Finish Type and Lead
The finishing process on the shaft will affect the sealability of the system. Recall the Hydrodynamic Effect and the Dynamic Sealing Mechanism discussed earlier. The microscopic effects of how the fluid media reacts at the contact width determines the hardening rate of the elastomer materials. The lead angle present on the shaft affects how the fluid transfers itself along the surface of the shaft. Lead acts as a screw during rotation. If the lead angle is along the direction of rotation and the angle is larger than 0.05°, leakage may occur.
Using the steps listed below, a simple process for determining the lead direction and angle can be followed. Interpret the data to determine the type of lead and the maginitude of the angle.
The type of finishing process is what determines the presence of lead. The roughness specification (See Shaft Specifications) for a radial shaft seal is relatively low requiring a secondary machining of the shaft. The ideal finishing process leaves circumferential lines with no lead. See a variety of finishing processes profiled and graded according to their ability to eliminate lead.
Procedure to Detect Shaft Lead
|1||Mount shaft in holding chuck|
|2||Eliminate any wobbling or runout, level shaft|
|3||Loop a thin thread over the shaft|
|4||Attach a 1 oz (30 g) weight to thread with 2/3 contact|
|5||Set shaft rotation to 60 RPM|
|6||Observe thread movement in axial direction|
|7||Record results in both directions of rotation|
Interpretation of Thread Movement
|Interpretation||CW Rotation||CCW Rotation|
|Right Hand Lead||Moves from chucked end to free end||Moves from free end to chucked end|
|Left Hand Lead||Moves from free end to chucked end||Moves from chucked end to free end|
|On-Level Shaft||Moves in same direction no matter shaft rotation.
Remounting the shaft end-for-end doesn’t help.
|Same as CW|
|Tapered Shaft||Moves in same direction no matter shaft rotation.
Remounting the shaft reverses direction.
|Same as CW|
|Cusped Shaft||Moves toward center||Moves toward center|
|Crowned Shaft||Moves away from center||Moves away from center|