Hi to all,
recently I've been doing on site balancing at full speed with excellent results. the unit I'm using also can be attached to our workshop balancer(IRD 50) soft bearing type directly to the tranducers.If I run a rotor up at 300 rpm on the balancer, is the velocity mm/sec going to be similar at the motors rated speed,say 1500 rpm, and would the correction weight change? Most workshop balancer computers have an allowance for this, my hand held is designed for full speed balancing, so I'm relying mainly on the velocity readings,unbalance in grams and pedastall movement to perform slow speed balancing.
Any input would be apreciated.

Max,
Unbalance will increase with the square of speed. So if you have 1 mil at 300 RPM, it will not be 1 mil at 1800 RPM. I also do shop balancing, in a balancing machine, as well as field balancing. When we balance in the stand, we balance to a tolerance such as G2.5 or 4w/n which is oz/in of unbalance. 1 oz/in of unbalance at 300 RPM is 1 oz/in of unbalance at 1800 RPM. I like to balance things as fast as I can within safe limits but I have balanced rotors at 600 RPM that run 3600 RPM and have had no vibration problems on the test stand.
Hope this is helpful.

Ronnie

Thanks ronnie for your input,I agree with what you have said but I'm having trouble trusting this field balancing unit with shop balancing, reason being is the ISO value its calculating is not correct because it only caculates it at the balance speed not rated speed. ie: for a rotor weight of 250kg, balanced at 260rpm, shows residual unbalance of 60grams, .8mm/sec movement and an ISO of G .07.
I know for this rotors rated speed of 1500 rpm 63 grams of unbalance is allowed for G1.0 ISO.
Are there any calculations I can use to check that the mm/sec reading is converted to grams of unbalance correctly?
if I know that the grams of unbalance is definatly correct then I can do the ISO calculation manually.

Unbalance units would be m-l, such as gram-cm, not just grams.

Our company has done both, but the majority of our balancing jobs are now done in the field. you can also shop bal., then trim in the field at operating speed.

Assuming the rotor is "stiff", if it is 'balanced' in the shop, it will be balanced in the field. Imbalance is a physical condition.

If given the time, I like to balance until I can no longer get a stable phase reading (i.e., there's not enough 1x for the meter to lock onto) -- then I know it can't get any better. Usually, this is an extra run or two.

Max,

The permissible residual unbalance G1.0 for a rotor weighting 250 kg. turning at 1500 rpm. must be 63 gr. (assuming the correction radius are at 25 mm.)

This 63 gr. must be the reached target when balancing the rotor on the balancing stand when it's turning at 260 rpm. So after the full process of balancing, you put correction weights in two planes I supposed. (assuming this rotor is rigid and long enough) Next, if you perform a trim balancing, the result of calculations demands new correction weights. If the amount of these weights are lower or equal to 63 gr. means that the quality grade G1.0 is achieved. The amplitudes are then usually very low. When the rotor is reinstalled in the field, generally the amplitudes increase a bit but are acceptable. There is no link of what will be predicted to be concerning the vibration on the balancing stand vs in the field since it depend on many variable conditions like damping, temperature, resonances etc...

Max,
I think Alec stated it very well. If your final corrections is less than or equal to your allowable unbalance then you should be in tolerance for actual run speed. Just make sure you divide your allowable amount by the radius you are balancing at. For example if your correction is at 10 inch radius, your allowable unbalance will be .1 of your calculated allowable oz/in tolerance. Sorry, my mind doesn't work in metrics.

Thanks,
Ronnie

Ronnie,
I realize now after rereading that my english is not very good and a correction radius of 25 mm. is nonsense. You're not alone to have problem with metrics!

Alec,
Actually 25 mm is approximately 1 inch (.984249) and all calculations should be related to 1 inch when considering oz/in tolerances. Although 25 mm is not a probable radius to add weights in most cases, your statement was correct for calculating the allowable unbalance as long the correction is made for a different radius by dividing the larger radius into the oz/in tolerance or in Max's case the oz/25mm.<-- Nor sure about that one. So if you are making your correction at 250 mm then divide the allowable tolerance by 10. Now I am sure that is all as clear as mud.

Thanks,
Ronnie

No no, you're right, all is clear. The allowable unbalance is 63 gr-in or 2.22 oz-in. And as you said: If the correction is at 10 inches radius, the allowable unbalance will be .1 of the calculated allowable oz-in tolerance, ie .222 ounce.

Thanks for all your input guys, I'm pretty sure I can work around this problem with the advice you all have given.
Regards
max