Rolling Element Bearings: Selection, Fits and the Failure Modes You Can Prevent

Bearing failures are rarely bearing defects. Survey after survey puts manufacturing defects at under 3 per cent of failures, with the rest split between lubrication, contamination, mounting and misapplication. That is good news, because all four are within your control.

Rating life, and what it does not tell you

L10 = (C / P)^p  million revolutions

  C = basic dynamic load rating (from the catalogue)
  P = equivalent dynamic bearing load
  p = 3 for ball bearings, 10/3 for roller bearings

In hours:  L10h = (10^6 / (60 x n)) x (C / P)^p     [n = rev/min]

L10 means 90 per cent of a large population survives at least that long under ideal conditions. It is a comparison tool, not a prediction for your specific bearing. The modified rating life applies factors for reliability, material and lubrication, and the lubrication factor can move the answer by an order of magnitude in either direction — which tells you where to spend attention.

Equivalent load

For combined radial and axial loading, P = X x Fr + Y x Fa, with X and Y from the catalogue depending on the ratio Fa/Fr relative to the limit value e. Using P = Fr when there is meaningful axial load is a common shortcut that silently halves calculated life.

Fits: the part that gets skipped

The rule is straightforward and violating it is the most common mounting error. The ring that rotates relative to the load direction gets an interference fit. The stationary ring gets a transition or clearance fit.

An interference fit consumes internal clearance. A C3 bearing pressed onto an m6 shaft may end up with near-zero operating clearance once thermal expansion is added, and a preloaded bearing that was not designed to be preloaded runs hot and fails quickly. If you specify a tight fit, check the residual clearance.

Mounting without damaging what you just bought

• Never press through the rolling elements. Force must be applied to the ring being fitted — inner ring when pressing onto a shaft.
• Heat mounting to 80-100 degrees C for interference fits above roughly 70 mm bore. Never exceed 120 degrees C, which begins to affect the heat treatment.
• Never strike a bearing directly. A hammer blow through a drift produces brinelling that shows up as noise within weeks.
• Mount in a clean area. A single 20-micron particle in a bearing with a 10-micron lubricant film is an indentation waiting to happen.

Lubrication in one page

Grease quantity for a re-lubricatable bearing: G = 0.005 x D x B, where G is grams, D is outside diameter in mm and B is width in mm. Over-greasing is as damaging as under-greasing — excess grease churns, raises temperature and degrades the base oil.

The relubrication interval falls sharply with speed and temperature. As a rule of thumb, every 15 degrees C above 70 halves grease life. A bearing running at 100 degrees C needs relubrication roughly four times as often as the catalogue interval implies.

Reading a failed bearing

Photograph every failed bearing before it goes in the bin, keep the photographs against the equipment record, and the pattern in your plant becomes obvious within a year.