This page covers the 8 mm bore size of LM Series Linear Ball Bushings to JIS B 1516 / ISO 10285. It reproduces the dimensional row, then works outward from it: the fit for the load case, the internal clearance group, the shoulder and fillet the corner radius allows, and where this bearing type should and should not be used.
8 mm Bore Dimensional Row (JIS B 1516 / ISO 10285)
| Designation | Shaft diameter d (mm) | Outer diameter D (mm) | Length L (mm) | Snap ring groove diameter (mm) | Snap ring groove position (mm) |
|---|---|---|---|---|---|
| LM8UU | 8 | 15 | 24 | 14.3 | 17.5 |
What a LM Series Linear Ball Bushings Does
A linear ball bushing recirculates its balls through tracks in the sleeve, so the load-carrying balls run along a hardened round shaft while the rest return unloaded. Motion is along the shaft rather than around it, which makes this the bearing of a linear axis, a printer gantry or a machine slide.
Radial load only, taken perpendicular to the shaft, and the load capacity depends on where the ball tracks sit relative to the load line. There is no axial capacity: the whole point of the bushing is that it slides freely along its shaft.
Speed at 8 mm Bore
There is no limiting speed in rev/min for a linear bearing: the limit is a linear velocity in metres per second, together with the acceleration the ball recirculation can stand. The two quantities are not interchangeable, so no shaft speed is quoted here.
Shaft & Housing Fit for a 8 mm Seat
With the inner ring rotating under a load whose direction is fixed, the inner ring sees a circumferential load and must be an interference fit, or it will creep on the shaft and fret it. The outer ring sees a point load and takes a transition or clearance fit so that it can shift slowly and spread the wear — and so that one of the two bearings on the shaft is free to take up axial expansion.
| Shaft, light load (P ≤ 0.06 C) | h5 |
|---|---|
| Shaft, normal load (P = 0.06–0.12 C) | j5 |
| Housing — Axial displacement of the outer ring required | H7 |
| Housing — Normal load, outer ring may still shift | J7 |
| Housing — Normal to heavy load, quiet running, outer ring not displaceable | K7 |
ISO 286-2 tolerance grades, applied per the conventional bearing seat recommendations of ISO 1132-2 and DIN 5425-1, for a SOLID STEEL shaft in a cast iron or steel housing. A hollow shaft, a thin-walled housing or a light alloy housing needs a tighter fit than these, because the seat itself deflects.
Mounting a 8 mm Bore LM Series Linear Ball Bushings
The ring that carries the rotating load takes an interference fit; the ring under a stationary point load takes a transition or clearance fit so that it can creep slowly and spread the wear, and so that one ring is free to take up axial expansion. With a rotating shaft under a fixed load direction, that means an interference fit on the shaft and a looser fit in the housing. Press only against the ring being fitted — force driven through the rolling elements leaves brinell marks in the raceway before the bearing has run a revolution.
At a 8 mm bore the interference is small enough that a cold press or a light hydraulic press is normally enough; heating the bearing in an induction heater or an oil bath to 80–100 °C above the shaft is the alternative, and never above 120 °C for a standard-stabilized bearing. Keep the heated bearing pressed against the shoulder while it cools, because it shrinks axially as well as radially.
Where a 8 mm Bore LM Series Linear Ball Bushings Is Used
At a 8 mm bore, lm series linear ball bushings are typically found in small motors and fans, instruments, dental and hand tools, printers, encoders and model machinery — sizes at which the bearing is usually chosen for smooth, quiet running and low starting torque rather than for its rating.
Failure Modes and What They Look Like
- Subsurface fatigue (spalling) — a flake of raceway lifts out and the bearing gets noisy and rough. This is the failure the L10 life calculation predicts — if it arrives near the calculated life the bearing did its job, and if it arrives early the load, the fit or the lubrication was not what the calculation assumed.
- Inadequate lubrication — a polished, then discoloured, then torn raceway, running hot with a darkened grease. It is the most common cause of early failure by a wide margin, and the interval, not the grease, is usually what was wrong.
- Contamination — dents in the raceway with raised edges, then premature spalling from those dents. Wear particles, casting sand and washdown water all do it; sealing and filtration are what prevent it.
- False brinelling and fretting — evenly spaced dull marks at ball or roller pitch, produced by vibration while the shaft is stationary — a machine that travels on a truck, or a standby pump beside a running one. It is not caused by overload and cannot be cured by fitting a bigger bearing.
- Seal failure — grease escaping, then water and dirt getting in. A contact seal is a wearing part with a temperature limit of its own, and a washdown at 80 °C will get past a standard nitrile lip long before the bearing itself is in trouble.
Where NOT to Use a LM Series Linear Ball Bushings
- Do not run a linear bushing on a shaft it was not sized for. The bushing runs on the shaft directly, so the shaft has to be hardened, ground and within the specified tolerance — a soft or undersized shaft gets ploughed by the balls.
- Do not use one where the load rotates around the shaft, and do not apply a moment across a single bushing. Linear bushings take radial load through their ball tracks; overhung moment load needs two bushings at a spacing, or a profiled rail.
- Do not run one unsealed in swarf and grit. The ball recirculation tracks pick up debris and grind it into the shaft; a sealed (UU) version and a wiper are the cheap prevention.
- Do not treat a sealed bearing as sealed for life in a wet or hot environment. A contact seal has a temperature and a pressure limit of its own, and once it is past them the grease leaves and the water arrives.
Stepping Up or Down from 8 mm
Within the same series, the next bore up in this series is 10 mm and the next bore down is 6 mm. Changing bore changes the shaft, so it is the last thing to change: when the calculated life at 8 mm is short, the first move is to a heavier diameter series at the SAME bore — the 6300 envelope instead of the 6200, for instance — which raises the rating without touching the shaft. The second is to change type: a roller bearing carries far more radial load than a ball bearing of the same envelope.
