A practical buyer's guide to linear shaft selection for B2B procurement. Covers chrome steel vs stainless steel, the HRC 60–64 hardness standard, ISO precision grades (g6, h6), and how to match shafts with linear bearings to avoid premature failure and rework.
BUYER GUIDE — LINEAR MOTION COMPONENTS
Linear Shaft Selection: Material, Hardness, Precision Grades, and Bearing Matching
If you are sourcing linear shaft (linear shafts) for an automation line, a packaging machine, or a custom linear motion system, the specification sheet you send to a supplier matters more than the price per meter. A shaft that is one grade off, or matched with the wrong bearing type, will fail early — and the failure will show up at your customer's site, not in your QC room.
This guide covers the four decisions that determine whether a linear shaft performs as intended: material selection (chrome steel vs stainless), hardness (why HRC 60–64 is the industry benchmark), precision grade (what g6 and h6 actually mean), and bearing compatibility (how to avoid the most common mismatches).
- Chrome steel (GCr15 / SUJ2 / 52100) covers 90% of indoor applications; stainless (440C, 304, 316) is for washdown, food, and corrosive environments — but hardness differs significantly between grades.
- Specify HRC 60–64 for load-bearing shafts with ball-type linear bearings. Softer shafts (HRC 50–55) may work with plain bearings but will brinell under ball contact.
- Precision grade g6 is the default for general automation; h6 for tighter fits. Do not specify h6 unless your bearing clearance requires it — it costs more and adds no benefit in most assemblies.
- Match shaft hardness to bearing type: hardened shafts for ball bearings, softer shafts only for polymer/plain bushings.
1. Material: Chrome Steel vs Stainless Steel
The material decision is usually the first filter. It is driven by the operating environment, not by preference.
Chrome Steel (GCr15 / SUJ2 / 52100)
Chrome steel is the standard material for linear shafts in dry, indoor, or lightly lubricated environments. The full designation is high-carbon chromium bearing steel, and it is the same material family used in bearing races and balls.
- Hardness after induction hardening: HRC 60–64 (surface), with case depth typically 1.5–2.5 mm depending on diameter.
- Wear resistance: Excellent against hardened steel balls in linear ball bearings.
- Corrosion resistance: Poor. Surface rust forms quickly in humid or washdown conditions unless plated (chrome-plated shafts are common) or greased regularly.
- Cost: Lower than stainless. Raw material cost is roughly 30–40% less than 440C, and machining is easier.
Stainless Steel (440C, 304, 316)
Stainless shafts are specified for food processing, pharmaceutical, semiconductor wet processes, and any environment with periodic washdown or high humidity. But "stainless" is not one material — the grade determines hardness and load capacity.
| Grade | Hardness (typical) | Corrosion Resistance | Best For |
|---|---|---|---|
| 440C | HRC 58–60 (through-hardened) | Good | Load-bearing shafts in washdown or mildly corrosive environments |
| 304 | HRC 20–25 (annealed) | Excellent | Guides, supports, non-load-bearing rails |
| 316 | HRC 20–25 (annealed) | Superior (marine grade) | Saltwater, chemical exposure, non-hardened applications |
Critical point: If you need both corrosion resistance and load capacity, 440C is the only stainless grade that approaches chrome steel hardness (HRC 58–60 vs 60–64). If you specify 304 or 316 for a shaft that will run in a linear ball bearing, the bearing balls will indent the shaft surface quickly — the shaft is too soft to support point contact loads.
For applications where corrosion resistance is required but load is low, some buyers use 304 shafts with polymer plain bearings. This works, but it is a design choice, not a default.
2. Hardness: Why HRC 60–64 Is the Benchmark
Hardness is not a marketing number. It directly determines whether the shaft surface can support the contact stress from bearing balls without permanent deformation.
Linear ball bearings create point contact between the recirculating balls and the shaft. The contact stress at that point is very high — often exceeding 1,500 MPa under moderate load. If the shaft surface hardness is below roughly HRC 58, the balls will brinell (indent) the shaft, creating a small depression. Once that depression forms, each ball drops into it and out again on every pass, producing vibration, noise, and accelerated wear. The bearing fails, and the shaft is usually scrap.
The HRC 60–64 range is not arbitrary. It matches the hardness of the bearing balls themselves (typically HRC 60–66). Matching hardness between the two contacting surfaces minimizes wear on both. Softer shafts wear faster; harder shafts (above HRC 64) become brittle and risk cracking under impact or edge loading.
Induction Hardening vs Through-Hardening
Chrome steel shafts are typically induction hardened on the surface, leaving a tough, ductile core. This is desirable: the hard case resists wear, and the softer core absorbs shock and bending loads. Stainless 440C is usually through-hardened, meaning the entire cross-section is hard — this makes it more brittle but gives consistent hardness even if the surface is machined after hardening.
When you request HRC 60–64, confirm with the supplier whether this refers to surface hardness (case) or core hardness. For chrome steel, it is surface hardness. For 440C, it is through-hardness. Both are valid, but they behave differently under impact loads.
3. Precision Grades: g6, h6, and What They Mean
Precision grade refers to the diameter tolerance of the shaft. It is the single most important dimension for bearing fit. The ISO tolerance system defines grades from coarse to fine; for linear shafts, g6 and h6 are the two you will encounter most.
| Grade | Tolerance (for 20 mm shaft) | Typical Fit with LM Bearing | Common Use |
|---|---|---|---|
| g6 | −0.007 / −0.020 mm | Sliding fit (small clearance) | General automation, most LM bearing applications |
| h6 | 0 / −0.013 mm | Close sliding fit (minimal clearance) | High-precision positioning, preloaded assemblies |
For a 20 mm shaft, g6 allows a diameter between 19.980 and 19.993 mm. h6 allows between 19.987 and 20.000 mm. The difference is about 7 microns — roughly 1/10th of a human hair. In most applications, this difference is invisible to the machine's performance, but it affects cost and manufacturability.
When to Choose g6 (Default)
g6 is the standard grade for linear shafts paired with standard LM bearings. The slight clearance (a few microns) allows the bearing to self-align slightly and accommodates thermal expansion. It is the grade you should specify unless you have a specific reason not to.
When to Choose h6 (Tighter)
h6 is for applications requiring minimal play: precision stages, measuring equipment, or assemblies with preloaded bearings. The tighter fit reduces clearance but also requires tighter control of shaft straightness and surface finish. Expect higher cost and longer lead time.
Straightness and Surface Finish
Precision grade covers diameter tolerance only. Two other specifications matter just as much:
- Straightness: Typically specified as mm per 1000 mm length (e.g., 0.05/1000 for standard, 0.02/1000 for precision). Straightness affects smoothness of travel and bearing life more than diameter tolerance in long-stroke applications.
- Surface finish (Ra): For ball-type linear bearings, Ra 0.4 µm or better is recommended. Rougher surfaces increase friction and wear; smoother surfaces (Ra 0.2) are for high-speed or low-noise applications.
4. Bearing Matching: The Interaction That Determines Life
The shaft does not work alone. Its performance is defined by the bearing it runs in. Here are the three bearing types and what they require from the shaft.
Linear Ball Bearings (LM Series)
These use recirculating balls in point contact with the shaft. Requirements:
- Shaft hardness: HRC 60–64 minimum. Softer shafts will brinell.
- Shaft finish: Ra 0.4 µm or better.
- Grade: g6 standard, h6 for precision.
- Lubrication: Required. Grease or oil film between balls and shaft.
Plain Bearings (Polymer or Bronze)
These use sliding contact over a larger area. Requirements:
- Shaft hardness: HRC 50–55 is often sufficient. Polymer bearings can run on softer shafts (even 304 stainless) because the contact area distributes load.
- Shaft finish: Ra 0.4–0.8 µm. Too smooth can reduce lubrication retention in some polymer bearings.
- Grade: g6 or h8 (looser) — plain bearings are more forgiving of diameter variation.
Ball Spline / Roller Types
Higher load capacity, used in torque transmission or heavy-duty linear motion. Requirements are similar to LM bearings but with stricter straightness requirements. Confirm with the bearing manufacturer before specifying.
| Bearing Type | Shaft Hardness | Shaft Finish (Ra) | Shaft Grade |
|---|---|---|---|
| Linear ball (LM) | HRC 60–64 | ≤ 0.4 µm | g6 (h6 for precision) |
| Plain polymer | HRC 50–55 (or softer) | 0.4–0.8 µm | g6 or h8 |
| Roller / spline | HRC 60–64 | ≤ 0.4 µm | g6, check straightness |
5. Procurement Checklist: What to Put in Your RFQ
When you send an inquiry for linear shaft, include these specifications explicitly. Ambiguity leads to mismatched product and rejected shipments.
- Material: State the grade (GCr15/SUJ2/52100 for chrome steel; 440C/304/316 for stainless).
- Hardness: HRC 60–64 surface hardness (for chrome steel), with case depth if diameter exceeds 30 mm.
- Diameter and tolerance: e.g., Ø20 mm, g6 (−0.007/−0.020).
- Straightness: e.g., ≤ 0.05 mm per 1000 mm.
- Surface finish: Ra ≤ 0.4 µm.
- Length: Exact cut length, or specify if you need cut-to-length with end machining (chamfer, thread, cross-hole).
- Coating (optional): Chrome plating for additional corrosion resistance on chrome steel; confirm plating thickness (typically 10–20 µm).
6. Common Specification Errors
Based on typical RFQ issues, these are the mistakes that cause the most rework:
- Specifying 304 stainless for a load-bearing shaft. It will fail under ball contact. Use 440C or switch to chrome steel.
- Specifying h6 without a reason. It increases cost by 10–20% and adds no benefit in most assemblies. g6 is the default.
- Ignoring straightness for long shafts. A 2-meter shaft with poor straightness will cause vibration and premature bearing failure even if the diameter tolerance is perfect.
- Matching a hardened shaft with a plain polymer bearing unnecessarily. It works, but you are paying for hardness you do not need. If corrosion is not a concern, a chrome steel shaft at HRC 50–55 is cheaper.
7. Questions to Ask Your Supplier
Before placing an order, confirm these with the supplier — they affect both performance and lead time:
- What is the case depth for the specified hardness? (For shafts above 30 mm diameter, this matters.)
- What is the straightness measurement method? (V-block vs laser — results differ.)
- Do you offer cut-to-length with end machining? What is the tolerance on length?
- What is the surface finish measurement? (Ra is standard; confirm the cutoff length.)
- Is the hardness test done on the finished surface or on a test coupon? (Finished surface is the correct method.)
Summary
Linear shaft selection is a four-variable decision: material, hardness, precision grade, and bearing match. Get the first three right, and the fourth follows naturally. Get any one wrong, and the assembly fails prematurely — usually at your customer's site.
For most indoor automation applications, the safe specification is: chrome steel (GCr15), HRC 60–64, g6 grade, Ra ≤ 0.4 µm, paired with standard LM bearings. Deviate from this only when the environment demands it — corrosion, washdown, or food contact — and in those cases, choose 440C, not 304.
If you are sourcing linear shafts and need to confirm specifications against your application, send your drawings or operating conditions to our team. We will review the load, environment, and bearing type and confirm whether the proposed shaft specification is adequate — or suggest an alternative before you commit to production.
Specifications and recommendations in this article are general guidelines. Final selection should be confirmed with your bearing manufacturer and shaft supplier based on your specific load, speed, and environmental conditions.