Clamps, eccentric clamps. Eccentric clamps

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.

Consider an eccentric clamp with a clamp.

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

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

where 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<12°.

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 on different sides of the 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.

Eccentric clamping devices are fast-acting and are widely used in large-scale and mass production with low clamping forces (Fig. 2). To determine the main dimensions of the eccentric design, it is necessary to have: tolerance to the base surface of the workpiece during its installation; angle of rotation of the eccentric β p from the initial position; the force applied at the end of the handle Q of the hands, and the length of the handle L of the hands.

Rice. 2. Elements of a circular eccentric used in calculations

The clamping force developed by the eccentric is

where Q hands is the force applied on the eccentric handle, N; e – eccentricity, mm; f t.p – friction coefficient on the eccentric surface; f t.o – friction coefficient on the surface of the axis, f t.o = 0.12 ... 0.15; g o – axis radius, mm.

Eccentric stroke

The most convenient angle of rotation for the worker is β p = 90° ... 120°. The eccentric stroke can be determined by the relationship. The outer diameter of the eccentric is determined from the condition D ≥ 20 ∙ e, and the radius of the axis r o is selected depending on the width of the working part of the eccentric for design reasons or is calculated using the formula.

Self-braking of the eccentric clamp must comply with the condition D/e ≥ 14, where the ratio D/e is a characteristic of the eccentric.

All design parameters of the round eccentric must be taken into account GOST 9061–68*, where D ec = 32 ... 70 mm, e = 1.7 ... 3.5 mm.

Example. Determine the structural elements of the round eccentric for clamping the workpiece according to the dimensions of the working drawing and calculate the clamping force of the workpiece being processed.

Solution. Let us determine the tolerance of the base surface being processed; workpieces, where δ = 0.34 mm. Let's set the eccentric stroke

We take the eccentricity e = 2 mm.

Determine the diameter of the round eccentric

D ≥ 20 ∙ e = 20 ∙ 2 = 40 mm.

Let's determine the clamping force of the eccentric

The length of the eccentric handle L of the arms is determined from the condition

L arms = 2.5 ∙ D = 2.5 ∙ 40 = 100 mm.

The rotation angle is assumed to be β p = 90°. The friction coefficient on the eccentric surface f t.p = 0.12. The friction coefficient on the surface of the axis f t.o = 0.15. The axis radius is taken constructively to be r o = 6 mm. We check the self-braking of the eccentric clamp according to the condition D/e ≥ 14 (where 40/2 = 20). Self-braking satisfies our condition.

Easy to manufacture, with a high gain, fairly compact eccentric clamps, being a type of cam mechanisms, have another, undoubtedly, main advantage - speed.

The working surface of the cam is most often made in the form of a cylinder with a circle or Archimedes spiral at the base. In this article we will talk about the more common and more technologically advanced round eccentric clamp.

The dimensions of standardized round eccentric cams for machine tools are given in GOST 9061-68. The eccentricity of the round cams in this document is set to 1/20 of the outer diameter to ensure self-braking conditions over the entire operating range of rotation angles at a friction coefficient of 0.1 or more.

The figure below shows the calculated geometric diagram of the clamping mechanism. The fixed part is pressed against the supporting surface as a result of turning the eccentric handle counterclockwise around an axis rigidly fixed relative to the support.

The position of the mechanism shown is characterized by the maximum possible angle α , while the straight line passing through the axis of rotation and the center of the eccentric circle is perpendicular to the straight line drawn through the point of contact of the part with the cam and the center point of the outer circle.

If you turn the cam 90° clockwise relative to the position shown in the diagram, then a gap is formed between the part and the working surface of the eccentric equal in magnitude to the eccentricity e. This clearance is necessary for free installation and removal of the part.

CALCULATION FORMULAS

Find the friction angle (°) “part - eccentric”:

φ 1 = arctan (f 1),

Where,
f 1- coefficient of friction "part - eccentric";
0.15 - the value of the friction coefficient “part - eccentric” corresponding to the case “steel on steel without lubrication”.

Find the friction angle (°) "axis - eccentric":

φ 2 = arctan (f 2),

Where,
f 2- coefficient of friction "axis - eccentric";
0.12 - the value of the coefficient of friction “axle - eccentric” corresponding to the case “steel on steel with lubrication”.

Reducing friction in both places increases the power efficiency of the mechanism, but reducing friction in the area of contact between the part and the cam leads to the disappearance of self-braking.

Find the maximum angle (°) of the circular wedge:

α = arctan (2 e / D),

Where,
e- cam eccentricity, mm;
To ensure self-braking on steel surfaces, it is desirable to fulfill the condition: D/e>15.
In GOST 9061-68: D/e=20.
D- eccentric diameter, mm.

Then the radius vector (mm) of the contact point will be equal to:

R = D / (2 cos (α)),

And the distance from the eccentric axis to the support (mm) will accordingly be:

A = s + R cos(α),

Where,
s- thickness of the clamped part, mm.

The condition for self-braking is the fulfillment of the relation:

e ≤ R f 1 + d/2 f 2,

If the condition is met, self-braking is ensured.

The clamping force (N) can be found using the formula:

F = P L cos (α) / (R tg (α + φ 1) + d/2 tg (φ 2)),

Where,
P- force on the handle, N;
L- handle length, mm.

The force transfer coefficient is:

k = F/P

The position of the eccentric clamp chosen for calculations and shown in the diagram is the most “unfavorable” from the point of view of self-braking and gain in strength. But this choice is not accidental. If in such a working position the calculated power and geometric parameters satisfy the designer, then in any other positions the eccentric clamp will have an even greater force transmission coefficient and better self-braking conditions.

When designing, moving away from the considered position towards reducing the size A if other dimensions are kept unchanged, it will reduce the gap for installing the part.

Increase in size A can create a situation where the eccentric wears out during operation and significant fluctuations in thickness s, when it is simply impossible to clamp the part.

GOST 9061-68 recommends using wear-resistant surface-cemented steel 20X with a surface hardness of 56...61 HRC at a depth of 0.8...1.2 mm as the material for making the cam. But in practice, an eccentric clamp is made from a wide variety of materials, depending on the purpose, operating conditions and available technological capabilities.

Using a small table in MS Excel created on the basis of these formulas, you can learn to quickly and easily determine the main parameters of clamps for cams made of any materials, just remember to change the values of the friction coefficients in the initial data.

In the example shown in the screenshot, based on the given dimensions of the eccentric and the force applied to the handle, the mounting size from the axis of rotation of the cam to the supporting surface is determined, taking into account the thickness of the part, the self-braking condition is checked, the clamping force and the force transfer coefficient are calculated.

This calculation file can be found on the website www.al-vo.ru.

Related documents:

GOST 12189-66: Machine tools. Cams are eccentric. Design;
GOST 12190-66: Machine tools. Double eccentric cams. Design;
GOST 12191-66: Machine tools. Eccentric fork pads. Design;
GOST 12468-67 - Double-support eccentrics. Design.

The initial data for calculating the main dimensions of a round eccentric (Fig. 8.3) are: δ - tolerance on the size of the workpiece from its mounting base to the place where the fastening force is applied, mm; α - angle of rotation of the eccentric from the zero (initial) position; Q- workpiece fixing force, N.

Rice. 8.3. Eccentric clamps:

A - disk eccentric, b - eccentric with L-shaped clamp

If the angle of rotation of the eccentric is not limited, then

2e=s 1 +d+s 2 +

where s 1 is the gap for free entry of the workpiece under the eccentric; s 2 - eccentric power reserve, protecting it from passing through the dead center (takes into account eccentric wear); J- rigidity of the clamping device, N/mm.

The last term of the formula characterizes the increase in the distance between the eccentric and the workpiece as a result of elastic deformation of the clamping system. With s 1 = 0.2÷0.4 mm and s 2 = 0.4÷0.6 mm

e= +(0.3÷0.5) mm

If the rotation angle α is significantly less than 180°,

e= (8.4)

We find the radius of the eccentric pin (mm) by taking the width d;

r=Q/2bσ cm, (8.5)

Where σ cm - permissible bearing stress (15-20 MPa).

At b = 2r

Eccentric radius R we find from the self-braking conditions. From the diagram of forces acting on the eccentric (Fig. 8.4, A) it follows that the resultant T reactions Q and friction forces F should be equal to the reaction from the axle passing tangentially to the friction circle of radius ρ, and directed opposite to it:

where j = static friction angle.

At e≤ p R min = e+ r+ Δ, where Δ is the thickness of the jumper (Fig. 8.4, b).

Rice. 8.4. Scheme for power calculation of eccentrics

The radius ρ of the friction circle is determined from the equality ρ = f"r, Where f" - coefficient of static friction in the axle. Values j and f"should be taken at the smallest limit. For semi-dry surfaces, j = 8° can be taken and f" = 0.12÷0.15.

Angle of rotation α 1 (see Fig. 8.4, A) for the least favorable position of the eccentric we will find using the formula α 1 = 90° - j.

Width of the working part of the eccentric IN we determine from the formula

σ=0.565

where σ is the permissible stress at the point of contact of the eccentric with the workpiece. For hardened steel, you can take σ = 800÷1200 MPa; E 1 E 2 - elastic moduli, respectively, of the materials of the eccentric and the element in contact with it (intermediate part or workpiece), MPa; µ 1, µ 2 - Poisson's ratios for the materials of the eccentric and the element in contact with it.

At E 1 =E 2 =E and µ 1 =µ 2 = 0.25 we get

from where (at R in mm)

B= 0.17 mm. (8.6)

Eccentric dimensions e, r, R And IN coordinated with GOST.

To establish the relationship between the fastening force Q and the moment on the eccentric handle at the end of securing the workpiece, we will use the diagram shown in Fig. 8.4, b. During the fastening process, three forces act on the eccentric: the force on the handle N, workpiece reaction T and the reaction of the axle S. Under the influence of these forces, the system is in equilibrium. Reaction T represents the resultant force Q and friction forces F. The sum of the moments of all acting forces relative to the eccentric rotation axis

Nl - Qe sin α" - fQ(R- e cos α") - Sρ = 0,

Where f- coefficient of friction between the eccentric and the workpiece.

Force S differs little in magnitude from normal force Q. Taking S" Q, we get the moment on the eccentric handle

Nl= Q[fR+ ρ + e(sin α" +f cos α")].

To simplify the resulting expression, we accept:

1) fR = tg j R"sin jR(at j= 6° the error is less than 1%);

2) expression sin α" +f cos α" replace sin (α" +j) (error 1%). After substitutions we get

Nl=Q(8.7)

Given the expression for R, we get

Nl= eQ. (8.8)

According to this formula, the moment Nl found with an accuracy of 10%.

Moving the point of contact of the eccentric with the plane when it is rotated through an angle α from the initial position (Fig. 8.5, a)

x = e- With= e- e cos α = e(1 - cos α).

Rice. 8.5. Schemes for calculating the movement of the point of contact of the eccentric with the plane when it rotates

In Fig. 8.5 b change shown X from α. Considering that

x=s 1 +d+ ,

cos α = 1- ; α "=180 o - α

Substituting the found value α " into formula (8.8), we can express the moment on the eccentric handle through the initial values.

Calculation of wedge clamps

Wedge clamps are used as an intermediate link in complex clamping systems. They are easy to manufacture, compact, easily placed in the device, and allow you to increase and change the direction of the transmitted force. At certain angles, the wedge mechanism has self-braking properties. For the most common single-bevel wedge in devices (Fig. 8.6, a) under the action of forces at a right angle, we have the following dependence obtained from the force polygon:

. . (8.9)

With a minus sign in the formula, we have a dependence for detaching the wedge. Self-braking occurs at α< φ 1 + φ 2 . Если φ 1 = φ 2 .= φ 3 = φ. то зависимость упрощается:

Rice. 8.6. Action of forces in the wedge mechanism:

a - with an angle of 90°; b - with an angle of more than 90°

When transmitting forces at an angle β > 90° (Fig. 8.6, b) the relationship between P and Q from the force polygon has the form (at 90 + α > β)

If the friction angle is constant and equal to φ, then

Calculation of lever clamps

Lever clamps, similar to wedge clamps, are used in combination with other elementary clamps, forming more complex clamping systems. Using a lever, the magnitude and direction of the transmitted force are changed, and the workpiece is simultaneously and uniformly secured in two places.

Eccentric clamps are made in combination with and without clamps.

Consider an eccentric clamp with a clamp.

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.

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
deα - 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

α > 2 ρ

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<12°.

Due to the fact that decreasing the angle leads to increased

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<α 1 +α 2 Если α 1 =α 2 =α 3 =α the dependence is simpler P = Qtg(α+2φ)

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 the conditions of their 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 pressure 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.

Collet clamping mechanisms operate in a wide range of sizes. The practically minimum acceptable size for fastening is 0.5 mm. On multi-spindle bar machines, the diameters of the bars, and

therefore, the collet holes reach 100 mm. Collets with a large hole diameter are used to secure thin-walled pipes, because... relatively uniform fastening over the entire surface does not cause large deformations of the pipes.

The collet clamping mechanism allows for securing workpieces of various cross-sectional shapes.

The durability of collet clamping mechanisms varies widely and depends on the design and correctness of 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. The positive qualities of this type of collet are that they are structurally quite simple and work 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, there is a harmful effect on

d) there is unsatisfactory centering of the collet in the spindle, since the head is centered in the nut, the position of which 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.

Collets of the third type They also have a reverse cone, but they work due to the axial movement of a sleeve with a conical hole, while the collet itself remains stationary.

This design avoids most of the disadvantages inherent in collets of the first and second types. However, one of the existing disadvantages of collets of this type is the increase in overall dimensions of the entire clamping unit in diameter.

For the manufacture of medium and large collets, steel grades 65G, 12KhNZA, U7A, U8A are mainly used. It is considered advisable to use low-carbon case-hardening steels. Experimental data show that case-hardened steels perform no worse than carbon steels. The presence, for example, of nickel in case-hardened steel 12ХНЗА ensures the resistance of the collet to abrasion, and case-hardening gives it relatively good plastic properties. Nevertheless, most factories prefer 65G steel.

R
Let's look at what forces arise when the collet operates in the absence of an axial stop.

P = (Q+Q")tg( α + φ )

Q - clamping force over preparations VCI is calculated using the formula

M - cutting moment M = P z V let’s substitute the values of the cutting moment

Where - V is the distance from the axis to the point of application of the cutting force R is the radius of the workpiece into the clamping areas.

q is the component of the force that shifts the workpiece along the axis.

ƒ - deflection arrow. k - safety factor

Q 1 - the force required to compress all the collets until they come into contact with the workpiece.

φ - friction angle between the collet and the body

where E is the elastic modulus.

1 - moment of inertia of the sector in the collet.

f - deflection arrow.

l is the length of the collet blade from the contact point to the middle of the cone.

Vacuum clamping devices

Vacuum clamping devices operate on the principle of direct transmission of atmospheric pressure to the workpiece being clamped.

Vacuum devices can be used to hold workpieces made of various materials with a flat or curved surface. The clamping force is sufficient for finishing and finishing operations. Vacuum devices are very effective for securing thin plates. The base surfaces of the workpiece can be either cleanly processed or black, but fairly smooth without any visible depressions or protrusions.

If there are polished surfaces, installation of workpieces without compaction is allowed. The workpieces are detached by connecting the cavity from which the air with the atmosphere is pumped out.

The force pressing the workpiece is calculated using the following formula

Q = F(l,033-P) kg.

where F is the area in cm 2, the boundaries of which are taken along the seal line P is the vacuum created in the cavity of the device by the suction device.

In practice, a vacuum of 0.1 0.15 kg/cm 2 is used

Using a deeper vacuum is expensive and the clamping force increases only slightly.

For uniform multi-point clamping of the workpiece to the plate, a large number of evenly spaced holes are made on the mounting plane.

In this case, the fastening takes place without local buckling and warping of the workpiece. The vacuum for individual installations is created:

a) centrifugal pumps P = 0.3 kg/cm 2

b) single-stage piston P = 0.005 kg/cm 2

two-stage R= 0.01 kg/cm 2