What is the correct way to balance a over hung fan that is 18" wide 40" diameter? Dynamic or static and couple (add same correction weight 180* on static plane as you did to couple plane)I have had some tell me dynamic is OK and others said to static and couple mode! Thanks Gary

Gary,
I have used dynamic balancing for two planes in our balance machine on similar deminsions for overhung fans with good success. I first tried single plane method which was unsuccesful before attempting two plane. I have read about static couple and attempted it years ago when balancing with a strobe light but was not successful. At the time my analytical skill were lacking considerably and I am not so sure I wasn't fighting some other problem than unbalance.
I can only assume that if dynamic balancing with an IRD 290 machine works then field balancing with an equivalent computer balancing program should give satifactory results.

Hope that helps,
Ronnie

Gary, it depends on whether or not your rotor has a "static" imbalance, or a "couple" imbalance.

IF I can put weights in both of the end-planes of the wheel, if the vibration is highest at the wheel-end bearing, then I put my first trial weight nearest that bearing (i.e., the 'near' plane). If the vibration is highest at the drive-end bearing (farthest from the fan wheel), then I put my trial weight at the 'far' plane of the wheel. Usually (90% of the time) a single plane balance will work fine. If the 'near' weight (nearest the wheel-end bearing) gives a good effect at the near bearing, but the far bearing is still too high, then leave the trial (static) weight where it is. You can start a new job using data from the trial run as the new reference. Add a "couple" to the wheel (in addition to the 1st static weight)... be sure you use exactly the same weight and that they are exactly 180 out from each other. You don't have to use any special solutions. Just do a single-plane balance, entering only the weight you put in the far plane (yes, you ignore the near, couple weight). The couple should not have much effect on the near bearing, so you don't use near bearing data in calculating the solution... just data from the far bearing. I have done this successfully a number of times.

Keep in mind that you don't usually have to commit to any one method when you are balancing. Most software today lets you 'save' and 'recall' balance jobs on-the-fly, so you can use several techniques at once.

For instance, start with a single plane balance as described above. Save this job. Then rename the job, and add a weight in the 2nd plane... you are now doing a two plane balance, or you can use the data to calculate 2 different single plane solutions (i.e., the 1st trial run becomes the 'reference' run for the 2nd trial run you did. You can learn a lot by calculating all possible solutions.