radial cam nomenclature

RADIAL CAM NOMENCLATURE

The terminology of a radial cam with reciprocating roller follower is illustrated in Fig.3.11.

With reference to Fig.3.11, the various terms used in the study of cam are presented in Table 3.1.

Table 3.1. Terms used in radial cams

1. Cam profile:

• The working surface of cam which comes into contact with follower is known as cam profile.

• In simple terms, the cam profile gives outline of the cam.

• The cam profile is represented by the curve in a two-dimensional space.

2. Base circle:

• Base circle is the smallest circle that can be drawn tangential to cam profile from the centre of rotation of cam.

• Base circle diameter decides the overall size of the cam and thus it is a fundamental parameter of the cam.

• The radius of base circle is also called the least radius of the cam.

3. Trace point:

• Trace point is the theoretical reference point on the follower which is used to generate pitch curve.

• Location of trace point:

• For a knife-edge follower, the trace point is at the knife-edge.

• For a roller follower, the trace point is at the roller centre.

• For a flat-faced follower, the trace point is at the point of contact at which the follower is in contact with cam on base circle.

4. Pitch curve:

• For the purpose of laying out the cam profile, it is assumed that the cam is fixed and the follower rotates around the cam.

• The locus or path of trace point is called pitch curve.

• In other words, when the cam is held fixed and the follower is rotated in a direction. opposite to that of the cam, then the curve generated by the locus of the trace point is called the pitch curve.

• For a knife-edge follower, the pitch curve and the cam profile are same, whereas for a roller follower, they are separated by the radius of the roller.

5. Prime circle: The smallest circle that can be drawn tangent to the pitch curve is known as prime circlė.

6. Pressure angle (ϕ):

• Pressure angle is the angle between the line of action of the follower and corresponding normal to the pitch curve through trace point.

• The pressure angle is very important in cam design as it measures the effectiveness of cam to transfer driving force to the follower.

• The pressure angle varies from maximum to minimum during complete rotation.

■ Higher value of pressure angle results in higher value of side thrust at the guides and hence higher the chances of jamming the translating follower in its guide ways.

■ The pressure angle should be as small as possible within the limits of design.

■ The maximum pressure angle should be less than 30° for cams with reciprocating followers, whereas it should be less than 45° for low speed cams with oscillating followers.

• The pressure angle can be reduced by increasing the cam size or by adjusting the offset. For more details, refer to Section 3.16.

7. Pitch point: Pitch point is a point on pitch curve at which the pressure angle is maximum.

8. Pitch circle: Pitch circle is the circle passing through the pitch point and concentric with the base circle.

9. Lift (or stroke) of follower: Lift, also known as stroke or throw, is the maximum distance (or angle) through which the follower moves (or rotates).

10. Cam angle (θ);

• Cam angle is the angle of rotation of the cam for a definite displacement of the follower.

■ Angle of rise (or angle of ascent or outstroke angle) (θ₀): The angle through which cam rotates when follower begins to rise till it reaches its highest position, is known as angle of rise or outstroke angle (θ₀).

■ Angle of dwell (θ_d): The angle through which cam rotates while follower remains stationary at the highest or the lowest position, is known as angle of dwell (θ_d).

■ Angle of return (or angle of descent) (θ_r): The angle through which cam rotates during the time follower returns to the initial position, is known as the angle of return or angle of descent (θ_r).