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Thread: Dynamic v static balancing
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20th Jun 2015, 11:08 PM #1Philomath in training
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Dynamic v static balancing
As many of you know I'm plodding on with my Jones & Shipman 520 rebuild. I have one shaft on and hope very soon to have a motor spinning (VFD currently being fitted) and then the second shaft mounted. The shaft currently fitted should turn at around 560rpm and the other shafts at 915rpm and 400rpm respectively. The product information I have proclaims that the shafts are dynamically balanced. I'm not sure how necessary that will be for my shafts - they are machined not cast and I've tried to turn them on a dummy shaft, so any out of balance should () be minor I hope. (I'll find out when I spin up I guess). I've never thought much about dynamic balancing something like this so would be interested in the thoughts of people familiar with the process. Making up a shed suitable process is possible although tricky - it's a matter of being able to relate an out of balance force to an angular position. As the v belt pulleys are small and light compared to the main flat belt pulleys (large diameter and heavier but the mass could be approximated to a single plane as they are 'thin'), would static balancing be acceptable? Making up some knife edges should not be difficult.
I do know that washing machines and high end power tools can be fitted with balancing rings (rings that either have fluid or balls in them - think of a ball bearing with no cage so the balls can accumulate on one side). I've never seen anything off the shelf though that I could just add onto the shaft. Again, does anyone know if they are available off the shelf?
Michael
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20th Jun 2015, 11:34 PM #2Golden Member
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I'm not sure about balancing rings, but expect that static balancing of the individual pulley sheaves would get you close (I think that's what you were suggesting), and much better than static balancing of the assembly. Of course that means you need to make up a suitable arbor for each sheave.
Cheers,
Bill
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21st Jun 2015, 07:39 AM #3
Hello Michael,
A lot of years back I was involved in the design of equipment for "On Car" wheel balancing ! I soon discovered how very difficult it was to actually dynamically balance a car wheel whilst still fitted to the vehicle. The equipment used a xenon strobe light and a linear displacement transducer to determine where to place the balance weight. The transducer, placed under the wheel pivot, who's output was used to trigger the strobe and determine the amount of weight to use. It turned out that this equipment was no better than doing a static balance with the wheel off the vehicle.
Modern off vehicle, wheel balancing equipment does dynamic balancing in a similar way but uses two transducers. One measuring the centripetal force and the other the bending moment of the drive shaft. In this way the balancing can account for both directions of deflection.
I would think that since steel density is pretty constant any out of balance would be due to any holes or keys that you put into them. Your knife edges should do the trick adequately.
Good luck with your rebuild.Best Regards:
Baron J.
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22nd Jun 2015, 09:10 PM #4Novice
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Ball Balancers or Liquid Ring Balancers
Hi Michael,
In order for ball balancers to function, the structure supporting the rotating shaft must be soft-mounted so that the structure's natural vibration frequency (at right angles to the shaft) is well below the rotational speed of the shaft. Some examples: Washing machine spin dryers, which have liquid ring balancers, are soft-mounted by hanging the whole structure on suspension rods and springs. The rotating parts of CD-ROM drives (now obsolete) had a ball balancer on the shaft. The whole mechanism was soft-mounted, within the casing, on soft polymer mounts. (It is interesting to disassemble one to see the mounts and find the ball balancer. The attached image shows one removed from the CD-ROM drive with its cover removed. It is about 30mm diameter. ).
Hence, it is not likely that a ball or ring balancer will fulfil its purpose on a normal machine-mounted shaft. The shaft's own natural vibration frequency is likely to be too high.
ball_balancer.jpg
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22nd Jun 2015, 09:14 PM #5Most Valued Member
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So, is dynamic balancing a means of achieving an even higher level of balancing compared to static since it's done at speed, the centrifugal forces "amplify" the amount of unbalance in the system and allows for greater accuracy in the position and amount of mass needed to added/removed?
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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22nd Jun 2015, 10:01 PM #6Philomath in training
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I tried to find a picture that would make this easy for me but...
Static balancing is relatively simple to do. Years and years ago car tyres were statically balanced by floating them in a tub of water and adding weights to the rim until they floated level. If you have a shaft you can balance it on knife edges and adjust the weights until it does not stop at a particular place. On both cases you have adjusted the centre of mass so that it is in the middle of the object. When it rotates around an axis passing through the centre it should be in balance, but...
Imagine now the tyre that you balanced in the tub of water has two heavy spots in it, directly opposite each other and the same weight but on opposite side walls. Statically the tyre may balance but when the tyre starts rotating on the axle, because the two masses are not in the same plane perpendicular to the rotation, there will be a (dynamic) out of balance situation. To determine what it is usually requires you to know the angular position of the shaft at the same time you measure the reaction force
In my situation I have a relatively heavy but thin disc on the shaft so static balancing may work but because I also have keyways, keys and pulleys at one or both ends I have potential for out of balance masses to be present. The pulleys on the shaft are basically symmetrical around the rotational axis and because all the keyways are in line (admittedly purely by accident), I'm hoping any out of balance will be easily found and corrected.
Michael
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22nd Jun 2015, 10:43 PM #7Member: Blue and white apron brigade
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I have use the static balancing method for squirrel cage fans (that rotate up to 1440 rpm) a number of times and it has worked quite well.
The weights were just 5mm wide pieces of sheet metal folded over the vanes.
ike this
Usually just a couple of pieces are all that is needed to get a reasonable balance.
One time I dropped the squirrel cage itself and being a flimsy swiss cheese model it sort of bent over like parallelogram, dropping it and then tapping it lightly the other way got it sort of back into shape but even after static balancing it still had a wobble. This fan ended up with about eight pieces of metal on it with the static test.
What I should have done is thrown it away but curiosity got the better of me and I persisted
The beauty of this type of fan is the weights can be slid along the vanes to effectively do what Michael G wrote about in a previous post. It took a while but eventually, through trial and error, I worked out that if some pieces were closer to the motor and others further away that the fan would balance better. I did have to remove and add a couple of pieces. It took ages and I would probably never do it again. Some sort of vibration sensor would have helped.
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23rd Jun 2015, 12:16 AM #8Golden Member
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Michael,
I'm rusty on this, but here goes.
The out of balance forces relate to the size of any unbalanced masses, their distance from the axis of rotation and the square of the rotational speed. The "square of the rotational speed" bit simply means out of balance forces gets much worse as the speed increases.
So for a fixed rpm, the factors are the size of unbalanced masses and their distance from the axis of rotation. Keys are generally light compared with other masses, and close to the axis so the unbalance forces they generate are generally small. Also, if the key is the same length as the keyway in shaft and disc there will be miniscule unbalance force (assuming the disc has the same density as the shaft). With aluminium pulleys there will an unbalance force due to the difference in density between the key and the pulley but unless the application is critical or speed very high it is hard to imagine vibration being a problem due to that.
If your application has the typical setup with discs separated by a length of shaft, dynamic balancing would be necessary if the whole thing was in one piece (eg made in one piece, or brazed together). However since it is separable, and it is unlikely that the shaft alone will contribute significantly to vibration problems when running, you will likely get a good result simply by statically balancing the discs. If you did this by mounting one disc on the shaft at a time on the knife edge rig, you could negate the effect of other keyway(s) by fitting a half-thickness key(s) taped in place.
I hope that makes sense.
Cheers,
Bill
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23rd Jun 2015, 05:57 AM #9
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23rd Jun 2015, 09:39 AM #10Most Valued Member
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