Gearbox Bearing Arrangement: Fixed End vs Floating End, and How Gear Loads Pick the Type

Does this gearbox need a locating end and a floating end at all?

A shaft on two or three bearings needs one locating bearing to fix its axial position, even when thrust is negligible¹⁰. Where the supports are far apart, the shaft and housing grow by different amounts, and one bearing must float so that differential growth does not create parasitic thrust¹⁰.

The same applies to a stack-up of axial tolerances between the two bearing locations¹⁰. Thermal movement is absorbed in one of two ways.

A float bearing lets the shaft slide axially, or designed-in internal clearance takes up the change; higher radial play suits a hotter inner ring, and that play closes as the shaft expands⁴. Cylindrical NU/N bearings can absorb the displacement inside the bearing itself²³.

A long gearbox shaft with significant distance between supports is the case where one locating and one float bearing are required¹⁰. A short shaft whose shrinkage does not greatly affect operation is the case where the two sides need not be distinguished.

A paired angular contact or tapered arrangement then carries the axial load, with clearance set by nuts or shims²². That is a genuine disagreement in the sources, not a threshold you can read off: no source gives the shaft length or spacing at which the float becomes necessary^10,12,22.

Before you fix the layout, ask your supplier or gear designer for the expected temperature difference between shaft and housing at the bearing seats, and the axial tolerance stack-up between the two locations. Neither figure is in this source set, and both decide whether the float is needed.

Which bearing type belongs on the floating end

The float position is best served by a cylindrical roller bearing with one straight race, because the lubricated rollers slide along the straight path and absorb axial displacement inside the bearing¹⁰. SKF describes the same N and NU designs for non-locating positions where thermal expansion of the shaft must be accommodated²³.

If you use a non-separable bearing on the free side instead, the usual practice is a clearance fit between outer ring and housing, or in some cases between shaft and inner ring. So the shaft can move through the bearing¹².

Deep groove ball, double-row angular contact, TDO tapered and spherical roller bearings are all handled this way when they sit on the free side¹⁰.

One detail that catches people out: a cylindrical roller used as the float bearing still needs its outer ring retained, because it is not attached to the inner ring and can walk².

The same source notes that the wide flat roller can churn oil badly if too much oil sits in that area, so oil management matters if efficiency is a driver². Check the housing drawing for the fit you intend at the floating position before the RFQ goes out.

A separable NU/N needs a locating feature for the outer ring; a non-separable alternative needs the clearance fit written on the drawing, not left to the machinist.

Choosing the fixed-end type from the axial load it must carry

The fixed side sets the shaft's axial position and must take axial load in both directions, so its type is selected for strength under that load¹².

The candidate list is broad: deep groove ball, matched pair or stacked angular contact, double-row angular contact, self-aligning ball, cylindrical roller with a rib (NUP and NH), double-row tapered, and spherical roller¹².

Which one you pick depends on load magnitude, speed and the rigidity you need, and no source names a single required type for a gearbox fixed end¹². Where thermal growth produces an axial reaction force, tapered rollers in an opposed pair carry the axial and radial components together by design²¹.

Angular contact ball bearings generate an axial component force of their own, so a pair is arranged face-to-face or back-to-back¹⁸. Spherical roller bearings self-align while carrying heavy combined radial and axial load, and are cited as common in gearboxes¹¹.

On a vertical shaft with heavy axial load, the fixed side still takes a bearing that carries both radial and axial load. And a thrust bearing can be added alongside a radial bearing when the axial load is large²².

Give the supplier the axial load in both directions and the required rigidity, not just the bore size. The candidate list is long enough that the load case, not the catalogue page, has to narrow it.

Matching the gear-mesh load direction to the bearing type

Bearings see radial loads perpendicular to the shaft axis and axial loads along it, and many designs carry both at once, with the internal structure optimised for the dominant direction and magnitude¹⁷. Load type, magnitude and direction are the first selection factor, ahead of speed, temperature and space⁹.

Straight spur gears generate no axial load, so a gearbox can run all cylindrical bearings with shoulders whose only job is to hold the shaft in place². That is the case where over-specifying a combined-load bearing buys nothing.

When the mesh does produce axial load, the fixed side needs a type that carries it: tapered rollers take radial and unidirectional axial load together and are typically used in pairs, while cylindrical rollers take pure radial load only¹⁴. Cylindrical roller variants differ here, and the difference matters at the RFQ.

NU and N designs take radial load only; NJ takes axial load in one direction; NUP or NJ+HJ takes it in both²³. For combined loading, the dynamic equivalent load is P = XFr + YFa, with the e limit deciding whether the axial term counts at all¹⁸.

Work out whether your gear mesh produces axial force before you choose the fixed-end type. If it does not, the all-cylindrical arrangement is a cited practice; if it does, the fixed side must carry that force in both directions.

Setting the life and reliability basis before you commit to a size

Life is calculated per ISO 281 as L10 from the bearing's dynamic load rating and the equivalent dynamic load, using peak loads that include startup, steady state and emergency stop^5,24. Reliability above 90% is handled by an a1 factor applied to that result: 1.00 at L10, 0.64 at L5 and 0.25 at L1²⁶.

If the adjusted life is unacceptable, the answer is a bearing with greater dynamic capacity, not a different arrangement²⁶. The target itself is application-specific.

It is set by industry standards or company policy and by how often the equipment runs — intermittent versus full-time, shifts per day, days per week³¹.

No source in this set states a required L10h target, a static or dynamic safety factor, or an aISO value for a gearbox, so those have to come from your own duty specification. Run the L10 calculation on the candidate you have chosen, then apply the a1 factor for the reliability you actually need.

If the number falls short, step up the dynamic capacity before you revisit the arrangement.

Lubrication and sealing for the arrangement you have chosen

Sealing configuration sets the contamination protection and the maintenance burden. Open bearings run fastest with no protection, metal shields are non-contact and keep out moderate dust, and 2RS rubber seals give maximum protection at a speed cost³⁰.

A supplier of precision bearings describes non-contact labyrinth seals as the first line of protection, with CF seals claimed to give full sealing efficiency against splashing liquids without extra space²⁹. Lubrication method can change the outcome more than the seal.

A paper mill converted its press roll bearings from grease to circulating oil after repeated failures. Since the change there have been no bearing failures in that section and vibration readings that signal early skidding damage were reduced⁷.

The same source notes the switch requires more training for lubrication and maintenance personnel, including oil disposal and level monitoring⁷. For a float position using a cylindrical roller, remember the oil churning point from the float section: too much oil in that area costs efficiency².

Decide the lubrication method and the seal type together, because the seal choice caps the speed and the lubrication choice sets the maintenance routine. Neither oil flow rate nor grease life is established in this source set, so those come from the lubricant supplier.

What to fix at assembly and what to leave adjustable

Where the arrangement needs higher rigidity under heavier or impact load, the fixed-side bearings are mounted back-to-back with preload, and the shaft and housing must be made to accurate dimensions¹⁹. Where bearing intervals are short, the axial clearance can instead be set after mounting with nuts or shims²².

The checks below separate the two cases before the drawing is released.

• ✓Confirm whether the fixed side needs preload for rigidity, or whether clearance adjustment by nuts or shims is acceptable.
• ✓If preload is required, verify the shaft and housing dimensions are toleranced for accurate mounting.
• ✓Check the bearing spacing against the short-interval case where fixed and free sides are not distinguished.
• ✓Confirm the load case has been characterised before the arrangement is frozen.
• ✓Check the required speed against the candidate bearing's limiting speed and dN rating.
• ✓Confirm misalignment tolerance, lubrication and mounting against the actual shaft, housing and environment.

Where cost and lead time are actually driven

There is no universally best arrangement; the application drives the gear design, which drives the bearing system⁸. A high-performance quiet gearset may justify preloaded shafts, while a design driven by cost and efficiency can use bearings with less running accuracy and stiffness and accept secondary NVH⁸.

In short-run production of custom bearings, cage design can become the deciding factor on bearing type, because unique pitch diameters and rolling element complements are often unrealistic once tooling cost is counted⁶. Material grade feeds the same decision: low-grade plastics sell for around US$1/kg while more sophisticated polymers cost around US$60/kg³².

Standard versus custom also moves the commercial terms. One supplier states that minimum order quantity for C3 bearings varies by type, specification and whether customisation is needed, and that a quote and lead-time assessment require the model, clearance code, quantity and application³³.

If your volumes are low, check whether the cage you have specified is a standard pitch diameter and complement before you assume the bearing is available off the shelf. That single question often decides whether the custom route is worth quoting.

Where the sources disagree

What the sources do not establish

• No source provides a method or data to compute gear-mesh tangential, radial and axial force components, load spectrum, or duty cycle for a gearbox.
• No source quantifies required axial positioning accuracy or allowable axial float.
• No source provides numeric radial or axial load ratings, limiting speeds, misalignment limits, or stiffness values for candidate bearing types.
• No source gives thermal growth magnitudes or a calculation method for shaft and housing expansion in a gearbox.
• No source states a required L10 or L10h target, static or dynamic safety factors, or aISO values for this application.
• No source gives assembly or disassembly times, or tooling requirements for adjusted versus fixed mounting.
• No source gives oil flow rates, oil level, grease life, or contamination protection specifications for a gearbox.
• No source gives bearing unit cost, housing machining cost, assembly labor, or volume thresholds for custom versus standard bearings.
• No source addresses gearbox-specific load cases such as gear misalignment, dynamic mesh forces, or bearing preload effects on gear contact.