A rolling bearing only performs if its inner ring is secured to the shaft and its outer ring sits correctly in the housing. Choose the fits wrong and the ring creeps, frets the seat, runs hot, or comes loose. The tolerances for the shaft and housing bore look like a confusing set of letters and numbers, but they follow a clear rule based on which ring rotates and which stays still. Getting the fits right is cheaper than replacing bearings every few months.
The two rings and the load
A bearing has an inner ring on the shaft and an outer ring in the housing. The key question is which ring rotates relative to the radial load. On most machines the inner ring turns with the shaft while the outer ring is stationary in the housing, and the load direction stays fixed in space. The ring that rotates under the load needs an interference fit so it does not creep around its seat; the stationary ring can be a looser fit so it can be installed and sometimes float axially.
Why the rotating ring needs interference
If the rotating ring is loose on its seat, it slips microscopically as the load passes around it. That creep wears and frets the seat, generates heat and metal particles, and eventually lets the ring walk. An interference fit locks the ring to the shaft so they turn as one. The stationary outer ring, by contrast, sees the same point loaded all the time and does not need to be clamped; a slightly loose fit also lets the bearing accommodate thermal expansion and, on non-locating positions, slide.
Shaft tolerances
For a normal rotating inner ring, common shaft tolerances are k5 for light loads and precision, k6 or m6 for general loads, and n6 or p6 for heavy or shock loads. The number is the ISO tolerance grade, lower numbers tighter, and the letter sets the position relative to nominal; lowercase letters are for the shaft. A k6 shaft is slightly larger than the bearing bore over part of its range, giving light interference. Heavier interference spreads load and prevents creep but makes the bearing harder to press on and can reduce internal clearance.
Housing bore tolerances
For a stationary outer ring, the housing bore is often H7, which gives a close sliding fit with little interference, or J7 for a touch more grip under moderate load. When the outer ring rotates or sees heavy load, tighter fits like P7 are used. Uppercase letters describe the housing bore. An H7 housing lets a non-locating bearing slide axially and simplifies installation; that matters because one bearing on a shaft usually must float to avoid clamping the rotor between two fixed bearings as it heats.
Locating and non-locating bearings
A typical shaft is located axially at one bearing, which fixes the shaft position, and supported at the other end by a non-locating bearing that can move. On the float end, the outer ring must slide in its H7 housing, or thermal growth loads the bearings against each other and the shaft binds and overheats. On the fixed end, both rings are clamped axially. Decide which end locates before choosing the housing fit, because the two positions need different treatment.
Internal clearance
Pressing a ring onto an interference seat expands or contracts the rings and reduces the bearing’s internal radial clearance. A heavy interference can remove too much clearance, making the bearing run hot or preloaded. That is why bearings come in clearance classes, normal, C3, C4, with C3 giving extra room for interference and temperature rise. If you use a tight shaft fit or expect high temperatures, a C3 bearing often keeps working clearance after mounting. Match the clearance class to the interference and the thermal conditions.
Split plummer blocks
Split pillow-block housings with a cap make bearings easy to fit and remove on long shafts, common on conveyors. The outer ring in a split housing is usually a relatively loose fit, and the cap clamps it; you should not rely on a tight bore interference here. Follow the maker’s torque for the cap bolts, because over-clamping can pinch the outer ring. Split housings suit moderate loads and spherical roller bearings that tolerate misalignment, not high-precision positions.
Mounting and the seats
The shaft seat and housing bore must be clean, smooth, and to size with a proper shoulder for the ring to locate against. The shoulder must be high enough to support the ring but low enough to let a puller grip it for removal. Burrs, a tapered seat, or a shoulder too small all cause trouble. Heat the bearing for mounting rather than hammering through the balls, and support the ring being pressed so force never crosses the rolling elements. Measure the seats before assembly.
Sealing and lubrication
The housing also carries the seals and the grease space. A sealed-for-life bearing needs no relubrication, but an open bearing in a housing needs a grease cavity and a path that does not trap over-packed grease, which churns and heats. Fit seals or shields suited to the contamination, and keep the grease nipple feeding the right zone. The fit gets the bearing in; the seal and lubrication keep it alive.
Common mistakes
Leaving the rotating ring loose so it creeps, clamping both bearing ends so the shaft binds, over-tightening interference and killing clearance, using a normal-clearance bearing where C3 is needed, over-clamping a split housing, and making shoulders too small or too high are recurring errors. Identify the rotating ring, set shaft and housing fits, and plan the float end.
Fit selection tables and how to read them
Bearing makers publish fit tables indexed by load size, ring rotation, and bearing bore diameter. The same tolerance grade behaves differently across sizes: a k6 on a 20 mm shaft and a k6 on a 100 mm shaft do not give the same micrometers of interference, because the grade band widens with size. Read the table for the actual bore, not just the letter code, and check the resulting interference against the bearing’s mounting spec. For solid versus hollow shafts the effective fit changes too; a hollow shaft needs a slightly tighter tolerance to grip the same way. When in doubt, use the manufacturer’s online fit tool rather than extrapolating from a smaller machine.
Thermal effects on fits
Temperature shifts the fits during operation. A hot inner ring expands onto the shaft, which usually helps grip, while a hot housing bore grows and can loosen the outer ring. On machines where the housing runs hotter than the shaft, an H7 outer fit that was snug cold may creep when hot. Consider the steady temperatures of each part, and tighten the housing fit or add a snap ring if the outer ring could lose grip. Conversely, a shaft much hotter than the inner ring can add interference that eats clearance. The fits specified cold must still be correct at operating temperature.
Inspection and verification
Verify the seats with the right instruments before assembly. Measure the shaft diameter with a micrometer at several points and the housing bore with a bore gauge, checking size, roundness, and taper. Confirm the shoulder is square to the axis, because a tilted shoulder loads one side of the ring. After mounting, the bearing should turn freely by hand, and on a running machine a steady temperature rise that then levels off is normal, while a bearing that keeps climbing in temperature is too tight or misaligned. A quick check of seat size and post-start temperature catches most fit errors before they destroy the bearing.
Read fit tables for the actual bore diameter, because the same grade gives different interference as size grows. Consider temperatures: a hot housing can loosen the outer ring, while a hot shaft adds interference that eats clearance. Before assembly, measure the shaft with a micrometer and the bore with a bore gauge at several points, and check the shoulder is square. After mounting the bearing should turn freely by hand. On start-up a temperature rise that levels off is normal; a bearing that keeps climbing is too tight or misaligned. These checks catch fit errors before they destroy the bearing.
Read fit tables for the actual bore, since the same grade gives different interference as size grows. Consider temperatures: a hot housing loosens the outer ring, a hot shaft eats clearance. Measure seats before assembly, check the shoulder is square, and after mounting the bearing should turn freely. On start-up a temperature rise that levels off is normal; one that keeps climbing means too tight or misaligned.
Bottom line
Give the rotating ring an interference fit, like k6 or m6 on the shaft, and the stationary ring a close but workable fit, often H7 in the housing. Locate the shaft at one end and float the other, choose a C3 bearing for tight fits or heat, and torque split-housing caps properly. Clean, correctly sized seats and shoulders finish the job. The right fits stop creep and heat and let the bearing reach its life.
