SKF
Application Parameters
Critical application parameters include shaft surface finish (typically Ra 0.2-0.8 µm), hardness (minimum 45 HRC), dynamic runout, housing bore tolerance, and the pressure differential across the seal. SKF defines acceptable ranges for each parameter. A shaft that is too rough wears the lip prematurely; too smooth can prevent adequate lubrication and cause overheating. Excessive runout or misalignment causes uneven lip contact and leakage. Defining these parameters accurately and ensuring they are met during machining and assembly is just as important as selecting the seal itself. SKF provides guidelines for shaft and housing design to achieve long seal life.
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Shaft Surface Finish Ra
0.2‑0.8 µm for NBR/FKM; 0.1‑0.4 µm for PTFE.
Shaft Hardness
Minimum 45 HRC; for abrasive environments, ≥55 HRC or wear sleeve.
Dynamic Runout (TIR)
≤0.3 mm for standard; ≤0.1 mm for PTFE lips.
Housing Bore Tolerance
H8 for metal case; H7 for rubber‑covered OD.
Lead‑In Chamfer
15‑20° and polished to prevent lip cutting during assembly.
Misalignment Static
Maximum permissible offset between shaft and housing centre.
The Danger of a Shaft That Is Too Smooth
A mirror‑finished shaft (Ra < 0.05 µm) cannot maintain an oil film under the lip because the surface lacks the microscopic valleys that hold lubricant. The lip runs dry, heats up, and hardens. SKF’s specification calls for a controlled roughness — not too rough, not too smooth — optimised for the seal material.
Measuring Shaft Lead Angle and Its Effect on Seal Leakage
Thread‑like grinding marks (lead) act like a pump, screwing oil past the lip. SKF recommends a plunge‑ground or super‑finished shaft with no lead. A simple thread‑detection test with a strip of paper on the rotating shaft reveals lead. If present, the shaft must be re‑ground or a wear sleeve installed.