Listening to a Machine: Noise, Heat and Play Are All Telling You Something

A RUMBLE or growl that rises with speed is usually a rolling bearing with damaged raceways — contamination, brinelling from bad mounting, or simple fatigue. A steady WHINE at a constant pitch for a given speed is usually gears, and a whine that appears only under load is a gear mesh problem rather than a bearing. A regular CLICK or knock once per revolution is a single defect passing a point — a chipped tooth, a flat on a roller, a loose key. A SQUEAL from a belt drive is slip. A squeal from a bearing is usually lack of lubricant, and it is the last warning before seizure. Use a screwdriver handle or a length of rod against the housing and your ear at the other end: it localises a noise remarkably well.

A bearing housing normally runs warm — comfortable to keep a hand on, typically 20 to 40 degrees above ambient. Too hot to touch means something is wrong, and it will get worse quickly. Measure it rather than judging it, at the same place every time, and write it down with the ambient temperature. "Hotter than usual" is only meaningful against a record of usual, and the repeat-and-look-at-the-spread rung applies here exactly. A NEW bearing often runs warm for the first hours as the grease distributes, and then settles. Temperature that rises and then falls is normal; temperature that rises and keeps rising is over-greasing, over-tight fits, misalignment or impending failure.

With the machine stopped and isolated, grip the shaft and try to move it radially and axially. A rolling bearing has a small measurable clearance and should feel tight and smooth; a plain bearing has more, by design. Measure it with a dial indicator rather than by feel where it matters. Feel finds gross play; an indicator gives a number you can compare against last time, which is what tells you whether it is getting worse. Turn the shaft slowly by hand and feel for roughness and notches. A bearing that is smooth in one position and notchy in another has a local defect, and a bearing that feels gritty throughout is contaminated or has lost its lubricant.

A failed bearing is a report. Even wear all round means normal fatigue. Wear on one side of the raceway means misalignment or a bent shaft. Dents evenly spaced at ball pitch mean it was hammered on. A polished, scored band means a ring turning on its seat — the fit was too loose. Brown staining and pitting mean water. Fluting — regular marks across the raceway — means electrical current passing through, which needs an insulated bearing or a shaft grounding brush rather than another bearing. Fitting a new part without reading the old one means fitting a new part into the same conditions that destroyed the last one. Photograph it, write down what you saw, and put it in the machine's log — which is where this batch ends, and where the next repair starts.