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

Step	Description
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
No Lead	Stationary	Stationary
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