why centrifugal pump free end bearing gets heated when replaced ?

Answer 1

I am considering that you have used a genuine bearing.

Free end bearing will get heated due to :
*misalignment (radial/axial) of shafts-prime mover and pump.
Better to use a double dial indicator method for alignment.
*coupling used are worn/rubber spider is damaged.
*shaft is having run-out more than the norms.
[maximum axial / radial runout allowed is 50 micron(0.05mm)
for best performance.
*Excessive / insufficient lubrication is the major source of heating.
*Dynamic balancing of impeller is not proper.
*Bearings are improperly mounted.Freeness of the bearings needs to be checked prior to further assembly.
*foundation is loose or damaged.
*All mounting bolts are improperly tightened.
*Piping stresses are more dangerous than shaft run-out.
Better to use a flexible hoses in suction and discharge lines of the pump.
*Use the practice of bearing monitoring.
Check the vibration of both end bearing by using vibrometer and ensure that it should exeed 0.3mm/sec depending upon rpm of pump.
You can use stethescope to check the condition of bearing.
*if it is a pully driven , then pulley alignment and pulley condition should be good enough.
*rubbing of impeller with front casing give thrust to bearing excessively.
*It is a general analysis that most of the bearing are faiing due to lubrication problem.(more than 80%).
*there should not be relative movement of bearing and shaft and fits and tolerences are maintained adequately.
*it is also possible that the bearing selected is not suffice to the application.

Answer 2

Listen to the bearing with a mechanic’s stethoscope. (Use a screwdriver if you don’t have one: put a thumb between your ear and the end of a screwdriver handle, then press the other end to the housing to couple the noise into your ear.) If there is a high pitch grinding tone, the bearing may be damaged or dried of lubricant. If the bearing sounds a smooth lubricated rotation, the load on the bearing may be too excessive, two possible causes:

(1) Shaft misaligment: Maybe the two parts of the pump housing did not come together properly making the two axes of the two ends’ bearings not exactly aligned with one another. Try loosening the two ends of the housing from one another, spin the shaft to let the two realign themselves, then evenly torque the halves together while letting the shaft standing upright so to free the bearings from alignment bias caused to the weight of the housing.

(2) Load unbalanced. Maybe either the center of mass of the pump rotor is not on the axis of rotation, or the plane of radial mass symmetry of the rotor disc is not on the plane of rotation. Check if any part of the rotor blades was broken and if the rotor was properly mounted onto the shaft.

Answer 3

Please give more info. Why does it heat up to failure after replacement?, why is it heated prior to being installed?. For the first one probably because of poor lubrication, poor design, or misalignment. For the second one you heat the bearing to cause it to expand. this allows the bearing hub to be fitted over the shaft. As the bearing cools it will contract onto the shaft, forming a very tight fit.