Rotor balancing assembly errors

Two major sources of vibration due to mechanical imbalance in equipment with rotating parts or rotors are (1) assembly errors and (2) incorrect key length guesses during balancing. 

Even when parts are precision balanced to extremely close tolerances, vibration due to mechanical imbalance can be much greater than necessary due to assembly errors. Potential errors include relative placement of each part s center of rotation, location of the shaft relative to the bore, and cocked rotors. 

Assembly errors are not simply the additive effects of tolerances, but also include the relative placement of each part s center of rotation. For example, a perfectly balanced blower rotor can be assembled to a perfectly balanced shaft and yet the resultant imbalance can be high. This can happen if the rotor is balanced on a balancing shaft that fits the rotor bore within 0.5 mil (0.5 thousandths of an inch) and then is mounted on a standard cold-rolled steel shaft allowing a clearance of over 2 mils. 

To prevent this type of error, the balancer operators and those who do final assembly should follow the following procedure. The balancer operator should permanently mark the location of the contact point between the bore and the shaft during balancing. When the equipment is reassembled in the plant or the shop, the assembler should also use this mark. For end-clamped rotors, the assembler should slide the bore on the horizontal shaft, rotating both until the mark is at the 12 o clock position and then clamp it in place. 

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