Balancing How To #6 – Balancing Tolerances

By now, if you’ve kept up with the Acoem balancing “how-to” blogs, you should see that balancing is mostly just a math problem – counteracting forces. But we haven’t discussed when the balance job is through.

When I started balancing fans many years ago, I stopped balancing when:

  1. The fan owner said, “that’s much better”, because the machine wasn’t bothering them anymore
  2. When I got the fan below 2 mils (PK-PK)
  3. When I couldn’t get it any better

None of these are very scientific, so let’s look at some specifications.

ISO 1940/1 covers balance grade qualities for many different types of machines – from slow-speed reciprocating engines, all the way down to gyroscopes. In the cases of most industrial machines, the specification is G 6.3. The 6.3 denotes the amount of residual vibration at running speed, in mm/sec.

Some high speeds applications, or electric motors operating “stand alone” might specify a G 2.5, or 2.5 mm/sec.

  • G 6.3 = 6.3 mm/sec, or 0.248 in/sec (PK), or 0.175 in/sec (rms).
  • G 2.5 = 2.5 mm/sec, or 0.098 in/sec (PK), or 0.069 in/sec (rms).

These are quite achievable, in most cases, IF unbalance is the only vibration at 1 times the rpm, and the machine is not operating in resonance. But there are other sources of vibration that manifest themselves at 1 x rpm, such as misalignment, a bent shaft, looseness, and so on.

So, how do you know “when to stop”?

Balancing is both a mathematical, and a logical process. If you add a correction weight to the right spot, you should expect the vibration to drop, and the phase angle to stay relatively steady. Even a trim run should be predictable. But if the vibration amplitude or phase does not perform as expected, look for:

  1. Any looseness in the system, either in the machine, or in your sensor or tachometer. If none is found,
  2. Check for resonance. Is the machine operating at a natural frequency? You can test for resonance in multiple ways, from a bump test, to a coast down test, to just changing the input on a VFD drive.
  3. Are there other components that could cause vibration at 1 x rpm? Balancing can only effect balance quality, not misalignment, or other causes.

Lastly, most manufacturers of machinery specify balance quality for their machine. You should balance to the manufacturer’s recommendations.

by Acoem contributor | September 6, 2022
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