A machine builder in Stuttgart ordered 32 pcs of LM20UU for a packaging line, paid USD 1,840 to an Alibaba supplier, and got parts that would not slide on his hardened 20 mm shaft. The bore measured 20.4 mm instead of the ISO-specified 20.000–20.020 mm range. Three days of downtime cost him close to EUR 9,000. That mess traces back to one step skipped: he never checked the linear bearing standard sizes on the drawing against the supplier's tolerance table.
If you source linear bearings for automation, medical devices or CNC equipment, read the standard size chart before you send drawings. This guide walks through what the linear bearing standard sizes cover, which LM / LMF / LMFL dimensions you can expect off the shelf, and where buyers get burned.
What "Linear Bearing Standard Sizes" Really Covers
When engineers talk about linear bearing standard sizes, they usually mean the dimensional series under ISO 1206, mirrored by JIS B 1556. These are caged ball bushings that slide on a hardened round shaft, not profile rail linear guides. The standard fixes bore diameter (d), outer diameter (D), overall length (L) and the chamfer at both ends. It does not fix radial clearance class, seal type or cage material.
Off the shelf you get the UU version (rubber seals both sides), the optional 2RS double-seal version and the adjustable AD type for preload tuning. The linear bearing standard sizes that stock distributors quote are almost always the UU series - LM3UU to LM60UU, bore 3 mm through 60 mm. Anything outside that range is a custom part with a longer lead time.
LM Series Standard Bore Sizes and Dimensions
The plain LM series is the workhorse. The most common linear bearing standard sizes buyers ask for:
LM6UU: bore 6 mm, OD 12 mm, length 19 mm
LM8UU: bore 8 mm, OD 15 mm, length 24 mm
LM10UU: bore 10 mm, OD 19 mm, length 29 mm
LM12UU: bore 12 mm, OD 21 mm, length 30 mm
LM16UU: bore 16 mm, OD 28 mm, length 37 mm
LM20UU: bore 20 mm, OD 32 mm, length 42 mm
LM25UU: bore 25 mm, OD 40 mm, length 59 mm
LM30UU: bore 30 mm, OD 45 mm, length 64 mm
LM40UU: bore 40 mm, OD 60 mm, length 80 mm
LM50UU: bore 50 mm, OD 80 mm, length 100 mm
LM60UU: bore 60 mm, OD 90 mm, length 110 mm
These are the linear bearing standard sizes that keep showing up on 3D printers, CNC router builds and small automation slides. If your shaft falls between two sizes (say 14 mm), you either step up to LM16UU with a clearance fit or switch to a custom bore. The product page for a linear ball bearing lists each LM model with its exact d × D × L and load rating.
Flange Linear Bearing Standard Sizes (LMF / LMFL / LMK)
Once the bearing has to bolt to a plate instead of press into a housing, the flange series kicks in. Three shapes dominate:
LMF (round flange): same d × D × L as plain LM, plus a round bolt flange at one end. LMF8UU, LMF12UU, LMF20UU are the volume sellers.
LMFL (long round flange): longer and thicker flange, used on thin plates where you want more bolt engagement.
LMK (square flange): square plate with four mounting holes, common in gantry axes and XY tables.
The linear bearing standard sizes for flange variants are dimensionally identical to the LM bore on the inside. The only drawing differences are the bolt-hole circle, number of bolt holes and overall length including flange. When you ask for a linear flange bearing in our showroom, the entry repeats the LM bore size plus a flange mounting dimension table.
Tolerances, Clearance and Material Specs
Most sourcing problems start here. The linear bearing standard sizes only lock the envelope. The tolerance class is a separate line item.
Bore d is H7 on the inner ball path. The shaft itself must be ground to g6 or h5 and hardened to at least HRC 60.
Outer diameter D is h6, pressing into an H7 housing bore.
Radial clearance splits into C1 (interference for preload), C2 (light clearance) and the default C3 (normal clearance). High-precision CNC axes usually ask for C1; general automation runs C3.
Cage material is usually aluminum alloy or stamped steel. High-temperature runs over 120 °C need a stainless steel cage.
If a supplier only quotes "LM20UU standard" without clearance class, you are rolling the dice. The Stuttgart case is exactly that: the bore was out of spec because the supplier ran a loose C4 clearance to cut cost.
How to Match Standard Sizes to Your Shaft and Housing
A practical workflow for buyers:
Measure your hardened shaft. The bore must match the shaft diameter within the clearance class you need.
Measure the housing bore you press into. It should land on the D dimension of the same LM series.
Pick plain press-in vs flanged bolt-on. Through-plate mounts go LMF / LMK; blind bores stay plain LM.
Pick the length. Longer L (LM25UU at 59 mm) handles higher moment loads; shorter versions ride better on short-stroke slides.
Specify seal type and clearance class on the PO. "LM20UU C3" is a complete line item; "LM20UU" is not.
If you are unsure whether plain or flanged fits your envelope, the introduction to linear bearings on our blog walks through the housing-pressure versus bolt-mount tradeoff with drawings.
Common Sizing Mistakes and How to Avoid Them
Three mistakes we see every quarter:
Ordering LM20UU for a 20 mm commercial shaft that is not hardened. The balls will gouge the shaft within weeks.
Mixing LM series with LME (Europa series) by mistake. LME20UU has a different OD than LM20UU and will not fit the same housing.
Skipping clearance class on the PO and getting C3 when the axis needs C1 preload for tight positioning.
A buyer in Milan ordered 80 pcs of LM12UU, assumed they were C3, and found 0.15 mm of play on the slides. Rework to C1 took four weeks and ate the margin. That is why we publish the linear bearing standard sizes with a full tolerance table on every product page, not just the d × D × L.
For compact axes where a standard LM length does not fit, the small linear bearings showroom covers LM3UU through LM13UU in open and sealed versions, plus the 6 mm LMH6UU flange variant for miniature medical equipment.
FAQ
Are linear bearing standard sizes universal? The envelope d × D × L is standardized under ISO 1206, so any LM20UU from a reputable maker should slide on a 20 mm hardened shaft. Clearance class, seals and cage material are not standardized and must be specified.
Do LM and LME series interchange? No. LME20UU has a 28 mm OD versus LM20UU at 32 mm. They share the 20 mm bore but the housing bore differs.
If you are sourcing a trial batch or production run and want the linear bearing standard sizes cross-checked against your drawing, send us your d, D and required clearance class. We will quote the matching LM, LMF or LMK model with the tolerance table attached, usually within one working day.
