If you have been sourcing linear bearings for a project and noticed that two part numbers look almost identical except for an extra "L", you are not alone. The LM20UU and LM20LUU sit side by side in most bearing catalogs, share the same shaft diameter, the same outer diameter, and the same material. On the surface they look interchangeable. In practice, picking the wrong one affects how your carriage moves, how well it handles side loads, and how long the bearing lasts under real operating conditions.
This article breaks down exactly what separates these two parts, when each one makes sense, and how to make the call for your application without overcomplicating it.
SQ bearings
What the Part Numbers Actually Mean
The naming convention follows the standard LM series format. "LM" stands for Linear Motion. "20" is the bore diameter in millimeters, meaning both bearings fit a 20mm shaft. "UU" means the bearing is sealed on both sides with rubber contact seals, which keeps dust and contaminants out and retains the factory lubrication.
The only difference in the name is the "L" inserted before "UU" in the second part number. That single letter means Long type, and it refers specifically to the overall length of the bearing body.
That is the entire naming difference. One letter, one dimension. Everything else in the designation is the same.
The Numbers Side by Side
Here is where the practical gap becomes visible:
| Specification | LM20UU | LM20LUU |
|---|---|---|
| Bore diameter (ID) | 20 mm | 20 mm |
| Outer diameter (OD) | 32 mm | 32 mm |
| Length (L) | 42 mm | 80 mm |
| Ball circuits | 5 | 6 |
| Dynamic load rating (C) | ~882 N | Higher |
| Static load rating (C0) | ~1370 N | Higher |
| Seal type | Double rubber (UU) | Double rubber (UU) |
| Material | Chrome steel GCr15 | Chrome steel GCr15 |
The LM20LUU is roughly 90% longer than the LM20UU. It is not a minor increment. That added length changes the contact geometry between the bearing and the shaft, increases the number of load-bearing ball circuits from five to six, and raises both the dynamic and static load ratings as a result. The LM20UU size in isolation tells you the bore and outer dimensions, but it does not tell you anything about how the bearing handles moment loads. That is where the long type starts to matter.
Why Length Affects More Than Just Load Rating
A linear bearing carries load through rolling contact between steel balls and the hardened shaft. In pure radial loading, where the force acts perpendicular to the shaft axis and passes through the center of the bearing, a standard-length bearing handles it cleanly. The balls on the loaded side take the force and transfer it to the outer housing.
Most real applications are not quite that clean. A carriage carrying a tool, a workpiece, or a sensor usually has its center of mass offset from the bearing contact zone. That offset creates a moment, a rotational force that tries to tilt the bearing relative to the shaft. With a short bearing, that tilting tendency is resisted by a relatively narrow contact span. With a longer bearing, the same moment force is distributed over a wider span, the leverage against tilting is greater, and the ball circuits at each end of the bearing are less likely to carry disproportionate loads.
This is why THK, the original developer of the LM series, specifically describes the LM20LUU as optimal for applications subject to moment loads. More contact length means more resistance to rocking and tipping under offset or cantilevered loading conditions.
The practical consequence is bearing life. A standard LM20UU running under a steady moment load will show premature wear at the ends of the ball circuits before the rest of the bearing is anywhere near worn out. Switch to an LM20LUU in the same installation and the load distributes more evenly across six circuits instead of five, the end circuits take less abuse, and the bearing runs longer between replacements.
SQ LM20UU Stainless Steel
When the Standard LM20UU Is the Right Choice
The standard type is not a compromise. It is the correct choice for a large number of applications and comes with real advantages in situations where space is a limiting factor.
If your carriage design fits two bearings spaced apart along the shaft, a pair of LM20UU bearings with adequate separation between them handles moment loads well. Two bearings working together over a long span behave similarly to a single longer bearing, sometimes better because the span between them can exceed what any single bearing body provides. Most 3D printer linear axes, small CNC router carriages, and automation transfer slides fall into this category. The LM20UU size fits neatly into standard SC20UU pillow block housings and similar off-the-shelf hardware.
The standard bearing is also lighter and shorter, which matters in applications where moving mass needs to be minimized or where the housing length is constrained. If you are building something that needs to fit into a tight enclosure and you are running two bearings per axis anyway, the standard length is usually the better starting point.
When the LM20LUU Makes More Sense
The long type starts to earn its cost when one of the following conditions applies.
First, if you are using a single bearing per carriage position rather than two. A single LM20UU under a moment load is a recipe for accelerated wear. A single LM20LUU still carries the moment better than the standard type, though for critical applications two standard bearings in a well-designed carriage will usually outperform a single long one.
Second, if the carriage carries a significant offset load and you cannot easily widen the spacing between two standard bearings. A machine tool spindle head, a camera slider with a heavy lens assembly, or a pick-and-place head with substantial reach all generate moment loads that grow with the offset distance. Adding bearing length directly addresses this without requiring a redesign of the bearing spacing.
Third, in applications where the shaft support span is long and shaft deflection is a concern. Longer bearing contact provides better guidance and reduces the sensitivity of the carriage to small amounts of shaft sag, which is relevant in horizontal spans above about 300 to 400mm without intermediate support.
Fourth, where vibration or high-speed reciprocating motion is involved. The longer contact patch damps micro-movements between bearing and shaft more effectively, which shows up as lower noise and smoother operation in fast-cycling equipment.
What Does Not Change Between Them
Both bearings fit the same 20mm hardened shaft. Both require the same housing bore for press-fit installation, H7 tolerance is the standard recommendation. Both come pre-lubricated with light oil from most suppliers and are ready to use on arrival. Both use the same chrome steel construction and the same double rubber seal design. Maintenance requirements are identical. Neither bearing should be run on an unground or unhardened shaft. The minimum shaft hardness recommendation is 58 HRC, and the shaft surface should be ground to a finish of Ra 0.4 micron or better. Running either bearing on a mild steel shaft will accelerate wear on both the shaft and the bearing, regardless of which type you choose.
Interchangeable part numbers across different brands also apply to both types equally. The LM20LUU can be cross-referenced against SM20GWUU, TBW20LUU, LD20LUU, and LMUW20L depending on the supplier. The LM20UU cross-references to SM20GWUU (without the L suffix), LD20UU, and LMUW20. These cross-references are mostly consistent in dimensions but can vary slightly in load ratings between Japanese-origin manufacturers like THK and IKO and their lower-cost alternatives. For precision equipment, the spec sheet from the specific supplier you intend to use is worth checking before finalizing a design.
Housing Considerations
Switching from LM20UU to LM20LUU in an existing design is not always a drop-in replacement, even though the outer diameter is identical. The housing bore length needs to accommodate the longer body. A standard SC20UU pillow block, for example, is sized for the 42mm standard bearing. Installing an 80mm LM20LUU into it is not possible without modification or a different housing.
If you are designing from scratch, specify the housing at the same time as the bearing. If you are retrofitting, measure the available housing depth before ordering the long type. This is the most common error in specifying the LM20LUU as an upgrade to an existing installation.
A Simple Decision Framework
If you are unsure which to pick, run through these questions in order.
Are you using two bearings per shaft position, spaced at least 60 to 80mm apart? If yes, start with LM20UU and check whether your load calculations show adequate moment resistance. Most light to medium applications will be fine.
Are you using a single bearing per position, or is your spacing severely constrained? Move to LM20LUU.
Does your carriage carry an offset load, a cantilevered arm, or a heavy asymmetric payload? LM20LUU is the safer choice.
Is housing length or moving mass a hard constraint? Stay with LM20UU and address moment loading through wider bearing spacing if possible.
For anyone sourcing replacement parts on existing machinery, match whatever is in the machine. Upgrading from standard to long type on one axis without redesigning the housing is rarely straightforward, and the performance benefit does not justify the engineering complication in most maintenance scenarios.
Final Thought
The difference between LM20UU and LM20LUU comes down to one dimension and the mechanical consequences that flow from it. Longer body, more ball circuits, better moment resistance, higher load ratings. The tradeoff is housing length, slightly higher cost, and more weight. Neither is universally better. They solve different problems, and knowing which problem you have is the only thing that makes the choice straightforward.
