CARB KILLS vibration

Medium and heavy industrial fan shafts are often supported by two spherical roller bearings that are frequently mounted in standard bearing housings. This economical arrangement has been regarded as a standard solution in the fan market for many years.

The arrangement itself is based on the principle of one locating bearing and one floating bearing that compensates for the thermal expansion of the fan shaft, which normally has a higher temperature than the surrounding pedestal.

It is usually assumed that the outer ring of the non-locating bearing is floating in the housing. In practise, however, the shaft of a large industrial fan is normally a considerable weight, often as a consequence of the dynamics of the large diameter of the rotor. This means that the required force needed to move the outer ring of the floating bearing has to overcome part of the shaft weight. And this, in turn, implies that quite heavy thrust loads are required to move the outer ring.

After a while in service, fretting corrosion may build up between the outer ring and the housing. Fretting corrosion occurs even under ideal conditions and is almost impossible to avoid. The result is that the coefficient of friction increases and that an extensive axial load has to be transmitted over the floating bearing before the outer ring is repositioned in the housing.

Furthermore, there is always a risk that the outer ring is tilted in the housing, thereby obstructing any repositioning of the outer ring. In practise, the margin is very small for a successful repositioning of the outer ring and any obstacles in the housing have a detrimental effect on the floating performance of the bearing.

One energy plant in Sweden experienced severe problems in a fan, due to problems with a floating bearing. The fan in question is an adjustable impeller type that is used to transport exhaust gases of various temperatures. The fan was supported by two SKF 22244 CCK/C3 bearings at a rotational speed of 950 rpm. The bearings were lubricated by circulating oil.

Extensive vibration levels occurred while using the fan. During short periods, vibration velocities of 15mm/sec and more were recorded. These were shown to cause severe endurance cracks on the fan casings and on the large pedestal of the fan.

The vibrations emanated from the floating bearing while the locating bearing behaved normally with low levels of vibrations during running. A study showed that the high peaks of vibrations appeared under short periods, and coincided with an adjustment of the fan blade angle and/or a change in gas temperature.

The peaks occurred because of an interaction between the located and the floating bearing, which resulted in a momentary change of the rotor dynamics. Each time there was a change of the thrust load, there was a corresponding deformation in the locating bearing and consequently, a change of the axial position of the fan shaft, and a change of the internal loads in the floating bearing.

After tests at customer sites, it was decided to mount two SKF CARB bearings. Corresponding vibration tests were carried out and it was shown that the vibration level was reduced considerably. As a consequence of the tests, the fan manufacturer is now introducing the CARB bearings as a standard solution for the non-locating position in their large fans. As a locating bearing, a spherical roller bearing will be used as before.

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