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In this post, the third in a series on factors influencing PTFE seal behavior, we are going to look at
dimensional effects on PTFE rotary seal behavior. We will be focusing on industry standard
recommendations for radial clearance and shaft/bore tolerances.

Clearance betweeen the Piston and the Shaft

One of the major issues with seals is seal extrusion, which refers to the seal material forcing its way into available clearances. Extrusion is directly related to system pressure and temperatures. If there is too much clearance, the seal will extrude into that open space when the pressure and temperatures increase. In the table below is a summary of typical pressures and the maximum recommended radial clearance. 


You will notice the clearance between the piston and the shaft gets smaller as the pressure increases.

Piston/Shaft and Bore Tolerance

In spite of the fact that PTFE Flourolon spring-energized seals can compensate for wider tolerances,
attention still must be paid to the piston/shaft and bore tolerance. The table below shows the recommended shaft tolerance for a range of diameters.


The recommended housing bore tolerances are summarized in the next table.


The housing bore should also have a lead-in chamfer between 15° and 30° with a transition radius of
0.031” to prevent damage during installation.


To maximize the life and performance of your PTFE seal, follow the industry standard guidelines for
clearance between the piston and the shaft and tolerances for shaft diameter and housing bore.

Missed Part 1 or Part 2 in the Series? View them here:

Factors Influencing PTFE Seal Behavior: The PV Limit – Part 2 in a 3 Part Series

Factors Influencing PTFE Seal Behavior: The Mating Surface – Part 1 in a 3 Part Series 


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