January 7, 2019

SMC Touch Tip-Using the SMC When Unbalance is Diagnosed

By Stan Riddle

SMC Unbalance one a

The Fixturlaser Smart Machine Checker (SMC) is a great balancing tool (for me personally, it’s the best balancer I’ve ever used).  Unbalance is one of the most common machinery problems on rotating equipment, especially fans.

If you get a diagnosis of unbalance, be aware of these tips:

  • Be mindful that a rotating mass, once balanced, should never get out of balance, unless something has been added to it, or something has been taken away from it. Check for:
    • Shiny places, indicating a balancing weight, or piece of blade has been thrown off.
    • Dirt build-up. I’ve balanced as many fans with a wire brush or pressure washer, as I have with balancing weights.  Clean the fan and re-check.
  • Check for a bent shaft. A bent shaft displaces center mass, making it appear to be unbalanced.  It’s best to check for a bent shaft with a dial indicator, or run out probe.  If the shaft is bent, it should be replaced.
  • Make sure nothing has been pulled into the fan wheel. I once went to balance a fan, only to discover that a box filter had been pulled into the fan wheel.
  • If the blades are hollow, make sure that the inside of the fan blade is clean. Typically, on outdoor units, hollow fan blades have a weep hole, to allow water to run out.  Make sure they are not plugged up.
  • If the fan is belt-driven, inspect the sheave as well. Large sheaves can often weigh almost as much as the fan itself.
  • If the machine is sitting on isolator springs, make sure they are set for their proper pre-load deflection. Just because a fan is “bouncing” on isolator springs does not automatically mean an unbalance problem.

If a good visual inspection does not reveal one of these problems, balancing the fan is probably the next step.

Lastly, balancing at resonance is difficult, if not impossible.  If a speed change happened right before the machine had a sudden unbalance problem, suspect resonance.  If you have a VFD, try balancing at a different speed.  If the machine isn’t variable speed, you have a problem.  The machine will either need a speed change, or a design change to get it out of resonance.  You can also perform a Coast Down test to determine resonant frequencies.

Balancing is typically very easy-just a simple math process.  If you’d like more information on how to balance with the SMC, please give us a call!