A hollow piston rod is a thick-walled tube bored through, not a solid bar. The bore saves weight and moving mass; on a double-rod cylinder it is also the oil path, so no external line has to flex with the stroke. We supply the tube before plating – cold drawn seamless, stress relieved, straight and round – in 1045, 40Cr or 37Mn5, with the outside diameter set to leave the grinding and chrome allowance your process needs.
Price: quoted per specification. Steel prices move with the raw material market, so every quotation is confirmed at the time of order.
Why the rod is hollow
Moving mass is the reason on fast-cycling machines: every kilogram on the rod has to be accelerated and stopped twice per cycle, and the inertia shows up as heat, noise and seal wear. Oil path is the reason on double-rod cylinders: the bore feeds the piston from the rod end, which removes a hose that would otherwise have to travel with the stroke. Cost is the third reason, because a bored tube starts closer to the finished wall than a solid bar.
The trade is wall thickness. A hollow rod has to carry the same push and pull load as a solid one, so the wall is sized from the buckling and tensile case rather than from the bore you would like.
The pre-plate dimension decides the job
Hard chrome is deposited on the finished diameter, so the tube has to arrive smaller than the rod by the plating thickness plus the grinding allowance. When an enquiry states only the finished rod diameter, the tube will be ordered oversize or undersize and the error is only found after plating.
Write both numbers on the order: the tube outside diameter before plating, and the finished rod diameter with its tolerance from the drawing. If the drawing gives only one, tell us the plating thickness and the grinding allowance and we will confirm the pre-plate size in writing before production.
Grades and their nearest equivalents
| Chinese grade | Nearest foreign designation | Character and use |
|---|---|---|
| 1045 | C45 / S45C / AISI 1045 / GOST 45 – direct equivalents | Induction hardenable, the common rod grade |
| 40Cr | 41Cr4 (1.7035) / AISI 5140 / SCr440 / GOST 40Kh – approx. | Deeper hardening and higher strength after quench and temper |
| 37Mn5 | DIN 36Mn5 (1.1167) approx.; no direct AISI equivalent | Manganese steel with good hardening response; not a standard western rod grade, so confirm it against the drawing |
| E355 | S355 / St52 / 16Mn (Q345, now Q355); 09G2S approx. | Lower strength, weldable, used where the rod is not hardened |
Two of these need a warning. 37Mn5 is widely used in Chinese tube making for casing and cylinder shells, and it is not a grade a European rod drawing will recognise; if your customer’s drawing says 37Mn5, the substitute discussion has to happen before the order. 16Mn was renamed Q355 in GB/T 1591-2018, so an old drawing and a new mill certificate can carry two different names for the same steel.
What to specify in the order
| Item | What to write | If left out |
|---|---|---|
| Outside diameter | Tube OD before plating, and finished rod OD with tolerance | Plating and grinding cannot hit the drawing |
| Bore | Inside diameter and tolerance; oil passage or not | Wall, weight and oil flow are all guessed |
| Grade | 1045, 40Cr, 37Mn5 or E355 | Hardening response and certificate may not match |
| Condition | +SR stress relieved | Straightness moves after machining and hardening |
| Straightness and roundness | Deviation per metre; roundness for the seal | Seal leaks and the rod binds in the gland |
| Length and ends | Cut length, chamfer, thread allowance if any | Extra machining and scrap |
Plating and hardening are downstream
We supply the tube to the pre-plate size. Hard chrome plating, grinding and induction hardening are normally done by the cylinder maker or a plating shop, and they are quoted as separate operations with their own tolerances. Keeping them separate is what lets you compare tube prices honestly: a plated rod price mixes steel, machining and plating, and hides which of the three is expensive.
Testing and packing
Chemistry per heat, tensile and elongation per lot, dimensions, wall, straightness and roundness by inspection. Tubes are oiled, end-capped and bundled with padded strapping, because the outside diameter is the surface that will be ground and plated. Certificates to EN 10204 3.1 carry the heat number and the measured values.