And I don't mean trying to stand up straight after a night on the booze!
Probably one of the most important parts of building any turbo is to ensure the rotating parts are properly balanced. This is critical to ensure the long life of the turbo and also to make sure it runs quietly.
So what do I mean by unbalanced? Well it's basically and excessive amount of weight at a given radius. Turbo balance is expressed as milligrams inches, Ie if you took a perfectly balanced turbo and put 1 milligram at 1 inch on a blade it would then be 1 milligram-inch out of balance.
There are 2 types of balancing techiques, the first is component balancing and the other is assembly balancing.
Component balancing is literally the individual balancing of the main rotor parts. These are the turbine shaft and the compressor wheel. It is critical that these items are balanced, Items such as the nut and thrust spacer are not required to be balanced as they are machined at manufacture to specific limits well within the spec of a balanced turbo. However, when all these items are put together a certain amount of new unbalance is created and this is called "stackup" Literally meaning stacking up all the components unbalance "milligram-inches" together to make a combined milligram-inch unbalance.
Stackup balance isn't really a major concern as the turbine and compressor wheels are balanced well within the tolerance needed for an assembled turbo, so even by adding the nut and thrust spacer the stackup balance is still well within limits required for the turbo and wont cause a problem.
Unfortunately with small turbo's, the light mass of the components and high speeds it isn't sometimes enough to balance the components and allow for stackup balance and an effective way to overcome this is to trim balance the components. This is basically taking the complete rotating assembly: turbine, compressor, nut and thrust spacer and balancing it further to over come the stackup balance.
Here is a picture of a trim balance.
While this method is great for balancing a turbo, it still requires the turbo rotor to be dismantled and reassembled in the CHRA after the balance has been completed. Marking up of the components is critical and correct assembly is also vital. This is the method that is being used in my turbo builds, and with a little skill and a steady hand very good results can be achieved this way. The main plus point of this method is that it is considerably cheaper and for the small time builder offers the best way to achieve a balanced unit. And from having spoken to well known turbo builders they have used this method very successfully in their builds. I would also like to thank Slappy-Dunbar as well for some very useful information he provided and almost being gracious enough in sharing it. I know many of turbo builders wouldn't have, trade secrets and all that nonsense!
The other type of Balancing is the CHRA balance, this can be done in 2 ways the VSR ( vibration sort rig ) or low speed ( balancing Machine)
The VSR machine is EXPENSIVE! You basically place your unbalanced CHRA assembly in the machine, and it spins the turbine wheel at high speed, Oil is fed to the journals and the machine senses the vibration. The computer then tells machine operator where it is unbalanced and he will grind small amounts of metal off the nut or nose of the compressor wheel to bring it into balance. The only down side of the VSR is that it is only a single plane IE just the compressor is worked on to achieve balance, it does have the advantage that as it is spun at high speed and any noises or oil leaks can be found easily.
The Balancing machine or Low speed rig again balances the complete CHRA but this time does so at a relatively low speed normally belt driven. No oil is needed and 2 Planes can be balanced Ie turbine and Compressor, offering a more accurate balance in theory
Either way will eliminate the stackup balance as well/
One thing definatly worth noting is that, balancing a turbo at high RPM doesn't give you a better balance than balancing it at slow rpm... Confused??? Remember we are talking about a weight at a given distance! 5milligram - inches at 1000 rpm is still 5milligrams - inches at 100,000 rpm, the weight hasn't change but the force has. The most important factor in balancing is to have a machine that is accurate for it's given speed. It's better to have a machine more accurate at 1000 rpm than one less accurate at 100,000 rpm.
So why choose VSR over Component or trim balancing? well the simple answer is cost and time.
A VSR machine is very fast and efficient way of producing mass produced items accurately. CHRA's can be put together quickly, with no need to take special care in assembly, the balancing is all done in one go and the item is finished.
Trim balancing can offer a more accurate balance due to the 2 plane balance but relies on human skill to balance, assemble, trim balance, disassemble and reassemble in the CHRA. For a manufacturer or large turbo builder this is time consuming and impractical, but for the bespoke builder it is ideal as balancing costs are reduced and varying spec'd turbos can all be built separately with out the need for expensive VSR machines.
I hope this has covered it and helps put some peoples minds at rest who may think that component balancing is an inferior technique to VSR.
Here's a picture of a VSR machine. There are also plenty of youtube clips on VSR, Trim and Component balancing if your that way inclined ;-)