THKLMK25UUSquare-Flange Linear Ball Bushing for 25 mm Round Shaft, 40 mm OD x 59 mm, Sealed Both Ends, 6 Ball Rows

Product Description

What this part is

This is a THK Linear Bushing — a recirculating linear ball bushing — with an integral square mounting flange. Balls run in point contact against a round, hardened, ground LM shaft and recirculate through six ball rows inside the bushing body, giving low-friction linear travel along that shaft. The flange lets you bolt the bushing straight to a plate or carriage face rather than pressing it into a bored housing.

It is the flanged, bolt-down version of the plain LM linear bushing, and the square-flange form specifically is the low-profile option in the family.

Decoding the designation

LM — THK Linear Bushing, metric series.
K — square (rectangular) flange, cut square across the flats so it sits lower and consumes less panel width than the round-flange LMF equivalent.
25 — 25 mm nominal shaft diameter.
UU — seals fitted at both ends. A single U means one end only; no U means unsealed.

The 25 mm figure is the shaft diameter, not a bore tolerance class and not the bushing outer diameter. That is a common misreading on linear bushing part numbers. The bushing body outer diameter is 40 mm.

What these specs mean in practice

A linear bushing on a round shaft is the low-cost, misalignment-tolerant way to get linear travel. Round shaft is cheap, easy to source, easy to cut and easy to support, and the bushing is far more forgiving of parallelism error between two axes than a profiled rail is.

The trade-off is capacity. Because the balls sit in point contact against a round shaft rather than line or Gothic-arch contact against a ground raceway, the load rating is roughly an order of magnitude below a profiled-rail LM guide of comparable size. A single bushing also has essentially no moment capacity worth designing around — it will not resist a twisting or tipping load on its own. The standard answers are two bushings spaced apart on one shaft, or two parallel shafts with a bushing on each; the spacing between bushings is what actually generates your moment resistance.

The UU double seals matter in any environment with chips, grinding dust or airborne swarf. A linear bushing that ingests debris drags it under the balls and scores the shaft, and the shaft is the expensive half of the pair. Seals are cheap insurance on a machine that makes chips.

Published ratings and mounting data

Shaft diameter dₑ: 25 mm (mating shaft ground to g6 or f6)
Bushing outer diameter D: 40 mm
Overall length L: 59 mm
Flange outer diameter D₁: 62 mm
Flange across flats K: 50 mm
Flange thickness H: 8 mm
Mounting: 4 holes on a 51 mm PCD, counterbored 5.5 × 9.2 × 5.4 mm
Number of ball rows: 6
Maximum eccentricity: 15 µm
Radial clearance tolerance: −9 µm
Basic dynamic load rating C: 980 N
Basic static load rating C₀: 1,570 N
Mass: 270 g
Recommended housing fit: H7 for a loose fit, J7 for a transition fit

Quantity and configuration

One bushing. No shaft, no shaft support blocks, no housing and no carriage plate. THK LM round shafts are a separate catalogue item and are sold separately, as are the shaft support rails and end blocks. If you are building an axis from scratch, budget for the shaft as well — it is usually the bigger line item.

Fitting notes

The mating shaft must be a proper hardened and ground LM shaft in g6 or f6. Running a bushing on cold-drawn bar or an unhardened shaft will flatten the shaft surface quickly.
If you are pressing the body into a bore rather than bolting through the flange, use H7 for a loose fit or J7 for a transition fit. Squeezing the outer body distorts the ball tracks and kills the running feel.
Bolt the flange to a face that is square to the shaft axis. A cocked flange preloads the ball rows unevenly and shortens life.
With two bushings on one shaft, set the spacing as wide as the design allows; moment capacity scales with the span, not with the bushing.

Interchange guidance

Written variously as LMK25UU, LMK-25-UU and LMK 25 UU; the base number is LMK25. Related THK variants that are not the same part and will not drop in interchangeably:

LM25UU — plain body, no flange at all.
LMF25UU — round flange, larger footprint across the flats.
LMK25L — long-body version, greater length and higher load rating, different overall length.

All of these share the 25 mm shaft size, so they will run on the same shaft — but the mounting envelope and hole pattern differ, so check the flange form and length before substituting.