Question

The radius of a friction circle for a shaft of radius r rotating inside a bearing is

(a) r sin φ

(b) r cos φ

(c) r tan φ

(d) r cot φ

I had been asked this question in a dream while sleeping

My question is taken from Friction and Double Hooke’s Joint in section Friction of Machine Kinematics

Answer 1

Right option is (a) r sin φ

Easiest explanation: If a circle is drawn with centre O and radius OC = r sin φ, then this circle is called the friction circle of a bearing. The force R exerted by one element of a turning pair on the other element acts along a tangent to the friction circle.