Keyway Surface Roughness

1. Keyway Width Sides (Flanks): Ra = 3.2 μm

• Rationale: This precision grade ensures that the key and the keyway sides can be assembled smoothly and have a sufficiently large actual contact area for transmitting torque. This effectively prevents stress concentration, fretting wear, and the initiation of fatigue cracks.

• Achieving the Finish: This surface roughness can typically be achieved directly through precision milling, broaching, or shaping without the need for additional finishing processes (such as grinding or honing), making it very cost-effective.

• Application: This is the standard selection for the vast majority of general-purpose machinery (e.g., gearboxes, transmission shafts, pulleys, couplings) under conventional load and speed conditions.

2. Keyway Bottom Surface: Ra = 6.3 μm

• Rationale: The bottom surface is a non-working face. Its primary function is to provide radial support and positioning for the key during assembly. An Ra=6.3μm requirement is smooth enough to ensure the key can seat properly at the bottom without interference, yet economical enough as it does not directly contribute to torque transmission.

• Achieving the Finish: This roughness is easily achieved in a single pass through conventional milling.

• Critical Point: For the bottom surface, the quality of the root fillet radius is more critical than the roughness. Machining must ensure the fillet is smooth, free of sharp corners, tool marks, or micro-cracks. These defects are primary sources of stress concentration and can easily lead to shaft failure initiating from the end of the keyway under alternating torque.