Linear Bearings, Rods & Shafts: Complete Pairing & Sizing Guide

A linear bearing is only as good as the shaft it runs on.

Pair an LM20UU with an undersized, un-hardened rod and you'll get sloppy motion and early bearing failure.

Pair a THK profiled rail guide with a bent, unmachined extrusion and you'll spend the rest of the project chasing alignment problems.

This guide covers how to select, pair, and size linear bearings with their support rails or shafts — from basic round-shaft systems to profiled rail setups.

If you're still deciding between a round-shaft and profiled rail system, start with our Linear Bearings vs Rotary Bearings overview first.

The Two Main System Types

System 1: Round-shaft (cylindrical) bearings

A hardened, ground steel shaft supports one or more bearing blocks or bushings. The bearing has ball elements (or polymer) that ride directly on the shaft OD. This is the simpler, lower-cost system.

Examples:

LMxxUU bearings on Thomson or igus shafting, LM8UU + 8 mm smooth rod in 3D printers.

System 2: Profiled rail guides

A precision-machined rail with hardened raceways bolted to a flat surface. Carriages with recirculating ball or roller elements run along the rail. This system is stiffer and more precise.

Examples:

THK HSR series, Hiwin HG/EG series, Rexroth STAR series, SBR/SC open-rail systems.

Round-Shaft Systems: Pairing Rules

Shaft (Rod) Requirements

The shaft is the foundation. If it's wrong, nothing else matters.

Material:

The shaft must be hardened steel — minimum 60 HRC surface hardness for precision ball-type bearings.

Standard cold-rolled steel is too soft; ball elements will indent the shaft surface within hours at any meaningful load.

Acceptable shaft types:

• 60-case steel (Thomson standard)

• Induction-hardened carbon steel (most Asian suppliers)

• Stainless steel 440C hardened (for corrosive environments, but lower load rating)

• Chromed carbon steel (widely available, economical)

Do not use:

mild steel (A36, 1018), aluminum rod, 316 stainless (too soft), or plastic rod with standard ball-type bearings.

Straightness tolerance: Shaft bow should not exceed 0.03–0.05 mm per 300 mm. Most precision-ground shafting meets 0.02 mm/300 mm or better.

Surface finish: Ra 0.2–0.4 μm (8–16 μin) for standard ball bearings.

Diameter tolerance: LMxxUU bearings use an h5 shaft fit — for a 20 mm shaft, that means 19.987–20.000 mm. Oversize = tight fit and high friction. Undersize = play and poor accuracy.

Shaft Length and Support Span

This is where most DIY and entry-level machine designs go wrong. The midspan deflection for a simply supported shaft under a central point load is:

δ_max = (F × L³) / (48 × E × I)

Where: F = load (N) · L = span (mm) · E = 200,000 N/mm² · I = π×d⁴/64 (mm⁴)

This formula assumes worst-case central loading. If the carriage traverses the full span, actual deflection is lower — but always use central loading for conservative design.

Worked example — 20 mm diameter shaft, F = 70 N (2 kg carriage + 5 kg payload), I = 7,854 mm⁴:

Rule of thumb:

keep span-to-diameter ratio under 30:1 for light loads, under 20:1 for anything requiring sub-0.1 mm accuracy. Doubling the span increases deflection by 8× (deflection scales with L³).

Matching Bearing to Shaft Diameter

Load ratings vary by manufacturer. Always verify against the actual product spec sheet.

LM vs. LME vs. LMB — What's the Difference?

• LMxxUU — standard metric bore, closed type, medium length. Correct for most applications.

• LMxxLUU — long type (~1.5× standard length). Higher load and moment capacity; use when cantilevered or offset loads are present.

• LMxxOP — open type. Allows installation onto an existing shaft without disassembly, and accommodates minor shaft misalignment. Not a compliance bearing — for true misalignment compensation, use a self-aligning housing.

• LMxxAJ — adjustable preload type; clearance can be reduced by tightening a clamp to remove play.

Shaft Support: Fixed vs. Floating End

One end of the shaft should be fixed (locked against axial movement) and the other should be floating (free to expand thermally).

For a 600 mm steel shaft subjected to a 20°C temperature rise: thermal expansion = 0.6 m × 12 × 10⁻⁶/°C × 20°C = 0.144 mm. If both ends are rigidly clamped, the shaft bows. On a machine with ±0.05 mm accuracy requirements, this matters from day one.

Profiled Rail Systems: Pairing Rules

Matching Rail Width to Carriage

Profiled rail systems use matched rail-and-carriage pairs. The size designator (15, 20, 25, 30, 35, 45 mm) refers to rail width, and carriages are designed for one rail width only:

• 15 mm rail → 15-series carriage (HSR15, HG15, etc.)

• 25 mm rail → 25-series carriage (HSR25, HG25, etc.)

Can You Mix Brands?

The short answer: often yes for profiled rail, never for round-shaft bearings.

Major profiled rail brands (THK, Hiwin, NSK, IKO) conform to shared dimensional standards within each size class — mounting hole patterns and carriage envelopes are largely compatible. However, load ratings, preload class, and running fit may differ from a matched-brand pair. For precision applications, buy rail and carriage from the same manufacturer.

Rail Length and Mounting

Minimum rail length: carriage length + required stroke + 20 mm safety margin.

Mounting surface flatness is the single most overlooked installation requirement. THK specifies 0.02–0.05 mm over the rail length for standard accuracy class (H) rails. If you bolt a precision rail to a warped plate, the warp becomes part of your motion system — it shows up as cyclic positioning error that looks like an encoder problem.

Rail splice joints: For axes longer than 4 m, joint gap should be ≤0.01 mm. Use manufacturer-supplied precision splice joints.

How Many Carriages Per Rail?

Use a minimum of 2 carriages per rail whenever moment loads are present. For a cantilever load: Moment M = F × d. Each carriage's moment rating must exceed M/2, with a safety factor of at least 2.0.

Sizing Example: CNC Router Z-Axis

Given: 8 kg spindle (78 N), 500 N peak cutting force, 150 mm stroke, ±0.05 mm accuracy, 6 hr/day, 250 days/year.

1. System type: 500 N + ±0.05 mm accuracy → profiled rail guide.

2. Design load: P = √(500² + 78²) ≈ 505 N. With 1.5× service factor → P_design ≈ 758 N.

3. Rail selection: Each Hiwin HG15 carriage rated 8.5 kN. With 2 carriages = 17 kN. Safety factor = 17,000/758 = 22.4. One HG15 rail + 2 carriages works.

4. L10 life: (8,500/758)³ × 50 ≈ 70,700 km. Travel/year: 0.15 m × 2 × 10 × 360 × 250 = 27 km/yr. L10 ≈ 2,600 years.

This extreme result is normal for Z-axes — they accumulate very little travel per day.

The practical limit is lubrication interval and seal condition, not bearing fatigue.

See our Noise & Failure Guide for the maintenance schedule that matters more than this L10 figure.

1. Rail length: 150 + 60 + 20 = 230 mm minimum → use standard 300 mm rail.

The Most Common Pairing Mistakes

1. Using un-hardened shaft with ball-type bearings — the shaft surface indents within hours

2. Over-spanning the shaft — deflection scales with L³; doubling the span = 8× more sag

3. Mounting the rail on an unflat surface — causes cyclic positioning error often misattributed to the encoder

4. Single carriage under a moment load — carriage rocks, ball circuit ends wear prematurely

5. Mismatching bearing bore to shaft diameter — too tight causes binding; too loose causes chatter

6. Mixing round-shaft brands — shaft hardness and diameter tolerances differ enough to cause early failure

FAQ

What's the difference between LM8UU and LM8LUU?

LM8LUU is the long type — roughly 1.5× the standard length, with an extra ball circuit. Use it when you have cantilevered or offset loads. For straightforward horizontal motion like a 3D printer axis, standard LM8UU is fine.

Can I mix brands on a round-shaft system?

No. Shaft diameter tolerances and surface hardness specs differ enough between manufacturers that you'll get either binding or slop. Profiled rail is more forgiving — THK, Hiwin, and NSK carriages are dimensionally compatible within the same size class — but for precision work, stick to matched pairs.

What's a pillow block, and is it different from a linear bearing?

A pillow block is a housing that holds the bearing and gives you bolt-down flanges for easy mounting. The LMxxUU inside is still the same bearing — same shaft requirements, same load ratings. You're just buying the bearing pre-packaged in an aluminum block instead of pressing it into your own housing.

Is there a speed limit for these bearings?

Round-shaft ball bearings (LMxxUU) are typically rated to 1.0–2.0 m/s. Push past that and you get noise, heat, and sharply reduced life. Profiled rail handles 3–5 m/s comfortably. In practice, your drive system — leadscrew, belt — usually becomes the limiting factor before the bearing does.

My bearings started making noise after a few hours. Bad batch?

Check the shaft first. If the surface has visible pitting or you can feel roughness under your fingernail at the ball contact lines, the shaft wasn't hardened. Soft steel (A36, cold-rolled 1018) indents within hours under any real load. Replace the shaft with properly hardened stock — the bearings themselves are usually fine.

What's the point of a flanged linear bearing?

The flange lets you push the bearing into a bore from one side and locate it from the other — no retaining ring needed. Useful when you're mounting into thin plate or extrusion profiles where there's no room for circlips. Performance is identical to the standard type.

Quick Reference: Pairing Checklist

Before you order:

• Shaft material is hardened to ≥60 HRC surface hardness

• Shaft diameter matches bearing bore to h5 tolerance

• Span-to-diameter ratio ≤30:1 (standard), ≤20:1 (precision)

• One shaft end fixed, one floating (thermal expansion)

• Profiled rail: rail and carriage are the same brand and size class

• Mounting surface flatness verified (≤0.05 mm/300 mm minimum)

• Minimum 2 carriages if moment loads are present

• L10 life calculated and meets the application's service requirement

• Lubrication method selected and interval scheduled

Need help sizing a specific axis? Share your load, stroke, accuracy, and duty cycle requirements in the comments, and we'll walk through the calculation.