Linear Bearing

What is a Linear Bearing

A linear bearing is a mechanical guide element that constrains a moving member, the carriage or table, to a single translational degree of freedom along a straight axis, while supporting the load that acts perpendicular to that axis. Its defining characteristic is that the contact path is open ended rather than closed: as the carriage travels, any rolling element under load must eventually reach the end of the loaded zone and return through an unloaded channel to re-enter the load zone. This recirculation is the feature that separates a linear bearing from a rotary one, where the same ball orbits the bore forever. The recirculation loop, not the rolling contact, is what sets the practical speed and noise limits of a linear bearing.

The simplest and oldest linear bearing is the plain bearing or slide bushing, in which the carriage slides directly on a low-friction surface such as bronze, filled PTFE, or a self-lubricating polymer. It has no moving parts inside, tolerates dirt, and is cheap, but it has high friction and wears with distance. The rolling linear bearing replaced sliding contact with recirculating balls or rollers to cut friction by one to two orders of magnitude. The round-shaft ball bushing, patented in the mid twentieth century, packaged recirculating balls into a sleeve that runs on a hardened cylindrical shaft. The profile rail guide, which matured industrially in the 1970s and 1980s through makers such as THK in Japan, moved the raceways onto a ground square rail to gain rigidity and moment capacity that a round shaft cannot match.

Functionally, every rolling linear bearing has the same four parts: a hardened raceway surface (the shaft or rail), recirculating rolling elements (balls or rollers), a carriage or sleeve body that contains the return channels, and a retainer or cage plus end caps and seals that keep the elements spaced and the contamination out. In a ball bushing the raceway runs along the shaft and the sleeve carries the return tracks; in a profile guide the raceway is ground into the rail and the block carries the loaded and return tracks. The geometry differs, but the load path is always carriage to rolling element to raceway to the structure beneath.

The scale of linear bearing duty spans many orders of magnitude. A desktop 3D printer rides on an 8 mm round shaft and a handful of LM8 ball bushings carrying a few newtons. A vertical machining center carries a several-tonne table on 45 mm profile rails rated for hundreds of kilonewtons of dynamic load. A semiconductor wafer stage may demand nanometer positioning on air bearings or crossed roller ways. No single linear bearing serves all of this, so selection is the discipline of mapping stroke, load, speed, accuracy, and environment onto a specific family and size.

Four engineering metrics dominate linear bearing quality and total cost: load capacity (the dynamic rating C that sets life and the static rating C0 that sets the brinelling limit), rigidity (deflection under load, which governs machining accuracy), running accuracy (travel parallelism and repeatability), and friction (which sets drive sizing, heat, and wear). As with any precision component, the cheapest bearing rarely has the lowest lifecycle cost: an undersized or under-hard system that wears or develops play within a year can cost far more in scrap, downtime, and replacement than a correctly rated assembly bought once.

The Three Linear Bearing Families

Industrial linear bearings divide into three mechanically distinct families, and choosing the wrong family is the most consequential early mistake in a motion design. A round-shaft ball bushing, a profile rail guide, and a plain polymer bushing can all carry the same nominal load on paper, yet they differ by an order of magnitude in rigidity, contamination tolerance, and price. The table below contrasts the families on the dimensions that decide selection.

Round-shaft ball bushings, also called linear bushings or ball bushings, recirculate balls inside a cylindrical sleeve that runs on a hardened round shaft. Because the ball contacts the round shaft at essentially a point, the bushing is compact, low friction, and forgiving of shaft droop, but its load capacity depends on the angular position of the load relative to the loaded ball tracks, and point contact gives it lower rigidity than a profile rail. The de facto interchangeable metric series is the LM series, standardized in envelope by ISO 10285. Typical closed-type sizes are LM8 (8 mm shaft, 15 mm outer diameter, 24 mm length), LM12 (12 by 21 by 30 mm), LM16 (16 by 28 by 37 mm), LM20 (20 by 32 by 42 mm), and LM30 (30 by 45 by 64 mm). Across the THK LM range, basic dynamic load ratings span roughly 139 N at the smallest bore to about 7,650 N at 60 mm, with static ratings from about 108 N to 10,000 N, so the family covers both gram-scale printer axes and tonne-scale gantries by size selection alone.

Within the round-shaft family there are three mounting variants that recur in catalogs. The closed type is a full sleeve that surrounds the shaft and is used with end-supported shafts. The open type (suffix OP) has a longitudinal slot so it can clear a support rail that holds the shaft along its whole length, which is mandatory for long unsupported spans where the shaft would otherwise sag. The adjustable type (suffix AJ) has a thin axial slot so that, when clamped in a split housing, the clearance between balls and shaft can be tightened to zero or to a light preload, which is the cure for play on a worn shaft or an imperfect bore. THK additionally offers flanged bushings (LMF, LMK, LMH) that integrate a mounting flange, and self-aligning variants that tolerate shaft misalignment.

Profile rail guides, also called square rail or linear motion guides, move the raceways onto a precision-ground rail with a rectangular cross-section, and run two or four rows of balls (or rollers) in matched grooves inside a steel block. Because the contact is a conforming groove rather than a point, and the block wraps the rail, a profile guide carries load and moment in all four directions, deflects far less, and accepts preload to remove play. Industry references put profile rail systems near ten times stiffer than a round shaft of comparable size, so a 15 mm profile rail can be stiffer than a 30 mm shaft. This is why machine tools, CNC routers, and precision stages almost always use profile rails despite their higher cost and tighter mounting-surface requirements.

Plain polymer bushings abandon rolling elements entirely and slide on a self-lubricating composite, typically a base polymer loaded with reinforcing fiber and solid lubricant particles, such as the igus DryLin and iglidur families. The solid lubricant is distributed through the whole wall thickness rather than applied as a film, so it cannot be washed out like grease and the bearing runs maintenance-free, which makes it the natural choice for dirty, wet, washdown, food, and clean-room duty where grease is unwelcome. The trade is friction one to two orders of magnitude higher than a ball bushing and a finite wear life proportional to travel and load, plus susceptibility to the 2:1 binding ratio on cantilevered loads.

Rolling Technologies and Contact

Among rolling linear bearings, the rolling element and the contact geometry determine load capacity, rigidity, and rated life. Four configurations cover almost all industrial use: recirculating balls on a round shaft, recirculating balls in a profile rail, recirculating rollers in a profile rail, and non-recirculating crossed rollers in a short-stroke way. The table below compares them on the metrics that drive an engineering decision.

Recirculating ball bearings dominate by volume because balls give the lowest friction and are cheap to make to a high finish. A ball in a round-shaft bushing contacts the shaft at a point, so contact stress is high and load capacity per element is modest, which is why these bushings deflect more and are rated direction-dependently. A ball in a profile rail rides a ground groove cut to a slightly larger radius (a Gothic-arch or circular-arc raceway), spreading the contact into an ellipse, raising capacity, and allowing the block to be preloaded so that play is eliminated and rigidity rises. For ball bearings the ISO 14728-1 rating-life exponent is 3, meaning halving the applied load multiplies travel life eightfold.

Recirculating roller bearings replace balls with cylindrical rollers, converting point contact into line contact. The larger contact footprint sharply increases load capacity and rigidity for a given envelope, so roller profile guides are the choice for heavy machine tools, grinding machines, and high-moment gantries where deflection must be minimal. The penalty is higher cost, lower maximum speed, and tighter sensitivity to mounting-surface flatness. The roller life exponent is 10/3 rather than 3, reflecting the way line contact spreads load; this exponent was set empirically at 10/3 rather than the theoretical 4 because real rollers do not maintain pure line contact across all loads.

Crossed roller ways use cylindrical rollers alternately crossed at ninety degrees, held in a cage that travels but does not recirculate, so stroke is limited to roughly the cage length. Within that short stroke they offer extremely high rigidity and the smoothest, most repeatable motion of any rolling guide, which makes them the standard for optics positioning, metrology stages, and semiconductor inspection where stroke is short but accuracy is everything. Because there is no recirculation, there is no end-of-track impact, so motion is exceptionally quiet and free of cyclic ripple.

A note on friction across these technologies: a recirculating ball bushing runs with a coefficient of friction of roughly 0.001 to 0.004, a ball profile guide around 0.002 to 0.01 once seals and light preload are included, and a roller guide slightly higher again. All of these are one to two orders of magnitude below the 0.05 to 0.20 of a sliding polymer bushing. Low friction is not merely an efficiency benefit: it directly governs drive motor sizing, servo following error, self-generated heat, and, on cantilevered loads, whether the bearing binds at all.

Shafts, Rails, and Materials

A rolling linear bearing is only as good as the surface it rolls on. Because the loaded balls or rollers transmit force through point or line contact, the shaft or rail surface must be hard enough to resist permanent indentation (brinelling), straight enough to preserve positioning accuracy, and finished smoothly enough to keep friction and wear low. Getting the mating surface wrong silently destroys the catalog life rating even when the bearing itself is perfect.

Shaft hardness is the first lever. Catalog dynamic load ratings for ball bushings assume a raceway hardness of about 60 HRC. When the shaft is softer, the rating must be reduced by a hardness correction factor, because softer surfaces deform under the concentrated ball contact and shed life. Bearing-grade chromium steel such as 100Cr6 (52100) reaches about 58 to 62 HRC, and case-hardening carbon grades like 1050 to 1060 are commonly held to a minimum of 60 HRC, which is why running on a fully hardened shaft can extend bearing life substantially over a soft or partly hardened one. The same logic applies to profile rails, whose raceways are induction- or through-hardened and ground.

Shaft and rail materials follow the duty. Through-hardened or case-hardened bearing steel such as 100Cr6 (52100) and case-hardening grades like 1050 to 1060 carbon steel are the default for dry indoor service. For corrosion resistance, hard-chrome-plated carbon steel keeps a hard, smooth running surface over a tough core, while martensitic stainless such as 440C suits wet or washdown service. Note the trade: 440C typically reaches only about 50 to 55 HRC, which can cut shaft life by roughly 20 to 50 percent against fully hardened carbon steel, so the corrosion benefit is paid for in load rating. The table below maps common environments to the practical shaft and bearing choices.

Straightness and finish set the accuracy ceiling. Linear shafting is commonly held to a straightness on the order of 0.001 in per foot, with 0.0005 in per foot available for accuracy-critical work, because any waviness in the shaft maps directly into carriage motion. Diameter tolerance is typically held to a ground class such as h6, and the running surface is finished smoothly to limit friction and seal wear. Profile rails add a flatness and parallelism specification for the mounting surface, because the rail conforms to whatever it is bolted to, so a wavy machine bed transfers straight into the guide.

Retainers, seals, and lubrication complete the picture. Recirculating bearings use a cage or retainer, often glass-filled polyamide, to space the elements; polyamide is robust and quiet but limits the upper temperature, with typical assemblies rated to roughly -20 to +80 degrees C and short excursions higher. End seals and side seals keep grit out and grease in, at a small friction cost. Rolling bearings need periodic grease or oil unless specified as lubricated-for-life, while polymer plain bearings carry their lubricant internally and need none, which is precisely their appeal in service-averse installations.

Key Specification Parameters

Comparing linear bearings across catalogs requires reading the same eight numbers consistently, because makers express them on different bases. The parameters that actually drive selection are the dynamic load rating, the static load rating, the rated life and its travel basis, rigidity or deflection, running accuracy, preload, speed and acceleration limits, and the friction coefficient. Each is explained below.

Basic dynamic load rating (C) is the constant load under which a bearing achieves its rated travel life. Per ISO 14728-1 the reference travel is 100 km (100,000 m), but some makers, notably older North American catalogs, rate to 2 million inches (about 50 km), so a higher C value can simply mean a shorter rating basis. Always confirm the travel basis before comparing C across two brands, and convert if needed before sizing.

Basic static load rating (C0) is the static load that produces a permanent deformation at the most heavily loaded contact equal to a small fraction of the element diameter. It guards against brinelling from shock, clamping, or a stationary overload, and it is the rating that matters when the carriage is parked and absorbing an impact rather than traveling. A bearing can be fine on C yet fail on C0 if it sees a heavy stationary shock.

Rated life (L10) is the travel distance that 90 percent of a bearing population reaches before fatigue, computed from L10 = (C / P) to the power n times the rated travel, where P is the equivalent load and the exponent n is 3 for ball bearings and 10/3 for roller bearings. Because of that exponent, modest oversizing buys disproportionate life: doubling C at fixed load multiplies ball-bearing life eightfold. Apply hardness and temperature correction factors to C before the calculation, and add an application or safety factor for shock and vibration.

Rigidity (deflection under load) is the elastic displacement of the carriage per unit load, and it is where round shafts and profile rails diverge most. A point-contact ball bushing deflects more than a groove-contact profile block, and a roller guide deflects least of all. For machining and metrology, rigidity often matters more than rated load, since deflection translates directly into part error. Preload trades a little life and friction for a large gain in rigidity and the elimination of lost motion; profile-rail makers commonly supply preload in the range of about 2 to 8 percent of the dynamic rating.

Running accuracy covers travel parallelism (how straight the carriage runs relative to its mounting reference), height tolerance, and running repeatability. Profile rails are graded into accuracy classes from normal through high, precision, super precision, and ultra precision, with travel-parallelism tolerances tightening from tens of micrometers down to a few micrometers over the rail length. Round shafts inherit their accuracy mostly from shaft straightness rather than a graded class.

Speed, acceleration, and friction close the list. Ball bushings and ball profile guides typically run up to about 5 m/s, with the recirculation zone, not the rolling contact, setting the limit because balls must decelerate and turn around at the ends of each track. Roller guides run slower. The friction coefficient, roughly 0.001 to 0.004 for ball bushings rising with seals and preload, sets drive sizing, heat, and following error, and is one to two orders of magnitude below a sliding polymer bushing. The list below summarizes the eight parameters and what each governs.

• Dynamic load rating C: sets travel life; confirm the 100 km versus 2 million inch basis.
• Static load rating C0: brinelling limit for parked or shock loads.
• Rated life L10: (C/P) to power 3 (ball) or 10/3 (roller) times rated travel.
• Rigidity: deflection per unit load; governs machining accuracy.
• Running accuracy: travel parallelism and repeatability, graded by class on profile rails.
• Preload: removes play and raises rigidity at a cost in friction and life.
• Speed and acceleration limit: set by recirculation, around 5 m/s for ball types.
• Friction coefficient: 0.001 to 0.004 for ball bushings; sizes the drive and sets heat.

Selection Decision Factors

To convert the preceding chapters into a specific part number, follow the decision sequence below. Most selection failures come not from a single wrong number but from deciding the family or the preload before the load case and environment are pinned down. These eight steps work as a fixed RFQ template for linear motion.

1. Define the load case and orientation: resolve the worst-case force into radial, axial, and moment components at the carriage, and note whether the load is centered or overhung. Overhung loads invoke the 2:1 binding ratio on plain bushings and bias a round-shaft bushing toward its weak direction; both push you toward a profile rail.
2. Choose the family: profile rail for rigidity, moment capacity, and precision; round-shaft ball bushing for long strokes, deflecting shafts, and low cost; self-lubricating polymer plain bearing for dirty, wet, washdown, food, or grease-free duty. This choice constrains everything downstream.
3. Set stroke, span, and shaft support: long unsupported spans sag, so either size the shaft for acceptable droop, switch to open-type bushings on a fully supported rail, or move to a profile rail bolted to a rigid bed. Verify carriage spacing against the moment arm.
4. Size load rating and life: compute the equivalent load P, apply hardness and temperature corrections to C, then solve L10 = (C/P) to power n times rated travel for the required travel distance, with a safety factor for shock and vibration. Confirm C0 against parked shock loads.
5. Specify accuracy and preload: pick a profile-rail accuracy class (normal to ultra precision) and a preload (clearance, light, or medium) that match the rigidity and repeatability the process needs. Tighter classes and heavier preload cost life and price.
6. Specify the shaft or rail material and hardness: case-hardened carbon steel at about 60 HRC for dry service, hard-chrome or 440C stainless for wet and washdown, ground and held to straightness and flatness tolerances that support the target accuracy.
7. Set environment, sealing, and lubrication: select end and side seals for the contamination level, choose grease, oil, or lubricated-for-life, and confirm the temperature window against the polyamide retainer limit. Specify a polymer plain bearing where grease is forbidden.
8. Total cost of ownership: add purchase price, mounting-surface machining, alignment labor, relubrication intervals, spare blocks, and downtime risk. A profile rail costs more upfront but its rigidity and graded accuracy often pay back in scrap reduction and longer service intervals.

One dimension that is routinely underweighted at the purchasing stage is serviceability and interchangeability: whether the bushing conforms to ISO 10285 so a replacement drops into the same bore, whether the profile-rail block can be reordered separately from the rail, whether preload class and accuracy class are documented for reorder, and whether the maker has local stock and technical support. THK, SKF, Thomson, Bosch Rexroth, HIWIN, NSK, Schaeffler INA, NB/Nippon Bearing, IKO, PBC Linear, and igus all maintain catalog interchangeability and regional distribution, which is what determines repair response time after five to ten years of production-line service.