Unbalance is one of the most common causes of excessive vibration in rotating equipment. Components such as bearings, couplings, seals, and motors are brought to premature failure every day due to excessive vibration. Balancing is usually the first thing that comes to mind when these increases are noticed. But what causes a machine to become “out of balance”?
Here are the four most common reasons:
1) Build-up
Foreign debris or build-up from the process that is taking place at the facility can accumulate on rotating parts. At first this build-up may be unimportant, as the material builds up evenly across the surface of a rotating component. However, as build-up increases, there will eventually be pieces that fly off. At this point, the machine immediately becomes out of balance.
Although the increase in vibration may seem sudden when material lets go, the process of accumulating material build-up had been occurring for quite some time in most cases. We could assume that the rotating component was balanced at one time. Therefore, adding correction weights to restore the balance may not be the best option. Further loss of material build-up is bound to occur. The best option in these situations is to thoroughly clean the rotating components that have built up material contamination to restore balance.
The higher the RPM, the greater the effect the build-up will have on the equipment. It is always a good idea to make sure rotating equipment remains clean and clear of build-up, especially high-speed machines.
2) Loss of Previous Balance Weights
It is not uncommon for rotating equipment to throw a weight that was once added by a balance technician. These weights come in many forms such as C-clamp style with a set screw, clips, or nuts and bolts. On larger machines some technicians will use beam clamps and other makeshift components to achieve balance. Sometimes plate steel that was welded on during the balance process can give way and fly off.
The most worrisome part of this problem is the safety concern that comes with weights flying off rotating equipment. Furthermore, a sharp increase in vibration occurs when a weight lets go. Always be sure to use proper weights for each scenario. Welding, drilling and tapping, drilling and bolting, or using weights designed specifically for your equipment are usually a safe bet. Weights should always be applied in such a manner as to avoid flying off from centripetal force or extreme wear from any process material.
3) Material Loss
In many situations, rotating components can be susceptible to material loss. This loss can come from physical damage, chemical damage, corrosion, cavitation, abrasion or other damaging processes in your facility.
As machines run continually it is not uncommon for material loss to occur. As long as the component is inspected and deemed safe to run, it can be re-balanced to restore vibration levels to normal. However, it is always best to eliminate material loss by improving the process. Sometimes coatings can be applied to parts to reduce wear. Cavitation and other flow issues can be avoided by properly sizing equipment and positioning valves correctly.
It is always best to address the root cause rather than simply performing “band-aid” repairs.
4) Run-out
This is where it gets tricky. Sometimes a machine can be called out by a vibration analyst for a high 1× vibration that looks like imbalance. Technicians will be assigned to make the repair with a balance process. If run-out is the cause of the high 1×, balance weights will rarely correct the issue.
Run-out can also be thought of as a bend, bow, or eccentric bore.
Bend: Sometimes rotating equipment gets bent from physical damage or excessive belt tension. This will cause an increase in 1× vibration that looks like imbalance.
Bow: This can occur in large rotors that sit still for too long and develop a bow in the shaft from gravity. A bow can also be caused by temperature changes if the shaft material isn’t tempered or stress-relieved correctly.
Eccentric Bore: This is very common on new components. If the machinist bores a coupling or hub slightly off center, it will naturally be out of balance as it swings around. This too will cause an excessive amount of 1× vibration that looks like imbalance.
If you’re trying to balance a machine and can’t seem to make it better, consider using a dial indicator to identify run-out issues. Be sure to check each component in several different locations. If run-out is found, the correct course of action is repairing the damaged component, not adding balance weights to it.
As you can see, unbalance in rotating equipment isn’t always as simple as adding (or removing) mass. To improve the reliability of a machine, it is always best to think outside the box and look for the root cause of the issue.
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Balancing by Collin Mann