Technical Foundation
What Is a Unmounted Linear Bearing?
An unmounted (or plain) linear bearing is a ball bushing sold as the bearing element alone, without a pillow block or flange housing — designed to be pressed into a customer-machined bore or held in a custom-designed housing, giving the designer full control over the mounting geometry. Internally, recirculating ball bearings ride in a linear path along a hardened, precision-ground shaft, providing very low-friction, smooth linear motion compared to a plain bushing sliding directly on the shaft.
Closed-type bearings form a full cylindrical bore around the shaft and support radial load in all directions, the standard choice for most single-axis motion applications. Open-type bearings have a slot along their length, allowing the bearing to pass over a shaft support rail or bracket that interrupts the shaft — necessary in long-travel applications where the shaft itself needs intermediate support to avoid sagging or deflection.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Bore Diameter | Matched exactly to the shaft OD, metric or inch sizes | Must match shaft diameter precisely — even small mismatches affect ball bearing preload and smoothness |
| Closure Type | Closed type vs. open type (slotted) | Open type is required where the shaft has intermediate support brackets the bearing must pass over |
| Load Rating | Dynamic and static load capacity, by bearing size | Must exceed actual radial load including any moment loading from off-center mounting |
| Precision Class | Standard vs. super precision (tighter radial clearance) | Super precision suits applications needing minimal play, like CNC and metrology equipment |
| Lubrication Type | Grease-lubricated vs. self-lubricating (oil-impregnated) | Self-lubricating types reduce maintenance in hard-to-access or continuous-duty installations |
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Applications
Where Unmounted Linear Bearings Get Used
Unmounted linear bearings are specified wherever a custom housing design or direct press-fit mounting is preferred over a pre-packaged pillow block unit.
CNC & Machine Tool Axes
Precision linear bearings guiding tool and workpiece positioning axes requiring minimal play and smooth motion.
Automated Assembly & Packaging Equipment
Linear bearings on pick-and-place and transfer mechanisms requiring repeatable, low-friction motion.
Robotics & Automation
Compact linear bearings in robotic linear actuators and guided motion subsystems.
Medical & Laboratory Equipment
Precision linear bearings in diagnostic and lab automation equipment requiring smooth, low-vibration motion.
3D Printing & Additive Manufacturing
Linear bearings guiding print head and bed motion axes on 3D printers and CNC routers.
Semiconductor Equipment
Super precision linear bearings in wafer handling and inspection equipment requiring minimal radial play.
FAQ
Unmounted Linear Bearings Questions, Answered
What's the difference between an unmounted linear bearing and a pillow block linear bearing unit?
An unmounted bearing is the ball bushing element alone, meant to be pressed into a customer-designed or machined housing. A pillow block (or flange) unit comes pre-assembled with the bearing already mounted in a housing with mounting holes, ready to bolt directly to a machine frame. Unmounted bearings give more design flexibility for custom or space-constrained applications; pillow block units are faster to specify and install for standard applications.
Should I choose an open or closed type linear bearing?
Closed type bearings form a complete cylindrical bore and support radial load from all directions — the standard choice unless there's a specific reason to use open type. Open type bearings have a slot allowing the bearing to slide past a shaft support rail's mounting brackets, which is necessary in longer-travel applications where the shaft needs intermediate support to prevent sagging under its own weight or load.
How precisely does the bearing bore need to match my shaft diameter?
Very precisely — linear ball bushings are designed for a specific shaft tolerance class, and a shaft outside that tolerance range will affect the ball bearing's preload, running smoothly either too loosely (excess play) or too tightly (added friction and heat). Use a shaft ground to the tolerance the bearing manufacturer specifies, not just a shaft that measures approximately the right diameter.
When do I need a super precision linear bearing instead of a standard one?
Super precision bearings have tighter radial clearance specifications, reducing play in the motion path, which matters in applications like CNC machining, metrology, and semiconductor equipment where positioning accuracy and minimal vibration directly affect output quality. Standard precision bearings are adequate for general automation and machinery where this level of precision isn't required, and cost less.
What's the benefit of a self-lubricating linear bearing?
A self-lubricating (oil-impregnated) bearing releases lubricant gradually during operation, reducing or eliminating the need for scheduled re-greasing, which is valuable in hard-to-access installations or continuous-duty equipment where maintenance downtime is costly. Standard grease-lubricated bearings may offer higher load capacity in some cases but require a periodic relubrication schedule to maintain performance.
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