AN INTRODUCTION TO CAMS (TOM Shortnotes)

CAMS

Introductory points about CAMS:

• Cam is a mechanical member used to impart desired motion to a follower by direct contact.
• Cam may be rotating or reciprocating whereas follower may be rotating, reciprocating or oscillating,
• Widely used in automatic machines. internal combustion engines, machine tools, printing control mechanism, and so on.
• Usually manufactured by die-casting/milling/punch-presses.
• A cam and follower pair belong to category of higher pair.

Necessary elements of a CAM mechanism:

1. Driving member -CAM

2. Driven member - FOLLOWER

3. A frame which supports the cam and guides follower

Types of CAMS:

Cams are classified according to:

1. Shape      2. Follower movement       3. Manner of constraint on the follower

According to shape:

A) Wedge and Flat Cams:

CAM - Translational motion

Follower - Translation/Oscillation

• Spring is used to maintain the contact between cam and the follower

• Instead of using a wedge type cam, a flat plate with a groove can also be used (positive drive is achieved without the use of spring)

B) Radial or Disc Cams:

• Simple & Compact
• It's a cam in which the follower moves radially from the centre of rotation of the cam

C) Spiral CAMS:

• A Face CAM in which a groove is cut in the form of a spiral.

• Consists of teeth which mesh with pin gear follower.

• Velocity of the follower is proportional to the radial distance of the groove from the axis of the cam.

• Generally used in computers.

• It's usage is limited since CAM has to be reversed in direction to reset the position of the follower.

D) Cylindrical CAMS:

Also called as barrel or drum CAM

In a cylindrical cam, cylinder which has circumferential contour cut in the surface rotates about its axis. The follower motion can be of two types:

1. Constrained oscillating motion (when on CAM surface Groove is cut on the)

2. Translation along or parallel to the axis of the rotating cylinder (here CAM is an end cam i.e. the end of the cylinder is working surface.

 

E) Conjugate CAMS:

It is a double disc cam, the two discs being keyed together and are in constant touch with the two rollers of a follower. Hence follower has a positive constraint

Preferred when the requirements are low wear, low noise, better control of the follower, high speed, high dynamic loads etc.

F) Globoloidal CAM:

For moderate speeds with large angle of oscillation for the follower.

Two types of surfaces: Concave and Convex (in the figure below convex surface is shown)

A circumferential contour is cut on the surface of rotation of the cam to impart motion to the follower (oscillations)

G) Spherical CAMS:

The follower oscillates about an axis perpendicular to the axis of the cam rotation.

According to follower movement:

Note: Dwell = zero displacement or absence of motion of the follower during the motion of the CAM

A) Rise-Return-Rise (R_R-R):

There is alternate rise and return of the follower with no dwells (very limited use in industry)

B) Dwell-Rise-Return-Dwell (D-R-R-D)

Rise and return of the follower after the dwell

C) Dwell-Rise-Dwell-Return-Dwell (D-R-D-R-D):

Most widely used in industry

According to manner of constraint on the Follower:

To reproduce exactly the motion transmitted by the CAM to the follower, it is necessary that the two remain InTouch at all the speeds and at all the times.

A) Pre-loaded spring CAM:

B) Positive drive CAM:

Roller follower operating in groove of a cam used. The follower can't go out of this groove under the normal working operations

C) Gravity CAM:

Rise of the follower achieved by rising surface of a cam and the return by the force of gravity or due to weight of the follower.

References:

"Theory of Machines" by SS RATTAN