A rod is four specifications wearing one part
Piston rod is usually bought by diameter and length. Four other numbers determine whether it survives: diameter tolerance, surface roughness, chrome layer and hardness, and straightness. A rod that hits diameter and misses the rest will pass incoming inspection and fail at three months.
ISO f7 limit deviations
Shaft tolerances use a lowercase letter. f7 is the conventional class for a chrome-plated piston rod running in a standard rod seal and guide bushing. All values are micrometres and both limits are negative, so an f7 rod is always at or below nominal.
| Diameter range (mm) | f7 upper | f7 lower | Band width |
|---|---|---|---|
| 18 – 30 | -20 | -41 | 21 µm |
| 30 – 50 | -25 | -50 | 25 µm |
| 50 – 80 | -30 | -60 | 30 µm |
| 80 – 120 | -36 | -71 | 35 µm |
| 120 – 180 | -43 | -83 | 40 µm |
| 180 – 250 | -50 | -96 | 46 µm |
Why f7 rather than h6 or g6. The rod has to slide through a seal and a bushing with a film of oil between them. An f class guarantees clearance at all times, which matters because a rod that is even slightly oversize will wipe the seal lip on assembly or generate friction heat in the first hours. h6 would give a line-to-line condition and g6 a tighter sliding fit, both used in specific designs, but f7 remains the general hydraulic default.
Surface roughness on the rod
Rod finish is specified tighter than bore finish, because the rod seal lip is softer than the bore side and more sensitive to abrasion. Ra 0.2 µm maximum is the figure most commonly published for chrome rod, achieved by grinding and superfinishing after plating. Roundness is typically held to half the diameter tolerance band, and straightness to 0.2 mm per 1000 mm.
One subtlety worth writing into the drawing. The rod is polished, and polishing is the mirror-finish problem in reverse. A rod polished to a glassy Ra 0.05 µm can starve the seal lip and increase breakout friction. Where stick-slip is a known issue, ask your seal supplier for their recommended rod finish rather than defaulting to the smoothest number.
Chrome layer and hardness
| Property | Commonly published range |
|---|---|
| Layer thickness | 20 µm minimum; 20 – 30 µm typical; up to 150 µm for rework allowance |
| Surface hardness | 800 – 1000 HV, some suppliers to 1150 HV |
| Induction hardened case | 55 – 60 HRC at 1.2 – 2.5 mm depth |
| Core hardness (C45 Q&T) | 190 – 220 HB |
| Salt spray (NSS, ISO 9227) | 96 hours rating 9 minimum; 120 hours common; 500+ hours for multi-layer systems |
Straightness, the specification people forget
A rod that is straight at the factory and bent on arrival has usually been stored wrong, but a rod that is bent at the factory is a process failure. Straightness is normally quoted as 0.2 mm per 1000 mm. On a long-stroke cylinder this matters more than it appears, because a bowed rod loads the guide bushing on every stroke and the bushing wear debris then attacks the rod surface.
Long rods need to be supported in storage at multiple points and shipped in cradles or individual sleeves. If your incoming inspection measures straightness, measure the rod supported on vee blocks at the ends and rotate it, because a single-point dial reading on a sagging rod will show twice the real error.
Hollow rod: when the hole is worth it
Hollow chrome rod is used on telescopic cylinders and on long strokes where weight and buckling resistance compete. Removing the core reduces mass while the outer diameter keeps the second moment of area high, which improves the slenderness ratio at a given weight. The penalty is the bore of the rod itself, which must be protected against internal corrosion, and the end closures, which have to seal it.
One example from our own floor: a hollow chromed piston rod at OD 125 mm f7, ID 90 mm, 3516 mm long, in C45 quenched and tempered with a 0.03 mm chrome layer. That combination of tight f7 on a large diameter, a deep through-bore and a four-metre-class length is the kind of job where supplier capability becomes the selection criterion rather than price.
What to put on the enquiry
Send diameter and tolerance class, length, material and delivery condition, chrome thickness after grinding, surface hardness target, roughness limit, straightness limit, salt spray requirement with test standard, and annual volume. Six of those eight fields are the difference between a quote you can compare and a quote you cannot.
Our rod scope is on the piston rods page and our heat treatment routes under heat treatment. Measurement capability is listed on inspection equipment. Send full specification to the RFQ page.