Key Principle: Power Transmission via Friction
The question asks to identify the key type that transmits power exclusively using frictional resistance. This mechanism relies on the friction generated between the surfaces of the key and the mating components (hub and shaft) to transfer torque.
Analysis of Key Types
- Saddle Key: This type of key fits into a groove on the shaft and rests in a keyway in the hub. It has no keyway cut on the shaft. Power is transmitted solely by the frictional grip between the top surface of the key and the hub, and the bottom surface of the key and the shaft. It is suitable for light loads and non-positive drives.
- Barth Key: Often a type of tangential key, Barth keys are designed to transmit heavy torques. They typically rely on shear strength and bearing pressure, not just friction.
- Kennedy Key: This is generally a type of tangential key or sunk key that fits into matching keyways in both the shaft and the hub. Power transmission involves shearing and bearing stresses, providing a positive drive.
- Tangent Key: Tangent keys are arranged in pairs, tangential to the shaft. They transmit torque through shear and bearing stresses on the sides of the key, acting in pairs tangential to the shaft's circumference. They are not primarily friction-based.
Conclusion on Frictional Transmission
Based on the operating principle, the Saddle key is uniquely designed to transmit power primarily, and often solely, through frictional resistance between the contacting surfaces.
Important Questions from Keys
A key having a square cross-section of side d/4 and length l is used to transmit torque T from the shaft of diameter d to the hub of a pulley. Assuming the length of the key to be equal to the thickness of the pulley, the average shear stress developed in the key is given by
The forces experience by a Key used in a gear train for power transmission is
A key of 14 mm width, 9 mm height and 100 mm length is mounted on a shaft of 50 mm diameter. If the allowable shear stress for the key material is 50 MPa, what is the maximum torque that can be transmitted?
- A key of width and height of 6 mm each is used to fix a gear on a shaft of 20 mm diameter. The shaft is used to transmit 10 kW power at 600 rpm to the gear. Permissible shear stress in the key is 80 N/mm$^2$, while compressive stress in the key is neglected. The minimum length of the key, in mm, is _____(round off to 2 decimal places).
Feather keys are generally _______
