Cams and Followers(1)

INTRODUCTION

The cam is rotating machine element parts which give reciprocating or oscillating motion to another machine element known to a follower. The cam and follower have a line contact and constitute them with a higher pair. The cams are usually rotated at a uniform speed by shaft but follower motion is already predetermined and will be according to the shape of the cam

Classification of Followers

According to the surface in contact.
(a) Knife edge follower: When the contacting end of given follower has sharp knife edge it is called a knife edge follower as shown in fig (a). The sliding motion takes between contacting surfaces i.e. the knife edge and the cam surface. It is seldom used in practice because the small area of that contacting surface results in more wear. In knife edge followers there is a considerable side thrust that remains between the follower and guide.

(b) Roller follower: When the contacting end of the follower is a roller then it is called roller follower as shown in fig (b). Since the rolling motion takes place between the contacting surfaces, therefore, the rate of wear is reduced greatly. In roller followers also the side thrust remain between the follower and guide. The roller followers are mostly used where more space is available such as in oil engines and aircraft engines.

(c) Flat faced or mushroom follower: When the contacting end of the follower is a perfectly flat face then it is called a flat-faced follower as shown in fig (c). It may be noted that side thrust between the follower and guide is much reduced in the case of flat faced followers. The only side thrust is due to the friction between the contact surfaces of follower and cam. The relative motion between these surfaces is highly of sliding nature but wear can be reduced by offsetting the axis of the follower as shown in fig (f), so that when the cam rotates follower then it also rotates about its own axis. The flat faced followers are generally used where space is limited such as in cams which operate the valves of automobile engines. When the flat faced follower is circular then it is then called a mushroom follower.

(d) Spherical faced follower: When the contacting end of the follower is of spherical shape then it is called a spherical faced follower as shown in fig (d). It may be noted that when a flat-faced follower is used in automobile engines but high surface stresses are produced. In order to minimize these stresses, the flat end of the follower is machined to a spherical.

Fig: Classification of followers

According to the motion of the follower
(a)Reciprocating or translating follower: When the follower reciprocates in guides as the cam rotates at uniformly then it is known as reciprocating or translating follower. The followers all shown in fig (a) to (d) are all reciprocating or translating followers.

(b)Oscillating or rotating follower: When the uniform rotary motion of cam is translate into predetermined oscillatory motion of the follower then it is called oscillating or rotating follower. The follower, as shown in fig (e), is an oscillating or rotating follower.

According to the path of motion of follower.
(a) Radial follower: When the motion of the follower is along an axis passing through the centre of the cam then it is known as a radial follower. The followers as shown in fig (a) to (e) are all the radial follower.

(b) Off-set follower: When the motion of the follower is along the axis that away from the axis of the cam centre then it is called off-set follower. The follower as shown in fig ( f ) is an off-set follower.

Based on modes of Input and Output motion

• Rotating cam – Translating follower
• Rotating cam – Oscillating follower
• Translating cam – Translating follower

As the cam rotates the follower moves upward and downward.
a) The upward movement of follower is known to rise i.e Outstroke
b) The downward movement is called fall i.e. Return stroke

Types of follower motion

• Uniform motion
• Uniform acceleration,retardation motion
• Cycloidal motion
• Simple harmonic motion (SHM)

Classification of Cams

i) Radial cam: In radial cams, the follower oscillates in a direction normal to the cam axis. The cams are as shown in fig are all radial cams.

ii) Cylindrical cam: In cylindrical cams, the follower oscillates in a direction parallel to cam axis. The follower rides in the groove at its own cylindrical surface. A cylindrical grooved cam of a reciprocating and an oscillating follower is shown in fig (a) and (b) respectively.

iii) End cams: It is also similar to cylindrical cams but the follower makes contact at the periphery of the cam as shown in fig.

Fig: Cylindrical cam.

Terms Used in Radial Cams

• Base circle: It is the smallest circle that can be drawn to cam profile.
• Tracepoint: It is a reference point on the follower and is used to generate pitch curve. In the case of knife edge, follower knife edge denotes the trace point and pitch curve corresponding to the cam profile. In a roller follower, the centre of the roller denotes trace point.
• Pressure angle: It is the angle between the direction of the follower motion and a perpendicular to pitch curve. This angle is very important in most designing a cam profile. If the pressure angle is large then a reciprocating follower will jam in its bearings.
• Pitch point: It is a point on that pitch curve having the maximum pressure angle.
• Pitch circle: It is that circle drawn from the centre of the cam by the pitch points.
• Pitch curve: It is that curve generated by the trace point as the follower moves relative to given cam. For a knife edge, follower the pitch curve and the cam profile are same where for a roller follower they are separated by the radius of the roller.
• Prime circle: It is the smallest circle the smallest circle that can be drawn from the centre of the cam and tangent to the pitch curve. For a knife edge and a flat face follower, the prime circle and the base circle are identical. For a roller follower, the prime circle is larger than the base circle by the radius of the roller.
• Lift or stroke: It is the maximum travel of the follower from its lowest position to the topmost position.
• Dwell: When the follower is not moving upward and downward even when the cam rotates, it is called dwell.

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Fig: Terms used in radial cams

References:
1. H.H. Mabie and C. F. Reinholtz, “Mechanism and Dynamics of Machinery”, Wiley.
2. J.S. Rao & R.V. Dukkipati Mechanisms and Machine Theory, New Age International (P) Limited..
3. J.E. Shigley and J.J. Uicker, Jr., “ Theory of Machines and Mechanisms”, McGraw Hill.
4. B. Paul, “Kinematics and Dynamics of Planar Machinery”, Prentice Hall.
5. C. E. Wilson, J.P. Sadler and W.J. Michels, “Kinematics and Dynamics of Machinery”, Harper Row.