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Balancing Safely - Balancing Stand Zero Speed Safety System


Balancing Safely

Safety Interlock System - Fan Blade Balancer safety system retrofit

Our customer received a safety violation from WorkSafe BC (WCB)

  • The Operator was exposed to sharp rotating edges when adding or adjusting balancing weights.
  • Fans were spun at up to 1200 rpm during balance testing
  • The operator manually adjusted the belt tensioning pully to accelerate or decelerate the fan very close by the rotating blades
  • There was no guarding around the balancer to prevent loose weights from being thrown into adjacent work areas
  • Blade fans could rotate for 1 minute, cage fans could rotate for several minutes after the motor stopped due to inertia

Typical Fan

Balancing Stand

Industrial fan
Belt driven fan balancing stand

(click for a larger view)

Identified Hazards
Moving Parts

  • fan blades
  • drive belt
  • thrown weights


  • None


  • None


  • None

Solution and Equipment Installed

A 3 sided wire mesh cage was installed against a wall, with a top to prevent weights from being thrown into adjacent work areas. The front included a hinged door that was interlocked with a Control Reliable solenoid release door tongue switch. The balancer control console was moved outside the cage and modified to include a Control Reliable E-stop button. A manual Reset button was mounted nearby. E-stop buttons were added inside the cage as well.

Couldn't use an encoder to sense zero speed
The biggest challenge was to determine how to sense zero speed of the fan as a condition to release the door solenoid tongue lock.
To keep costs down, the customer kept the existing smooth flat fabric belt drive system vs. retrofitting a new motor and VFD. The balancing shaft is minimally coupled by the fabric belt to the motor through a tensioning pully. When the motor is turned off the flat fabric belt provides almost no braking friction. The inertia of the fan can keep it spinning for a long time after the motor has stopped. Traditional methods of sensing zero speed using the motor's Back EMF or an encoder coupled to the motor shaft could not be used. The customer uses dozens of different balancing shafts of different diameters, so an encoder could not be mounted to the shaft either.
WIS designed a simple optical solution to sense shaft speed and worked with 2 different product vendors to source an appropriate safety relay and a pair of optical sensors.

Call us if you have a Machine Safety challenge.

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