TOOLROOM 2 - Types of cams - additional
DEFINATION OF A CAM
A cam may be defined as a rotating, reciprocating or oscillating machine part, designed to impart reciprocating and oscillating motion to another mechanical part, called a follower.
A cam and follower have, usually, a line contact between them and as such they constitute a higher pair. The contact between them is maintained by an external force which is generally, provided by a spring or sometimes by the sufficient weight of the follower itself.
Classification of Cams
Cams are classified according to :
(a) Shape
(b) Follower movement
(c) Type of constraint of the follower
According to Shape
Wedge and Flat Cams
It is shown in Figures 1.48(a), (b), (c) and (d).
In Figure 1.48(a), on imparting horizontal translatory motion to wedge, the follower also translates but vertically in Figure 1.48(b), the wedge has a curved surface at its top. The follower gets an oscillatory motion when a horizontal translatory motion is given to the wedge. In Figure 1.48(c), the wedge is stationary, the guide is imparted translatory motion within the constraint provided. This results in the translatory motion of the follower in Figure 1.48(d), instead of a wedge, a rectangular block or a flat plate with a groove is provided. When horizontal translatory motion is imparted to the block, the follower is constrained to have a vertical translatory motion.
Further, there is no need to provide a spring in this case as in case (a) and (b). In this case the path of the groove, which causes motion to the follower, constrains the follower to move upward and downward.
Radial or Disc Cam
In radial or disc cams the shape of the working surface (profile) is such that the followers reciprocate in a plane at right angles to the axis of the cam as shown in Figure 1.49(a). It is called radial cam because the motion of the followers obtained is radial (Figure 1.49). A differently shaped radial cam is also shown in Figure 1.49(b).
It is pointed out that the radial cams are very popular due to their simplicity and compactness,
Cylindrical Cams
Cylindrical cams have been shown in Figures 1.50(a) and (b). In Figure 1.50(a) the follower reciprocates whereas in Figure 1.50(b) the follower oscillates. Cylindrical cams are also known as barrel or drum cams.
Spiral Cams
It is shown in Figure 1.51. The cam comprises a plate on the face of which a groove of the form of a spiral is cut. The spiral groove is provided with teeth which mesh with pin gear follower.
This cam has a limited use because it has to reverse its direction to reset the position of the follower. This cam has found its use in computers.
Conjugate Cams
As the name implies, the cam comprises two discs, keyed together and remain in constant touch with two rollers of a follower as shown in Figure 1.52.
This cam is used where the requirement is of high dynamic load, low wear, low noise, high speed and better control of follower.
Globoid Cams
This cam has two types of surfaces: convex and concave. A helical contour is cut on the circumference of the surface of rotation of the cam as shown in Figures 1.53(a) and (b). The end of the follower is constrained to move along the contour and then oscillatory motion is obtained. In this cam, a large angle of oscillation of the follower is obtained.
Spherical Cams
In this cam, as shown in Figure 1.54, the cam is of the shape of a sphere on the peripheral of which a helical groove is cut. The roller provided at the end of the follower rolls in the groove causing the oscillatory motion to the follower in an axis perpendicular to the axis of rotation of the cam.
According to Follower Movement
Rise-return-rise (RRR)
In this type of cam, its profile or contour is such that the cam rises, returns without rest or dwell, and without any dwell or rest, it again rises. Follower displacement and cam angle diagram for this type of cam is shown in Figure 1.55(a).
Dwell, Rise-return Dwell (DRRD)
In this type of cam after dwell, there is the rise of the follower, then it returns to its original position and dwells for sometimes before again rising. Generally, this type of cam is commonly used. Its displacement cam angle diagram is shown in Figure 1.55(b).
Dwell-rise-dwell-return
It is the most widely used type of cam. In this, dwell is followed by a rise. Then the follower remains stationary in the dwell provided and then returns to its original position [Figure 1.55(c)].
Dwell-rise-dwell
As may be seen in the follower-displacement verses cam angle diagram, shown in Figure 1.55(d) in this cam, the fall is sudden which necessities an enormous amount of force for this to take place.
According to Type of Constraint of the Follower
Pre-loaded Spring Cam
For its proper working there should be contact between the cam and the follower throughout its working, and it is achieved utilizing a pre-loaded spring as shown in Figures 1.48(a) and (b), etc.
Positive Drive Cam
In this case, the contact between the cam and the follower is maintained by providing a roller at the operating end of the follower. This roller operates in the groove provided in the cam. The follower cannot come out of the groove, as shown in Figures 1.52 to 1.54.
Gravity Drive Cam
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