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Eccentric clamp |
The initial data for the design of a round eccentric clamp are tolerance delta (Greek) on the dimension of the work from its locating surface to the point of clamping force application; angle a of clamp rotation; Clamping force FQ.
In order to keep the size of this article small the below calculations are excluded:
- Bearing stress on the surface of the eccentric pin
- Surface pressure in the region of the eccentric’s contact with work
- Maximum tangential force which the eccentric can take up
The actuating force FN at point (B) produces through distance L a torque which is converted by means of eccentricity between center of rotation and center of contact surface to a high ratio force FQ at point (A). At first we calculate the diameter of eccentric pin r or we can choose one from standard (stock material). For the given material we know the coefficient of friction of rest and from formula (1) we calculate the radius of the friction circle. The friction circle is always tangent to the friction forces due to rotation of the eccentric pin.
In order to have a self-locking condition we use formula (2). The angle phi (Greek) is the friction angle between the work piece and eccentric clamp (usually for half dry surface is 8 degrees). From formula (2) for a limited radius R we calculate eccentricity e (or from formula (4) we calculate eccentricity and substitute at formula (2)).
Formula (3) gives the required displacement of the point of eccentric-plane contact in order to clamp the work piece, where s1 the clearance for a free passage of the work beneath the eccentric and delta (Greek) the tolerance on the dimension of the work.
Substituting values at formula (4) we can calculate eccentricity or the angle of clamp rotation a. In the eccentric’s figure is shown the angle a1 which depends on the position of the eccentric pin center. When we calculate rotation angle we perform a few iterations by solving both parts of formula (4) simultaneously in order to have them equal. After calculating rotation angle, from formula (5) and (6) we have the force ratio.
The formula (6) is rounded with a total error up to 10%.
The formula (3) gives the required displacement without considering elastic deformation of the fixture. The dimensions of the eccentric can be reduced by increasing the stiffness (decrease elastic deformation) of the system and diameter of eccentric pin.
