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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 23, Centerline Shift Between Balancing Machine and Final Assembly

Even when parts are precision balanced to extremely close tolerances, the vibration due to unbalance can be several times higher than necessary due to the way parts are assembled. "Assembly errors" are not always due to the stacking of tolerances but to the relative repositioning of each part's centerline of rotation. For example, a "perfectly" balanced blower rotor can be assembled to a "perfectly" balanced shaft and, yet, the resultant unbalance can be high. This can also happen if the rotor was balanced on a balancing shaft that fit the rotor's bore within ½ mil and then assembled to a cold-rolled steel shaft, allowing a clearance of over 2 mils. Shifting the mass of a rotor from the rotational center on which it was balanced can cause an assembly unbalance error four to five times greater than its balance tolerance.

A rotor should always be balanced on a shaft having a diameter as nearly the same as practical to the shaft on which it will finally be assembled. If not practical, balance the rotor on its own shaft rather than on a separate balancing shaft. This may require balancing some rotors in an overhung position, which is often resisted by the balancing shop. However, it is easier to learn how to do this than it is to make too many balancing shafts.

For rotors that don't use setscrews to locate the shaft relative to the bore, such as end-clamped rotors, problems of unbalance often result. On the balancing machine, the balancing shaft is usually horizonal. The operator slides the rotor on the shaft, with gravity causing the rotor's bore to make contact at the 12:00 o'clock position on the top surface of the shaft. In this position, the rotor is end-clamped in place and then balanced. If the operator removes the rotor from the balancing shaft without marking the point of bore and shaft contact, then upon assembly, the contact point between bore and shaft may be at an entirely different position. This often shifts the rotor radially by several mils as compared to the axis on which it was originally balanced, causing further unbalance. Ignoring this does not usually produce high vibration. Instead, it produces mediocre vibration levels, even when the rotor has been balanced to precision tolerance. If the rotor is resonant with some part of the machine or structure, then a more serious vibration results.

For such rotors, a "rule" should be followed between the balancer operators and those who do final assembly. That is, the balancer operator should be required to permanently mark the location of the contact point between bore and shaft while preparing it for balancing. Then the assembler should follow the same assembly procedure. For end-clamped rotors, the assembler should slide the bore on the horizontal shaft, rotating both until the mark is at the 12:00 o'clock position and then clamp.


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