Seal journals · Ra

Shaft finish for lip seals

The surface finish and diameter tolerance the configurator applies to a rotary lip-seal journal: 16 µin Ra at h8, 32 µin Ra at h11, with the computed limits at common seal diameters and the processes whose roughness range reaches each target.

A rotary lip seal runs on the shaft, so the journal under it is a wear surface as much as a size. Too rough and the lip is abraded and weeps; the finish target on a seal journal is therefore tighter than on a bearing seat, where the surface is clamped rather than rubbed. Both the size tolerance and the finish go on the same diameter, and they are chosen together.

The configurator offers two fit classes on a seal journal, h8 and h11, and pairs each with a finish target: h8 carries 16 µin Ra, h11 carries 32 µin Ra. Both are h-classes, so the upper deviation is zero and the shaft is never over nominal; the grade sets how far under it may run. At 1″ that is -.0013 for h8 and -.0051 for h11.

Set either class on a seal diameter and the drawing carries the computed ISO 286 limits for that exact nominal together with the finish target; the tables below are those numbers at the seal diameters a shaft shop meets most, and the processes whose published roughness range reaches each target.

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Finish targets by application and fit class

Ra in microinches, from the configurator finish targets. The seal journal is listed first; bearing seat and bushing journal are shown for contrast.

h8 and h11 limits at common seal diameters

Deviations from nominal in inches at 0.0001 in resolution; max and min are the finished diameter limits. Metric nominals are converted and the limits shown in inches.

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Processes whose published Ra range reaches each target

Ra ranges in microinches from the same process table as the surface finish guide; a process is marked when its finest published Ra is at or below the target.

Two targets, and what each is for

h8 is the closer class and carries the finer finish (16 µin Ra); h11 is the wider class and carries 32 µin Ra. For contrast, every bearing-seat class in the configurator targets 32 µin Ra, the same as h11 on a seal journal: a bearing ring is pressed on and stays put, so size matters more than texture. A bushing journal, which does slide, targets 16 µin at g6, 32 µin at f7, 32 µin at e8.

The finish target is applied by the configurator when it recognises the application; it is a target the drawing states, and you can override the Ra on any diameter. A seal maker's installation sheet may call for a window with a floor as well as a ceiling; where it does, specify that window on the diameter and the drawing carries it.

Processes that reach the target

Of the processes in the surface finish guide, 8 publish a range that reaches 16 µin Ra (Turning / boring, Broaching, Cylindrical grinding, Honing, Polishing, Lapping, Superfinishing, Electrolytic grinding) and 10 reach 32 µin Ra (the same plus Milling, Reaming). Turning spans 16 to 250 µin, so both seal-journal targets sit at its fine end: a finishing pass with a sharp tool, not a routine finish turn. The ranges are the published spans for each process, from Machinery's Handbook; which process a shop uses on a given journal is the shop's call, and the drawing states only the Ra.

Related: Surface finish reference · Shaft fits and tolerances · Pump shafts · Mixer shafts.

Frequently Asked Questions

These references are for specifying a part; every value here is one the configurator applies when you build one. Numbers are representative of the standard; check them against your part and its mating components before ordering. See all guides on the guides page.