Clamping elements must provide reliable contact the workpiece with the installation elements and prevent it from being damaged 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. Cam 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 permanent and large clamping forces, reduction of clamping time, implementation 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 workpieces. There are plates that use ferrites as permanent magnets. These plates are characterized by high holding force and smaller distance between poles.
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- one-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 complex profile, whose task is to guide cutting tool along the workpiece surface to be processed to obtain the specified accuracy of the trajectory of their movement.
MINISTRY OF EDUCATION AND SCIENCE OF UKRAINE
Donbass State Academy of Construction
and architecture
METHODOLOGICAL INSTRUCTIONS
for practical classes in the course "Technological Fundamentals of Mechanical Engineering" on the topic "Calculation of Devices"
Minutes No. of 2005 were approved at a meeting of the department "Cars and Automotive Industry"
Makeevka 2005
Methodological instructions for practical classes in the course "Technological fundamentals of mechanical engineering" on the topic "Calculation of devices" (for students of specialty 7.090258 Automobiles and automotive industry) / Comp. D.V. Popov, E.S. Savenko. - Makeevka: DonGASA, 2002. -24 p.
Basic information about machine tools, design, main elements are presented, and a methodology for calculating devices is presented.
Compiled by: D.V. Popov, assistant,
E.S. Savenko, assistant.
Responsible for the release S.A. Gorozhankin, associate professor
Devices4
Elements of devices5
Installation elements of devices6
Clamping elements of fixtures9
Calculation of forces for securing workpieces12
Devices for guiding and determining the position of 13 cutting tools
Housings and auxiliary elements of devices14
General methodology for calculating devices15
Calculation of jaw chucks using the example of turning16
Literature19
Applications20
DEVICES
All devices based on technological characteristics can be divided into the following groups:
1. Machine tools for installing and securing workpieces, depending on the type of machining, are divided into devices for turning, drilling, milling, grinding, multi-purpose and other machines. These devices communicate the workpiece with the machine.
2. Machine tools for installing and securing the working tool (they are also called auxiliary tools) communicate between the tool and the machine. These include cartridges for drills, reamers, taps; multi-spindle drilling, milling, turret heads; tool holders, blocks, etc.
Using the devices of the above groups, the machine-workpiece-tool system is adjusted.
Assembly devices are used to connect mating parts of a product, used for fastening base parts, ensuring the correct installation of connected elements of a product, preliminary assembly of elastic elements (springs, split rings), etc.;
Control devices are used to check deviations in size, shape and relative position of surfaces, mating of assembly units and products, as well as to control design parameters obtained during the assembly process.
Devices for capturing, moving and turning heavy, and in automated production, GPS and light workpieces and assembled products. Devices are the working parts of industrial robots built into automated production and GPS systems.
There are a number of requirements for gripping devices:
reliability of gripping and holding the workpiece; basing stability; versatility; high flexibility (easy and fast changeover); small overall dimensions and weight. In most cases, mechanical gripping devices are used. Examples of gripping diagrams for various gripping devices are shown in Fig. 18.3. Magnetic, vacuum and elastic chamber gripping devices are also widely used.
All described groups of devices, depending on the type of production, can be manual, mechanical, semi-automatic and automatic, and depending on the degree of specialization - universal, specialized and special.
Depending on the degree of unification and standardization in mechanical engineering and instrument making in accordance with the requirements of the Unified System of Technological Preparation of Production (USTPP), approved
seven standard machine fixture systems.
In the practice of modern production, the following systems of devices have developed.
Universal prefabricated devices (USF) are assembled from finally processed interchangeable standard universal elements. They are used as special reversible short-acting devices. They provide installation and fixation of various parts within the dimensional capabilities of the USP kit.
Special prefabricated devices (SRP) are assembled from standard elements as a result of their additional mechanical processing and are used as special irreversible long-term devices made from reversible elements.
Non-separable special devices (NSD) are assembled using standard parts and assemblies for general purpose as long-term irreversible devices made from irreversible parts and assemblies. They consist of two parts: a unified base part and a replaceable nozzle. The devices of this system are used for manual processing of parts.
Universal non-adjustment devices (UPD) are the most common system in mass production conditions. These devices provide installation and fixation of workpieces of any small and medium-sized products. In this case, the installation of a part is associated with the need for control and orientation in space. Such devices provide a wide range of processing operations.
Universal adjustment devices (UNF) provide installation using special adjustments, fixation of workpieces of small and medium dimensions and performance of a wide range of processing operations.
Specialized adjustment devices (SAD) provide, according to a certain basing scheme, with the help of special adjustments, and fixation of parts related in design to carry out a typical operation. All of the listed device systems belong to the unified category.
ELEMENTS OF DEVICES
The main elements of devices are installation, clamping, guides, dividing (rotary), fasteners, housings and mechanized drives. Their purpose is as follows:
installation elements - to determine the position of the workpiece relative to the fixture and the position of the processed surface relative to the cutting tool;
clamping elements - for securing the workpiece;
guide elements - to implement the required direction of movement of the tool;
dividing or rotating elements - to accurately change the position of the workpiece surface being processed relative to the cutting tool;
fasteners - for connection individual elements between themselves;
housings of devices (as base parts) - for placing all elements of devices on them;
mechanized drives - for automatic securing of the workpiece.
Elements of devices also include gripping devices of various devices (robots, GPS transport devices) for gripping, clamping (unclamping) and moving workpieces being processed or assembled assembly units.
1 Installation elements of devices
Installation of workpieces in fixtures or on machines, as well as assembly of parts includes their basing and fastening.
The need for fastening (force closure) when processing a workpiece in fixtures is obvious. For precise processing of workpieces it is necessary: to carry out its correct location in relation to equipment devices that determine the trajectories of movement of the tool or the workpiece itself;
ensure constant contact of the bases with the reference points and complete immobility of the workpiece relative to the fixture during its processing.
For complete orientation in all cases, when fastening, the workpiece must be deprived of all six degrees of freedom (the six-point rule in basing theory); In some cases, a deviation from this rule is possible.
For this purpose, main supports are used, the number of which must be equal to the number of degrees of freedom that the workpiece is deprived of. To increase the rigidity and vibration resistance of the workpieces being processed, auxiliary adjustable and self-aligning supports are used in the fixtures.
To install a workpiece in a fixture with a flat surface, standardized main supports are used in the form of pins with spherical, notched and flat heads, washers, and support plates. If it is impossible to install the workpiece only on the main supports, auxiliary supports are used. As the latter, standardized adjustable supports in the form of screws with a spherical bearing surface and self-aligning supports can be used.
Figure 1 Standardized supports:
A-e- permanent supports (pins): a- flat surface; b- spherical; V- notched; G- flat with installation in the adapter sleeve; d- support washer; e- base plate; and- adjustable support - self-aligning support
The mating of supports with spherical, notched and flat heads with the body of the device is carried out according to the fit 
or 
. Installation of such supports is also used through intermediate bushings, which are mated with the housing holes according to the fit 
.
Examples of standardized main and auxiliary supports are shown in Figure 1.
To install a workpiece along two cylindrical holes and a flat surface perpendicular to their axes, use
Figure 2.Schemebased on the end and hole:
a – on the high finger; b – on the low finger
standardized flat supports and mounting pins. To avoid jamming of the workpieces when installing them on the fingers along the exact two holes (D7), one of the installation fingers must be cut off and the other cylindrical.
Installation of parts on two fingers and a plane has found wide application in the processing of workpieces on automatic and production lines, multi-purpose machines and in GPS.
Schemes for basing on a plane and holes using mounting fingers can be divided into three groups: on the end and hole (Fig. 2); along the plane, end and hole (Fig. 3); along a plane and two holes (Fig. 4).
Rice. 19.4. Scheme of basing on a plane and two holes
It is recommended to install the workpiece on one finger according to the fit 
or 
, and on two fingers - each 
.
AND 
From Fig. 2 it follows that installing the workpiece along the hole on a long cylindrical uncut pin deprives it of four degrees of freedom (double guide base), and installation on the end deprives it of one degree of freedom (support base). Installing the workpiece on a short pin deprives it of two degrees of freedom (double support base), but the end in this case is an installation base and deprives the workpiece of three degrees of freedom. For complete basing it is necessary to create a force closure, i.e. apply clamping forces. From Fig. 3 it follows that the plane of the base of the workpiece is the installation base, the long hole into which the cut finger with an axis parallel to the plane enters is the guide base (the workpiece is deprived of two degrees) and the end of the workpiece is the support base.
Figure 3. Based onplane, Figure 4 Based on
end and hole of the plane and two holes
In Fig. Figure 4 shows a workpiece that is installed along a plane and two holes. The plane is the installation base. The holes centered with the cylindrical pin are the double support base, and those centered with the cut pin are the support base. The applied forces (shown by the arrow in Fig. 3 and 4) ensure alignment accuracy.
The finger is a double support base, and the cut finger is the support base. The applied forces (shown by the arrow in Fig. 3 and 4) ensure alignment accuracy.
To install workpieces with the outer surface and the end surface perpendicular to its axis, support and mounting prisms (movable and fixed), as well as bushings and cartridges are used.
Elements of fixtures include settings and probes for setting the machine to required size. Thus, standardized settings for cutters on milling machines can be:
high-rise, high-rise end, corner and corner end.
Flat probes are made with a thickness of 3-5 mm, cylindrical ones with a diameter of 3-5 mm with an accuracy of 6th grade (h6) and subjected to hardening 55-60 HRC 3, ground (roughness parameter Ra = 0.63 µm).
The actuating surfaces of all installation elements of devices must have high wear resistance and high hardness. Therefore, they are made from structural and alloy steels 20, 45, 20Х, 12ХНЗА with subsequent carburization and hardening to 55-60 HRC3 (supports, prisms, mounting pins, centers) and tool steels U7 and U8A with hardening to 50-55 HRG, ( supports with a diameter of less than 12 mm; mounting pins with a diameter of less than 16 mm; installations and probes).
The designs of clamping devices consist of three main parts: a drive, a contact element, and a power mechanism.
The drive, converting a certain type of energy, develops a force Q, which is converted into a clamping force using a power mechanism R and is transmitted through contact elements to the workpiece.
The contact elements serve to transfer the clamping force directly to the workpiece. Their designs allow forces to be dispersed, preventing crushing of the workpiece surfaces, and distributed between several support points.
It is known that rational choice of devices reduces auxiliary time. Auxiliary time can be reduced by using mechanized drives.
Mechanized drives, depending on the type and source of energy, can be divided into the following main groups: mechanical, pneumatic, electromechanical, magnetic, vacuum, etc. The scope of application of manually controlled mechanical drives is limited, since a significant amount of time is required for installation and removal of workpieces. . The most widely used drives are pneumatic, hydraulic, electric, magnetic and their combinations.
Pneumatic actuators work on the feed principle compressed air. Can be used as a pneumatic drive
pneumatic cylinders (double-acting and single-acting) and pneumatic chambers.
for cylinder cavity with rod
for single acting cylinders
The disadvantages of pneumatic drives include their relatively large overall dimensions. The force Q(H) in pneumatic cylinders depends on their type and, without taking into account friction forces, it is determined by the following formulas:
For double-acting pneumatic cylinders for the left side of the cylinder
where p - compressed air pressure, MPa; compressed air pressure is usually taken to be 0.4-0.63 MPa,
D - piston diameter, mm;
d- rod diameter, mm;
ή- efficiency, taking into account losses in the cylinder, at D = 150...200 mm ή =0.90...0.95;
q - spring resistance force, N.
Pneumatic cylinders are used with an internal diameter of 50, 75, 100, 150, 200, 250, 300 mm. Fitting the piston in the cylinder when using o-rings 
or 
, and when sealed with cuffs 
or 
.
The use of cylinders with a diameter of less than 50 mm and more than 300 mm is not economically profitable; in this case, it is necessary to use other types of drives,
Pneumatic chambers have a number of advantages compared to pneumatic cylinders: they are durable, withstand up to 600 thousand starts (pneumatic cylinders - 10 thousand); compact; They are lightweight and easier to manufacture. The disadvantages include the small stroke of the rod and the variability of the developed forces.
Hydraulic drives compared to pneumatic ones they have
the following advantages: develops great forces (15 MPa and above); their working fluid (oil) is practically incompressible; ensure smooth transmission of the developed forces by the power mechanism; can ensure the transfer of force directly to the contact elements of the device; have a wide range of applications, since they can be used for precise movements of the working parts of the machine and moving parts of devices; allow the use of working cylinders of small diameter (20, 30, 40, 50 mm v. more), which ensures their compactness.
Pneumohydraulic drives have a number of advantages over pneumatic and hydraulic ones: they have high labor force, speed of action, low cost and small dimensions. The calculation formulas are similar to the calculation of hydraulic cylinders.
Electromechanical drives are widely used in CNC lathes, aggregate machines, and automatic lines. Driven by an electric motor and through mechanical transmissions, forces are transmitted to the contact elements of the clamping device.
Electromagnetic and magnetic clamping devices They are carried out mainly in the form of plates and faceplates for securing steel and cast iron workpieces. Magnetic field energy from electromagnetic coils or permanent magnets is used. The technological capabilities of using electromagnetic and magnetic devices in conditions of small-scale production and group processing are significantly expanded when using quick-change setups. These devices increase labor productivity by reducing auxiliary and main time (10-15 times) during multi-site processing.
Vacuum drives used for fastening workpieces made of various materials with a flat or curved surface, taken as the main base. Vacuum clamping devices operate on the principle of using atmospheric pressure.
Force (N), pressing the workpiece to the plate:
Where F- area of the cavity of the device from which air is removed, cm 2;
p - pressure (in factory conditions usually p = 0.01 ... 0.015 MPa).
Pressure for individual and group installations is created by one- and two-stage vacuum pumps.
Power mechanisms act as amplifiers. Their main characteristic is the gain:
Where R- fastening force applied to the workpiece, N;
Q - force developed by the drive, N.
Power mechanisms often act as a self-braking element in the event of a sudden failure of the drive.
Some typical designs of clamping devices are shown in Fig. 5.
Figure 5 Clamping device diagrams:
A- using a clip; 6 - swinging lever; V- self-centeringprisms
Clamping elements hold the workpiece workpiece from displacement and vibrations arising under the influence of cutting forces.
Classification of clamping elements
The clamping elements of devices are divided into simple and combined, i.e. consisting of two, three or more interlocked elements.
Simple ones include wedge, screw, eccentric, lever, lever-hinge, etc. - called clamps.
Combined mechanisms are usually designed as screw-type
lever, eccentric-lever, etc. and are called tacks.
When to use simple or combined
mechanisms in arrangements with mechanized drive
(pneumatic or other) they are called mechanisms - amplifiers. Based on the number of driven links, the mechanisms are divided: 1. single-link - clamping the workpiece at one point;
2. two-link - clamping two workpieces or one workpiece at two points;
3. multi-link - clamping one workpiece at many points or several workpieces simultaneously with equal forces. By degree of automation:
1. manual - working with a screw, wedge and others
buildings;
2. mechanized, in
are divided into
a) hydraulic,
b) pneumatic,
c) pneumohydraulic,
d) mechanohydraulic,
d) electric,
e) magnetic,
g) electromagnetic,
h) vacuum.
3. automated, controlled from the working parts of the machine. They are driven by the machine table, support, spindle and centrifugal forces of rotating masses.
Example: centrifugal-energy chucks for semi-automatic lathes.
Requirements for clamping devices
They must be reliable in operation, simple in design and easy to maintain; should not cause deformation of the workpieces being fixed and damage to their surfaces; fastening and unfastening of workpieces should be carried out with a minimum expenditure of effort and working time, especially when securing several workpieces in multi-place devices; in addition, clamping devices should not move the workpiece during the process of securing it. Cutting forces should, if possible, not be absorbed by clamping devices. They should be perceived as more rigid installation elements of devices. To improve processing accuracy, devices that provide a constant clamping force are preferred.
Let's take a short excursion to theoretical mechanics. Let's remember what is the coefficient of friction?
If a body of weight Q moves along a plane with a force P, then the reaction to the force P will be a force P 1 directed in the opposite direction, that is
slip.
Friction coefficient
Example: if f = 0.1; Q = 10 kg, then P = 1 kg.
The coefficient of friction varies depending on the surface roughness.
Method for calculating clamping forces
First case
Second case
The cutting force P z and the clamping force Q are directed in the same direction
In this case Q => O
The cutting force P g and the clamping force Q are directed in opposite directions, then Q = k * P z
where k is the safety factor k = 1.5 finishing k = 2.5 roughing.
Third case
The forces are directed mutually perpendicularly. The cutting force P counteracts the friction force on the support (installation) Qf 2 and the friction force at the clamping point Q*f 1, then Qf 1 + Qf 2 = k*P z
G 
de f, and f 2 - sliding friction coefficients Fourth case
The workpiece is processed in a three-jaw chuck
In this direction, P tends to move the workpiece relative to the cams.
Calculation of threaded clamping mechanisms First case
Flat head screw clamp From equilibrium condition 
where P is the force on the handle, kg; Q - clamping force of the part, kg; R cp - average thread radius, mm;
R - radius of the supporting end;
Helix angle of thread;
Friction angle in threaded connection 6; 
- self-braking condition; f is the friction coefficient of the bolt on the part;
0.6 - coefficient taking into account the friction of the entire surface of the end. The moment P*L overcomes the moment of the clamping force Q, taking into account the friction forces in screw pair and at the end of the bolt.
Second case
■ Bolt clamp with spherical surface
With increasing angles α and φ, the force P increases, because in this case, the direction of the force goes up the inclined plane of the thread.
Third case
This clamping method is used when processing bushings or disks on mandrels: lathes, dividing heads or rotary tables on milling machines, slotting machines or other machines, gear hobbing, gear shaping, radial drilling machines, etc. Some information from the directory:
The Ml6 screw with a spherical end with a handle length L = 190 mm and a force P = 8 kg, develops a force Q = 950 kg
Clamping with a screw M = 24 with a flat end at L = 310 mm; P = 15kg; Q = 1550mm
Clamp with hex nut Ml 6 wrench L = 190mm; P = 10kg; Q = 700kg.
Eccentric clamps are easy to manufacture and for this reason they are widely used in machine tools. The use of eccentric clamps can significantly reduce the time for clamping a workpiece, but the clamping force is inferior to threaded clamps.
Eccentric clamps are made in combination with and without clamps.
Let's consider eccentric clamp with grip.
Eccentric clamps cannot work with significant tolerance deviations (±δ) of the workpiece. For large tolerance deviations, the clamp requires constant adjustment with screw 1.
Eccentric calculation
M
The materials used for the manufacture of the eccentric are U7A, U8A With
heat treatment to HR from 50....55 units, steel 20X with carburization to a depth of 0.8... 1.2 With hardening HR from 55...60 units. Let's look at the eccentric diagram. The KN line divides the eccentric into two? symmetrical halves consisting, as it were, of 2
X wedges screwed onto the “initial circle”.
The eccentric rotation axis is shifted relative to its geometric axis by the amount of eccentricity “e”.
Section Nm of the lower wedge is usually used for clamping.
Considering the mechanism as a combined one consisting of a lever L and a wedge with friction on two surfaces on the axis and point “m” (clamping point), we obtain a force relationship for calculating the clamping force.
where Q is the clamping force
P - force on the handle
L - handle shoulder
r - distance from the eccentric rotation axis to the point of contact With
workpiece
α - angle of rise of the curve
α 1 - friction angle between the eccentric and the workpiece
α 2 - friction angle on the eccentric axis
To avoid the eccentric moving away during operation, it is necessary to observe the condition of self-braking of the eccentric
Condition for self-braking of the eccentric. = 12Р
about chyazhima with expentoik
G
de α -
sliding friction angle at the point of contact with the workpiece ø -
friction coefficient For approximate calculations of Q - 12P, consider the diagram of a double-sided clamp with an eccentric
Wedge clamps
Wedge clamping devices are widely used in machine tools. Their main element is one, two and three bevel wedges. The use of such elements is due to the simplicity and compactness of the designs, speed of action and reliability in operation, the possibility of using them as a clamping element acting directly on the workpiece being fixed, and as an intermediate link, for example, an amplifier link in other clamping devices. Typically self-braking wedges are used. The condition for self-braking of a single-bevel wedge is expressed by the dependence
α >2ρ
Where α - wedge angle
ρ - the angle of friction on the surfaces G and H of contact between the wedge and the mating parts.
Self-braking is ensured at angle α = 12°, however, to prevent vibrations and load fluctuations during the use of the clamp from weakening the workpiece, wedges with an angle α are often used.
Due to the fact that decreasing the angle leads to increased
self-braking properties of the wedge, it is necessary when designing the drive to the wedge mechanism to provide devices that facilitate the removal of the wedge from the working state, since releasing a loaded wedge is more difficult than bringing it into the working state.
This can be achieved by connecting the actuator rod to a wedge. When rod 1 moves to the left, it passes path “1” to idle, and then, hitting pin 2, pressed into wedge 3, pushes the latter out. When the rod moves back, it also pushes the wedge into the working position by hitting the pin. This should be taken into account in cases where the wedge mechanism is driven by a pneumatic or hydraulic drive. Then, to ensure reliable operation of the mechanism, different pressures of liquid or compressed air should be created with different sides drive piston. This difference when using pneumatic actuators can be achieved by using a pressure reducing valve in one of the tubes supplying air or liquid to the cylinder. In cases where self-braking is not required, it is advisable to use rollers on the contact surfaces of the wedge with the mating parts of the device, thereby facilitating the insertion of the wedge into its original position. In these cases, it is necessary to lock the wedge.
Let us consider the diagram of the action of forces in a single-skew, most often used in devices, wedge mechanism
Let's construct a force polygon.
When transmitting forces at right angles, we have the following relationship
+ pinning, - unpinning
Self-braking occurs at α
Collet clamps
The collet clamping mechanism has been known for a long time. Securing workpieces using collets turned out to be very convenient when creating automated machines because to secure the workpiece, only one translational movement of the clamped collet is required.
When operating collet mechanisms, the following requirements must be met.
The clamping forces must be ensured in accordance with the emerging cutting forces and prevent movement of the workpiece or tool during the cutting process.
The clamping process in the general processing cycle is an auxiliary movement, so the response time of the collet clamp should be minimal.
The dimensions of the clamping mechanism links must be determined from their conditions normal operation when securing workpieces of both the largest and smallest sizes.
The positioning error of the workpieces or tools being fixed should be minimal.
The design of the clamping mechanism should provide the least elastic compression during the processing of workpieces and have high vibration resistance.
The collet parts and especially the collet must have high wear resistance.
The design of the clamping device must allow its quick change and convenient adjustment.
The design of the mechanism must provide protection for the collets from chips.
The practically minimum acceptable size for fastening is 0.5 mm. On
multi-spindle bar automatic machines, bar diameters, and
therefore, the collet holes reach 100 mm. Collets with a large hole diameter are used for fastening thin-walled pipes, because relatively uniform fastening over the entire surface does not cause large deformations of the pipes.
The collet clamping mechanism allows you to secure workpieces various shapes cross section.
The durability of collet clamping mechanisms varies widely and depends on the design and correctness technological processes in the manufacture of mechanism parts. As a rule, clamping collets fail before others. In this case, the number of fastenings with collets ranges from one (breakage of the collet) to half a million or more (wear of the jaws). The performance of a collet is considered satisfactory if it is capable of securing at least 100,000 workpieces.
Classification of collets
All collets can be divided into three types:
1. Collets of the first type have a “straight” cone, the top of which faces away from the machine spindle.
To secure it, it is necessary to create a force that pulls the collet into the nut screwed onto the spindle. Positive traits This type of collet is structurally quite simple and works well in compression (hardened steel has a higher permissible stress in compression than in tension. Despite this, collets of the first type are currently of limited use due to disadvantages. What are these disadvantages:
a) the axial force acting on the collet tends to unlock it,
b) when feeding the bar, premature locking of the collet is possible,
c) when secured with such a collet, harmful effects on
d) there is unsatisfactory centering of the collet in
spindle, since the head is centered in the nut, the position of which is on
The spindle is not stable due to the presence of threads.
Collets of the second type have a “reverse” cone, the top of which faces the spindle. To secure it, it is necessary to create a force that pulls the collet into the conical hole of the machine spindle.
Collets of this type ensure good centering of the workpieces being clamped, since the cone for the collet is located directly in the spindle and cannot
jamming occurs, the axial working forces do not open the collet, but lock it, increasing the fastening force.
At the same time, a number of significant disadvantages reduce the performance of collets of this type. Due to the numerous contacts with the collet, the conical hole of the spindle wears out relatively quickly, the threads on the collets often fail, not ensuring a stable position of the rod along the axis when fastened - it moves away from the stop. Nevertheless, collets of the second type are widely used in machine tools.
