A linear shaft and a piston rod are both ground steel bars with a hard-chromed surface. They are not interchangeable by default. A linear shaft can serve as a piston rod only where the rod is a plain guiding element, carries light to moderate load, and needs little or no end machining. Where the ends need heavy machining or the load calls for a quenched-and-tempered base, the substitution fails.
Are a linear shaft and a piston rod the same part?
They are made on similar equipment and share the same chrome finish, but they are specified against different duty. In bar form, the difference is the base condition under the chrome and the condition of the ends.
A linear shaft is a guiding element. Its job is to hold diameter, straightness and surface finish so that a linear bearing or bushing runs on it, and it is normally supplied cut to length with the ends left as sawn. Yixin Precision Machinery (Changzhou) makes the 45 Steel Chrome-Plated Linear Shaft on the same route as its 45 Steel Induction-Hardened Chrome Shaft: 45 steel, an induction-hardened case of at least HRC55 to about 2 mm deep, then hard chrome. A diameter band of 0/−0.02 mm is typical for linear-bearing fits.
A piston rod works inside a cylinder. Its ends are usually machined — threaded, shouldered or fitted with an attachment — and the load is often axial and reversing. That combination is met by a quenched-and-tempered base rather than an induction-hardened case. In the Yixin range it is covered by the 45 Steel Q&T Chrome-Plated Bar, the 40Cr Q&T Chrome-Plated Bar and the 42CrMo Q&T Chrome-Plated Bar.
So the two bars can be dimensionally identical and still not be the same part. The question is not what the bar looks like, but what the base has to do and how much machining follows supply.
When can a linear shaft replace a piston rod?
When the rod works as a plain guide, the load is light to moderate, and little or no machining follows supply.
Yixin states the split plainly. The 45 Steel Induction-Hardened Chrome Shaft is chosen when the rod surface takes the wear and guidance load and little machining follows after supply. If the rod ends need heavy machining, or the load calls for a quenched-and-tempered base, the catalogue points to the Q&T chrome-plated bars in 45 steel, 40Cr and 42CrMo. The 45 Steel Chrome-Plated Linear Shaft follows the same process route as the induction-hardened chrome shaft, so the same logic applies to it: it is a guiding bar, supplied cut to length with sawn ends.
The substitution holds where the drawing shows a plain cylindrical bar, the running surface takes the wear, and the ends stay sawn or receive only light work. It stops holding as soon as the drawing shows machined features at the ends, or the duty is one the catalogue places in the heavy or shock load group.
| Yixin product | Base condition | Surface | Where it fits rod work |
|---|---|---|---|
| 45 Steel Chrome-Plated Linear Shaft | 45 steel, induction hardened | case of at least HRC55, about 2 mm deep, then hard chrome; 0/−0.02 mm typical for linear-bearing fits | plain guiding duty with little end machining |
| 45 Steel Induction-Hardened Chrome Shaft | 45 steel, induction hardened | case of at least HRC55, about 2 mm deep, then hard chrome | rod surface takes the wear and guidance load, little machining after supply |
| 45 Steel Chrome-Plated Soft Shaft | 45 steel, not induction hardened, base about HRC17 | hard chrome | light-duty rods whose ends need substantial machining |
| 45 Steel Q&T Chrome-Plated Bar | 45 steel, quenched and tempered | surface HRC25–28 or HRC28–32 per order, then hard chrome; core is softer | rods that need a quenched-and-tempered base |
| 40Cr Q&T Chrome-Plated Bar | 40Cr, quenched and tempered | HRC28–35, then hard chrome; made to order | higher base strength with a machined-end drawing |
| 42CrMo Q&T Chrome-Plated Bar | 42CrMo, quenched and tempered | HRC30–38, then hard chrome; made to order | heavy or shock loads |
What changes when the load is heavy or shock?
The base condition changes, not the chrome. Heavy or shock loading moves the specification away from an induction-hardened case and onto a quenched-and-tempered bar.
For the 45 Steel Q&T Chrome-Plated Bar, the figure to work to is surface hardness, HRC25–28 or HRC28–32 chosen per order. That range is a surface figure. The core is softer, because grade 45 has limited hardenability, so it should not be read as core, through or uniform hardness. Where more base strength is needed, the 40Cr Q&T Chrome-Plated Bar is quoted at HRC28–35, and the 42CrMo Q&T Chrome-Plated Bar at HRC30–38.
The 42CrMo grade is made to order, from 1 t, with a 7–10 day lead time, and it is the bar listed for heavy or shock loads. Both the 40Cr and 42CrMo chrome-plated bars are made to order. A useful check at quotation stage: if the rod is a pressure-carrying member of a cylinder, or the drawing calls out a quenched-and-tempered base, treat the linear shaft as out of scope and specify a Q&T bar instead. If the rod only guides, the linear shaft remains a valid starting point.
Does the chrome layer decide the answer?
No. Chrome is applied to all of these bars, so the presence of chrome tells you nothing about whether a bar suits rod duty or linear-bearing duty. What the chrome layer provides is a wear surface. What decides the application is the base under it.
Chrome thickness is 10–50 μm per side and is chosen per order, with a chrome hardness capability of HV800+. Surface finish is Ra ≤ 0.4 μm on chrome, and Ra ≤ 0.8 μm on ground, unplated bar.
The practical consequence for a buyer is the order of comparison. When two quotations for a substitute rod are placed side by side, compare the base condition and the end allowance first, then the chrome. A quotation that lists only chrome thickness is not yet a rod specification. Plating is carried out by long-term partner plants to Yixin's specification, followed by final polishing, and chrome thickness, diameter and surface are checked before packing. If a rod end will be machined after plating, say so at enquiry stage so the base and the plating allowance can be set together.
How do I verify a substitute bar on receipt?
Check the items the drawing already fixes — diameter and tolerance, chrome thickness, straightness, length and end condition, and packing — against the mill certificate and the confirmed order.
| Check | Target | Method |
|---|---|---|
| Diameter | Ø4–11: −0.02/−0.05 mm; from Ø12: 0/−0.02, −0.01/−0.03 or −0.02/−0.05 mm | micrometer at several positions along the bar |
| Chrome thickness | 10–50 μm per side, chosen per order | plating record checked against the confirmed order |
| Straightness | about 0.1 mm/m from Ø25; about 0.2 mm/m below | dial gauge on a surface plate or in V-blocks |
| Length and ends | cut to length from 200 mm; ends left as sawn | tape or rule at delivery |
| Packing and documents | bubble film and wooden cases for export; no steel straps on finished bars | visual at unpacking, against the packing list |
Two points deserve attention when a linear shaft is standing in for a rod. First, the ends are sawn, not finished, so any thread or shoulder is your operation and has to be planned before the bar is ordered. Second, straightness and finish matter most where a seal or a bearing runs on the surface; where the rod is a plain guide, diameter and straightness carry the specification.
What to put in your RFQ
- The product you want, named with its base condition: 45 Steel Chrome-Plated Linear Shaft, an induction-hardened chrome shaft, or a Q&T chrome-plated bar in 45 steel, 40Cr or 42CrMo.
- Diameter and length, with the tolerance band from the drawing.
- Chrome thickness per side, within the 10–50 μm range, and the chrome hardness requirement.
- Straightness and surface finish, where the bar runs in a bearing or under a seal.
- End condition: sawn ends as supplied, or a description of the machining you will carry out, with the finished length you need from 200 mm upward.
- Order quantity; minimum order is 1 t, and samples of 20–50 kg are available.
- Packing and documentation requirements: export packing is bubble film in wooden cases, with no steel straps on finished bars.
Summary
A linear shaft and a piston rod are not the same part, even when the bar dimensions match. The linear shaft is a guiding element with an induction-hardened case on 45 steel, a diameter band of 0/−0.02 mm typical for linear-bearing fits, and sawn ends. Piston-rod duty usually combines machined ends with axial, reversing or shock loading, which points to a quenched-and-tempered base — 45 steel at HRC25–28 or HRC28–32 surface hardness, 40Cr at HRC28–35, or 42CrMo at HRC30–38 for heavy or shock loads.
The substitution is valid in one direction and under conditions: a linear shaft can replace a piston rod where the rod only guides, the load is light to moderate, and no machining follows supply. It is not valid where the ends must be machined or the load calls for a quenched-and-tempered base. Decide on the base condition first, then fix chrome thickness, tolerance and straightness on the drawing, and verify those same items on receipt.