CAM Mechanisms

CAM Mechanisms

CAM Mechanisms -  Elementary earn mechanism is shown in Fig. The axis of cam rotation is offset from the line of action of the follower.

The following notation has been accepted in this diagram (and sub sequent):

θ = pressure angle the angle between the normal to the earn curve (profile) at the point of contact and the line of action of the follower.

δ= offset angle the angle between the radius vector and the line of action of the follower

θ s = slope angle of the cam curve the angle between the radius vector and the normal to the cam curve (or between the perpendicular to the radius vector and the tangent to the cam curve at the point of contact)

P = force acting in the direction of follower motion

N = force normal to the cam curve.

The force growth factor ε = serves as the force characteristic of a cam mechanism. The normal force N, the bearing stress on the working surface of the cam, and earn wear increase with E. Factor E increases with the pressure angle θ and slope angle θ s of the cam curve. As a rule, E = 1.35 to 2.

Upon an increase in angle θ s' there is a danger of the cam mechanism being jammed as a result of the increased friction forces in the follower guides and on the earn surface, reducing the efficiency η c of the cam mechanism to η c = 0.

In the case of an elementary cam mechanism with no offset between the axis of cam rotation and the line of follower action, i.e., when δ = 0 and θ s = θ, jamming begins at the value where f1 and f2 = coefficients of friction on the earn surface and in the follower guides, respectively

θ jam = pressure angle at which jamming occurs. Thus a reduction in the pressure angle θ leads to a reduction in the normal force N, acting on the working surface of the cam, and in the danger of jamming of the mechanism.

On the other hand, a reduction in angle θ leads to an increase in the central angle of cam rotation required to obtain the given length of follower motion for the given overall size of the earn. As can be seen if the cam rotates through a small angle Δ α, with sufficient accuracy for practical purposes.

Elementary Cam Mechanism

With a reduction in the pressure angle θ, the central angle of rotation Δ α increase. At a definite speed of rotation of the earn, an increase in angle Δ α will require more time for the given displacement of the follower, i.e., it .will lead to a reduction in the production capacity of the automatic machine tool.

This is of especial importance for rapid idle motions of the main working members (for example, slide approach and withdrawal). It is also taken into consideration when auxiliary working members have a cam drive.

The magnitude of the central angle of rotation of the cam serves as the principal index of the time required for the working and idle motions of the main and auxiliary working members of the machine. This should be taken into account in assigning the pressure and slope angles of the cam curve.

It can be seen from equation (3) that Consequently, an increase in the pressure angle enables the overall size of the cams mechanism to be reduced. In common practice, e = 10' to 40' for the curves of work cams while 9max = 45' to 60' for the curves of idle travel or return cams. These data concern the working and idle sections when their curves are on a single cam.

When the cam mechanism being employed differs from the elementary cams mechanism, the following equivalent coefficients of friction are introduced into equations (1) and (2) where the reduction factors A, and are determined by comparison with the elementary cam mechanism.

To increase the safety factor in respect to jamming, and to increase the pressure angle with a consequent reduction in the central angle of cam rotation, measures should be taken to reduce the friction of the follower shoes and in the follower guides.

Instead of sliding shoes (flat face followers), follower rolls (Fig.) are used, in which case or rolls mounted on rollers, needle bearings or antifriction bearings (Fig.). Value of the central angle of rotation can be varied by offsetting the axis of rotation of the cam, since an increase in the offset angle 5 (Fig.) reduces the central angle of rotation A ex [see equation (3)].

In practice, the maximum overall size of the cams has been established for each existing model of machine tool and is dictated by the maximum diameters of the cam blanks. In designing setups for machine tools under these conditions, it is good practice to use curves with the maximum possible radii for the working travel.

Follower Roll With a Plain Bear	Follower Roll With an Antifriction Bearing

This leads to a reduction in the pressure angle 9 and in the normal force growth factor E on the cam surface. Adjusting the length of follower travel. To change over a earn controlled automatic machine tool for machining workpieces of different sizes, it is usually necessary to change the lengths of travel of the main and sometimes the auxiliary working members.

With the general trend being to extend the field of application of an automatic machine tool and to simplify and speed up changeovers, the choice of a method for  adjusting the stroke depends, not so much on the type and volume of production, as on the conditions under which cam mechanisms are being employed for driving the various working members.