auto balancer 1

[jclough]

I'm trying to design an automatic balancer to balance a fan attached to an electric motor. I've made a disc withh a groove containing a few steel balls which are free to move around to conpensate for the fans unbalance. This assembly seems to do a pretty good job of reducing vibration but there's one problem. Since the motor is mounted horizontally, the balls all drop to the bottom of the disc when the motor stops. This causes a grest deal of vibration at start up until the assembly can re balance itself. I've considered filling the groove with a grease or fluid to help the balls retain their position while the motor is stopped. Any suggestions as to what material to use? The assembly must operate outdoors year round.


Thanks

jon

 

[MintJulep]

Wouldn't it be easier and cheaper to actually balance the fan?

 

[Highspeed]

Check this thread...
thread237-73625

 

[jclough]

Hi Mint,

We've been doing that for years now but they only stay balanced for a few months. Seems the air that we're moving have vapors which build up on the blades. I suppose one could clean and rebalance it but shutting the machine down costs money it terms of lost productivity.

Jon

 

[ietech]

Am I possibly understanding that you run your machinery 24/365 without preventative or planned maintenance. If so then shame on you. Just the opinion of an old timer who still has some respect for the machines that make our life easier.

My advice --- Get Real --- AND DO APPROPRIATE MAINTENANCE.

 

[Neilmo]

Jon,

The idea you describe of using balls in a groove to balance a fan. I have seen this before in an article in SKF bearings magazine `Evolution` a couple or so years back and was fascinated by it. In the article they were actually using a ball bearing with no cage, this allowed the balls to float free, to balance the disc of a disc grinder. The theory behind the idea was explained and from what I remember only works when there is no oil or grease present. This is because the forces moving the balls to the `balance` position are very small and any contamination with oil or grease would prevent this.

You might try SKF's web site to look for the article -

http://www.skf.com

or

http://evolution.skf.com

Regards,
Neilmo

 

[Neilmo]

Jon,

Hi again, just had a look on the `Evolution` web site, try this for size -

http://evolution.skf.com/gb/article.asp?articleID=10

Regards,
Neilmo

 

[strokersix]

Here's a link:

http://www.skf.co.uk/News/skfnews/press_211.htm

 

[MintJulep]

If the vapor condenses and builds up on the fan blades why do you think that it will not condense and build up inside your auto-balancer, or in the grease?

Sounds like you are only deferring your problem.

How about getting a float of spare fans. Periodically you could shut down very briefly, pull off an out-of-balance fan and put on a cleaned and balanced fan. Take the old fan back to the shop, clean it, balance it and put it on the shelf for the next go around.

Also, follow ietech's advice. It is well proven that maintenance only in response to failures is more disruptive and costly than scheduled preventive maintenance.

 

[EnglishMuffin]

jclough:
I seem to recall that this balancing method, or something like it, was used on Cincinnati centerless grinders many decades ago, although I could be wrong about the exact operating principle. There were three balls immersed in a lubricant, and you could see them through a transparent window at the center of the wheel. I also seem to recall that the balls were locked in position until the grinding wheel got close to its operating speed. It only works if you operate above the critical speed of the rotor you are trying to balance. There was also a guy that tried to sell round plastic tubes with balls inside them, which were supposed to automatically balance car wheels. I tested one on a balancing machine (rather perfunctorily) and it didn't work - probably the critical speed was wrong.
Now it might be possible in your case to come up with a device that released the balls automatically above a certain speed, utiizing centrifugal force. It doesn't have to be anything complicated - possibly just a very flexible dish shaped plastic disc, for example, arranged so that it was in light contact with the balls at low speed, but made such that it would become flatter at high speed and release the balls.

 

[ischgl99]

Have you checked to see if you are running near a resonace of the fan? If so, stiffening the base might give you longer times between cleanings.

Mike Bensema

http://www.dutchmenservices.com

 

[jclough]

Hi english,

I seem to remember something about the wheel balancers too. Any idea as to why that product didn't take off? All I could find on the web was something called balancemasters. Looks like some sort of a mercury filled device. While mercury seems like the perfect material (functionally) I don't feel comfortable using it. I can still remember seeing photos of a mercury damaged human brain in National Geographic magazine when I was a kid.

Anyway... I first tried my auto balancer on one of my balancing machines and had the same problem as you. As you suggested the device doesn't work until speed is above the first critical. In fact, because all of the balls fall to the bottom when the motor is stopped, vibration was much worse.

What do you think oof this idea... Rather than have the balls in a groove, have an assembly similar to a jar lid. Iside of the lid recess would be blades radiating in a spiral from the center to the outside. One ball would be captive in each segment. I'm thinking that at start up the spiral blades would scoop the balls toward the center there by reducing vibration. As speed increased the balls would move outward until the first critical shook things up and it started to balance.


Mint and IE,

I would truly love to have the luxury of doing as you suggest.

The machine on which this machine mounts uses 24 if these assemblies. They are all mounted on twhat is basically a large piece of sheet metal. During development, the customer found that often the fan unbalance would cause a beat frequency on the sheet metal. It was quite a thing to see. You could actually see the waves moving across the mounting plate. Anyway, rather than stiffen the mounting plate and/or isolate each motor they decided that to have us balance the fans. End of story right? Wrong...Even after balancing the fans to a smooth G2.5 it didn't work. Seems that the variation in our shaft runout (a respectable .0005 over 12 inches)still left too much vibration in thier system. We are able to balancce the fan/motor assembly well enough but now we're being asked to guarantee it for five years. They want to sell thier machine on quality (reads reliability). All thier customers will have to do for the first five years is pay the electric bill and cash the checks. Sounds like the same idea as GM has been doing with thier 100,000 first tune up deal.

Rant off,

Jon

 

[Cooky]

To take MintJulep's idea a stage further. Why don't you have a run & standby arrangement, so you can change-out the fan when vibration occurs and keep your process operational.

Cooky

 

[ietech]

I understand your problem. The customer wants a lot --- five-years is a pretty long stretch to expect things not to wear or have a change in balance dynamics.

You are right .0005" over 12" is very respectable. Since the customer will not allow stiffening of the mount plate the next best suggestion I can think of is to use vibration dampeners at the mount location of each unit. These have been sucessful over the years but still not sure if they would be useful over 5-years under constant use. PM is still desirable. Of course this would be considered isolating each unit and the customer doesn't like that either. HMMM

From what I have learned Autobalancing is relatively unproven except on grinders and then over the long haul--- questionable.

I haven't seen any published reliability data for any marketed products. I would be very reluctant to accept liability for five-years.


AHHH GM yep -- their tune ups are 100k but everything else needs constant attention. Tune-ups are cheap compared to trannies and other stuff.

Good Luck

ietech

 

[EnglishMuffin]

jclough :
The wheel balancer didn't take off because it didn't work!
But it was patented - just shows you how much that is worth sometimes - and as I recall, the patent did not mention critical speed. It definitely had balls in it, not mercury, but the name "balancemaster" seems familiar. Enough mercury to balance four car wheels would also cost a fortune and give the EPA guys heart failure. Maybe the mercury has damaged the inventor's brain (just kidding)! Devices like these also require some precision in regard to the concentric positioning of the tube and the center of rotation. There have been a number of active autobalancers developed over the years, (rather than passive ones like you are considering) but they are all rather complex. I believe I read that there is an ongoing investigation to incorporate them in jet engines, but my memory could be at fault.
I personally don't think your spiral vane idea would work, although I may not understand it correctly and could be wrong. Is your fan horizontal or vertical? In the horizontal case, it's not clear what would keep the balls at the center of the vanes when stationary. In the vertical case, what would be the function of the vanes, and what is the function of the spiral? The balls would get flung right to the outside immediately, with or without vanes. The balls have to take up certain angular positions all at the same radius, and hopefully remain there. I repeat my suggestion that I think you need some means of mechanically locking them in the most recent optimal position until you are close to the operating speed (which I think is the way the Cincinnati grinder worked). I don't see why this should be so difficult. Whether it's worth it is another matter, as others have said.

 

[EnglishMuffin]

lclough :
As usual, I don't think enough before dashing off these replies. Perhaps what you are proposing is that the spiral vane angle is chosen such that during start up, the angular acceleration produces enough inward radial force to overcome the centrifugal force. I'm talking relative to the rotating frame of course - I don't want to get into any arguments from people about centrifugal force being fictitious etc. But then what happens during coast down (or deceleration)?

 

[wmccracken]

My experience is somewhat different from others in this thread I guess. I bought and used an "auto balancer" for my car many (about 25) years ago with great success. The device consisted of a plastic circular channel attached to a steel plate shaped to fit between the brake drum and the steel wheel & tire. It had 5 holes to be placed over the wheel studs and then the wheel & tire place over it and bolted in place. The circular steel was shaped to fit in the recessed area on the back side of the wheel.

The operating principle was basically similar to that described by others. The channel was filled with oil and with some steel shot (small balls). When at rest, the shot fell to the bottom of the channel. As the car started to move it was possible to hear the shot tumbling until maybe 5 mph or so after which centrifugal force held it in place around the circular channel. I guess the tumbling ended up distributing the shot fairly evenly around the circumference of the channel. Since the channel was concentric with the desired center of rotation of the wheel, any shift in the center of rotation (due to excess weight on that side of the wheel) resulted in the shot rolling inside the channel to the opposite side (effectively "downhill from the perspective of the shot). This thereby would bring the center of rotation back to the desired location.

It worked!! I used to get new tires, have them balanced without the device, and then install the auto balancer. I never needed to rebalance for the life of the tire. The car (a Volvo) was susceptable to unbalance and before this I had to rebalance the tires periodically. I would have continued using the devices but they were shaped for the specific wheel for my car. I was never able to find a replacement for other cars I have owned since.

 

[EnglishMuffin]

The ones that I tested over twenty years ago (presented to me by my boss at the time - don't know where he got them) -did not have any oil in them. Perhaps the oil, which presumably introduces damping, is an essential feature - the Cincinnati grinding wheel balancer that I recall contained oil as well as steel balls. But the thing is that the principle can only work when operating above the critical speed of the rotor. Probably cars differ in regard to this critical speed, but it's hard for me to believe that the critical speed of most car wheel axles, particularly solid rear ones, can be that low. It presumably depends partly on the spring rate of the suspension and its multi-directional uniformity.

 

[wmccracken]

I guess I didn't say it in my post but I've always felt that a critical feature in a self balancing system like the car wheel balancer I described above, is compliance or the ability of the center of rotation to shift. If the centre of rotation doesn't shift there is nothing to cause the balls to move to the other side to cause the balancing effect.

A freely suspended wheel would be the ideal case, allowing the centre of rotation to move to the centre of gravity of the rotating mass. A car wheel is less than ideal but still has a bit of compliance due to the compressibility of inflated tire against the road surface. I guess it is enough but not the ideal case.

If this system is to be applied to rotating equipment there must be a resillent suspension system to allow the centre of gravity to shift. Depending on the application this should be easily designed in.

 

[jclough]

Hi English,

Did the Cincinatti grinder balancer operate above critical?
I would think that the first critical on that grinder would be rather high. Grinders are really stiff.

Jon