Understanding Kinematic Pairs

In the study of kinematics, a kinematic pair is a joint between two links in a mechanism. Pairs are classified based on the type of contact between the two links.

Types of Kinematic Pairs

Kinematic pairs are broadly classified into two types:

• Lower Pair: A pair that has surface contact between the two links.
• Higher Pair: A pair that has point or line contact between the two links.

Analyzing the Given Options

Let's examine each option provided to determine if it is a lower pair or a higher pair:

1. Prismatic pair:

   This type of pair allows relative sliding motion between the links. The contact between the links is over a surface (e.g., a piston sliding in a cylinder). This is an example of a lower pair.

2. Helical and cylindrical pair:

   A helical pair, like a screw and nut, allows relative helical motion (a combination of rotation and translation). The contact is between the threads, which is a surface contact. A cylindrical pair allows both rotation and translation along the axis, again with surface contact (e.g., a shaft in a cylindrical bearing). Both helical and cylindrical pairs are examples of lower pairs.

3. Revolute pair:

   This pair allows relative rotational motion about an axis. The contact between the links is over a surface (e.g., a pin joint). This is an example of a lower pair.

4. Rolling ball on a flat surface:

   When a ball rolls on a flat surface, the theoretical contact is at a single point (or a line if considering a cylinder rolling). In reality, there might be slight deformation leading to a small area of contact, but kinematically, it is treated as point contact. This type of contact classifies it as a higher pair.

Conclusion

The question asks for the pair that is NOT an example of a lower pair. Based on our analysis, the prismatic pair, helical/cylindrical pair, and revolute pair all involve surface contact and are thus lower pairs. The rolling ball on a flat surface involves point/line contact and is therefore a higher pair.

Thus, the pair that is NOT an example of a lower pair is the rolling ball on a flat surface.