January 6, 2022
A Handy Guide to Balancing Rotating Machinery
By Stan Riddle
Balancing rotating machinery is mostly math, with a bit of common sense and experience thrown in. The math is constant, so here are some tips to help with the common sense and experience part.
Preparing to balance
- Inspect before you balance
I once balanced a fan by pulling an air filter out of the fan wheel! Look for clues of where unbalance is coming from before you balance. In some instances you may fix the vibration without balancing. Go through the following questions to inspect your machine:
- Are all of the blades (or elements) in place?
- Do you see where a welded, clipped, or clamped weight might have come off?
- Is the rotor dirty?
- Are bolts missing?
- Is the rotor loose on the shaft?
2. Clean the machine
A balanced rotating mass, such as a fan, will always be balanced unless something is added to it or something is taken away. Something added is usually dirt build-up. Something taken away is typically throwing a weight or a blade. I’ve balanced a lot of fans with a wire brush, shop vac, and a pressure washer, or by replacing a bolt or nut. Cleaning the machine may balance it.
3. Get out of resonance
A rotating mass can be balanced practically 100% of the time unless it is operating in resonance. Since resonance is an amplification of a force, balancing a machine in resonance can be difficult. If you are running at resonance, you will need to change the speed, the mass, or the stiffness of the machine.
4. Pick a reference mark
Find a keyway, or other reference mark, and place your reflective tape on, or in line with, this reference. This way, if your reflective tape comes off, you know where to put it back. On fans, pick a blade in line with the key (or close to it). The reflective tape should also be placed toward the rotor’s outside edge if possible.
Set up your equipment
5. Reference your laser photo tachometer
If you use a phototachometer, place it in a position you know such as 3:00, 12:00, or so on. This is beneficial because if your tach moves, calculating the correction weight location will be off. By placing the beam in a position on the rotor you know, you can see if it moves.
6. Mark the blades
Use a marker, chalk, or something else to number the blades consecutively. You will also need to know if your balancer measures with rotation or opposite rotation.
Gather your information
7. Pick a radius
Balancing a rotor requires you to place weights in a reference radius. As an example, if you place a weight close to the center of the rotor’s rotation, it will have less effect. If the weight is farther out, it will have more effect. Calculating a balance correction will not work if you change the radius during the balance job.
8. Make a sketch
Old school, I know. But for me, a quick sketch of the rotor and a note of the blade counts, sensor location, and phototach location, helps me keep up with what’s going on.
9. Determine whether you need a single plane or a two plane balance
Typically, if the rotor width is less than half of the diameter a single plane balance will be sufficient. If the width is more than half the diameter, a two plane balance may work best.
10. Balancing by angles or blades
Historically, balancing was calculated in degrees. But some balancing tools will allow you to input the number of blades or locations where weight can be placed. As an example, if you have a 64 blade fan, you have 64 places to place weights (5.625 degrees apart).
Balancing the machine
11. Place your vibration sensor where the vibration is the highest.
Take a full set of measurements, or at least a quick check of vibration on the rotor bearings. For the balance job, place your accelerometer where the vibration is highest.
- On a center hung fan, if the vibration is about equal on both bearings, placing the weights in the center of the fan blade may make both ends better. If one side is considerably higher than the other, you may want to keep your weights toward the side with the highest vibration.
- On an overhung fan, use the near-near, far-far principle. If the bearing closest to the fan is highest, place your weights on the side of the fan closest to that bearing. If the vibration is highest on the bearing opposite, place your weight closer to the opposite side.
12. If vibration is really high, try a static balance test
In some instances, the rotor may be so unbalanced that the heavy spot will roll to the bottom. If this happens, place your trial weight on top. On a belt driven unit, you may have to remove the belts to do this.
13. Be smart with your trial weight
There are many ways to calculate a trial weight. But all of them require you to know the rotor weight. If you know the weight, here’s a simplified formula:
56375 x rotor weight
(rpm x 2) x radius of weight placement = equals trial weight
If you don’t know the rotor weight, use some common sense. A 5,000 lb fan turning at 200 rpm may take quite a bit of weight, while a 10 pound rotor turning at 3,600 rpm may take a tiny weight.
A good rule of thumb is a trial weight should make either a 30% change (up or down), or a 30 degree change in the phase angle. If you don’t get at least one of these, either move the trial weight about 90 degrees or so or change to a bigger weight.
If you don’t have enough information to calculate a trial weight start small. If you don’t get a sufficient change in amplitude of phase, double the weight. Continue this until you do (within reason).
Be smart with positioning the weight. If possible, place the weight where centrifugal force will work in your favor such as the inside or leading edge of the fan blade. This way, centrifugal force will tend to hold the weight on, not throw it off.
Lastly, on the first start after you’ve placed a trial weight, make sure to keep your hand close to the E-stop or disconnect! If the machine starts shaking dangerously, shut it down and move the trial weight 180 degrees.
DISCLAIMER: Balancing a rotor can be dangerous or deadly if proper safety procedures and common sense are not followed.
14. Remove your trial weight once the trial is over
The trial weight is just used to make a change in the vibration amplitude or phase. Once you do your trial weight run-remove the trial weight (unless you get lucky, and place the right amount of weight in the right location!).
15. What ifs
- If the amplitude doubles but the phase angle stays the same, you have the right amount of weight but are 180 degrees off the light spot.
- If the amplitude is halved and the phase angle stays the same, you are in the right spot but only have half the needed correction weight.
- If you can’t make a change in amplitude or phase even after doubling the trial weight a time or two, unbalance may not be the problem. Look for something else.
- If the fan has a large sheave it is possible the sheave is out of balance as well. It may need balancing separately, or you can do a two plane balance, with the sheave as one of the planes.
- On fans with multiple fan wheels on one shaft, a two plane balance is almost always needed. You may also need to try a trial weight on one fan wheel, then the next, until you find the rotor that is unbalanced.
16. Correction weights
Most modern balancing tools can calculate a correction weight, if you input the trial weight amount and location correctly. You should have a scale handy to weigh both the trial weights and correction weight(s).
- Get a good balancing weight set. There are many commercially available. Select the type(s) of weights you need based on what you are mounting the weight to.
- Unless you get lucky, you may have to split the correction across two locations (blades). Most modern balancing tools will do this as well. If yours will not, you can find formulas for this called the parallelogram method for splitting vectors.
- If weights must be welded in place, remember that the tack weld is a weight too. If the balancing weight has set screws to lock it on make sure to remove the screws once the weld is done.
17. When to quit
- A rotor can typically be balanced to low vibration levels, if unbalance is the only thing wrong. Balancing a fan will not fix:
- A bent shaft
- An unbalanced sheave
- If you continue trying to balance, and everything you do makes the vibration go up – that’s it. The remaining vibration is coming from something else.
- You can only correct unbalance by balancing. And you can only balance down until you don’t have a correction weight small enough to make the correction.
- Follow the rotor manufacturers instructions, or consult your engineering department, to know when to stop. Often the answer will be “as good as you can get it”. An old rule of thumb is if you can balance the rotor to less than 0.1 in/sec, you may be able to consider it balanced.
18. Remeasure vibration after the balance job is complete
A before and after vibration test will help justify the balance job, makes you look good, and is important to keep up with.
19. Document EVERYTHING!
Document your reference location for reflective tape, original unbalance, trial weight amount, correction weight amounts and locations, and everything else!
- Keep it in a file or folder
- If this rotor goes out of balance in the future, you’ll have a good reference of:
- How much weight, and where
- It may help you calculate a new correction weight. As as example:
- Your original unbalance was 0.5 in/sec.
- A 60 gram correction weight got the vibration to 0.05 in/sec (a change of 0.45 in/sec).
- 60 grams /.45 in/sec = 133 grams per 1.0 in/sec
- If the new unbalance vibration is 0.4 in/sec – 133 grams x .4 = 53.3. A 50 gram trial weight should get you pretty close to the correction weight amount.
If you have more questions on how to balance your machinery, connect with us so we can help!