Temperature, Fits and Bearing Clearance

Bearings are specified cold and run hot, and everything about them changes with temperature. The steel rings grow, the shaft grows differently from the housing if the materials differ, internal clearance shrinks or grows, and the lubricant ages faster with every degree. Overlooking this is behind a large share of premature failures: a bearing chosen with normal clearance and a tight fit that seizes at operating temperature, or an aluminium housing that lets its outer ring spin loose when warm. Temperature is not a side note; it is a design input.

This guide quantifies the expansions involved, tracks where the operating clearance actually goes, sets out the material limits of rings, seals and cages, and closes with the measurement practice that turns assumptions into numbers. The companion pages on clearance classes and shaft and housing fits supply the tables this article refers to.

Specified cold, judged hot

Every figure on a bearing drawing - bore, outside diameter, radial clearance - applies at about 20 °C. The machine, by contrast, may settle anywhere between 40 and 150 °C in service, and the bearing rings rarely sit at the same temperature as the parts around them. A rotating inner ring on an interference fit runs hotter than the shaft beneath it and the housing around the outer ring, so three different expansions happen simultaneously. A selection that ignores temperature is correct only by accident.

How fast bearing steel grows

Bearing steel expands at roughly 11 to 12 parts per million per kelvin. A 100 mm bore inner ring grows about 1.15 µm per kelvin, so a 50 °C rise adds almost 0.06 mm to its diameter. That may sound small, but the internal clearance of a small bearing is measured in tens of micrometres, so thermal growth and internal clearance are the same order of magnitude and cannot be treated separately. The same 50 °C rise on a 50 mm bore adds roughly 0.03 mm - which can consume an entire CN clearance band on its own before the fit has taken its share.

When shaft and housing disagree

When the housing is aluminium and the shaft is steel, the housing grows about twice as fast - roughly 23 parts per million per kelvin. The housing bore therefore enlarges more than the outer ring, reducing the grip on the outer ring and, in severe cases, allowing it to creep and wear the seat. Conversely, when the outer ring runs hotter than the housing, which is common when a hot process fluid surrounds it, the ring grows into the bore and the fit tightens further. Both directions must be checked, and the fix differs: a stiffer interference or a steel nest for the first case, more clearance or a looser housing fit for the second. The housing design guide covers the structural side of this trade.

Where the operating clearance goes

The clearance a bearing actually runs with is what remains after two consumers have taken their share: the interference fit applied at mounting, and the differential thermal growth in service. Clearance class ranges come from ISO 5753 and the tolerance grades from ISO 492, and the worked example below follows a mid-size deep groove bearing through both.

What the fit consumes

Pressing an inner ring onto a j6-class shaft seat closes the radial clearance by roughly 5 to 15  µm on a 50 mm bore, depending on where in the tolerance band the parts land. The consumption is fixed at mounting and can be measured immediately: if the cold bearing already turns with noticeable drag, the operating clearance has no future.

What the temperature consumes

The second consumer works while the machine runs. The inner ring, heated by friction and sitting on the warm shaft, typically runs 20 to 30 °C hotter than the outer ring. On a 50 mm bore that split removes about 11 to 17 µm more. Unlike fit closure, this consumption changes with duty: a cold start has it, full load does not, so the bearing cycles through its remaining band every run.

Illustrative arithmetic for a 50 mm bore; the exact numbers come from the fit tables and the measured temperature split.

What remains to run on

Selecting for operating clearance therefore means starting with more clearance when the temperature rise is large, which is exactly why hot applications specify C3 or C4. The aim is not the largest band but a small positive remainder at full temperature: enough that the rolling elements never pinch, little enough that load sharing and noise stay acceptable. The clearance class guide lists the numeric bands per class.

The clearance-heat feedback loop

Too little operating clearance means the rolling elements are pinched: friction rises, the temperature rises, the rings expand further, and clearance shrinks again. A bearing can run away thermally into seizure in minutes if the loop is unstable. Too much clearance means only a few elements carry the load, load sharing is poor, noise and vibration rise, and fatigue life falls. The target is a small positive operating clearance at full temperature - neither pinched nor loose - and the only way to aim for it is to estimate both the fit loss and the thermal loss before ordering.

Material limits: rings, seals and cages

Clearance is not the only temperature-sensitive property. Each material in the assembly has its own continuous limit, and the lowest one governs.

Ordering a hot-duty bearing in standard steel with polyamide cage defeats the purpose of the C4 clearance it carries: the ring may hold dimension, but the cage is the part that gives up first. Specify the stabilisation suffix and the cage material together.

Grease life and viscosity at temperature

Grease life falls steeply with temperature. A widely used rule of thumb is that usable life halves for every 10 to 15 °C above the base rating, so a grease rated for 70 °C that lasts years in a cool fan may last only months in a hot one. Beyond a point no conventional grease survives and the design must switch to oil or a high-temperature grease. Temperature also changes viscosity, not just life: a grease that is thick and protective when cold can become thin and film-poor when hot, reducing the separating film at the contacts and accelerating wear before the grease chemically degrades at all. Choosing the lubricant by its base-oil viscosity at the operating temperature, using the grease selection guide, prevents a large class of heat-related failures, and the relubrication interval guide turns the temperature into a schedule.

Axial growth and the floating bearing

Thermal expansion does not only affect the fit. A long shaft grows axially as it warms - a 300 mm steel shaft grows about 0.2 mm over a 60 °C rise - and if both bearings are located rigidly, that growth is absorbed as extra preload or as axial load on the rings. The fix is a floating arrangement: one bearing located, the other free to slide axially in its housing, so growth is not turned into force. This is standard practice in gearboxes, motors and any machine with a long, hot shaft. Cylindrical roller NU designs are the classic floating choice, as the cylindrical roller guide explains, and self-aligning pairs for heavier duty are covered in the self-aligning roller category.

Measuring and designing for the real temperature

Design estimates of temperature rise are useful but never final. The bearing's actual operating temperature depends on load, speed, lubrication, cooling and the surrounding machine, and the only reliable way to know it is to measure it: a contact thermometer on the housing near the bearing, or a probe in the lubricant. If the measured temperature is higher than assumed, revisit the clearance and lubricant choices rather than leaving them as they were. For critical machines, continuous monitoring pays off - a steadily rising bearing temperature is one of the earliest warnings of a developing problem, whether loss of clearance, grease breakdown, contamination or a fit gone wrong, and catching the trend turns a scheduled replacement into an avoided emergency.

• Estimate the operating temperature rise, not just the ambient, before selecting clearance.
• Check shaft and housing growth separately when the materials differ, in both radial and axial directions.
• Select the clearance class for the operating condition, not the as-delivered one.
• Specify seals, cages and stabilisation to the real temperature, not the room.
• Provide one floating bearing so axial growth does not become axial load.
• Measure after commissioning and treat a rising trend as the first alarm.

Temperature also explains why two apparently identical machines behave differently: a fan that draws cool air through its own housing runs far cooler than the same fan in a cramped enclosure, even though the bearing, load and speed are the same. The environment is part of the thermal design. Mounted bearings for such duties, with their generous internal space, are compared in the ball bearing units category and the FYH listings, and the background to steel choice at temperature is in the bearing steel guide. Mounting practice that protects clearance during fitting is covered in the mounting guide.