Fixturlaser SMC Touch Tip-Using the SMC When Unbalance is Diagnosed

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!

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