Dynamically Balanced Within 2 Oz

[PETE1968]

good morning
I have a part that is calls out to be balanced dynamically with 2oz at about 2200 rpm. this part assly. to another part that is a bigger dia. my ? is i have to elongate a couple mounting of holes that where put in out of position will this throw the part out of balance . all remainingg mounting hole are correct

thanks heavy

[Geof]

It depends where the two holes are. If they are opposite each other the part will not be thrown out of balance; if they are side by side it will.

It also depends on how much material you remove and how far away the hole is from the center; a lot of material removed a short distance from the center will have less effect than a small amount removed further out.

However, having said that, I think you should do some deep thinking. If I sent a job to some shop and then a year or two later the part blew up at speed and an investigation found some of the mounting holes were oversize???? I think I might go looking for someone to crucify.

Not saying this is or is not likely but sometimes taking in in the teeth now and redoing the job is better than having the risk of taking it in the neck at a later date.

[NC Cams]

From the "2 oz at whatever rpm" spec, it sounds like they are specifying the amount of net reaction force at force application point, probably the bearing or bearing journals.

This force is usually the result of an unbalance load that is eccentrically mounted from the actualy axis of rotation.

Example. if you have a 2 oz imbalance that is 0.100" off center (or 0.2 oz-in), you'll end up with less net imbalance than if you have the same 2 oz located 0.5" off center (1.0 oz-in).

Basically, the imbalance force will go up in proportion to the amount eccentricity but it will go up in forc MUCH more and faster the higher up in speed you go. If memory serves correct,if you double the speed you'll quadruple the imbalance load. Thus, a shaft with 1.0 oz-in of imbalance can acually generated way less imbalance at 500 rpm than a 0.2 oz-in imbalance at say 4000 rpm.

Not only do you have to deal with the position of the imbalance but also the shaft speed as they interactively combine unless you are willing to take the chance of oversimplifying something that is really much more complex.

The subject of "static" versus "dynamic" balance was one of the hardest classes that I barely got thru in engineering school and cringe when guys ask for simiplified answers/explanations to what could be a lurking, complex problems

[Kiwi]

I'm no expert at this but here is a program I wrote for a local balancing specialist to show force and centre of gravity.
Total weight (Body Mass), RPM and a balancing weight (W) need to be entered.
You may get some indication of the forces etc.

If the amount of material you are removing is the same distance from the centre and evenly spaced eg. 180deg for 2 holes, 120deg for 3 holes etc. you shouldn't have any trouble.

[PETE1968]

thanks for the replies I will found out today what the customer is going to say.I hope the dont scrap the part. we have been real slow and it seems like more mistaked happen when you dont have any work than when your covered up. Go figure

[PETE1968]

well it from the customer saying i was going to have to eat it .then he called back and said weld it. then when i was down at the weld shop the custonmer called and said elongate it just in the nick of time. good thing is we were able t0 save the part. must be living right.

[Geof]

Originally Posted by PETE1968 well it from the customer saying i was going to have to eat it .then he called back and said weld it. then when i was down at the weld shop the custonmer called and said elongate it just in the nick of time. good thing is we were able t0 save the part. must be living right.

Get it in writing or make sure you have some proof that it was the customer's decision. Anyone who suggested welding to fix something like that in my opinion does not have any common sense.