Correct bearing mounting: method, fits and the mistakes that shorten service life

The bearing is the component most often blamed and least often investigated. When a machine stops because of a broken bearing, the conversation usually ends at the quality of the spare part. Yet ISO 15243 — the international reference for naming bearing damage — describes six failure mechanisms, and only one of them, subsurface fatigue, corresponds to the natural end of the component's life. The other five are the consequence of something done, not done, or done badly: mounting, lubrication, contamination, misalignment or electrical current passage.

Mounting is where the most damage can be done in the least time. A correctly selected, properly lubricated bearing can be condemned in the thirty seconds a badly aimed hammer blow lasts. This article sets out the procedure we apply in the field and the checks worth writing into the work order.

Before touching the bearing

Five preliminary checks prevent most failed mountings. None of them requires special instrumentation beyond a micrometer and a dial gauge.

• Measure shaft and housing with a micrometer and bore gauge, in two perpendicular planes and two axial positions. An oval or tapered seat transfers its shape to the ring and distorts the raceways.
• Inspect the seat: burrs, fretting corrosion left by the previous bearing, puller marks. Correct them with an oil stone and fine abrasive cloth, never with a file.
• Confirm the replacement bearing is the right one, suffix included. C3 and C0 are not interchangeable, and fitting a C0 where the design called for C3 removes the operating clearance at temperature.
• Have the tooling ready before opening the packaging: fitting sleeve kit, induction heater, hydraulic nut or pullers, as applicable.
• Define lubricant and quantity before mounting, not after. This does not apply to sealed-for-life bearings; it applies to everything else.

Fits: which ring goes tight, and why

There is one rule and it admits no practical exceptions: the ring carrying a rotating load is mounted with interference; the ring carrying a stationary load may use a transition fit or even clearance. In an electric motor or a centrifugal pump the load rotates relative to the inner ring and is fixed relative to the outer, so the inner ring goes tight on the shaft and the outer slides into the housing. Reverse that and the ring that should stay put starts creeping on its seat, with fretting corrosion appearing within weeks.

Fits are expressed using ISO 286 tolerance classes. These are the most common starting values for industrial applications on a solid steel shaft; the manufacturer's data sheet overrides any general table.

Interference has a price, and it is paid in internal clearance. Forcing the inner ring onto the shaft expands the ring and reduces the radial clearance available. As a design reference, roughly 80 % of the effective interference translates into lost radial clearance. That is why a bearing with a tight fit, in a machine whose shaft also runs hotter than its housing, has to start from a larger clearance: it is the reason the C3 and C4 classes exist.

The three mounting methods

1. 01

   Mechanical mounting

   For bores up to roughly 100 mm. Use a fitting sleeve that bears only on the interference-fitted ring and a soft-faced hammer — better still, a press. Force must never pass through the rolling elements: striking the outer ring to seat the inner one produces raceway indentations that ISO 15243 classifies as plastic deformation and that end in spalling.

2. 02

   Thermal mounting

   The preferred method above 100 mm and the only sensible one on large bearings. Heat the inner ring with an induction heater to some 80–90 °C above shaft temperature, never exceeding 120 °C. Fit the bearing in a single movement, keep it pressed against the shaft shoulder while it cools, and check contact with a 0.03 mm feeler gauge. The heater must demagnetise on completion; a magnetised ring attracts ferrous particles for the rest of its life.

3. 03

   Hydraulic mounting

   Applies to tapered-bore bearings and adapter or withdrawal sleeves. A hydraulic nut and oil injection let you control axial drive-up and therefore the exact interference. Here the fit is not measured in millimetres of diameter but in axial displacement or in internal clearance reduction measured with a feeler gauge, and that value comes from the manufacturer's table for the specific designation.

Alignment and preload: what happens after tightening

A flawless mounting does not compensate for coupling misalignment. Angular misalignment subjects the bearing to a load that rotates within its own geometry and produces a displaced load pattern, visible on the dismounted ring as a slanted running track. Aligning with a dial indicator or laser equipment after mounting — not before — is what closes the job.

In arrangements with tapered roller or angular contact bearings mounted back-to-back or face-to-face, preload is set to specification and verified through starting torque or axial displacement. Excessive preload raises operating temperature and shortens life just as much as excessive clearance does.

First start-up

• Fill with grease to between 30 % and 50 % of the bearing's free volume at normal speeds. Excess grease is the most common cause of overheating at start-up.
• Start unloaded and let temperature settle. A temperature peak during run-in is normal and drops as the lubricant distributes.
• Record the stabilised operating temperature and the baseline vibration reading. Without that initial value, later vibration measurements have nothing to be compared against.
• Write into the work order the full designation fitted, the method used, the heating temperature and the lubricant applied. That record is what makes failure analysis possible later.

ISO 20816 defines the evaluation criteria for mechanical vibration in rotating machinery and is the usual reference for setting alarm thresholds. Taking the baseline reading on mounting day turns the new bearing into its own comparison standard.

What to ask your technical supplier for

A distributor who only hands over the box leaves the job half done. When ordering a bearing, ask as well for the recommended fit table for that designation and that application, the starting internal clearance value, the lubricant compatible with the working temperature and speed, and the mounting tool suited to the bore. At Rodacaribe we supply SKF bearings, seals and greases and Q8 Oils lubricants, and that information travels with the order.