Mechanical Engineering
• # Cam Terminology and Displacement Diagram

(1 review)

A cam is a mechanical member used to impart desired motion or displacement to a follower by direct contact.  It is widely used inautomobile industries to direct opening and closing of inlet and exhaust valves at desire time. Cams are either in rotary or reciprocating or oscillating motion. The driver member in cam follower mechanism is called cam and driven member is called follower. Cam mechanism belong to higher pair mechanism because the cam and follower makes point contact. When we study about cam follower mechanism, some common terminology like prime circle, base circle, pressure angle etc. are used to describe a cam. These are as follow.

Cam Terminology and Displacement Diagram:

Terminology:

Base Circle:

Base circle is the smallest circle that can be drawn tangentially to the cam profile.

Trace Point:

A trace point is a theoretical point on the follower. It’s motion describing the movement of the follower. For example a knife edge follower, the trace point is at the knife edge.

Pitch Curve:

It is the curve drawn by the trace point assuming that the cam is fixed and the trace point of the follower rotates around the cam.

Pressure Angle:

It represents the steepness of the cam profile. The angle between the direction of the follower movement and the normal to the pitch curve at any point is called pressure angle.

Pitch Point:

A pitch point correspond to the point of maximum pressure angle. Its

Pitch Circle:

A circle drawn with its center at the cam center and pass through the pitch point is known as the pitch circle.

Prime Circle:

The prime circle is the smallest circle that can be drawn from the center of cam and it is tangential to the pitch curve.

Displacement Diagram:

Angle of Ascent:

It is the angle through which the cam turns during the time the follower rises.

Angle of Dwell:

Angle of dwell is the angle through which the cam turns while the follower remains stationary at the highest or the lowest point.

Angle of Descent:

Angle of descent is the angle through which the cam turns while follower returns to the initial position.

Angle of Action:

This is the total angle moved by the cam during the time between the beginning of rise and the end of return of the follower.

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1 of 1 member found this review helpful 1 / 1 member

Most of the terminology is correct for the type of cam shown, except that the prime circle is labelled incorrectly in the drawing. But, this is just a detail, so who cares? Right?

The displacement diagram show show that the cam angular position ranges through a full revolution for the curve shown, but again, only a detail. Right?

The stated definition for the Pressure Angle is "It represents the steepness of the cam profile. The angle between the direction of the follower movement and the normal to the pitch curve at any point is called pressure angle." The term "steepness" is not very meaningful. Steep compared to what? This fails to mention that important fact that the pressure angle controls the side thrust on the follower shaft and thus both the bending and friction in the system.

This post provides a lot of terminology for this particular cam type, but it gives no information at all about how to design such a system, either in terms of

(1) system dynamics

(2) system kinematic concerns, such as potential under cutting

(3) contact stress considerations

But again, those are all just details, not the sort of thing engineers are concerned about; right?

DrD