Chrome Rod Surface Finish for Seals: Ra, Rz and Rmr Explained

What Ra does a chrome rod need for seals?

Most seal data sheets place the dynamic working surface of a rod between Ra 0.05 µm and Ra 0.4 µm; the lower part of that band serves PTFE seals and the upper part serves elastomer and thermoplastic seals. In the seal-maker data we hold, one source recommends Ra 0.05–0.2 µm for PTFE seals and Ra 0.1–0.4 µm for TPU, TPE and rubber seals. Another gives PTFE seals Ra 0.05–0.2 µm and elastomer and thermoplastic seals Ra 0.1–0.4 µm. The two agree on the window even where their Rz and Rmr figures differ.

Conditions move the number as much as the seal does. Gas media such as air, nitrogen, hydrogen and helium call for tighter values than water and hydraulic oil, and the recommended values also shift with rod material, fluid viscosity and the surface coating. Pneumatic cylinder data is tighter again: Rz 1 µm and Ra 0.2 µm for PTFE and NBR seals, Rz 1.6 µm and Ra 0.4 µm for PU seals, with the bearing ratio Tp added on top.

Ra on its own is not enough to judge a surface, and the seal literature says so directly. Yixin chrome-plated bars are catalogued at Ra ≤ 0.4 µm on the chrome, and ground bars at Ra ≤ 0.8 µm. If your seal data sheet asks for Ra 0.05–0.2 µm, confirm the achievable value and the measurement conditions on the drawing before the order is placed.

Why does Rz matter as much as Ra?

Because a surface can meet an Ra target and still carry peaks that exceed the Rz limit, and Rz above the recommended upper limit increases seal wear. That is why both parameters normally appear together on a rod drawing.

Both seal makers we hold cap Rz. One source applies Rz 1.6 µm max to PTFE seals. The other gives PTFE seals Rz 0.40–1.60 µm and elastomer and thermoplastic seals Rz 0.6–2.50 µm. The numbers differ, but the intent is the same: keep the profile height down.

The failure mode on the rough side is mechanical. Where the rod surface is too coarse, roughness peaks make small cuts or scratches on the seal lip, and the seal fails early. Ra is a single average value and does not show those peaks; Rz is the limit that captures them. When two quotations both pass your Ra requirement, compare them on Rz before you compare anything else, and treat the seal maker's upper Rz limit as the number that decides acceptance.

What does Rmr add?

Rmr, the material ratio, is the parameter that shows whether the peaks on a rod are load-bearing or abrasive. Some seal makers write the same idea as Tp, and each defines its own evaluation depth, so the depth belongs in the specification.

At one extreme, an Rmr near 0% means the surface is mostly high peaks that act abrasively against the seal. At the other, an Rmr near 100% means the surface does not retain a suitable oil film; temperature rises, the seal can deform from heat, and low-pressure systems can leak. At least one of the seal makers we hold evaluates Rmr at a depth of 25% of Rz from a reference line at 5% material ratio. The pneumatic data evaluates Tp at its own depth and reference line. Either way, quoting an Rmr figure without its evaluation depth is meaningless.

These are recommendations, not standard requirements. They vary with rod material, fluid type, viscosity and coating, so the seal maker's data sheet and your drawing govern.

Can a chrome rod be too smooth for a seal?

Yes. A surface that is too smooth does not retain lubrication properly, friction heat builds up, and the seal wears excessively. On a rod, smoother is not automatically better.

The two failure modes sit at opposite ends of the same curve. Too smooth gives friction and heat; too coarse gives small cuts and scratches on the seal lip, and the seal fails early. In pneumatic systems, unsuitable surface roughness causes leakage, so sharp edges and burrs on the rod end have to be removed as well as the roughness controlled.

The parameter that separates the two cases is the material ratio, not Ra. A single Ra number cannot distinguish a surface with a few tall peaks from one that is uniformly flat, and Ra or any single parameter is not sufficient to judge whether a surface is suitable. That is the practical reason chrome rods for seals are bought against Ra, Rz and Rmr together, with the evaluation depth stated.

How should the finish be specified and inspected?

Write all three parameters with their evaluation settings, and inspect incoming bars on the same basis. A finish callout that names only Ra cannot be checked against a seal maker's data sheet.

Measurement direction, sampling length and filter conditions have to be written in the order, because the same surface reports different numbers under different settings. State whether Rmr is evaluated at a depth of 25% of Rz from a reference line at 5% material ratio, or whether you are working to a Tp figure with its own reference conditions. State whether the values you are quoting came from a gas or a liquid application, since gas media call for tighter values than water and hydraulic oil.

Yixin supplies hard-chromed bars with a catalogued chrome finish of Ra ≤ 0.4 µm, and ground bars at Ra ≤ 0.8 µm, across 45 steel, 40Cr and 42CrMo grades. For sealing duty the chrome-plated families are the ones to compare: the 45 Steel Chrome-Plated Soft Shaft, the 45 Steel Induction-Hardened Chrome Shaft and, for heavy or shock loads, the 42CrMo Q&T Chrome-Plated Bar. All figures are standard capabilities confirmed on the drawing and order.

What to put in your RFQ

• Seal material and type, and whether the service is a gas or a liquid; gas media call for tighter values than water and hydraulic oil.
• Target values for Ra, Rz and Rmr — not Ra alone — with the Rmr evaluation depth and reference line stated.
• Measurement direction, sampling length and filter conditions for the roughness check.
• The revision of the seal maker's data sheet the values were taken from.
• Grade and condition, diameter and tolerance, and length, if the application fixes them.
• The surface route required, for example a chrome finish at Ra ≤ 0.4 µm for the sealing surface and a ground finish elsewhere.
• Whether sharp edges and burrs must be removed or chamfered.
• Whether the roughness values should be recorded and reported with the order paperwork.

Summary

Chrome rod finish for seals is a three-parameter specification, not one number. Seal makers put the dynamic sealing surface in a low-Ra band, cap Rz, and ask for an Rmr range that depends on the source, evaluated at a stated depth. A surface that is too smooth loses its oil film; a surface that is too rough cuts the lip; Rmr shows which of the two you have. Specify all three parameters with their measurement conditions, and confirm the values against your seal maker's data sheet and the drawing.