Question

What is the primary reason for using a key in shaft and gear assemblies?

• A. To reduce friction between shaft and gear
• B. To transmit torque without relative rotation
• C. To allow axial movement of the gear
• D. To increase the shaft diameter

Hint:

A key is designed to act as a safety link.
In case of extreme torque overload, the key is designed to shear first, protecting the much more expensive shaft and gear from major mechanical failure.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary function of a mechanical key in rotating machine element assemblies such as shafts, gears, pulleys, and couplings.

Step 2: Key Formula or Approach: This is a standard machine design concept. A key is a temporary fastening element inserted into matching keyways cut into the mating shaft and hub.

Step 3: Detailed Explanation:

• When power is transmitted from a rotating shaft to a gear (or vice versa), there is a tendency for the gear hub to slip rotationally relative to the shaft.

• To prevent this relative rotation and ensure positive, slip-free drive transmission, a mechanical key is used.

• The key is placed half inside a slot cut on the shaft (shaft keyway) and half inside a slot cut in the gear hub (hub keyway).

• This positioning physically locks the shaft and gear together, allowing torque to be transmitted safely.

• While some specialized keys (like feather keys or splines) can permit relative axial movement along the shaft, the fundamental and primary reason for using any key is to transmit torque without relative rotational slippage.

Step 4: Final Answer:
The primary reason for using a key in shaft and gear assemblies is to transmit torque without relative rotation.