A linear bearing is a recirculating ball bushing that converts rotary shaft motion into smooth linear travel. This manufacturer's explanation covers the structure (outer ring, ball circuit, retainer, seals), how it works, and where it fits versus linear guides, with real specifications from Dongfeng Bearing's LM, LMF, and LMUU series.
Linear Motion Basics
A linear bearing is a recirculating ball bushing that lets a shaft move in a straight line with low friction. It converts the rotary motion of a shaft into linear travel through a closed circuit of steel balls. If you are specifying your first linear motion axis, this explanation covers the structure, how it works, and where a linear bearing fits compared with a linear guide.
Key takeaways
- A linear bearing is a recirculating ball bushing that converts rotary shaft motion into linear travel.
- Its main parts are an outer ring, a ball circuit, a retainer, and seals.
- Dongfeng Bearing's LMF series uses GCr15 chrome steel, HRC 60-64, UU double rubber seals, and shaft diameters from 6 to 50 mm.
- LMF16UU has a dynamic load rating of 1,370 N and follows JIS B 1185 interchangeability.
- Linear bearings are for point-load, low-friction linear motion; linear guides are for higher rigidity and moment load.
What is a linear bearing?
A linear bearing is a cylindrical bearing that runs on a hardened shaft. The most common type is the linear ball bearing, also called a linear motion bearing or a linear ball bushing. Inside the bearing, steel balls circulate in a closed loop. As the shaft moves, the balls roll along the shaft surface, carry the load, and then return through a return channel. This recirculation is what allows continuous travel rather than a limited stroke.
In automation equipment, you will find linear bearings in pick-and-place heads, sliding carriages, measuring fixtures, and light-duty gantries. They are often chosen when the design needs a simple round-shaft axis with low friction and easy mounting.
Structure: what is inside a linear ball bearing?
A linear ball bearing is not just a tube with balls. The working parts are:
- Outer ring: the hardened steel sleeve that holds the bearing in the housing and provides the running surface for the ball circuit.
- Ball circuit: rows of steel balls that contact the shaft and recirculate through return channels.
- Retainer: the cage that keeps the balls spaced and guides them through the return path.
- Seals: rubber or felt seals at each end that keep dust out and retain lubricant.
For example, the LM Series Linear Ball Bearing Bush is a standard cylindrical bushing for round-shaft applications. The LMF Series Square Flange Linear Ball Bearing adds a square flange to the same basic structure, so the bearing can be bolted directly to a flat mounting face. The LMUU sealed extended-length series uses a longer outer ring and seals for applications that need more support length.
How does a linear bearing work?
The working principle is straightforward. The shaft is hardened and ground. The balls contact the shaft along load-carrying rows. When the shaft moves axially, friction makes the balls roll. Because the balls are in a closed circuit, they leave the loaded zone, travel through the return channel, and re-enter the loaded zone. The retainer keeps this circulation orderly. The seals close the ends so contaminants do not enter the ball path.
Two points matter for buyers:
- Motion conversion: the bearing itself does not create motion. It supports and guides linear travel while the drive system (belt, screw, or cylinder) provides the force. The recirculating balls reduce friction so the drive does not have to overcome sliding resistance.
- Load direction: a standard linear ball bearing is designed mainly for radial loads. It can handle some torque, but if your axis has significant moment load, a linear guide is usually the better choice.
Real specifications: LMF series example
Specifications are where a manufacturer's explanation becomes useful for selection. The LMF series square flange linear ball bearing from Dongfeng Bearing has these confirmed facts:
| Parameter | Specification |
|---|---|
| Material | GCr15 chrome steel |
| Hardness | HRC 60-64 |
| Seals | UU double rubber seals |
| Shaft diameter range | 6 mm to 50 mm |
| Dynamic load (LMF16UU) | 1,370 N |
| Interchangeability | JIS B 1185 |
These values apply to the listed series and sizes. Confirm the exact rating for your selected bore size and mounting arrangement with the supplier.
What the seals and material mean for your machine
GCr15 chrome steel hardened to HRC 60-64 gives the raceway and balls the wear resistance needed for repeated rolling contact. The UU double rubber seals contact the shaft at both ends. In a dusty or coolant-exposed environment, these seals are the first line of defense for the ball circuit. If your application runs in a clean room, a sealed bearing still helps retain grease and extend relubrication intervals.
JIS B 1185 interchangeability means the mounting dimensions follow a recognized Japanese Industrial Standard. That matters when you are replacing a bearing from another supplier or designing a housing that must accept multiple sources.
Linear bearing vs linear guide: where each fits
Buyers often ask whether they should use a linear bearing or a linear guide. They solve different problems.
| Consideration | Linear ball bearing (round shaft) | Linear guide (profile rail) |
|---|---|---|
| Load type | Mainly radial; limited moment | Radial, reverse, and moment loads |
| Rigidity | Moderate | High |
| Mounting | Simple; shaft and housing | Requires machined rail mounting surface |
| Typical use | Light-duty slides, pick-and-place, fixtures | CNC axes, heavy gantries, high-precision stages |
| Cost profile | Lower for simple axes | Higher, but handles more load |
A linear bearing is often the pragmatic choice when the axis is short, the load is light, and you want a compact round-shaft design. A linear guide is better when you need to resist tilting, carry a cantilevered load, or hold tight straightness over a long stroke.
What about plain bushings?
The SC plain bush is a sliding bearing, not a recirculating ball bearing. It uses a sliding contact surface instead of rolling balls. Plain bushings can be suitable for low-speed or oscillating motion, but they generally have higher friction than a linear ball bearing. If your application needs smooth continuous travel with low driving force, a recirculating ball type is usually the better fit.
How to choose a linear bearing for your axis
Use these steps to narrow the selection:
- Define the load. Estimate the radial load and whether any moment load exists. If moment load is significant, consider a linear guide instead.
- Check the shaft. The shaft must be hardened and ground to match the bearing's internal clearance and hardness. A soft or worn shaft will damage the ball circuit.
- Select the bore size. The LMF series covers 6 mm to 50 mm shaft diameters. Choose the smallest size that meets your load and rigidity needs.
- Decide on mounting. Use a cylindrical LM bushing for a housing bore, or an LMF square flange bearing for a flat mounting face.
- Consider the environment. For dusty or wet conditions, specify the UU double rubber seal version. For longer support, consider the LMUU extended-length series.
- Confirm interchangeability. If you are replacing an existing bearing, check that the mounting dimensions follow JIS B 1185 or the standard your machine uses.
Frequently asked questions
Is a linear bearing the same as a linear motion bearing?
Yes. "Linear motion bearing" is a broad term that includes recirculating ball bushings, plain bushings, and other linear bearing types. "Linear ball bearing" specifically refers to the recirculating ball design.
Can a linear bearing carry a moment load?
A standard linear ball bearing is primarily designed for radial loads. It can tolerate limited moment, but for significant tilting or cantilever loads, a linear guide is the more appropriate choice.
What does UU mean on a linear bearing?
UU indicates double rubber seals at both ends of the bearing. These seals help keep contaminants out of the ball circuit and retain lubricant.
What is the dynamic load rating of LMF16UU?
The LMF16UU has a dynamic load rating of 1,370 N. Confirm the rating for your specific bore size and application conditions.
Next step
If you are selecting a linear bearing for an automation project, start with the shaft diameter, load direction, and mounting style. The LM Series Linear Ball Bearing Bush and LMF Series Square Flange Linear Ball Bearing cover common round-shaft applications with GCr15 chrome steel, HRC 60-64 hardness, and UU sealing. For extended support length, review the LMUU sealed extended-length series. Share your shaft size, load, and stroke with the supplier to confirm the right part number.