Balancing a rotating part

Fans, pumps, motors, impellers and wheels all turn on a shaft. Balancing is the work of adding or removing small weights until the part spins smoothly on that shaft.

What an imbalance is

A part is out of balance when its centre of gravity sits off the line it turns about. As it spins, the heavier side pulls outwards, and the pull turns round with the shaft.

A tutorial from the Turbomachinery Laboratory at Texas A&M University names causes that develop while a machine runs. They include dirt or other deposits on the rotor, a shaft bowed by uneven heating or damage, and material lost by rubbing.

Static, couple and dynamic imbalance

Static imbalance is an offset of the centre of gravity from the shaft's axis. It shows without running the part, because on free-turning supports the heavy side settles at the bottom. Correcting it takes a weight in one plane.

Couple imbalance is a tilt of the part's mass, with the centre of gravity still on the axis. The University of Almería's lesson on machine balancing says it only shows while the part spins. It needs correction in two planes.

Dynamic imbalance is both at once, and it also takes two correction planes. On a thin part such as a disc or pulley, a course on rotor balancing from the Indian Institute of Technology (published through NPTEL) says one correction weight in a single plane is enough in practice.

Signs a machine needs balancing

The Almería lesson names vibration, faster bearing wear and a poorer finish from machine tools as effects of imbalance.

The Texas A&M tutorial says imbalance shows as vibration at the running speed, once per turn. It notes that many other faults also vibrate at once per turn, including misalignment, loose parts and rubbing. So check for those before asking for a balance.

How good a balance needs to be

The British Standards Institution's catalogue lists BS ISO 21940-11:2016, which sets the procedures and tolerances for balancing rotors that behave as rigid bodies. It says the standard cancels and replaces ISO 1940-1:2003. The standard sets how much imbalance may remain and how many correction planes a rotor needs.

Balance quality is given as a grade, written G and a number. The Texas A&M tutorial, written under ISO 1940, the older standard, explains the grades as speeds in millimetres per second, starting at 0.4 and rising by a factor of 2.5 (0.4, 1.0, 2.5, 6.3 and on). It gives G6.3 as typical for pumps and fans, and G2.5 as typical for large motors. Ask the balancer which grade and which speed they will work to.

In the workshop or in place

A workshop balancing machine spins the part on the machine's own supports and can measure the out-of-balance force directly. Once a rotor is installed, the Texas A&M tutorial says, its imbalance can't be measured directly. Balancing it in place (field balancing) works from the machine's vibration instead.

The balancer runs the machine and records the size and timing of the vibration. They fit a trial weight, run it again at the same speed and work out the correction from the change. Two-plane balancing puts weights in two planes, with one near each bearing.

Ask for the vibration readings from before and after the work, so the change is on record.

The rules on maintenance and guards

In Great Britain, regulation 5 of the Provision and Use of Work Equipment Regulations 1998 (PUWER) says every employer must keep work equipment in an efficient state, in efficient working order and in good repair.

Regulation 11 requires measures that keep people away from dangerous parts of machinery, starting with fixed guards wherever that is practicable. HSE says moving plant must have stopped, and its power must be isolated, before maintenance starts.

Field balancing means running the machine. Where a hazard can't be avoided during maintenance, HSE lists temporary guarding and slow-speed hold-to-run controls among the measures to consider. It also says maintenance should only be done by people competent to do it.

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