting at the points of contact of the workpiece with the device - support reaction force and friction force
4) secondary forces, which include the forces that arise when the cutting tool (drills, taps, reamers) is removed from the workpiece.
During assembly, the assembled parts are subject to assembly forces and reaction forces that arise at the points of contact of the mating surfaces.
The following requirements apply to clamping devices::
1) when clamping, the position of the workpiece achieved by basing should not be disturbed. This is satisfied by a rational choice of the direction and places of application of the clamping forces;
2) the clamp should not cause deformation of the workpieces fixed in the fixture or damage (crushing) of their surfaces;
3) the clamping force should be the minimum necessary, but sufficient to ensure a fixed position of the workpiece relative to the installation elements of the devices during processing;
4) the clamping force must be constant throughout the entire technological operation; the clamping force must be adjustable;
5) clamping and detaching the workpiece must be done with minimal effort and worker time. When using manual clamps, the force should not exceed 147 N; Average duration of fastening: in a three-jaw chuck (with a key) - 4 s; screw clamp (key) - 4.5…5 s; steering wheel - 2.5…3 s; turning the handle of the pneumatic and hydraulic crane - 1.5 s; by pressing a button - less than 1 s.
6) the clamping mechanism must be simple in design, compact, as convenient and safe to use as possible. To do this, it must have minimum overall dimensions and contain a minimum number of removable parts; The clamping mechanism control device should be located on the worker's side.
The need to use clamping devices is eliminated in three cases.
1) the workpiece has a large mass, in comparison with which the cutting forces are small.
2) the forces arising during processing are directed in such a way that they cannot disturb the position of the workpiece achieved during basing.
3) the workpiece installed in the fixture is deprived of all degrees of freedom. For example, when drilling a hole in a rectangular strip placed in a box jig.
3.2. Classification of clamping devices
The designs of clamping devices consist of three main parts: a contact element (CE), a drive (P) and a power mechanism (SM).
The contact elements serve to directly transfer the clamping force to the workpiece. Their design allows forces to be dispersed, preventing the workpiece surfaces from being crushed.
The drive serves to convert a certain type of energy into initial force R and transmitted to the power mechanism.
A force mechanism is required to convert the resulting initial clamping force R and in clamping force R z. The transformation is carried out mechanically, i.e. according to the laws of theoretical mechanics.
In accordance with the presence or absence of these components in the fixture, clamping devices of fixtures are divided into three groups.
TO first The group includes clamping devices (Fig. 3.1a), which include all of the main parts listed: a power mechanism and a drive, which ensures the movement of the contact element and creates the initial force R and, converted by the power mechanism into clamping force R z .
In second group (Fig. 3.1b) includes clamping devices consisting only of a power mechanism and a contact element, which is actuated directly by the worker applying the initial force R and on the shoulder l. These devices are sometimes called manual clamping devices (one-off and small-scale production).
TO third This group includes clamping devices that do not have a power mechanism, and the drives used can only conditionally be called drives, since they do not cause movement of the elements of the clamping device and only create a clamping force R z, which in these devices is the resultant of a uniformly distributed load q, directly acting on the workpiece and created either as a result of atmospheric pressure or through a magnetic force flux. This group includes vacuum and magnetic devices (Fig. 3.1c). Used in all types of production.
Rice. 3.1. Clamping mechanism diagrams
An elementary clamping mechanism is a part of a clamping device consisting of a contact element and a power mechanism.
Clamping elements are called: screws, eccentrics, clamps, vice jaws, wedges, plungers, clamps, strips. They are intermediate links in complex clamping systems.
In table 2 shows the classification of elementary clamping mechanisms.
table 2
Classification of elementary clamping mechanisms
| ELEMENTARY CLAMPING MECHANISMS | SIMPLE | SCREW | Clamping screws |
| With split washer or strip | |||
| Bayonet or plunger | |||
| ECCENTRIC | Round eccentrics | ||
| Curvilinear involute | |||
| Curvilinear according to the Archimedes spiral | |||
| WEDGE | With a flat single bevel wedge | ||
| With support roller and wedge | |||
| With double bevel wedge | |||
| LEVER | Single-arm | ||
| Double-armed | |||
| Curved double arms | |||
| COMBINED | CENTERING CLAMPING ELEMENTS | Collets | |
| Expanding mandrels | |||
| Clamping sleeves with hydroplastic | |||
| Mandrels and chucks with leaf springs | |||
| Diaphragm cartridges | |||
| RACK AND LEVER CLAMPS | With roller clamp and lock | ||
| With conical locking device | |||
| With eccentric locking device | |||
| COMBINED CLAMPING DEVICES | Lever and screw combination | ||
| Combination of lever and eccentric | |||
| Articulating lever mechanism | |||
| SPECIAL | Multi-place and continuous action |
Based on the source of drive energy (here we are not talking about the type of energy, but rather the location of the source), drives are divided into manual, mechanized and automated. Manual clamping mechanisms are operated by the muscular force of the worker. Motorized clamping mechanisms are powered by a pneumatic or hydraulic drive. Automated devices move from moving machine components (spindle, slide or chucks with jaws). In the latter case, the workpiece is clamped and the processed part is released without the participation of a worker.
3.3. Clamping elements
3.3.1. Screw terminals
Screw clamps are used in devices with manual fastening of the workpiece, in mechanized devices, as well as on automatic lines when using satellite devices. They are simple, compact and reliable in operation.
Rice. 3.2. Screw terminals:
a – with a spherical end; b – with a flat end; c – with a shoe. Legend: R and- force applied at the end of the handle; R z- clamping force; W– ground reaction force; l- handle length; d- diameter of the screw clamp.
Calculation of screw EZM. With a known force P 3, the nominal diameter of the screw is calculated
where d is the screw diameter, mm; R 3- fastening force, N; σ р- tensile (compressive) stress of the screw material, MPa
The purpose of clamping devices is to ensure reliable contact of the workpiece with the mounting elements and to prevent its displacement and vibration during processing. Figure 7.6 shows some types of clamping devices.
Requirements for clamping elements:
Reliability in operation;
Simplicity of design;
Ease of maintenance;
Should not cause deformation of workpieces and damage to their surfaces;
The workpiece should not be moved during its fastening from the installation elements;
Fastening and detaching workpieces should be done with minimal labor and time;
The clamping elements must be wear-resistant and, if possible, replaceable.
Types of clamping elements:
Clamping screws, which are rotated with keys, handles or handwheels (see Fig. 7.6)
Fig.7.6 Types of clamps:
a – clamping screw; b – screw clamp
Fast acting clamps shown in fig. 7.7.
Fig.7.7. Types of quick release clamps:
a – with a split washer; b – with a plunger device; c – with folding stop; g – with a lever device
Eccentric clamps, which are round, involute and spiral (along the Archimedes spiral) (Fig. 7.8).
Fig.7.8. Types of eccentric clamps:
a – disk; b – cylindrical with an L-shaped clamp; g – conical floating.
Wedge clamps– the wedging effect is used and is used as an intermediate link in complex clamping systems. At certain angles, the wedge mechanism has the property of self-braking. In Fig. Figure 7.9 shows the calculated diagram of the action of forces in the wedge mechanism.
Rice. 7.9. Calculation diagram of forces in the wedge mechanism:
a- single-sided; b – double-skewed
Lever Clamps used in combination with other clamps to form more complex clamping systems. Using the lever, you can change both the magnitude and direction of the clamping force, as well as simultaneously and uniformly secure the workpiece in two places. In Fig. Figure 7.10 shows a diagram of the action of forces in lever clamps.
Rice. 7.10. Diagram of the action of forces in lever clamps.
Collets They are split spring sleeves, the varieties of which are shown in Fig. 7.11.
Rice. 7. 11. Types of collet clamps:
a – with a tension tube; b – with a spacer tube; c – vertical type
Collets ensure concentricity of workpiece installation within 0.02...0.05 mm. The base surface of the workpiece for collet clamps should be processed according to accuracy classes 2…3. Collets are made of high-carbon steels of type U10A with subsequent heat treatment to a hardness of HRC 58...62. Collet cone angle d = 30…40 0 . At smaller angles, the collet may jam.
Expanding mandrels, the types of which are shown in Fig. 7.4.
Roller lock(Fig. 7.12)
Rice. 7.12. Types of roller locks
Combination clamps– a combination of elementary clamps of various types. In Fig. 7.13 shows some types of such clamping devices.
Rice. 7.13. Types of combined clamping devices.
Combination clamping devices are operated manually or by power devices.
Guide elements of devices
When performing some machining operations (drilling, boring), the rigidity of the cutting tool and the technological system as a whole is insufficient. To eliminate elastic pressing of the tool relative to the workpiece, guide elements are used (guide bushings when boring and drilling, copiers when processing shaped surfaces, etc. (see Fig. 7.14).
Fig.7.14. Types of conductor bushings:
a – constant; b – replaceable; c – quick-change
Guide bushings are made of steel grade U10A or 20X, hardened to a hardness of HRC 60...65.
Guide elements of devices - copiers - are used when processing shaped surfaces of complex profiles, the task of which is to guide the cutting tool along the workpiece surface to be processed to obtain a given accuracy of the trajectory of their movement.
3.1. Selecting the location of application of clamping forces, type and number of clamping elements
When securing a workpiece in a fixture, the following basic rules must be observed:
· the position of the workpiece achieved during its basing should not be disturbed;
· the fastening must be reliable so that the position of the workpiece remains unchanged during processing;
· the crumpling of the workpiece surfaces that occurs during fastening, as well as its deformation, must be minimal and within acceptable limits.
· to ensure contact of the workpiece with the support element and eliminate its possible shift during fastening, the clamping force should be directed perpendicular to the surface of the support element. In some cases, the clamping force can be directed so that the workpiece is simultaneously pressed against the surfaces of two supporting elements;
· in order to eliminate the deformation of the workpiece during fastening, the point of application of the clamping force must be selected so that the line of its action intersects the supporting surface of the supporting element. Only when clamping particularly rigid workpieces can the line of action of the clamping force be allowed to pass between the supporting elements.
3.2. Determining the number of clamping force points
The number of points of application of clamping forces is determined specifically for each case of workpiece clamping. To reduce the compression of the surfaces of the workpiece during fastening, it is necessary to reduce the specific pressure at the points of contact of the clamping device with the workpiece by dispersing the clamping force.
This is achieved by using contact elements of appropriate design in clamping devices, which make it possible to distribute the clamping force equally between two or three points, and sometimes even disperse it over a certain extended surface. TO Number of clamping points largely depends on the type of workpiece, processing method, and direction of the cutting force. For decreasing vibration and deformation of the workpiece under the influence of the cutting force, the rigidity of the workpiece-device system should be increased by increasing the number of places where the workpiece is clamped and bringing them closer to the machined surface.
3.3. Determining the type of clamping elements
Clamping elements include screws, eccentrics, clamps, vice jaws, wedges, plungers, clamps, and strips.
They are intermediate links in complex clamping systems.
3.3.1. Screw terminals
Screw terminals used in devices with manual fastening of the workpiece, in mechanized devices, as well as on automatic lines when using satellite devices. They are simple, compact and reliable in operation.
Rice. 3.1. Screw clamps: a – with a spherical end; b – with a flat end; c – with a shoe.
The screws can be with a spherical end (fifth), flat, or with a shoe that prevents damage to the surface.
When calculating ball heel screws, only friction in the thread is taken into account.
Where: L- handle length, mm; - average thread radius, mm; - thread lead angle.
Where: S– thread pitch, mm; – reduced friction angle.
where: Pu 150 N.
Self-braking condition: .
For standard metric threads, therefore all mechanisms with metric threads are self-locking.
When calculating screws with a flat heel, friction at the end of the screw is taken into account.
For the ring heel:
where: D – outer diameter of the supporting end, mm; d – internal diameter of the supporting end, mm; – friction coefficient.
With flat ends:
For shoe screw:
Material: steel 35 or steel 45 with a hardness of HRC 30-35 and thread accuracy of the third class.
3.3.2. Wedge clamps
The wedge is used in the following design options:
1. Flat single-bevel wedge.
2. Double bevel wedge.
3. Round wedge.
Rice. 3.2. Flat single bevel wedge.
Rice. 3.3. Double bevel wedge.
Rice. 3.4. Round wedge.
4) a crank wedge in the form of an eccentric or flat cam with a working profile outlined along an Archimedean spiral;
Rice. 3.5. Crank wedge: a – in the form of an eccentric; b) – in the shape of a flat cam.
5) a screw wedge in the form of an end cam. Here, the single-bevel wedge is, as it were, rolled into a cylinder: the base of the wedge forms a support, and its inclined plane forms the helical profile of the cam;
6) self-centering wedge mechanisms (chucks, mandrels) do not use systems of three or more wedges.
3.3.2.1. Wedge self-braking condition
Rice. 3.6. Condition of self-braking of the wedge.
where: - friction angle.
Where: – friction coefficient;
For a wedge with friction only on an inclined surface, the self-braking condition is:
with friction on two surfaces:
We have: ; or: ; .
Then: self-braking condition for a wedge with friction on two surfaces:
for a wedge with friction only on an inclined surface:
With friction on two surfaces:
With friction only on an inclined surface:
3.3.3.Eccentric clamps
Rice. 3.7. Schemes for calculating eccentrics.
Such clamps are fast-acting, but develop less force than screw clamps. They have self-braking properties. The main disadvantage: they cannot work reliably with significant variations in size between the mounting and clamping surfaces of the workpieces.
where: ( - the average value of the radius drawn from the center of rotation of the eccentric to point A of the clamp, mm; ( - the average angle of elevation of the eccentric at the clamping point; (, (1 - sliding friction angles at point A of the clamp and on the eccentric axis.
For calculations we accept:
At l 2D calculation can be done using the formula:
Condition for eccentric self-braking:
Usually accepted.
Material: steel 20X, carburized to a depth of 0.8–1.2 mm and hardened to HRC 50…60.
3.3.4. Collets
Collets are spring sleeves. They are used to install workpieces on external and internal cylindrical surfaces.
Where: Pz– workpiece fixing force; Q – compression force of the collet blades; - friction angle between the collet and the bushing.
Rice. 3.8. Collet.
3.3.5. Devices for clamping parts such as bodies of rotation
In addition to collets, for clamping parts with a cylindrical surface, expanding mandrels, clamping bushings with hydroplastic, mandrels and chucks with disc springs, membrane chucks and others are used.
Cantilever and center mandrels are used for installation with a central base hole of bushings, rings, gears processed on multi-cutter grinding and other machines.
When processing a batch of such parts, it is necessary to obtain high concentricity of the external and internal surfaces and a specified perpendicularity of the ends to the axis of the part.
Depending on the method of installation and centering of the workpieces, cantilever and center mandrels can be divided into the following types: 1) rigid (smooth) for installing parts with a gap or interference; 2) expanding collets; 3) wedge (plunger, ball); 4) with disc springs; 5) self-clamping (cam, roller); 6) with a centering elastic bushing.
Rice. 3.9. Mandrel designs: A - smooth mandrel; b - mandrel with split sleeve.
In Fig. 3.9, A shows a smooth mandrel 2, on the cylindrical part of which the workpiece 3 is installed . Traction 6 , fixed on the rod of the pneumatic cylinder, when the piston with the rod moves to the left, the head 5 presses on the quick-change washer 4 and clamps the part 3 on a smooth mandrel 2 . The mandrel with its conical part 1 is inserted into the cone of the machine spindle. When clamping the workpiece on the mandrel, the axial force Q on the rod of the mechanized drive causes 4 between the ends of the washer , shoulder of the mandrel and the workpiece 3 moment from the friction force, greater than the moment M cutting from the cutting force P z. Dependence between moments:
where does the force on the rod of the mechanized drive come from:
According to the refined formula:
Where: - safety factor; P z - vertical component of cutting force, N (kgf); D- outer diameter of the surface of the workpiece, mm; D 1 - outer diameter of quick-change washer, mm; d- diameter of the cylindrical mounting part of the mandrel, mm; f= 0.1 - 0.15- clutch friction coefficient.
In Fig. 3.9, b shows a mandrel 2 with a split sleeve 6, on which the workpiece 3 is installed and clamped. The conical part 1 of the mandrel 2 is inserted into the cone of the machine spindle. The part is clamped and released on the mandrel using a mechanized drive. When compressed air is supplied to the right cavity of the pneumatic cylinder, the piston, rod and rod 7 move to the left and the head 5 of the rod with washer 4 moves the split sleeve 6 along the cone of the mandrel until it clamps the part on the mandrel. When compressed air is supplied to the left cavity of the pneumatic cylinder, the piston, rod; and the rod move to the right, the head 5 with the washer 4 move away from the sleeve 6 and the part is unclenched.
Fig.3.10. Cantilever mandrel with disc springs (A) and disc spring (b).
The torque from the vertical cutting force P z must be less than the moment from the friction forces on the cylindrical surface of the split sleeve 6 mandrels. Axial force on the rod of a motorized drive (see Fig. 3.9, b).
where: - half the angle of the mandrel cone, degrees; - friction angle on the contact surface of the mandrel with the split sleeve, deg; f=0.15-0.2- friction coefficient.
Mandrels and chucks with disc springs are used for centering and clamping along the inner or outer cylindrical surface of workpieces. In Fig. 3.10, a, b a cantilever mandrel with disc springs and a disc spring are shown respectively. The mandrel consists of a body 7, a thrust ring 2, a package of disc springs 6, a pressure sleeve 3 and a rod 1 connected to the pneumatic cylinder rod. The mandrel is used to install and secure part 5 along the inner cylindrical surface. When the piston with the rod and rod 1 moves to the left, the latter, with the head 4 and bushing 3, presses on the disc springs 6. The springs are straightened, their outer diameter increases and the inner diameter decreases, the workpiece 5 is centered and clamped.
The size of the mounting surfaces of the springs during compression can vary depending on their size by 0.1 - 0.4 mm. Consequently, the base cylindrical surface of the workpiece must have an accuracy of 2 - 3 classes.
A disc spring with slots (Fig. 3.10, b) can be considered as a set of two-link lever-joint mechanisms of double action, expanded by axial force. Having determined the torque M res on cutting force P z and choosing the safety factor TO, friction coefficient f and radius R mounting surface of the spring disc surface, we obtain the equality:
From the equality we determine the total radial clamping force acting on the mounting surface of the workpiece:
Axial force on the motorized actuator rod for disc springs:
with radial slots
without radial slots
where: - angle of inclination of the disc spring when clamping the part, degrees; K=1.5 - 2.2- safety factor; M res - torque from cutting force P z,Nm (kgf-cm); f=0.1- 0.12- coefficient of friction between the mounting surface of the disc springs and the base surface of the workpiece; R- radius of the mounting surface of the disc spring, mm; P z- vertical component of cutting force, N (kgf); R 1- radius of the machined surface of the part, mm.
Chucks and mandrels with self-centering thin-walled bushings filled with hydroplastic are used for installation on the outer or inner surface of parts processed on lathes and other machines.
On devices with a thin-walled bushing, the workpieces with their outer or inner surfaces are mounted on the cylindrical surface of the bushing. When the bushing is expanded with hydroplastic, the parts are centered and clamped.
The shape and dimensions of the thin-walled bushing must ensure sufficient deformation for reliable clamping of the part on the bushing when processing the part on the machine.
When designing chucks and mandrels with thin-walled bushings with hydroplastic, the following is calculated:
1. main dimensions of thin-walled bushings;
2. dimensions of pressure screws and plungers for devices with manual clamping;
3. plunger sizes, cylinder diameter and piston stroke for power-driven devices.
Rice. 3.11. Thin-walled bushing.
The initial data for calculating thin-walled bushings are the diameter D d holes or workpiece neck diameter and length l d holes or necks of the workpiece.
To calculate a thin-walled self-centering bushing (Fig. 3.11), we will use the following notation: D- diameter of the mounting surface of the centering sleeve 2, mm; h- thickness of the thin-walled part of the bushing, mm; T - length of the bushing support belts, mm; t- thickness of the bushing support belts, mm; - the greatest diametrical elastic deformation of the bushing (increase or decrease in diameter in its middle part) mm; S max- maximum gap between the mounting surface of the bushing and the base surface of the workpiece 1 in a free state, mm; l to- length of the contact section of the elastic bushing with the mounting surface of the workpiece after the bushing has been unclamped, mm; L- length of the thin-walled part of the bushing, mm; l d- length of the workpiece, mm; D d- diameter of the base surface of the workpiece, mm; d- hole diameter of the bushing support bands, mm; R - hydraulic plastic pressure required to deform a thin-walled bushing, MPa (kgf/cm2); r 1 - radius of curvature of the sleeve, mm; M res =P z r - permissible torque arising from the cutting force, Nm (kgf-cm); Pz- cutting force, N (kgf); r is the moment arm of the cutting force.
In Fig. Figure 3.12 shows a cantilever mandrel with a thin-walled sleeve and hydroplastic. The workpiece 4 is installed with the base hole on the outer surface of the thin-walled bushing 5. When compressed air is supplied to the rod cavity of the pneumatic cylinder, the piston with the rod moves in the pneumatic cylinder to the left and the rod through the rod 6 and the lever 1 moves the plunger 2, which presses on the hydraulic plastic 3 . The hydroplastic evenly presses on the inner surface of the sleeve 5, the sleeve expands; The outer diameter of the sleeve increases, and it centers and secures the workpiece 4.
Rice. 3.12. Cantilever mandrel with hydroplastic.
Diaphragm chucks are used for precise centering and clamping of parts processed on lathes and grinding machines. In membrane chucks, the parts to be processed are mounted on the outer or inner surface. The base surfaces of the parts must be processed according to the 2nd accuracy class. Diaphragm cartridges provide a centering accuracy of 0.004-0.007 mm.
Membranes- these are thin metal disks with or without horns (ring membranes). Depending on the effect on the membrane of the rod of a mechanized drive - pulling or pushing action - membrane cartridges are divided into expanding and clamping.
In an expanding membrane horn chuck, when installing the annular part, the membrane with horns and the drive rod bends to the left towards the machine spindle. In this case, the membrane horns with clamping screws installed at the ends of the horns converge towards the axis of the cartridge, and the ring being processed is installed through the central hole in the cartridge.
When the pressure on the membrane stops under the action of elastic forces, it straightens, its horns with screws diverge from the axis of the cartridge and clamp the ring being processed along the inner surface. In a clamping membrane open-end chuck, when installing a ring part on the outer surface, the membrane is bent by the drive rod to the right of the machine spindle. In this case, the membrane horns diverge from the axis of the chuck and the workpiece is unclenched. Then the next ring is installed, the pressure on the membrane stops, it straightens and clamps the ring being processed with its horns and screws. Clamping membrane horn chucks with a power drive are manufactured according to MN 5523-64 and MN 5524-64 and with a manual drive according to MN 5523-64.
Diaphragm cartridges come in carob and cup (ring) types, they are made from steel 65G, ZOKHGS, hardened to a hardness of HRC 40-50. The main dimensions of the carob and cup membranes are normalized.
In Fig. 3.13, a, b shows the design diagram of the membrane-horn chuck 1 . A chuck pneumatic drive is installed at the rear end of the machine spindle. When compressed air is supplied to the left cavity of the pneumatic cylinder, the piston with rod and rod 2 moves to the right. At the same time, rod 2, pressing on the horn membrane 3, bends it, the cams (horns) 4 diverge, and the part 5 opens (Fig. 3.13, b). When compressed air is supplied to the right cavity of the pneumatic cylinder, its piston with rod and rod 2 moves to the left and moves away from membrane 3. The membrane, under the action of internal elastic forces, straightens, the cams 4 of the membrane converge and clamp part 5 along the cylindrical surface (Fig. 3.13, a).
Rice. 3.13. Scheme of a membrane-horn chuck
Basic data for calculating the cartridge (Fig. 3.13, A) with horn-like membrane: cutting moment M res, seeking to rotate the workpiece 5 in the cams 4 of the chuck; diameter d = 2b base outer surface of the workpiece; distance l from the middle of the membrane 3 to the middle of the cams 4. In Fig. 3.13, V a design diagram of a loaded membrane is given. A round membrane rigidly fixed along the outer surface is loaded with a uniformly distributed bending moment M I, applied along a concentric circle of a membrane of radius b base surface of the workpiece. This circuit is the result of superposition of two circuits shown in Fig. 3.13, g, d, and M I = M 1 + M 3. M res
Powers P z cause a moment that bends the membrane (see Fig. 3.13, V).
2. With a large number of chuck jaws, the moment M p can be considered to act uniformly around the circumference of the membrane radius b and causing it to bend:
3. Radius A the outer surface of the membrane (for design reasons) are specified.
4. Attitude T radius A membranes to radius b mounting surface of the part: a/b = t.
5. Moments M 1 And M 3 in fractions of M and (M and = 1) found depending on m= a/b according to the following data (Table 3.1):
Table 3.1
| m=a/b | 1,25 | 1,5 | 1,75 | 2,0 | 2,25 | 2,5 | 2,75 | 3,0 |
| M 1 | 0,785 | 0,645 | 0,56 | 0,51 | 0,48 | 0,455 | 0,44 | 0,42 |
| M 3 | 0,215 | 0,355 | 0,44 | 0,49 | 0,52 | 0,545 | 0,56 | 0,58 |
6. Angle (rad) of the cams opening when securing a part with the smallest maximum size:
7. Cylindrical stiffness of the membrane [N/m (kgf/cm)]:
where: MPa - modulus of elasticity (kgf/cm 2); =0.3.
8. Angle of greatest expansion of cams (rad):
9. The force on the rod of the motorized drive of the chuck, necessary to deflect the membrane and spread the cams when expanding the part, to the maximum angle:
When choosing the point of application and direction of the clamping force, the following must be observed: to ensure contact of the workpiece with the support element and eliminate its possible shift during fastening, the clamping force should be directed perpendicular to the surface of the support element; In order to eliminate the deformation of the workpiece during fastening, the point of application of the clamping force must be selected so that the line of its action intersects the supporting surface of the mounting element.
The number of points of application of clamping forces is determined specifically for each case of clamping a workpiece, depending on the type of workpiece, processing method, and direction of the cutting force. To reduce vibration and deformation of the workpiece under the influence of cutting forces, the rigidity of the workpiece-fixture system should be increased by increasing the number of workpiece clamping points by introducing auxiliary supports.
Clamping elements include screws, eccentrics, clamps, vice jaws, wedges, plungers, and strips. They are intermediate links in complex clamping systems. The shape of the working surface of the clamping elements in contact with the workpiece is basically the same as that of the mounting elements. Graphically, the clamping elements are designated according to table. 3.2.
Table 3.2 Graphic designation of clamping elements