Question

A screw gauge is provided with a “ratchet” arrangement, in the form of a knob at the right end of the spindle. The screw should be rotated by this knob only. The purpose of the device is-
A. To reduce zero error.
B. To prevent backlash error
C. To control the rate of rotation of the screw.
D. To prevent damage to the pitch of the screw.

Answer 1

Hint: The ratchet prevents unnecessary spindle movement. The pitch of the screw depends on the spindle rotation.

Complete answer:
Screw gauge is basically a screw with a constant pitch. It consists of a U-shaped piece of solid steel one arm of which carries a stud while the other arm, the spindle, carries a hollow cylinder within which an accurate screw having a plane face at its send, can move. The spindle is moved and adjusted using the thimble. The ratchet is provided at the end of the cylindrical tube. It acts as a limiting device which applies pressure by slipping at a predetermined torque, thereby preventing the spindle from moving further. As the pitch of the screw depends on the rotation of the spindle. The ratchet restricts the spindle from moving more than necessary, preventing damage to the pitch of the screw.

So, the correct answer is “Option D”.

Note:
The ratchet can only stop backward motion at discrete points, so it does allow a limited amount of backward motion or backlash. Hence, option B is incorrect.
The zero error of a screw gauge is due to mis-ling of the zero-line of the circular scale and the reference line. This is due to wear of the device and cannot be correct by the use of ratchet. So, option A is incorrect.
The spindle and thereby the screw is rotated by using the thimble. So the rate of rotation of the screw is not controlled by the ratchet. Hence, option C is incorrect.