Unbalance and excess vibration
Most premature failures in rotating machinery are a result of imbalance, which is often caused by uneven mass distribution around the axis of rotation. Excess vibrations are a result of this unbalance in rotating machinery, and may cause unacceptable levels of noise and reduce the lifespan of shaft bearings. Dynamic balancing is among the main solutions to counteract this.
What is dynamic balancing?
One goal of manufacturers when designing any machine is to ensure that it will operate free from vibrations. The other goal is to ensure that the machine runs for the longest time possible. However, in reality, vibrations form part of rotating machinery, and some vibration levels may be unacceptable.
Instead of removing all vibration from machinery, the manufacturers try to balance it to the peachiest extent possible. One method of balancing is dynamic balancing, where machinery is used to identify the imbalance points and rectify it. Manufacturers perform dynamic balancing using sensors that they attach to bearing pedals. The sensors allow identification of unbalance on two planes to allow real correction.
Factors that can affect the efficiency of dynamic balancing:
Many manufacturing factors can affect the efficiency of dynamic balancing, including material issues such as porosity, density, blowholes, and voids. Fabrication issues include eccentric machining, misshapen castings, and poor assembly. Distortion issues include temperature and aerodynamics changes and rotational stresses. These problems arise during the manufacturing process, while others arise during the machine’s operational life.
Unbalance can occur during rotor fabrication due to many reasons – one of them is tolerances stack up. If a manufacturer combines a well-balanced shaft and rotor, the needed assembly tolerances may allow radial displacements. That leads to an imbalance condition. Keys and keyways additions contribute to the problem.
Rotors that have been in service for a very long time alongside some other factors may contribute to imbalance condition. The factors may include wear, corrosion, deposit build up, and distortion. Deposits may also unevenly break off leading to severe imbalance. That mostly applies to fans, compressors, blowers and some other rotating devices. To minimize the effect, you will need to inspect and clean your machine regularly. However, eventually, you will have to remove the machines from service due to imbalance.
Flexible and rigid rotors
The difference between the rotors in the machinery can also cause imbalance. There are two primary types of rotors, flexible and rigid. Rotors operating within 70-75% critical speed – the natural frequency – are flexible rotors. Those operating below the speed are rigid rotors. Rigid rotors are usually at balance at two end planes and remain in balance when in service.
Flexible rotors require multi-pane balancing. When a rotor is in balance on a low-speed balancing machine (assuming that it is rigid) and then it becomes flexible in service, unbalance will arise leading to high vibration. Machines in this group include compressors, multistage centrifugal pumps, and gas and steam turbines.
Extend the life of your machinery
A machine operating in a smooth, properly balanced order has many benefits. When operating the machinery, you will enjoy lower vibrations, low noise, lower operator fatigue, higher operator safety, and lower operational cost. The other benefits include more productivity, longer bearing life, and lower structural stress.