The problem: We recently rebuilt a 2-pole motor and the centrifugal blower it drives. When the customer reinstalled them, he reported high vibration levels. Everything runs smoothly for 10-15 minutes after a cold startup. Then the vibration starts to climb. We balanced the rotor and blower to G 1.0 tolerances. We even balanced each of the 7 blower impellers separately using a balancing mandrel. Shaft runout was less than 0.0002" on the motor and blower when we finished the job. The customer uses laser alignment. He is convinced that something is loose and wants us to rebuild the blower again. What did we do wrong? The solution: First, you probably did nothing wrong. The precision balance was a smart move, because the relatively small diameter shaft on this type of blower tends to be flexible. With several impellers stacked on a common shaft, multiple planes of imbalance exert radial force on the shaft in several directions at once.With the impellers shouldered against each other, it is possible to deflect the shaft when tightening the clamping nuts. "Stacked tolerances" (when several mating surfaces have slight imperfections) can add up to unacceptable total deviations. Your final shaft runout indicator readings make it unlikely that this is the case. The cause of the vibration is probably misalignment.
Shaft couplings are devices that connect two rotating shafts together. They efficiently transfer motion and power from the drive unit to the driven unit without adversely impacting either piece of rotating equipment. Under ideal conditions, both shafts should function as a continuous unit. The design of a flexible coupling is to accommodate small amounts of shaft misalignment. Coupling manufacturers have designed their couplings to withstand the forces resulting from excessive shaft misalignment. Unfortunately, shaft alignment tolerances have sometimes been governed by the coupling manufacturers' design specifications.These are maximum values that are dimensionally possible for a specific coupling. The coupling misalignment tolerances reported by coupling manufacturers apply ONLY to the coupling.
When vibration problems occur, the magnitude and direction of the vibration can give a good indication of where to look for the cause. When vibration is higher in the vertical plane, one of the first things we should examine is the base/foundation of the motor. If the high vertical readings are compounded by indications of an eccentric airgap, such as high axial vibration and a predominant twice-line-frequency vibration, a "soft foot" or twisted frame is often to blame.
End play in an electric motor is the amount of axial movement allowed by the motor's construction. This end play is limited by the motor's bearing design. The bearing's primary purpose is to locate the shaft radially so it can be aligned to the driven equipment shaft and efficiently transmit torque to the load. It is also important that the axial location be controlled such that the motor and driven equipment bearings are not subjected to excessive thrust or vibration and still have room for thermal growth of the shaft as it heats up during operation. This can be accomplished by a number of ways depending on the design of the motor. If the motor has sleeve bearings, axial movement is expected within the limits of the bearing design. Most rolling element bearings have much less axial clearance but must be contained in the bearing housing to control the end play. This article looks at:
- Sleeve bearings
- Axial hunting
- Ball bearings