Inappropriate handling of vibrations affecting the stable operation of SKF high-temperature bearings

Grease lubrication includes a sealing method where grease is pre-filled into sealed bearings, as well as a replenishment method where an appropriate amount of grease is filled into the housing and topped up or replaced at regular intervals. Furthermore, for machinery with multiple high-temperature bearings requiring lubrication, a centralised lubrication system connected via piping to each lubrication point is also employed. Grease lubrication allows for long periods without the need for replenishment after a single filling, and as the sealing mechanism is relatively simple in structure, it is widely used. Oil lubrication is relatively straightforward. Regular lubrication can extend the service life of SKF bearings. Imported bearings may produce slight operational noise during operation; however, excessive noise, abnormal sounds or knocking noises indicate a bearing fault.

The causes of noise in rolling bearings are complex; one factor is wear on the mating surfaces of the inner and outer rings. This wear disrupts the fit between the bearing and the housing, or between high-temperature bearings and the shaft, causing the shaft axis to deviate from its correct position and generating abnormal noise during high-speed rotation. When SKF bearings become fatigued, surface metal flaking can also increase the radial clearance of the bearing, resulting in abnormal noise. Furthermore, insufficient lubrication leading to dry friction, as well as bearing fracture, can all produce abnormal sounds. Once a bearing has worn and become loose, the loosening or damage to the cage can also produce abnormal noises.

During operation of the mechanism, a certain temperature is permissible at the bearing mounting location. When touching the mechanism housing with your hand, it should not feel hot to the touch; otherwise, this indicates that the bearing temperature is too high. Causes of excessive bearing temperature include: lubricant quality failing to meet requirements or having deteriorated, or the lubricant having too high a viscosity; excessive tightness in the assembly (insufficient clearance); SKF bearings fitted too tightly; the bearing race rotating on the shaft or within the housing; excessive load; or the fracture of the bearing cage or rolling elements. Selection and control of shaft and bearing housing tolerances: After the bearing has been pressed into place, it should rotate freely without any sense of resistance. If rotation is noticeably stiff, this indicates that the shaft dimensions are too large, and the tolerance must be reduced. If, after a high-temperature bearing has been pressed onto the shaft, there is a distinct ‘gritty’ sensation when turning it by hand, this may indicate that the shaft tolerance is too large or that the shaft’s roundness is poor. Therefore, whilst controlling the tolerances of the shaft and bearing housing, roundness must also be controlled. Currently, many domestic manufacturers only control tolerances and do not control roundness.

As SKF bearings are high-precision products, improper assembly can easily damage the bearing raceways, leading to bearing failure. Specialised tools must be used during bearing assembly; the bearing must not be struck arbitrarily. When pressing the bearing onto the shaft, force must be applied only to the inner ring; when pressing the outer ring, force must be applied only to the outer ring. Assembly must be carried out using pneumatic or hydraulic pressure. During press-fitting, the upper and lower dies must be kept level; any tilt may cause damage to the bearing raceways due to uneven force distribution, resulting in bearing noise. Previously, many manufacturers attributed this issue to the bearings themselves; however, following our ongoing awareness campaigns, motor manufacturers have now recognised that the primary cause lies with the insulating varnish. This problem arises mainly because acidic substances emitted by the varnish form corrosive compounds under certain temperature and humidity conditions, which corrode the bearing raceways at high temperatures and result in bearing failure. Currently, the only solution is to select high-quality insulating varnish and to allow the components to ventilate for a period after drying before assembly.