Trouble shooting bearing failures in a high speed gearbox

A client in the Netherlands had problems with the high speed plain bearings of a speed increasing gearbox, driven by an 2-pole E-motor and driving a Stork centrifugal compressor running at 16.630 rpm. Stork Gears & Services was invited to analyze the problem and come up with solutions and recommendations.

The challenge:

The gearbox is frequently started and stopped, with running cycles lasting maximum 2 hours. The high speed bearings failed for the second time prematurely:

When bearing temperatures exceeded 85°C measured with the temperature sensors at top dead center (a location determined by the OEM), the gearbox started to make noise and vibrations were noticed by the client. During inspection we discovered that the plain bearings suffered severe damage. The temperature settings mentioned in the operating manual are 100°C for alarm and 106°C for trip.

Our findings:

The ramp up time from 0 – 16630 rpm is roughly 1 minute. The amount of starts and stops (up to 4-times a day) is not ideal for these plain bearings. Consideration of thermal expansion of casing, bearing and journal is essential. Proper actions for pre-lubrication and pre-heating of the gearbox internals and casing during commissioning are crucial.

The bearing analysis showed that the original position of the temperature sensor was not measuring the hottest temperature (hotspot) of the bearing. The calculated bearing pad temperature at the original position was 70°C, while the highest temperature in the cavitation area of the bearing was 118°C. This is a temperature difference of about 50°C!

The material of the inspected high speed bearings was a so-called two-layer compound with a steel backing material and a Tin-based lining material (80%Sn). The actual pad temperature was considered to be high for the lining material.

Our solution:

Stork Gears & Services advised the client to change the commissioning parameters regarding pre-lubrication and pre-heating of the gearbox internals and casing.

We rejected the high speed bearings and replaced the bearings by a so-called three-layer compound. The composition of the intermediate lining material is Copper-based with a Tin-based running layer (thickness of about 25 µm). The higher fatigue strength of the lining material makes the bearings more resistant for the changing loads and speeds (e.g. repeated start-up and shutdown, dynamic misalignment).

As an extra measure, Stork Gears & Services advised to add embedded temperature sensors at the inboard side of the gearbox right at the hotspot of the bearings, to monitor the highest temperature.

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