Every machine that moves in a straight line eventually leans on a rolling element. A linear bearing does for translation what its rotary cousin does for rotation: it replaces sliding friction with rolling contact, and it carries a load that is often a moment rather than a simple force. The difference is the stroke. A rotary bearing runs in one spot; a linear bearing may travel 1 m to 2 m or more, thousands of times an hour, through dust, heat and reversal. That is why the family splits into distinct designs and why the mounting, not the part number, usually decides life.
Recirculating elements: the idea behind linear rolling
A linear rolling bearing cannot simply fill a track with balls, because they would run out at the end of the stroke. The recirculating design solves this with a closed circuit: balls roll between the raceway and the carriage, leave the loaded zone, pass through a return channel in the carriage, and re-enter at the other end. The circuit holds a fixed number of balls, so the bearing travels indefinitely.
Three details decide how well it works. The return channel must guide each ball gently, because a sharp turn makes the ball slide and generates noise and wear. The ball spacing set by the cage keeps the balls from crowding. The preload removes internal play so the carriage does not shift before it takes load. Get these right and friction stays low and stick-slip is effectively eliminated; get them wrong and the bearing is noisy, hot and short-lived.
Round-shaft ball bushings and their limits
The round-shaft linear ball bushing is the simplest recirculating design and the cheapest way into linear motion. A thin cylindrical sleeve holds several circuits of small balls that run on a hardened, ground shaft, often inside a light alloy or steel housing. It is compact, widely available in bore sizes from 4 mm to 150 mm, and adequate for light and moderate loads where the shaft can be made stiff.
Its limits follow from the geometry. A single bushing has a short contact length, so it tolerates very little moment; hang a load off the side and the sleeve tilts. The round shaft itself deflects, and because the bushing cannot compensate, two bushings on parallel shafts or a shaft supported along its length are the norm. Shaft quality is not optional: a soft or rough shaft brinells and wears the races within weeks, so it must be hardened and ground to bearing-raceway care. Where the deflection or moment is significant, the profile rail is the better answer.
Profile rails: four load directions in one carriage
A profiled rail guideway replaces the round shaft with a rail whose raceways are ground at about 45 degrees on both sides. The carriage carries recirculating balls or rollers on those angled raceways, so it can be loaded downward, upward and sideways and resists roll, pitch and yaw moments. Because the rail bolts flat to a machined datum and the carriage is a rigid block, the assembly is far stiffer and more accurate than a round-shaft bushing.
This is why profiled rails are the default for machine tools, robots and precision positioning. They can be preloaded to remove all play, which matters wherever a reversal must not lose position, and they come in accuracy grades that let a designer match rail runout to the tolerance budget. The price is a machined mounting surface: a rail bent to a curved casting wears out early.
| Linear system | Contact form | Moment capacity | Speed ceiling | Typical duty |
|---|---|---|---|---|
| Round-shaft ball bushing | Recirculating balls, point contact | Low | High | General automation, guides |
| Profile rail, balls | Balls at about 45 degrees | Moderate | High | Pick and place, light machining |
| Profile rail, rollers | Cylindrical line contact | High | Moderate | Heavy machine tools, gantries |
| Cam follower or track roller | Rolling wheel on a track | Position-dependent | Moderate | Conveyors, cam mechanisms |
| Plain sliding way | Boundary film, no rolling | Very high, well damped | Low | Slow, heavy machine tools |
The four rolling families trade stiffness, capacity and damping; the plain sliding way trades friction for damping and load capacity.
Roller carriages where rigidity outweighs drag
When the load and the rigidity requirement rise beyond what balls can supply, the carriage is fitted with cylindrical rollers instead. Line contact spreads the load over a rectangle rather than a point, so a roller carriage carries 3 to 5 times the load of a ball carriage in the same envelope and deflects far less. The rollers are arranged in crossed or alternating rows so the carriage still resists moment in every direction.
The trade is friction and speed. Line contact and the need to keep the rollers aligned add drag, so a roller carriage runs a little hotter and reaches a lower speed than a ball carriage of the same size. In heavy machine tools, large gantries and press feeds that is a price worth paying: the stiffness and the load capacity decide the cut, and the machine rarely needs the top speed. Above all, a roller carriage demands a stiffer, flatter mounting surface than a ball carriage, because misalignment shows up immediately as edge loading on the roller ends.
Preload classes and the reversal-error budget
Preload removes internal clearance so the carriage is already in contact when the load reverses. Without it a bi-directional axis loses position at every direction change. Preload is expressed in classes, from light for smooth running to heavy for maximum rigidity, and it is set to a measured value, often 10 to 30 µm of interference at the rolling element. The carriage envelope follows the ISO 15 boundary system for bore, rail width and height.
Light, medium and heavy preload
The choice follows the duty. A light preload suits a transport axis that moves a known load and values low drag. Medium is the general machine-tool setting, balancing accuracy against friction. Heavy is reserved for axes where deflection is forbidden, such as a grinding wheelhead, and is always checked against the heat and life it costs.
What over-preload costs
Preload is a load the bearing carries before the machine does any work. Too much raises drag torque, warms the carriage and shortens fatigue life, exactly as over-preloading a rotary pair does. The right setting is the lightest preload that holds the reversal error inside tolerance, recorded as a number rather than a feel.
| Preload level | Carriage fit | Stiffness effect | Drag and heat | Typical axis |
|---|---|---|---|---|
| Clearance | Play present | Lowest | Lowest | Non-critical transport |
| Light | Just in contact | Moderate | Low | General automation |
| Medium | Slight interference | High | Moderate | Machine-tool feed axes |
| Heavy | Firm interference | Highest | High, at a life cost | Grinding and measuring |
Rating a linear system by distance travelled
Linear bearings are rated like rotary bearings, but life is expressed as a distance rather than a number of revolutions. The basic dynamic rating C is the load at which 90 percent of a group survives a standard travel, and the L10 defined by ISO 281 is the distance 90 percent reaches before fatigue. On a typical profile rail a 1,000 mm stroke is common, so the equivalent load and the cleanliness of the environment dominate that distance.
Adding moments to the load
A load hung at a distance from the carriage produces a moment as well as a force, and the moment must be converted into an equivalent rolling load before the rating is checked. Two carriages on one rail share the moment; a single carriage under an offset load is the worst case and the usual cause of early failure.
Why contamination shortens travel life
A guideway that ingests grit abrades its raceways far faster than a rotary bearing, because the wiper meets every pass of the stroke. Sealed carriages, double-lipped wipers, bellows and a positive grease film are not accessories; on a dusty machine they separate a rated life from a short one. The harsh-environment guide covers the sealing options.
Speed, acceleration and the re-lubrication question
Linear axes often run at 1 m/s to 5 m/s with accelerations up to 100 m/s². High acceleration raises the force between ball and raceway and loads the return-channel entry, so the path design and the ball spacing set the noise and wear at speed. Lubrication arrives as grease through the carriage or as oil from a metered system, and sealed carriages keep the film in and the dust out. On a high-duty-cycle machine, automatic re-lubrication beats the classic failure of a carriage greased once and forgotten. Linear speed limits scale with the ball or roller size and the cage, not with a simple DN figure, so the manufacturer's curve governs the ceiling; the rotary speed and DN guide explains the thermal logic.
Aligning rails and shafts to the machine datum
A linear system is only as straight as the surface it bolts to. A rail that follows a curved casting forces the carriage against its own raceways, so the bearing wears out before it carries a working load. Alignment is the highest-value hour in the installation.
Checking a rail with a dial indicator
A dial indicator along the rail face measures straightness and parallelism to the datum, and a pass along the top face checks height. The rail is brought in with the bolts in stages, tightening and re-checking so it is not pulled into a curve. Parallel rails are checked for height and lateral offset, because a height error twists the carriage. The usual target is 0.01 mm over the rail length, checked every 200 mm.
Parallel round shafts
Round shafts must be parallel and at equal height to within the bushing's tolerance, or the sleeve binds and the balls skid. Straightedges, indicator stands and careful shimming get them true, and the effort pays back in life, because a misaligned bushing hears the same edge loading as a misaligned rotary bearing. The shaft and housing fit guide covers the seat tolerances that keep the alignment.
Choosing a linear system follows a short sequence; work it before the envelope is fixed.
- Define the load and its offset. Resolve the force and moment at the carriage into an equivalent rolling load.
- Set the stiffness target. The reversal error the machine tolerates decides the preload class and whether balls or rollers are needed.
- Pick the family. Round-shaft bushing for light duty, profile rail for accuracy, roller carriage for heavy or rigid duty, cam follower where a wheel runs on a track.
- Check the stroke and speed. Confirm the return channel, ball spacing and lubricant suit the speed, using the grease selection guide.
- Plan protection and alignment. Specify wipers, seals and bellows, and hold the mounting datum flat and straight within tolerance.
Common mistakes that shorten linear life
- Running a bushing on a soft shaft. The raceway brinells and spalls.
- Ignoring the moment. An offset load edge-loads the rolling elements.
- Over-preloading for stiffness. Extra preload warms the carriage and cuts life.
- Bolting a rail to an unmachined surface. A bent rail never runs straight.
- Skipping re-lubrication. A dry carriage fails early and quietly.
- Leaving the carriage unprotected. Dust and chips wear the guideway faster than load.
- Recirculation is the core idea. Balls or rollers loop through a return channel, so a carriage can travel without limit.
- Bushings, rails and rollers divide the duty. Round shafts for light loads, profile rails for accuracy, roller carriages for stiffness.
- Preload buys accuracy and costs life. Choose the lightest class that holds the reversal error inside tolerance.
- Travel life is dominated by cleanliness. Wipers, seals and lubrication matter more than the rating on paper.
- Alignment is the hidden specification. A rail or shaft off a poor datum wears out before it works.
Standard profile rail and round-shaft families are filterable by bore, rail width and accuracy grade in the bearing selector, and the linear range sits in the linear guideway catalogue. Where a carriage must also carry a rotary load, the combined-load guide and the cylindrical roller range show how the load splits. Round-shaft bushings from INA and recirculating units from IKO show how the principle is packaged, the L10 life guide turns the rating into a worked distance, and the background is collected in the technical library.
Frequently asked questions
What does 'recirculating' mean in a linear bearing?
It means the balls or rollers loop through a return channel built into the carriage. They roll through the loaded zone, leave it, pass through the channel and re-enter at the other end, so the bearing holds a fixed number of elements and can travel any distance without running out of balls.
When should I choose a profile rail over a round-shaft bushing?
Choose a profile rail whenever the load is off-centre, the deflection is significant, or the axis must resist moments and hold accuracy. A round-shaft bushing is cheaper and simpler but tolerates little moment, so it suits light loads on stiff, well-supported shafts.
Why does a roller carriage carry more load than a ball carriage?
Because its contact is a line rather than a point. The load spreads along the roller into a narrow rectangle, producing a much lower peak stress than a ball contact patch, so a roller carriage carries several times the load in the same envelope at the cost of higher friction and a lower speed ceiling.
Can I preload a linear guideway as much as a rotary bearing?
You can preload it heavily, but the same trade applies: heavy preload raises drag torque, heat and wear even when the axis is idle. Choose the lightest class that keeps the reversal error inside the machine's tolerance, and treat the setting as a documented value.
How is the life of a linear bearing expressed?
As a distance, not a number of revolutions. The basic dynamic rating is the load at which 90 percent of a group survives a standard travel, and the calculated L10 is the distance that 90 percent reaches before fatigue. Offset loads add a moment that must be converted into an equivalent load.
What is the biggest cause of short linear guideway life?
Contamination. A wiper meets every pass of the stroke, so grit between ball and raceway abrades the surfaces far faster than a rotary bearing in the same plant. Sealed carriages, bellows and a maintained grease film usually matter more than the catalogue rating.