A linear bearing is a cylindrical ball bushing running on a round hardened shaft, typically 6 to 60 mm in diameter, while a linear guide pairs a profiled steel rail with a carriage block whose raceways have circular-arc or Gothic-arch geometry [S2][S3]. The two systems both deliver low-friction straight-line motion, but they differ in contact geometry, load direction, rigidity, precision grade, and installation tolerance [S1][S3].
Engineers reach for round-shaft bearings where budget and self-alignment matter, and for profiled-rail guides where the axis must hold position under heavy or moment loads [S5][S9]. Selection is driven by load capacity, stiffness, accuracy, surface flatness available on the machine base, and total installed cost, not by the words "linear guide" and "linear bearing" being used interchangeably in casual spec sheets [S1][S3].
What Each Component Actually Is
A linear bearing is the rolling element: a closed cylindrical bushing with an outer housing, inner sleeve, and rows of recirculating steel balls that make point contact with a round shaft [S3][S4]. Ball spline and roller variants exist, but the canonical form is the ball bushing that wraps a shaft [S4]. Plain linear bearings use sliding contact instead of rolling elements and trade speed for higher shock tolerance and lower cost [S4].
A linear guide is the complete axis: a precision-ground steel rail with shaped raceways, mounted to the machine base, plus a carriage block that travels along it and supports the moving load [S3][S7]. The rail is bolted through machined holes directly to a flat reference surface, and the carriage handles radial, reverse-radial, lateral, and moment loads from a single rail [S3]. THK's reference definition describes a linear guide as a machine element that "utilizes bearings, which were developed for rotary motion, in order to move heavy objects easily in a straight line" [S7], so the bearing is the rolling sub-component inside the guide, not a separate system.
Contact Geometry and Load Capacity
Round-shaft linear bearings produce point contact between the balls and the cylindrical surface, so load capacity scales with shaft diameter and a single shaft resists loads well only in the primary radial direction [S3][S6]. Two parallel shafts are commonly used together to resist torque and side loads, which doubles the cost and the alignment burden [S3][S5].
Profiled-rail carriages use circular-arc or Gothic-arch raceways where the ball track radius is only slightly larger than the ball itself, cradling the balls in multi-row contact [S5][S6]. This geometry lets one rail resist radial, reverse-radial, lateral, and moment loads simultaneously, and it is why machine tools, semiconductor handlers, and CNC routers default to profiled-rail designs when deflection under cutting force matters [S3][S5]. For background on how these two families differ mechanically, see the entry on linear guides and the related reference on linear bearings.
Precision, Speed, and Repeatability
Round-shaft linear bearings typically deliver repeatability around 0.05 to 0.1 mm in general-tolerance builds, with excellent speed capability because the bushing mass is low and inertia is small [S3]. They are the right pick for conveyor transfers, pick-and-place shafts, and 3D-printer gantries where micron-level positioning is not required [S3].
Profiled-rail carriages are offered in precision grades up to UP (ultra-precision), with repeatability in the low-micron range and high acceleration capability, but the carriage mass is higher and the system demands a flat, ground mounting surface to hit those numbers [S3]. Misalignment on a profiled rail causes uneven ball loading and can halve service life, while a round-shaft bushing can absorb minor shaft-to-shaft elevation differences because the bushing is free to rotate about the shaft [S5].
Mounting, Alignment, and Maintenance
Guide rails bolt through the rail's mounting holes directly into the machine base, whereas linear bearings sit on shaft supports clamped to the frame and the shaft is fixed only at its two ends [S3]. Round-shaft systems tolerate flatness error greater than 150 μm/m and can span gaps of 12 to 24 shaft diameters, which makes them useful on welded frames and factory floors that were never precision-machined [S5].
Profiled-rail installations need a ground or milled reference surface, a defined bolt torque sequence, and preload verification, all of which add labour cost up front but pay back in stiffness and life [S3][S5]. Linear guides on production machinery are often difficult to access, and a bearing failure on a linear guide can shut down a line, so condition monitoring and lubrication intervals matter more than on a round-shaft axis that can be swapped bushing-by-bushing [S5]. For crossed-roller alternatives that sit between the two in rigidity, see the reference on crossed roller guides.
Selection Criteria: Round Shaft vs Profiled Rail
Use the matrix below to shortlist the right family before talking to a supplier; the criteria are drawn from the structural and performance comparison published in current manufacturer guidance [S3][S5].
<b>Decision criteria, round-shaft linear bearing vs profiled-rail linear guide:</b>
1) Load and moment: choose profiled-rail if the load is heavy, off-centre, or applies a moment; choose round-shaft if loads are light, primarily radial, and two parallel shafts can be used to carry torque [S3][S5].
2) Precision and repeatability: choose profiled-rail for low-micron or UP-grade repeatability; choose round-shaft for general-tolerance work in the 0.05 to 0.1 mm band [S3].
3) Mounting surface: choose round-shaft when flatness error exceeds 150 μm/m or the frame is welded tube; choose profiled-rail only when a ground or milled reference surface is available [S5].
4) Cost and lead time: choose round-shaft for the lowest system cost and shortest lead time on shafts from 5 mm to 60 mm in CF53, chrome-plated CF53, or X46 stainless [S2]; choose profiled-rail when total cost of ownership over the machine's life justifies the surface-prep and alignment work [S3][S5].
5) Environment: choose stainless or chrome-plated round shafts and corrosion-resistant bearings for washdown or medical use; choose profiled-rail with sealed carriages and stainless rails where debris exposure is high [S2][S4].
Material, Size, and Common Pitfalls
Linear shafts are commonly produced in CF53 steel, CF53 chrome-plated steel, and X46 stainless, induction-hardened and precision-ground to keep friction low and wear life high [S2]. Diameter range from 5 mm to 60 mm covers most automation, medical, and light-machine-tool shafts, while profiled rails are stocked in 15, 20, 25, 30, 35, 45, and 55 mm widths across the major suppliers, with custom sizes available [S2][S4].
The most common spec error is treating "linear guide" and "linear bearing" as interchangeable, then under-specifying a round-shaft axis for a moment load or over-specifying a profiled-rail system on a frame that cannot hold the flatness it needs [S1][S3][S5]. A second pitfall is ignoring self-alignment: a round-shaft bushing in pairs tolerates elevation differences between rails, while a profiled-rail carriage does not, so a fabricated frame that "looks flat" will still bind a profiled-rail axis [S3][S5]. Specifying the right family first, then the size and precision grade, prevents both failure modes. For a deeper look at how the surrounding machine drives the choice, the article on AGMA service factors for gearboxes walks through the same load-first, geometry-second method on a different component.
Trackable next signals: published precision-grade tables from profiled-rail makers (THK, NSK, Hiwin, Bosch Rexroth) for UP and P-grade repeatability figures, and ISO 14728 rolling-bearing fatigue-life methods applied to linear carriage L10 ratings, both of which decide whether a guide holds position over a 20,000-hour machine life [S3][S7].