Vibration analysis

I've never operated at the rpms listed but I did find a book on Mechanical Vibrations by Den Hartog that had a simple statement that went something like: If you have a vibrating rotating shaft that is rotating at some speed below the critical speed and approach that shaft carefully with a pencil, it will make a mark on the heavy side of the shaft. If however that same shaft is rotating at some speed above the critical speed the pencil will mark the light side of the shaft indicating a 180 phase shift. If however you are exactly on the critical speed the pencil will mark a point 90 lagged from the heavy spot. Now I can imagine that going from 5,000 to 60,000 one might encounter moding and several critical speeds that might just cause the strobe, which seems to be triggered on the equivalent of the pencil, to make phase shifts accordingly. I know with the electronic balancer I see the 180 degree phase shift for the same shaft when it rotates below and above the critical speed. There is some math in Den Hartogs book that might be useful in dealing with this phenomena. He further alludes to more than one critical speed.

Matt: We had a helicopter vibration whiz give us the No Phase method for balancing and he assumed a 3 bladed helicopter so he used the 120 degree separation for the test weigh locations. I modified his method and used just a 2 blade rotor system and a reader of our magazine used just a dial indicator on his tail rotor system and achieved a good balance. I've also used that graphical method on my tail rotor which, is somewhere above the critical speed and can give crazy phase angle information to my electronic balancer. BTW I liked your tutorial. If we would have known about you we would have asked your permission to print a single plain, no phase description in the Experimental Helo magazine which we publish. Good work. Stu

Wes,

The circuit may have some lag, it is a couple of IC. Will get son to check after he finishes tiling the bathroom floor:-)).

Stuart,

Thanks for your reply.

Judging by the amplitude of the vibrations at different rpm the rotor indeed appears to have several critical speeds.

If it weren't so small I'd place test weights like I do on grinding wheels, but at 5/8" dia. its difficult to do this. Drilling a small hole in the side of the rotor is possible, though.

Will try to borrow the book you recommended... thanks.

Wolfgang

Cool Stu, I consider those posts "placed in the public domain" so be my guest. You can use all or part of anything posted (I don't even care about notation),so feel free to edit.

As per your post, there are many ways to get the job done, the more ways you know, the better you get.

Matt

You have to "freeze" the rotor with the strobe to use manual balancing calculations. This means you fire the strobe by external means and then noting amplitude while phase is tracked by "eye" against a known reference.

You will also need to filter for the dominant vibration (think old radio tuner). A scope with a tuner on the input works very well for balancing, without it your vibration sensors will show "everything in the neighborhood" and your strobe could be firing at some frequency caused by the neighbors lawnmower... Moving coil transducers are the worst at this.

The balancing machines of "my day" that internally tracked phase were more analog than digital, meaning that the machines were tuned to known references and any part changing of fiddling with wires could get you in trouble.

Matt

Page seems to be down, is there a mirror floating around somewhere?

Jon

I went to another ISP a week or so after that post. I'll try to set it up someplace else. There has also been a lot of good information submitted by others

Hi Rick, thanks. Most of what was mentioned in the thread was wildly over my head, as I am still at the grease pencil marking stage of learning how to balance a spinning disk. It is something I do need to figure out one of these days, so as much as I can get from osmosis will help me towards my eventual goal.

Jon

Jon: I can sympathize. Although a retired E.E. with enough math for a B.S, the balancer confused me when I saw it applied in what looked like just a "Cook Book" method of plotting points on a pre-prepared polar chart where if your data landed out at the " 2' Oclock" position, you put a weight somewhere or you lagged a blade. There didn't appear to be any rational process to what was going on. I got a d.c. motor and made up a flywheel with holes around the edge where I could introduce wts to throw the flywheel out of balance and see just what the electronic balancer said. I actually compared two different balancers to see if there was any common methodology being used. With what I learned, I quit using the "Cook Book" approach that I didn't understand and developed my own method which has worked for balancing the rotor blades on my helicopter as well as several others. I'll see if there is someway to put an e-mail package of my write-up and zip it to you if you are interested, send me your e-mail address to: snipped-for-privacy@iwvisp.com

(snip)

Stuart: I'd like to see your solution (everything in the mental toolbox helps)! As to your post

Think of balancing with polar charts as similar to trying to bring a "load" to unity in an AC circuit problem.

The unbalanced rotor not only shakes, it also uses more power.

Vector math is used to visually explain leading/lagging load problems and real world effects.

The geometric construction methods allow you to solve a balance problem using only a compass and scale. You can use your hand or a pencil to determine amplitude. You can use anything for a trial weight as long as you are able to divide or multiply it.

Since the problem, trial weight observations and solution are expressed in front of your eyes, you can also quickly make weight divisions to correct for fans that have nothing there where the correction needs to be, again just using simple tools. So you are visually working math problems without knowing the math.

Matt

On its way, and thanks Stuart.

Jon

Try here.I hope it's of some use.

Thanks Rick, that's a great selection, it's nice to be able to flesh out some of the concepts I've come across.

Jon

You're welcome. As others have said, you can also compute the vector rather than use a graphical method. I'll try to scan the dual plane section and post that.

Dual Plane is now posted:

Awesome possum, thanks again, Rick!

BTW, I had better luck getting the second set at this URL:

Jon

Yeah, I noticed that after I sent the post; )

There is also a series on Youtube:

Here's a list of the entire series-something for everyone....