CAM & Followers

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CAMS AND FOLLOWERS:

- A cam is a rotating machine element, which gives reciprocating or oscillating motion to another element known as follower.
- The cam and the follower have a line contact and constitute a higher pair.
- The cams are usually rotated at uniform speed by a shaft, but the follower motion is predetermined and will be according to the shape of the cam.
- The cam and follower is one of the simplest as well as one of the most important mechanisms found in modern machinery today.
- The cams are widely used for operating the inlet and exhaust valves of internal combustion engines, automatic attachment of machineries, paper cutting machines, spinning and weaving textile machineries, feed mechanism of automatic lathes etc.
Classification of Followers:

1. According to the surface in contact.

o Knife-edged – not practically used because of extreme wear of the contact surface.
o Flat-faced – used where space is limited e.g. in automobile engines
o Spherical faced - used where space is limited
o Roller- extensively used in aircraft engines

1. According to the motion of the follower

o Translation – all followers are translatory followers i.e. followers reciprocates as the cam rotates uniformly.
o Oscillation – the cam’s motion is converted into osciallatory motion.

1. According to the path of motion

o Radial – passing through the center of the cam
o Offset – offset from the cam center
Classification of Cams:

1. Radial or Disc cams – the follower moves in the direction perpendicular to the cam axis.
2. Cylindrical cams – the follower moves parallel to the cam axis. The follower rides in a groove at its cylindrical surface.

Base circle:

It is the smallest circle that can be drawn to the cam profile.
Trace point:

It is the reference point on the follower and is used to generate the pitch curve. In case of knife-edge follower, the knife-edge represents the trace point and pitch curve corresponds to the cam profile. In a roller follower, the center of the roller represents the trace point.


Pressure angle:

It is the angle between the direction of the follower motion and a normal to the pitch curve. This angle is very important in designing of a cam profile. If the pressure angle is too large, a translating follower will jam in its bearings.
Pitch point:

It is a point on the pitch curve having the maximum pressure angle.
Pitch circle:

It is a circle from the cam center through the pitch points.
Prime circle:

It is the smallest circle from the cam center through the pitch curve.
Lift or Stroke:

It is the maximum travel of the follower from its lowest position to the topmost position.
Motion of the Follower:

During its travel,

1. Uniform velocity
2. Simple harmonic motion
3. Uniform acceleration and retardation
4. Cycloidal motion

Construction of Cam Profiles:

- In order to draw the shape of the cam profile, the displacement diagram is drawn first of all.
- The important point to remember is that the follower must rotate in the opposite direction to the cam rotation.
- Then by constructing the follower in its proper position at each angular position, the profile of the working surface of the cam is drawn.

Tangent Cam with Reciprocating Roller Follower:

When the flank of the cam is straight and tangential to the base circle and nose circle then the cam is known as a tangent cam. Such type of cams is used for operating the inlet and exhaust valves of internal combustion engines.