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what are balance speed depend on???

what are balance speed depend on???

(OP)
we have 16kg shaft of compressor with 40000 rpm. if in document balance grade Is written 2.5G , is it mean 2.5 is acceptable for balance in 40000 rpm?
can we Equivalent this balance speed with another rpm?
if yes, how? and what can we do this equivalent?

RE: what are balance speed depend on???

Hesam ,
If you ask about balance grade and speed I think 2,5 balance grade is defined for 40000 rpm .
If you don*t have possibility to balance shaft on that rpm which depends on the characteristics of your balance machine it can be balance at a lower speed but limits have to be stricter.
I think it is more severe in a ratio between the operating speed and the speed at which you can balance but it needs someone experienced to confirm.
I'm just an electrical engineer and I don*t have specially experience in balancing .
Good luck !

RE: what are balance speed depend on???

G 2.5 is a quality grade. Quality grade is based on the type of rotor.
Check out page 2 here -
http://www.irdbalancing.com/downloads/techpaper1ba...

G 16 might be agricultural equipment and crushing machinery.
G 6.3 is a general machinery grade.
G 2.5 is for fancier machinery like machine tool drives and aircraft gas turbines.

The expected possible range of service rpm for G 2.5 parts classification is 200 to 60,000 rpm. To decide on a balance tolerance, the actual service rpm must be used. That tolerance (based on actual max service speed) should be used to balance the part, regardless of the rpm used for balancing in a balance machine (or even in service).
Check out page 3 at the link I provided previously, repeated here -
http://www.irdbalancing.com/downloads/techpaper1ba...

This all assumes "rigid rotor" behavior, which can often be a big bad assumption.

RE: what are balance speed depend on???

(OP)
ok let me consider the conditions:
Rotor weight: 25 kg
shaft speed: 40000 rpm
balance Grade: 2.5
shaft radius: 50 mm
according the previous link and formula we can calculate Uper or permissible equal to : 149 g-mm
if we want add weight at shaft balance way (in 50 mm radius): so 149/50= 2.98 g
so what is this 2.98 g exactly mean?
it is our acceptable unbalance weight but it is main question: is it value (2.95) different when i balance it in 3000 rpm in balance machine? or when i balance it in 12000 rpm balance machine?
in the formula it dosnt consider balance speed.

RE: what are balance speed depend on???

I come up with a maximum allowable residual unbalance of a little less than 15 g mm total unbalance, which would be apportioned between balance correction planes.

That is the balance tolerance, whether balanced at 495 rpm or 40,000 rpm.

Note also the changes in centering between shop "balancing" and service conditions must be held to << 0.6 um or unbalance equal to the entire tolerance will used up.

NOTE !! As mentioned previously a few times "Rigid Rotor" behaviour is assumed.
Does the compressor manufacturer provide any critical speed information?
Generally for a rotor to qualify for the rigid rotor merit badge the first critical speed must be at least twice the maximum operating speed.

RE: what are balance speed depend on???

(OP)
thanks guys
its not rigid of course. let me check critical speed.
i just new it has 2 critical speed.

RE: what are balance speed depend on???

Are you having the rotor balanced, or responsible for having it balanced?
Or, wanting to become familiar with some of the specifications?

Is this a commercially available product?
New, or being overhauled?

The compressor manufacturer should have a thorough overhaul and assembly procedure, and it should include balancing information as well.
Although it might be "send the rotor back to the factory."


Special procedures are needed to successfully balance flexible rotors .
The process can be quite complicated, even when for a fully developed mature product.
Stuff like -
- low speed balancing of individual components to their 2.5 tolerance
- careful assembly paying special attention to component fits and runouts at every stage of assembly
- a low speed balance of the assembly using very specific correction planes inboard from the ends, and possibly non contact sensors at positions along the rotor to detect "whipping."

The final step is usually trim balancing the installed assembly at operating speed.

Pretty complete but cumbersome flexible rotor information here -
http://www.dtic.mil/dtic/tr/fulltext/u2/a181731.pd...

"Balancing to ISO G 2.5" is just the iceberg's tip. Which is not trivial by itself either to accomplish, or to inspect.
It is WAY more than a computer printout of the final readings in the balance machine.

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