CourseWiki/Engineering Fundamentals/Power Transmission

Shaft Support and Axial Adjustment

Locate one end, allow thermal movement at the other, and distinguish end play from preload.

Locate the shaft at one end and allow growth at the other

In a locating/non-locating arrangement, also called fixed/floating here, both supports carry radial load. The locating support also carries the applied axial load and establishes the shaft's axial position. The non-locating support allows axial movement as the shaft expands relative to the housing.

A locating bearing on the left has axial stops; the radial support on the right has room to move along the housing as the shaft grows. — “Fixed” means axially located, not prevented from rotating. “Floating” means axial travel, not loose radial support.

Location needs a load path from shaft to bearing to housing: shoulders, suitable spacers, nuts, retaining rings or covers can retain the rings. Which end is fixed follows the required position of the driven parts; it is not automatically the left end or the motor end.

The floating end can accommodate movement inside a suitable bearing, such as an NU cylindrical roller bearing, or by controlled sliding of a ring in its seat. Do not loosen an arbitrary fit: see Tolerances and Fits and the bearing maker's mounting guidance.

Why two locating supports can create unwanted load

A warming shaft wants to lengthen. If a housing prevents that length change, elastic deformation creates axial reactions. These can consume clearance, increase friction and cause heating. Actual force depends on shaft, bearing and housing stiffness, temperatures and initial clearance. A floating seat also has friction; “radial only” describes its intended support duty, not literally zero incidental axial force.

Keep shaft support separate from torque transmission. Bearings support the shaft relative to the housing; a keyed shaft-hub connection transfers torque to or from a pulley, sprocket or gear. Securing a hub against sliding along the shaft is a separate assembly requirement.

Heat the shaft and compare free travel with restraint; Interactive

Heat the shaft and compare free travel with restraint

Static comparison: at half the heating range, the shaft wants half the model's maximum free growth. A floating support with sufficient travel accommodates all of it. Two locating supports prevent it. With travel limited to 40%, half-range heating leaves 10% of maximum growth unaccommodated.

The drawing exaggerates growth. Heating level is a relative teaching scale, not a temperature or a material property. The left support and housing stay in place; only the shaft is heated. The dashed outline shows the length the shaft would reach if free. The solid outline shows the ideal constrained length. This model shows incompatible motion, not a binding force, stress or safe operating limit.

End play is free movement; preload is an initial internal load

End play is the axial free movement available before the bearing arrangement takes up its clearance. Preload is a deliberately applied internal load before the external working load arrives. Reducing end play to a specified value and applying preload are different adjustments.

Do not tighten a bearing until it merely “feels solid.” Excess preload increases friction and temperature. Some arrangements require positive operating clearance; others use specified preload for stiffness or running accuracy. Temperature and fits change the operating condition.

A shim is a thin, accurately sized spacer. Changing its thickness sets a position in a rigid stack. Whether adding a shim increases or decreases end play depends on where the shim sits and which faces it separates.

Avoid burrs and rubbing contact that could release debris. Use the material and placement specified for the assembly.

A spring maintains force over travel; a shim sets distance; Checklist; Practice

A spring maintains force over travel; a shim sets distance

A shim sets distance and a spring maintains force.

A Belleville washer is a conical disc spring. A wave washer has an undulating shape that compresses axially. Both can apply preload when selected for the required force and working deflection.

A spring can accommodate small dimensional changes while maintaining load. “Very light load” is not a universal specification. Use the bearing's required preload and the spring's load-deflection data; an ordinary lock washer is not a calibrated preload spring.

Position preload versus spring preload

Position preload sets the bearing rings at a fixed relative distance, for example with shims. Compared with that arrangement, a spring allows more axial movement under an added load. The spring can maintain preload as dimensions change, but the shaft position changes more under load.

Identify which support locates the shaft and trace its axial load path.
Find where the other support permits thermal movement and check available travel.
Read the specified end play or preload and its measurement conditions.
Choose the prescribed shim or spring arrangement and verify the final measured setting.

Manufacturer reading and open material questions

Further reading: SKF Railway Technical Handbook, bearing installation, describes both internal displacement and ring-seat sliding. NSK: Preload shows position adjustment using shims. Smalley: Bearing Preload covers wave springs. SPIROL: Precision Shims lists both stainless steel and brass among available materials.

No manufacturer restriction confining brass shims to a nonrotating ring was verified. Material and mounting instructions must come from the actual assembly documentation.

Continue with Linear Bearing Guides or practice locating a shaft.

Appears in

Bearings and Shaft Support DET 403 DET