Finish Type and Lead

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

1Mount shaft in holding chuck
2Eliminate any wobbling or runout, level shaft
3Loop a thin thread over the shaft
4Attach a 1 oz (30 g) weight to thread with 2/3 contact
5Set shaft rotation to 60 RPM
6Observe thread movement in axial direction
7Record results in both directions of rotation


Interpretation of Thread Movement

InterpretationCW RotationCCW Rotation
Right Hand LeadMoves from chucked end to free endMoves from free end to chucked end
Left Hand LeadMoves from free end to chucked endMoves from chucked end to free end
No LeadStationaryStationary
On-Level ShaftMoves in same direction no matter shaft rotation.
Remounting the shaft end-for-end doesn’t help.
Same as CW
Tapered ShaftMoves in same direction no matter shaft rotation.
Remounting the shaft reverses direction.
Same as CW
Cusped ShaftMoves toward centerMoves toward center
Crowned ShaftMoves away from centerMoves away from center

Parameters Affecting Sealing:

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