Linear bearings sit behind a lot of machinery that most people never think about — the slide on a CNC router, the vertical axis on a pick-and-place machine, the transfer carriage in a packaging line. They do one job: allow controlled, low-friction movement along a fixed path. Get the selection right and they run quietly for years. Get it wrong and you'll be replacing them inside a year under real load.
This guide covers what linear bearings are, how they work, the different types available, and how to select the right bearing for the shaft size, load, and environment. It's aimed at maintenance fitters, machine builders, and anyone sourcing replacements or specifying new linear motion components for industrial equipment.
What Is a Linear Bearing?
A linear bearing is a bearing designed to allow movement in a straight line — along an axis — rather than rotational movement. Unlike a radial ball bearing, which supports a rotating shaft, a linear bearing supports and guides a shaft or rail that moves back and forth in a straight path.
The standard recirculating ball linear bearing (the LM series used in automation, CNC, and industrial equipment) consists of an outer steel cylinder housing a recirculating track of ball bearings. As the bearing moves along a hardened steel shaft, the balls roll along the shaft surface and recirculate through internal return channels, maintaining continuous contact with minimal friction throughout the stroke.
The result is a bearing that can traverse a shaft repeatedly with very low rolling friction — typically a coefficient of friction of 0.001 to 0.004, compared to 0.10 to 0.15 for plain sliding contact. This makes recirculating ball linear bearings suitable for high-speed, precise, and repetitive linear motion applications.
How Does a Linear Bearing Work?
In a recirculating ball linear bearing, a series of ball bearings sit within internal raceways arranged in rows along the bearing body. As the bearing moves along the shaft, the balls in contact with the shaft roll and are channelled through a return passage inside the bearing housing, bringing them back to re-enter the loaded zone.
This recirculation means the bearing can traverse an unlimited stroke — unlike a simple ball cage, which can only travel as far as the balls allow. The recirculating design is what enables the LM bearing series to support high loads and long strokes in machinery.
The key to performance is the hardened shaft. Standard linear bearings require a case-hardened shaft of SUJ2 bearing steel (equivalent to AISI 52100) ground to an h6 tolerance and surface-hardened to HRC 60–62. A soft shaft or an incorrectly sized shaft will result in rapid wear and premature failure — the shaft, not the bearing, is often the first component to fail in poorly specified systems.
Types of Linear Bearings
Three main categories of linear bearing are in common use in industrial and automation applications, each with different performance characteristics.
Recirculating Ball Linear Bearings (LM Series)
The LM series — also referred to as ball bushings or linear ball bearings — is the dominant type for general industrial and automation use. These are the cylindrical bearings pressed into a housing bore, designed to slide along a round hardened steel shaft. They are available in standard (UU — double-sealed) and open configurations.
The LM designation follows a straightforward naming convention. The number indicates the bore diameter in millimetres (matching the shaft diameter). The suffix indicates the sealing and length variant:
- No suffix: open bearing — no end seals, requires regular lubrication
- U: single rubber seal on one end
- UU: double rubber seals — one on each end — the standard for most applications
- L: long version — extended length for higher load capacity and moment resistance
- LUU: long version with double seals
So an LM12UU is a 12mm bore, double-sealed, standard-length linear bearing. An LM25LUU is a 25mm bore, double-sealed, long version.
LM series bearings are precision components — housing bore and shaft diameter tolerances must be within specification for the bearing to perform correctly. A housing bore to H7 tolerance and a shaft to h6 tolerance is the standard pairing for a light interference fit in the housing and a sliding fit on the shaft.
Plain and Polymer Linear Bearings
Plain linear bearings — also called linear bushings or sleeve bearings — replace the recirculating ball mechanism with a sliding contact surface. The bearing slides directly along the shaft with no rolling elements.
Bronze and sintered bronze bearings are self-lubricating — oil is impregnated into the porous metal structure and is released under load. They are robust, tolerant of misalignment, and can operate in environments where contamination would destroy a recirculating ball bearing. Load capacity per unit length is lower than LM series, but they handle shock loads and misalignment better.
Polymer plain bearings — of which the igus drylin system is the most well-known in AU — are made from engineered polymers with embedded solid lubricants. They require no grease or oil, are corrosion-resistant, and tolerate dust, moisture, and light contamination. They are widely used in food processing, pharmaceutical manufacturing, and outdoor machinery where contamination of lubricants is a concern and where metal-on-metal contact is undesirable.
The trade-off with polymer bearings is load capacity (lower than recirculating ball) and dimensional sensitivity (coefficient of thermal expansion is higher than steel — clearances must account for temperature variation). They are also less suitable for high-speed applications where heat generation at the sliding contact becomes a limiting factor.
Linear Guide Rails (Profiled Rail Systems)
Linear guide rails — THK, Hiwin, Bosch Rexroth, and similar — are a fundamentally different architecture. Instead of a cylindrical bearing running on a round shaft, a profiled steel rail carries a precision-machined carriage block. Multiple rows of recirculating balls or rollers sit between the carriage and the rail, providing high load capacity in all directions (radial, reverse radial, and lateral).
Linear guide rails offer significantly higher load ratings than round shaft LM bearings, better moment capacity, and higher stiffness. They are the standard choice for machine tools, precision machining centres, and high-load automation where LM bearings would be undersized.
The trade-off is cost and installation precision — rail surfaces must be ground flat to within fractions of a millimetre, and the carriage preload must be correctly specified for the application. They are not a drop-in substitute for round shaft bearings; they require precision mounting surfaces and correct rail alignment.
LM Series Sizing Guide
The table below covers the standard LM series range with principal dimensions. All dimensions are in millimetres. Load ratings are approximate dynamic load ratings (C) for standard-length UU variants — actual ratings vary by manufacturer.
| Designation | Bore (mm) | OD (mm) | Length (mm) | Long (LUU) Length (mm) | Approx. C (kN) |
|---|---|---|---|---|---|
| LM6UU | 6 | 12 | 19 | 35 | 0.5 |
| LM8UU | 8 | 15 | 24 | 45 | 1.4 |
| LM10UU | 10 | 19 | 29 | 55 | 2.2 |
| LM12UU | 12 | 21 | 30 | 57 | 3.2 |
| LM16UU | 16 | 28 | 37 | 70 | 5.6 |
| LM20UU | 20 | 32 | 42 | 80 | 9.4 |
| LM25UU | 25 | 40 | 59 | 112 | 16.2 |
| LM30UU | 30 | 45 | 64 | 123 | 22.0 |
| LM35UU | 35 | 52 | 70 | 134 | 32.0 |
| LM40UU | 40 | 60 | 80 | 154 | 42.0 |
| LM50UU | 50 | 75 | 100 | 192 | 68.0 |
| LM60UU | 60 | 90 | 125 | 240 | 100.0 |
| LM80UU | 80 | 120 | 165 | 320 | 196.0 |
The most common sizes in Australian industrial maintenance and automation are LM8UU (used extensively in 3D printers, small CNC machines, and light automation), LM12UU through LM20UU (general automation, transfer mechanisms), and LM25UU through LM40UU (heavier machinery, industrial slides, and transfer carriages).
Load Ratings Explained
Linear bearing datasheets specify two load ratings: dynamic load rating (C) and static load rating (C0). Understanding the difference matters when specifying or replacing a bearing under real load.
Dynamic load rating (C) is the load under which a bearing will achieve a rated travel life — typically expressed in kilometres of travel. The ISO standard for linear bearings uses 50km as the reference life (L10 = 50km at 90% reliability). Dynamic load rating is used for applications with continuous or frequent reciprocating movement — conveyor slides, transfer mechanisms, robotic axes.
Static load rating (C0) is the maximum load the bearing can support without permanent deformation of the balls or raceway. Static load rating applies to applications where the bearing is stationary under load, or where shock loading occurs. For applications with infrequent movement and high static loads, C0 is the relevant figure — not C.
The basic travel life calculation follows the ISO formula: L = (C/P)³ × 50, where L is life in km, C is dynamic load rating in kN, and P is the applied load in kN. Halving the applied load increases travel life by approximately eight times — load management has a disproportionate effect on bearing life.
For critical applications, a safety factor of 2–3 applied to the calculated load is standard practice in industrial machine design. This accounts for shock loads, vibration, misalignment, and acceleration forces that are difficult to quantify precisely in real-world machinery.
What Rail Material Is Best for Linear Bearings?
For standard recirculating ball linear bearings (LM series), the shaft must be a hardened steel rod — not mild steel, not aluminium, not stainless. The minimum surface hardness requirement is HRC 58. The standard shaft material is SUJ2 bearing steel (JIS standard), equivalent to AISI 52100 / EN 31. It is case-hardened to HRC 60–62 and ground to h6 tolerance.
The practical answer is: buy matched shafts from the same supplier as the bearings. Using a mild steel rod as an improvised shaft will result in shaft wear, not bearing wear — the shaft surface will be scored within a short period of use. This is the most common installation error encountered in the field.
Chrome-plated shafts are also commonly available. Chrome plating adds corrosion resistance to the hardened steel core — useful for applications where condensation or light moisture is present. The chrome layer is typically 10–25 microns and does not significantly change shaft dimensions for standard bearing fit. Chrome-plated shafts are appropriate for food processing, marine, and washdown environments.
Stainless steel shafts are available for corrosive environments but require careful selection — standard austenitic stainless (304, 316) is too soft and will score. Martensitic stainless or specially hardened stainless grades are required for LM series bearings. Confirm hardness ≥ HRC 58 before specifying stainless shafts with standard LM bearings.
For polymer plain bearings (igus drylin), the shaft material options are broader — anodised aluminium shafts, hard-chrome steel, and stainless all work because the polymer sliding contact is self-lubricating and less demanding on shaft hardness than recirculating balls. This is one of the practical advantages of polymer systems in environments where sourcing and maintaining hardened steel shafts is difficult.
Sealed vs Open Linear Bearings
The sealing suffix on LM bearings indicates the type and number of end seals:
Open bearings (no suffix) have no end seals. Grease can be applied directly into the ball track from the ends. They are used in clean, controlled environments where regular maintenance is possible — precision machine tools, enclosed enclosures, applications where the bearing can be accessed frequently for relubrication. Open bearings are also used where compact installation length is critical.
Single-seal (U) bearings have one rubber lip seal on one end. Partial protection — useful where contamination approaches from one direction only. Less common in standard practice.
Double-seal (UU) bearings have rubber lip seals on both ends. This is the standard specification for general industrial use. The seals retain grease inside the bearing and exclude dust, swarf, and light contamination from entering the ball track. For most maintenance replacement applications, UU is the correct choice — it requires less frequent relubrication and is more tolerant of imperfect environments.
The UU suffix seals are contact lip seals — they provide good retention but add a small amount of friction compared to an open bearing. In high-speed applications (linear speed >2 m/s consistently), this friction can become relevant. For standard industrial speeds (typically <0.5 m/s in most maintenance applications), it is not a practical concern.
Can Linear Bearings Be Used Vertically?
Yes. Linear bearings can be used in any orientation — horizontal, vertical, or at any angle. The bearing mechanism functions identically regardless of orientation.
The considerations specific to vertical applications are:
Load direction: In a vertical application, the weight of the moving element (carriage, toolhead, gripper assembly) acts as a constant downward load throughout the stroke. The bearing must be rated for this load — check that the applied load is within the dynamic load rating under continuous operation. For heavy vertical loads with long strokes, the long LUU version increases load rating and improves moment resistance.
Grease retention: Gravity draws grease downward in a vertical orientation. In sealed (UU) bearings, this is largely managed by the end seals. For open bearings used vertically, more frequent relubrication of the upper end of the bearing may be required, as grease migrates away from the upper contact zone over time. This is a practical issue in long-service vertical applications — not a barrier to use, but a maintenance consideration.
Self-weight of the carriage under power loss: In vertical systems where the carriage is power-driven, consider what happens if power is lost and the drive disengages. The carriage will travel under gravity at whatever speed the linear bearing permits. If this is a hazard, a brake or counterbalance must be part of the system design — the linear bearing itself does not provide resistance to free travel. This is a system design issue, not a bearing limitation.
Polymer vs Recirculating Ball: Which Should You Choose?
The choice between polymer plain bearings and recirculating ball bearings depends on the operating environment, load, speed, and maintenance context. There is no universal answer — both types are in widespread use in Australian industry for valid reasons.
Choose recirculating ball (LM series) when:
- Precision of positioning matters — LM bearings have tighter running clearance and better repeatability
- High speed is required — rolling contact handles higher linear speeds with less heat generation
- Load capacity is critical — LM series outperforms polymer by a significant margin per unit size
- The environment is clean and controlled — lubricant contamination is not a concern
- The application is standard automation, CNC, 3D printing, or transfer machinery
Choose polymer plain bearings when:
- Maintenance access is difficult or infrequent — polymer runs dry indefinitely, no relubrication required
- The environment is wet, dusty, or contaminated with food products, cleaning agents, or fine particles that would contaminate lubricant
- Corrosion resistance is required — polymer and anodised aluminium shafts can be used where steel would corrode
- Noise is a constraint — polymer bearings are quieter than recirculating ball bearings in service
- The application is outdoor, agricultural, or food processing
The forum consensus among engineers on r/3Dprinting and r/robotics is that recirculating ball bearings (LM series) win on precision and speed, while polymer bearings win on reliability in contaminated or maintenance-inaccessible environments. Both assessments are correct for their respective contexts — the selection decision should be driven by the operating environment and maintenance reality, not by cost alone.
Moment Loading and Minimum Bearing Span
A linear bearing loaded purely in the radial direction (load perpendicular to shaft, no offset) is in its optimal loading condition. Moment loading — where the applied load creates a turning force about the bearing — reduces the effective load capacity significantly and must be accounted for in design.
Moment loads arise when: the load point is offset from the bearing centreline, a single bearing supports a cantilevered load, or acceleration forces act on a load with a centre of mass offset from the shaft axis. In engineering terms, moment load (M) = applied force (F) × offset distance (L).
The standard practice for managing moment loads is to use two bearings per shaft, spaced as far apart as the application allows. Increasing the bearing span by a factor of 2 reduces the effective moment on each bearing by a factor of 2. For heavily cantilevered loads, LUU (long) bearings are preferred over standard-length bearings — the longer bearing body distributes moment force over more ball contact points.
For critical applications with significant moment loading, a profiled linear guide rail system (THK/Hiwin style) is more appropriate than LM round shaft bearings — the four-way load capacity of a profiled rail carriage handles moment loading far more effectively than a round shaft bearing can.
Installation and Alignment
Correct installation is the single biggest factor in linear bearing service life after correct sizing. The most common causes of premature failure are not bearing defects — they are installation errors.
Housing bore tolerance: The housing bore must be machined to H7 tolerance for a standard LM bearing. A bore that is too tight will crush the outer race and reduce internal clearance, causing the bearing to run rough or seize. A bore that is too loose will allow the bearing to spin in the housing under load, causing housing wear and eventual loss of positional accuracy. Do not attempt to compensate for an oversized bore with adhesive alone — re-machine the housing or use an interference-fit sleeve.
Press fitting: Always press on the outer race — never the inner race or balls. Pressing on the inner race forces the load through the balls, which can indent the raceways and cause premature failure. Use a mandrel or press tool that contacts only the outer ring end face. A soft mallet against a properly fitting mandrel is acceptable for light-interference installations.
Shaft alignment: Two parallel shafts (as in a twin-shaft gantry or slide) must be parallel within the manufacturer's specified tolerance — typically 0.05 to 0.1mm over the full shaft length. Misalignment creates a pre-load on the bearings throughout the stroke, drastically reducing service life and increasing operating force. If the carriage feels stiff or jerky when moved by hand with no external load applied, misalignment or housing bore error is the cause.
For applications requiring precision shimming of shaft supports to achieve correct alignment, refer to the AIMS industrial shim guide — shim stock selection and material considerations apply directly to linear motion system alignment work.
Shaft support spacing: Support the shaft at intervals appropriate to its diameter and expected load. Unsupported shaft spans that are too long will allow the shaft to deflect under load, creating a curved travel path that overloads the bearing in the deflection zone. As a general guideline, the support span should not exceed 40–60 times the shaft diameter for standard industrial loads — shorter spans for heavier loads or higher speed applications.
Lubrication and Maintenance
Recirculating ball linear bearings require lubrication to protect the ball-to-raceway contact surfaces. Without adequate lubrication, the Hertzian contact stress at ball-to-raceway interfaces causes surface fatigue and early failure — typically spalling of the raceway surface.
Grease: NLGI 2 lithium-based grease is the standard specification for sealed LM bearings in general industrial applications. Apply a small amount of grease through the nipple fitting (if present) or by removing the end seal and applying directly. Grease quantity matters — over-packing creates churning resistance and heat; under-packing starves the contact. As a general guide, fill approximately one-third of the internal free space.
Oil: Light machine oil (ISO VG 32 or VG 46) is used in applications where grease would be displaced by high-speed recirculation, or where the bearing is part of an oil recirculation system. Oil-lubricated open bearings require more frequent replenishment than grease-lubricated sealed bearings.
Relubrication intervals: For general industrial applications with sealed UU bearings under moderate load and speed (linear speed <0.5 m/s, load <30% of rated capacity), a relubrication interval of 6–12 months is a reasonable starting point. Increase frequency for higher speeds, higher loads, elevated temperature, or contaminated environments. The symptom of inadequate lubrication is increased operating noise — a clicking or grinding sound that develops gradually as the bearing surface deteriorates.
For open bearings or applications in contaminated environments, a penetrating lubricant used as a maintenance flush (to clear contamination before regreasing) can extend bearing life between full replacements. See the AIMS penetrating oil guide for product selection by application context.
Polymer plain bearings: Require no lubrication — self-lubricating material releases lubricant under load from the bearing matrix. Do not apply grease or oil to polymer bearings — it attracts dirt, which then acts as an abrasive and accelerates wear. Keep polymer bearings dry and clean.
Common Failure Modes and How to Identify Them
Understanding how a linear bearing fails helps in diagnosing cause and preventing recurrence in the replacement bearing.
Spalling (raceway fatigue): Surface flaking of the raceway or ball surface. Appears as a rough, irregular texture in the ball track zone. Cause: fatigue under load — normal end-of-life mode if the bearing has reached its rated travel life. Premature spalling indicates overloading, contamination, or inadequate lubrication.
Scoring and scratching: Linear grooves in the raceway or shaft surface running parallel to the shaft axis. Cause: contamination — hard particles (swarf, grit, debris) trapped between balls and raceway. Prevention: sealed bearings (UU), shaft wipers, and cleaner operating environment. Replacement of the shaft may also be required if the scoring is significant.
Pitting and corrosion: Rust pitting on balls or raceway. Cause: moisture ingress into the bearing — condensation in a closed environment, washdown without sealed bearings, or inadequate sealing. Prevention: chrome-plated shafts, sealed UU bearings, stainless variants for extreme environments, and correct storage (bearings stored in factory packaging with desiccant until installation).
False brinelling: Evenly spaced indentations in the raceway matching ball spacing. Cause: vibration while stationary — the bearing oscillates slightly under vibration without full rolling motion, causing Hertzian contact damage at rest positions. Common in machinery shipped long distances or stored adjacent to vibrating equipment. Prevention: store and transport with shaft in place or with a dummy shaft through the bearing; isolate from vibration during storage.
Excessive noise: Clicking, rattling, or grinding during travel. Cause: contamination, inadequate lubrication, overloading, or worn raceways. If a bearing that previously ran quietly begins to produce noise under unchanged operating conditions, check lubrication first — then inspect for contamination. If noise persists after relubrication, replacement is the correct action.
Sourcing Linear Bearings in Australia
The LM series is globally standardised — an LM12UU from any reputable manufacturer (THK, NSK, Hiwin, IKO, PMI) will have the same external dimensions and is interchangeable with any housing machined to H7 bore tolerance. This standardisation means replacement sourcing is straightforward: you need the designation, not the brand. For rotating bearings in the drive systems that pair with linear motion assemblies — motor end-shield bearings, gearbox bearings, idler shafts — the AIMS Bearing Cross Reference Guide decodes SKF, NSK, NTN, FAG, Koyo, NACHI and other brand designations for those components.
Quality variation exists between manufacturers. Precision grade (P5 or P4) bearings from major Japanese and Taiwanese manufacturers hold tighter tolerances than standard-grade economy bearings. For precision CNC applications or medical/food processing machinery, specify the precision grade. For general industrial slides, transfer carriages, and maintenance replacements, standard grade is adequate and represents significantly better value.
For LM8UU through LM40UU, same-day or next-day availability from industrial bearing suppliers in Australia is typical for standard UU variants. LUU (long) versions and larger sizes (LM50UU and above) may require 2–5 working days. Linear guide rail systems (THK, Hiwin) generally require longer lead times if not held in local stock — confirm availability before committing to a design that depends on them.
Frequently Asked Questions
How does a linear bearing work?
A linear bearing allows controlled, low-friction movement along a straight path. In the most common type — the recirculating ball linear bearing (LM series) — a series of steel balls sits in internal raceways within the bearing body. As the bearing moves along a hardened steel shaft, the balls roll along the shaft surface and recirculate through internal return channels, maintaining continuous contact. This rolling contact produces very low friction (typically 0.001–0.004 coefficient of friction) compared to plain sliding contact, making it suitable for high-speed, precise, and repetitive linear motion applications.
What are the different types of linear bearings?
The three main categories are: recirculating ball linear bearings (LM series — cylindrical bearings running on round hardened steel shafts, the most common type in automation and industrial equipment), polymer or plain linear bearings (self-lubricating bushings for contaminated or maintenance-inaccessible environments), and linear guide rail systems (profiled steel rails with recirculating ball or roller carriages, used for high-load and high-precision machine tool applications). Each type suits different load, speed, precision, and environmental requirements.
Can linear bearings be used vertically?
Yes. Linear bearings operate correctly in any orientation — horizontal, vertical, or at an angle. In vertical applications, the bearing must be rated to support the weight of the moving element as a continuous load. Grease retention in sealed bearings (UU) is generally adequate for vertical use, though open bearings in vertical orientation may require more frequent relubrication of the upper end as gravity draws grease downward over time. The bearing itself does not resist free travel under gravity — if power loss would allow an unsupported carriage to fall, a brake or counterbalance must be part of the system design.
What does LM8UU mean?
LM8UU is the designation for a specific linear bearing. LM stands for Linear Motion. The number (8) is the bore diameter in millimetres — this must match the shaft diameter. UU indicates double rubber end seals on both ends of the bearing, which retain grease and exclude contamination. The standard LM8UU has a bore of 8mm, outer diameter of 15mm, and length of 24mm. An LM8LUU is the long version of the same bearing, with a length of 45mm for higher load capacity.
What is the difference between LM and LME bearings?
LME bearings are a metric variant of the LM series common in European machinery. They have the same bore diameter as the equivalent LM bearing but different outer dimensions — the outer diameter and length follow European metric standards rather than the JIS (Japanese Industrial Standard) used for the LM series. LM and LME bearings are not directly interchangeable if the housing bore has been machined to a specific series. When replacing a bearing, confirm whether the housing was designed for LM or LME dimensions before ordering. LM series is the more common format in Australian industrial equipment and automation.
How do I choose the right size linear bearing?
Start with the shaft diameter — the bore of the bearing must match the shaft. Then check the dynamic load rating (C) of the candidate bearing against your application load with a suitable safety factor (2–3× for industrial applications). If the standard-length bearing is marginal on load capacity, move to the long (LUU) version of the same bore size. For applications with significant moment loading (offset loads, cantilevered carriages), use two bearings per shaft spaced as far apart as practical. If your calculated load exceeds what the LM series can handle at the required bore size, consider a linear guide rail system instead.
What is the difference between polymer and recirculating ball linear bearings?
Recirculating ball bearings (LM series) use rolling ball contact for very low friction, high load capacity, and high precision. They require lubrication and are sensitive to contamination. Polymer bearings (igus drylin and similar) use a self-lubricating polymer sliding contact — they require no grease, tolerate contamination and moisture, and are corrosion-resistant, but have lower load capacity and are less precise. Choose recirculating ball for standard automation, CNC, and precision applications in clean environments. Choose polymer for food processing, outdoor, washdown, or maintenance-inaccessible applications where contamination of lubricant is a real concern.
How long do linear bearings last?
Service life depends on load, speed, lubrication, and contamination. The ISO standard reference life is 50km of travel at 90% reliability under rated dynamic load (C). Reducing the applied load significantly extends life — halving the load increases travel life by approximately eight times (life scales with the cube of the load ratio). In a well-maintained, correctly loaded industrial application, LM series bearings routinely achieve hundreds of kilometres of travel. Common causes of early failure are contamination (swarf, grit), inadequate lubrication, overloading, and misalignment — none of which are inherent bearing weaknesses.
Do linear bearings need lubrication?
Recirculating ball linear bearings (LM series) do require lubrication. Without lubricant, the ball-to-raceway contact stress causes surface fatigue and early failure. NLGI 2 lithium grease is standard for sealed bearings in general industrial use. Sealed UU bearings come pre-greased and require periodic relubrication (typically every 6–12 months under moderate conditions). Open bearings require more frequent attention. Polymer plain bearings (igus drylin) are self-lubricating and do not require — and should not receive — added grease or oil.
What causes linear bearings to fail early?
The most common causes of premature linear bearing failure are: contamination (swarf, grit, or abrasive particles entering the ball track — prevented by sealed UU bearings and clean installation), inadequate lubrication (dry contact causes rapid raceway fatigue — maintain correct relubrication intervals), incorrect shaft hardness (using a mild steel rod instead of hardened SUJ2 bearing shaft — the shaft wears rapidly and destroys the bearing), misalignment (parallel shafts out of alignment create a pre-load throughout the stroke, drastically reducing life), and overloading (exceeding the dynamic load rating — always apply a 2–3× safety factor).
What is a linear guide rail and how does it differ from a linear bearing?
A linear guide rail is a profiled steel rail paired with a precision carriage block — as used in machine tools, CNC machining centres, and precision automation. Multiple rows of recirculating balls or rollers between the carriage and rail provide high load capacity in all directions, including moments. A standard round-shaft LM linear bearing runs on a cylindrical shaft and handles radial loads and limited moments. Linear guide rails offer significantly higher stiffness, load capacity, and moment resistance than round shaft bearings, but require precision ground mounting surfaces and carry a higher cost. They are the correct choice for heavy machine tool applications; LM round shaft bearings suit lighter automation and general industrial use.
What is the correct housing bore tolerance for LM linear bearings?
The standard housing bore tolerance for LM series linear bearings is H7 (for example, an LM12UU with 21mm OD requires a housing bore of 21mm H7). H7 provides a light interference fit between the bearing outer race and the housing, preventing the bearing from rotating in the housing under load. A bore machined too tight will crush the outer race and cause the bearing to run rough or seize. A bore too loose allows the bearing to spin in the housing, wearing both components. Do not attempt to compensate for an oversized bore by applying adhesive alone — the housing must be correctly sized for the bearing to perform as specified.