Wheel balancer theory

[gregs]

I have another thread about using my wheel balancer yesterday. But I was wondering about how it calculates the amount of weight needed depending on the location. I have one of the generic import electronic balancers. You input the data it asks for and it initially spins up and gives you the weight needed. If its a wheel (steel) that you can hammer on weights to the inside and outside lips than thats all you need to do. If its an aluminum wheel that you need to use stick on weights in different locations or a combination of hammer on and stick on weights there is a button you press that moves it thru a display showing 4 different scenarios. As I was moving thru the different scenarios I notice a decent amount of difference between the amount of weight it was calling for. Its seems like on the initial spin where the weights would be attached to the farthest from center point, the weight was much less. As the weight moved closer to the center and closer to the face of the wheel it took much more. I am guessing because the mass of the wheel is the face and it would take more to make a change happen? Any other insight? Just trying to learn something.

 

[vavet]

Just like a longer wrench gives you more leverage, moving the weight farther away from the axis of rotation means you don’t need as much weight.

 

[rlitman]

The static balance part is pretty simple. They use a tachometer that tracks how fast the wheel spins. When the heavy side is headed down it spins faster, and when its going up the wheel slows down. Plot that on a graph, and the magnitude of the sinusoidal component will tell you how much torque the imbalance is applying, and it's phase shows you where the weight goes.

Then, as vavet pointed out, knowing the radius of the weight circle lets it calculate how much weight will counteract the imbalance when placed on the rim at that radius.

The dynamic balance part (where it decides how much weight goes on the inside vs the outside) is a little more complicated. It requires force sensors that pick up on when the wheel is pulling up or pushing down on the spindle (or side to side, it doesn't really matter). And this is where rim width and offset are used to calculate the weight needed. After this, the problem is the same as the above, matching a weight against the magnitude of the sine wave.

Calibration of the machine is how it matches it's sensors against a known weight, so it now can tell exactly how much weight to apply (as opposed to only knowing where is goes, and whether it needs a lot or a little in a relative sense). i.e. how big is the sine wave when the calibration weight is applied at a known radius.

Interestingly, calibration does NOT have anything to do with the machine knowing if your wheel is in balance or not. No matter what the calibration is set to (correct, or way off), if the wheel reads 000-000, it's balanced (at least within the limits of the hardware) and that's that, because when the wheel is balanced, those sine waves will effectively be flat.

 

[gregs]

Thats kinda what I figured. I know the calibration has been discussed here a few times. I sorta went thru some of this a few years back when I bought and repaired the balancer, but as time goes by you can forget things. Thanks

 

[jsaw]

I was told that the shaft the wheel mounts on is longenough so that it extends far into the machine, there is a sensor on the end of the shaft inside of the machine. it measures the imbalance of the wheel