Clamping elements of devices. Special clamping fixtures Clamping elements
Clamping elements must ensure reliable contact of the workpiece with the installation elements and prevent its disruption under the influence of forces arising during processing, fast and uniform clamping of all parts and not cause deformation and damage to the surfaces of the fastened parts.
Clamping elements are divided into:
By design - for screw, wedge, eccentric, lever, lever-hinge (combined clamping elements are also used - screw-lever, eccentric-lever, etc.).
According to the degree of mechanization - manual and mechanized with hydraulic, pneumatic, electric or vacuum drive.
The clamping bellows can be automated.
Screw terminals used for direct clamping or clamping through clamping bars, or holding one or more parts. Their disadvantage is that that fastening and unfastening the part requires a lot of time.
Eccentric and wedge clamps, just like screw ones, they allow you to fasten the part directly or through clamping bars and levers.
Circular eccentric clamps are the most widely used. An eccentric clamp is a special case of a wedge clamp, and to ensure self-braking, the wedge angle should not exceed 6-8 degrees. Eccentric clamps are made from high carbon or case hardened steel and heat treated to a hardness of HRC55-60. Eccentric clamps are fast-acting clamps because... required for clamping turn the eccentric at an angle of 60-120 degrees.
Lever-hinged elements used as drive and reinforcing links of clamping mechanisms. By design, they are divided into single-lever, double-lever (single- and double-acting - self-centering and multi-link). Lever mechanisms do not have self-braking properties. Most simple example Lever-hinged bellows are clamping bars of devices, levers of pneumatic cartridges, etc.
Spring clamps used for clamping products with little effort that occurs when the spring is compressed.
To create constant and high clamping forces, reduce clamping times, implement remote control clamps are used pneumatic, hydraulic and other drives.
The most common pneumatic drives are piston pneumatic cylinders and pneumatic chambers with an elastic diaphragm, stationary, rotating and swinging.
Pneumatic actuators are driven compressed air under a pressure of 4-6 kg/cm² If it is necessary to use small-sized drives and create large clamping forces, hydraulic drives are used, operating pressure oils in which reaches 80 kg/cm².
The force on the rod of a pneumatic or hydraulic cylinder is equal to the product of the working area of the piston in square cm times the air pressure or working fluid. In this case, it is necessary to take into account friction losses between the piston and the cylinder walls, between the rod and guide bushings and seals.
Electromagnetic clamping devices They are made in the form of slabs and faceplates. They are designed for holding steel and cast iron workpieces with a flat base surface for grinding or fine turning.
Magnetic clamping devices can be made in the form of prisms that serve to secure cylindrical blanks. There are plates that use ferrites as permanent magnets. These plates are characterized by high holding force and smaller distance between poles.
In serial and small-scale production, equipment is designed using universal clamping mechanisms (CLM) or special single-link ones with manual drive. In cases where large workpiece clamping forces are required, it is advisable to use mechanized clamps.
In mechanized production, clamping mechanisms are used in which the clamps are automatically retracted to the side. This ensures free access to the installation elements for cleaning them from chips and ease of reinstallation of workpieces.
Lever single-link mechanisms controlled by a hydraulic or pneumatic drive are used when securing, as a rule, one body or large workpiece. In such cases, the clamp is moved or turned manually. However, it is better to use an additional link to remove the stick from the workpiece loading area.
L-type clamping devices are used more often to secure body workpieces from above. To rotate the clamp during fastening, a screw groove with a straight section is provided.
Rice. 3.1.
Combined clamping mechanisms are used to secure a wide range of workpieces: housings, flanges, rings, shafts, strips, etc.
Let's look at some standard designs clamping mechanisms.
Lever clamping mechanisms are distinguished by their simplicity of design (Fig. 3.1), a significant gain in force (or in movement), constancy of the clamping force, and the ability to secure the workpiece in hard to reach place, ease of use, reliability.
Lever mechanisms are used in the form of clamps (clamping bars) or as amplifiers of power drives. To facilitate the installation of workpieces, lever mechanisms are rotary, folding and movable. According to their design (Fig. 3.2), they can be rectilinear and retractable (Fig. 3.2, A) and rotary (Fig. 3.2, b), folding (Fig. 3.2, V) with a swinging support, curved (Fig. 3.2, G) and combined (Fig. 3.2, 
Rice. 3.2.
In Fig. 3.3 shows universal lever CMs with a manual screw drive, used in individual and small-scale production. They are simple in design and reliable.
Support screw 1 installed in the T-shaped groove of the table and secured with a nut 5. Clamp position 3 The height is adjusted using screw 7 with a support foot 6, and spring 4. The force of fastening to the workpiece is transmitted from the nut 2 through the clamp 3 (Fig. 3.3, A).
In ZM (Fig. 3.3, b) workpiece 5 is secured with a clamp 4, and the workpiece 6 clamping 7. The fastening force is transmitted from the screw 9 for sticking 4 through the plunger 2 and adjusting screw /; to the clamp 7 - through the nut fixed in it. When changing the thickness of the workpieces, the position of the axes 3, 8 easy to adjust.
Rice. 3.3.
In ZM (Fig. 3.3, V) frame 4 clamping mechanism is secured to the table with a nut 3 via bushing 5 With threaded hole. Curved Clamp Position 1 but the height is adjusted with a support 6 and screw 7. Clamp 1 there is play between the conical washer installed iod the head of screw 7, and the washer, which is located above the locking ring 2.
The design has an arched clamp 1 while fastening the workpiece with a nut 3 rotates on an axis 2. Screw 4 in this design it is not attached to the machine table, but moves freely in a T-shaped slot (Fig. 3.3, d).
The screws used in clamping mechanisms develop a force at the end R, which can be calculated using the formula
Where R- the force of the worker applied to the end of the handle; L- length of the handle; r av - average thread radius; a - thread lead angle; cf is the friction angle in the thread.
The moment developed on the handle (key) to obtain a given force R
where M, p is the friction moment at the supporting end of the nut or screw:
where / is the sliding friction coefficient: when fastening / = 0.16...0.21, when unfastening / = 0.24...0.30; D H - outside diameter rubbing surface of a screw or nut; s/v - screw thread diameter.
Taking a = 2°30" (for threads from M8 to M42, angle a changes from 3°10" to 1°57"), f = 10°30", g avg= 0.45s/, D, = 1.7s/, d B = d u/= 0.15, we obtain an approximate formula for the moment at the end of the nut M gr = 0.2 dP.
For flat end screws M t p = 0 ,1с1Р+ n, and for screws with a spherical end M Lr ~ 0.1 s1R.
In Fig. 3.4 shows other lever clamping mechanisms. Frame 3 universal clamping mechanism with a screw drive (Fig. 3.4, A) secured to the machine table with a screw/nut 4. Sticking b during fastening, the workpiece is rotated on axis 7 with a screw 5 clockwise. Clamp position b with body 3 Easily adjustable relative to the fixed liner 2.
Rice. 3.4.
Special lever clamping mechanism with an additional link and pneumatic drive (Fig. 3.4, b) used in mechanized production to automatically remove the stick from the workpiece loading area. During workpiece/rod unfastening b moves downwards, while the sticking 2 rotates on an axis 4. The latter together with the earring 5 rotates on an axis 3 and occupies the position shown by the dashed line. Sticking 2 removed from the workpiece loading area.
Wedge clamping mechanisms come with a single-bevel wedge and wedge-plunger ones with one plunger (without rollers or with rollers). Wedge clamping mechanisms are distinguished by their simplicity of design, ease of setup and operation, ability to self-braking, and constant clamping force.
To securely hold the workpiece 2 in adaptation 1 (Fig. 3.5, A) wedge 4 must be self-braking due to the angle a of the bevel. Wedge clamps are used independently or as an intermediate link in complex clamping systems. They allow you to increase and change the direction of the transmitted force Q.
In Fig. 3.5, b shows a standardized hand-operated wedge clamping mechanism for securing the workpiece to the machine table. The workpiece is clamped with a wedge / moving relative to the body 4. The position of the moving part of the wedge clamp is fixed with a bolt 2 , nut 3 and a puck; fixed part - bolt b, nut 5 and washer 7.
Rice. 3.5. Scheme (A) and design (V) wedge clamping mechanism
The clamping force developed by the wedge mechanism is calculated using the formula
where sr and f| - angles of friction respectively on the inclined and horizontal surfaces wedge
Rice. 3.6.
In the practice of mechanical engineering production, equipment with rollers in wedge clamping mechanisms is more often used. Such clamping mechanisms can reduce friction losses by half.
The calculation of the fastening force (Fig. 3.6) is made using a formula similar to the formula for calculating a wedge mechanism operating under the condition of sliding friction on contacting surfaces. In this case, we replace the sliding friction angles φ and φ with the rolling friction angles φ |1р and φ pr1:
To determine the ratio of friction coefficients during sliding and
rolling, consider the equilibrium of the lower roller of the mechanism: F l - = T - .
Because T = WfF i =Wtgi p tsr1 and / = tgcp, we get tg(p llpl = tg the upper roller, the formula is similar. In the designs of wedge clamping mechanisms, standard rollers and axles are used, in which D= 22...26 mm, a d= 10... 12 mm. If we take tg(p =0.1; d/D= 0.5, then the rolling friction coefficient will be / k = tg 0,1 0,5 = 0,05 =0,05. Rice. 3. In Fig. Figure 3.7 shows diagrams of wedge-plunger clamping mechanisms with a two-ring plunger without a roller (Fig. 3.7, a); with a two-support plunger and a roller (Fig. 3.7, (5); with a single-support plunger and three rollers (Fig. 3.7, c); with two single-support (cantilever) plungers and rollers (Fig. 3.7, G). Such clamping mechanisms are reliable in operation, easy to manufacture and can have the property of self-braking at certain wedge bevel angles. In Fig. Figure 3.8 shows a clamping mechanism used in automated production. The workpiece 5 is installed on the finger b and fastened with a clamp 3.
The clamping force on the workpiece is transmitted from the rod 8
hydraulic cylinder 7 through a wedge 9,
video clip 10
and plunger 4.
Removal of the clamp from the loading zone during removal and installation of the workpiece is carried out by a lever 1,
which turns on an axis 11
projection 12.
Sticking 3
easily stirred by lever 1
or springs 2, since in the axle design 13
rectangular crackers are provided 14,
easily moved in the grooves of the clamp. Rice. 3.8. To increase the force on the rod of a pneumatic actuator or other power drive, hinged lever mechanisms are used. They are an intermediate link connecting the power drive with the clamp, and are used in cases where greater force is required to secure the workpiece. According to their design, they are divided into single-lever, double-lever single-acting and double-lever double-acting. In Fig. 3.9, A shows a diagram of a single-acting articulated lever mechanism (amplifier) in the form of an inclined lever 5
and roller 3,
connected by an axis 4
with lever 5 and rod 2 of pneumatic cylinder 1.
Initial strength R, developed by a pneumatic cylinder, through rod 2, roller 3 and axis 4
transmitted to the lever 5.
In this case, the lower end of the lever 5
moves to the right, and its upper end rotates the clamp 7 around the fixed support b and secures the workpiece by force Q. The value of the latter depends on the strength W and grip arm ratio 7. Strength W for a single-lever hinge mechanism (amplifier) without a plunger is determined by the equation Force IV, developed by a double-lever hinge mechanism (amplifier) (Fig. 3.9, b), equal to Strength If"2
,
developed by a double-lever hinge-plunger mechanism of one-sided action (Fig. 3.9, V), determined by the equation In the given formulas: R- initial force on the motorized drive rod, N; a - angle of position of the inclined link (lever); p - additional angle that takes into account friction losses in the hinges ^p = arcsin/^П;/- coefficient of sliding friction on the roller axis and in the hinges of the levers (f~ 0.1...0.2); (/-diameter of the axes of the hinges and roller, mm; D- outer diameter of the support roller, mm; L- distance between lever axes, mm; f[ - angle of sliding friction on the hinge axes; f 11р - friction angle rolling on a roller support; tgф pp =tgф-^; tgф pp 2 - reduced coefficient zhere; tgф np 2 =tgф-; / - the distance between the hinge axis and the middle of the friction, taking into account friction losses in the cantilever (skewed) plunger 3/ , the plunger guide sleeve (Fig. 3.9, V), mm; A- length of the plunger guide bushing, mm. Rice. 3.9. actions Single-lever hinged clamping mechanisms are used in cases where large workpiece clamping forces are required. This is explained by the fact that during fastening of the workpiece, the angle a of the inclined lever decreases and the clamping force increases. So, at an angle a = 10°, the force W at the upper end of the inclined link 3
(see Fig. 3.9, A) amounts to JV~ 3,5R, and at a = 3° W~ 1 IP, Where R- force on the rod 8
pneumatic cylinder. In Fig. 3.10, A An example of the design of such a mechanism is given. The workpiece / is secured with a clamp 2.
The clamping force is transmitted from the rod 8
pneumatic cylinder through a roller 6
and length-adjustable inclined link 4,
consisting of a fork 5
and earrings 3.
To prevent rod bending 8
a support bar 7 is provided for the roller. In the clamping mechanism (Fig. 3.10, b) The pneumatic cylinder is located inside the housing 1
fixture to which the housing is attached with screws 2
clamping Rice. 3.10. mechanism. While securing the workpiece, the rod 3
pneumatic cylinder with roller 7 moves upward, and the clamp 5
with link b rotates on an axis 4.
When unfastening the workpiece, the clamp 5 takes the position shown by the dashed lines, without interfering with the change of the workpiece.
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) cutting forces and 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 must 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 fastening: in three-jaw chuck(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 as possible in operation. To do this, he must have minimum 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 fixture clamping devices 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 atmospheric pressure, or by means of 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 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 operate from 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- length of the handle; 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 installation 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 must be done with minimum cost 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– combination of elementary clamps 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 operations machining(drilling, boring) rigidity of the cutting tool and technological system in general it turns out to be 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 profile, the task of which is to guide cutting tool along the workpiece surface to be processed to obtain the specified 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 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 clamping devices contact elements of appropriate design, which allow the clamping force to be distributed equally between two or three points, and sometimes even distributed over a certain extended surface. TO Number of clamping points largely depends on the type of workpiece, processing method, 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 thread self-braking.
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 – inner diameter support 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- screw cam profile;
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 revolution
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 cut 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 submitting compressed air into 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 cylindrical surface split bushing 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 outside or inner surface 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 clamp;
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; Smax- 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- it's thin metal wheels 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 diaphragm open-end chuck, when installing an annular part along outer surface the membrane bends 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 shown design diagram 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 b can be considered to act uniformly around the circumference of the membrane radius
and causing it to bend: A 3. Radius
the outer surface of the membrane (for design reasons) are specified. 4. Attitude T A radius b membrane to radius 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 | 1,25 | 1,5 | 1,75 | 2,0 | 2,25 | 2,5 | 2,75 | 3,0 |
| m=a/b | 0,785 | 0,645 | 0,56 | 0,51 | 0,48 | 0,455 | 0,44 | 0,42 |
| M 1 | 0,215 | 0,355 | 0,44 | 0,49 | 0,52 | 0,545 | 0,56 | 0,58 |
M 3 6. Angle (rad) of the cams opening when securing the 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):
When choosing the point of application and the 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. Form work surface clamping elements in contact with the workpiece are basically the same as the mounting elements. Graphically, the clamping elements are designated according to table. 3.2.
Table 3.2 Graphic designation clamping elements
