A recirculating linear ball bearing, including round-shaft ball bushings and profile rail guides, must have grease or oil present on the raceway at all times, because metal-to-metal contact between the rolling elements and the rail is otherwise unavoidable [S5].
Linear plain bearings, whether PTFE/Frelon-lined, polymer (iglide/drylin style) or oil-impregnated sintered bronze, are engineered to run dry: the lubricant is built into the bearing material itself, not added at the machine [S1][S3]. The two families are not interchangeable on the lubrication axis, and the wrong choice creates either a starved rolling bearing or an over-lubricated polymer liner.
Why rolling-element linear bearings cannot run dry
All metal-on-metal recirculating linear bearings require lubrication to function, with the lube serving four jobs: separating rolling elements from the raceway, removing frictional heat, inhibiting corrosion, and helping seals run [S5]. Thomson's support guidance breaks duty into three bands: light loads at high speed suit low-viscosity machine oil or soft grease, medium load/speed suits NLGI 1 or 2 grease, and heavy loads typically need a stiffer grease or EP-grade product [S4].
Without that film, the ball or roller makes direct contact with the shaft or rail, and failure modes are well documented: galling, brinelling, adhesive wear, and accelerated raceway spalling [S1]. For design context, a linear bearing running in boundary or mixed lubrication is the default state at low speeds, which is why grease is preferred for slow strokes and oil is reserved for high-speed, well-cooled service [S5]. Orientation also matters: for vertical or inclined axes, grease is typically specified because oil migrates downward and pools away from the contact zone [S5].
How self-lubricating plain bearings actually stay lubricated
Self-lubrication in a linear plain bearing is defined as the bearing material transferring microscopic amounts of itself to the mating shaft, building a low-friction film over the rail surface, and continuing that transfer for the full design life of the part [S1]. Five criteria define a "true" self-lubricating system: the lubricant is an integral part of the bearing material, no oil or grease is added at install, the lubricant does not age out, it is supplied consistently along the stroke, and no extra hardware is needed [S1].
PBC Linear's Frelon liner is a PTFE-based compound that bonds to the shaft during a break-in period of typically 50 to 100 strokes; after that the transfer process is continuous, and friction stays roughly constant for the life of the bearing [S1]. On the polymer side, igus drylin-style tribopolymer liners reach linear speeds of 3 m/s and above, with three functional ingredients: a base polymer for temperature and chemical envelope, reinforcing fibers and fillers (carbon, glass, glass beads) for strength, and embedded solid lubricants (PTFE, graphite, or similar) that drop the coefficient of friction and lift the PV limit [S3]. A practical overview of these dry-running designs is captured in the spherical plain bearing and linear bearing encyclopedia entries, which both contrast self-lubricated liners against rolling-element hardware.
Grease versus oil: the recirculating-bearing selection matrix
For a linear ball bearing, the lubricant choice is rule-driven rather than preference-driven, and four criteria dominate the decision [S4][S5]:
Speed. Below the boundary/mixed regime, grease wins because it stays in place; above it, oil's cooling effect is needed and an automatic lubrication system is usually specified to meter the dose [S5].
Load. Light duty is fine with low-viscosity oil or NLGI 1 grease, medium duty maps to NLGI 1 or 2, and heavy duty typically demands NLGI 2 to 3 or an EP-grade grease [S4].
Orientation. Vertical or inverted axes are grease territory, because oil migrates and pools in the lowest cavity, starving the upper contact zone [S5].
Additive package. Solid additives such as PTFE, MoS2, or graphite are forbidden in a rolling-element linear bearing, even though they are standard in plain bearings, because the solid particles damage raceways and rolling elements [S5]. Grease consistency is rated under NLGI 000 (fluid) to 6 (block), with classes 00 to 3 the usual operating window for linear ball bushings and profile rail guides [S5].
The same shaft and housing envelope can therefore host a ball bearing variant that must be greased on a defined interval, or a plain variant that is installed and forgotten; mixing the two lubrication philosophies is the most common field failure this engineer sees on retrofits.
Plain-bearings that still need external lube, and the "lubed for life" trap
Two common non-self-lubricating systems are still sold in the linear space, and both will eventually need re-lubrication [S1]. Oil-impregnated sintered bronze holds a finite oil charge in its porosity; under ideal conditions that oil is drawn to the surface, but in dirty, hot, or high-load service the oil is consumed and the bearing must be re-oiled or replaced [S1]. Rolling-element profile rail and round-rail linear bearings can be supplied with oil-impregnated felt seals at the ends of the block, but those seals only extend life; they do not eliminate the requirement for external grease at the relubrication interval [S1].
The marketing watch-out is the "self-lubricating" or "lubed for life" label applied to hardware where the lubricant is a charge inside a seal, not a material in the bearing liner. PBC Linear's own guidance: many of those parts are "lubed for a long time," not lubed for life, and the lube will eventually be used and need to be replaced [S1]. Prusa's MK3S/MK3 community guidance on acceptable consumer greases points the same direction: hobby-grade linear rails still get periodic re-grease even when shipped with a factory charge, because the lube starves off over thousands of strokes [S2].
Who should pick which: decision rules for the spec sheet
Pick a linear ball bearing when the duty cycle demands high speed (above the 1 to 2 m/s band where polymer liners start to heat-soak), high positional repeatability, low breakaway friction, and when the application already has a relubrication plan with a defined grease grade and interval [S4][S5]. Factory automation, machine-tool axes, and high-cycle pick-and-place are the natural fit, and a ball screw paired with a linear ball bearing is the standard axis package in those segments.
Pick a self-lubricating plain bearing when the duty cycle is slow to moderate, the environment is dirty or washdown-heavy (food, medical, outdoor), contamination from oil or grease is unacceptable (cleanroom, pharmaceutical, semiconductor), or maintenance access is restricted [S1][S3]. The same rule covers ball spline alternatives when the load is light and the stroke is short; for higher loads on a rotating-reciprocating shaft, the ball spline still requires grease.
Do not pick a self-lubricating plain bearing when the application demands tight tolerance, very high speed, very long life under continuous cycling, or very high static load, and do not pick a linear ball bearing when the system cannot be re-lubricated on a defined interval. In process plants where gasketed seals share the same equipment footprint, designers balancing elastomer choice against chemical exposure often make the same trade-off, and the EPDM modulus and stiffness inputs for gasket design reference covers the parallel decision on the sealing side.
Maintenance intervals, contamination, and what the OEM actually requires
For a linear ball bearing the relubrication interval is set by speed, load, temperature, and stroke length, and the OEM's published interval should be treated as a ceiling, not a floor [S4][S5]. Three failure patterns dominate when intervals are missed: starved-raceway brinelling at high static load, wash-out in wet or outdoor service where water displaces the grease, and contamination embedding when grit combines with old grease to form an abrasive paste [S5].
For a self-lubricating plain bearing, the maintenance task list collapses to a different set: shaft cleanliness (the transfer film builds on the rail, so a contaminated shaft contaminates the liner), periodic inspection for uneven transfer patterns, and verification that the break-in strokes have actually occurred before the machine is loaded to full duty [S1]. Where motion hardware sits inside a larger equipment package, the expansion anchor applications: equipment fixing to concrete article covers the parallel concern of keeping that package anchored when vibration and cyclic load are present.
Cross-checking with adjacent motion components: a linear actuator with a ball valve on the same skid is a common food and chemical plant assembly, and the valve stem's lubrication needs (silicone- or PTFE-based) do not contaminate the linear axis when the two are physically separated by a stem extension.
The next signal to track is the 2026 update to the major linear-bearing makers' relubrication calculators, where Thomson, Bosch Rexroth, NSK, and SKF have been publishing NLGI grade maps against their speed factor (dmN) curves; any published change to the upper oil-viscosity limit for high-speed profile rail, or to the EP additive allow-list for linear ball bushings, would be the single most consequential spec shift for designers in this space.