Boost Lab

TURBOCHARGER BALANCING

FACTS AND MISCONCEPTIONS

One of the most important parts of building a turbocharger that will operate quietly and efficiently for a long time is being sure that the rotating components are properly balanced. The problem is that many people do not know what “properly balanced” really means.

Unbalance is usually expressed as the product of weight and radius. If a one gram weight is placed at a one inch radius on a balanced part, the part is said to be unbalanced by one gram-inch. Modern turbocharger tolerances are typically expressed in milligram-inches, or thousandths of a gram-inch. For example, the Garrett T-3 and T-4 frame turbos generally have component balance tolerances around .010 gram-inches (10 milligram inches).

One area of confusion about balancing is component versus assembly balance. Turbocharger rotating groups are made up of several component parts that are assembled to make up the rotating assembly. Of these components, only the turbine and compressor wheels are component balanced. Balancing of these components is critical, and must be done prior to assembly. The axial thrust spacers and compressor locknut are not balanced, and the mechanical fit of these components are subject to machining tolerance limits. When these pieces are mated a certain amount of “stackup” unbalance is introduced into the completed turbo.

Stackup unbalance is not a major concern with larger turbochargers. Typically, the turbine and compressor wheels in these turbos are balanced to a tolerance substantially closer than required by the assembled turbo. This way, when the components are assembled, the stackup unbalance is not large enough to cause a problem with the complete turbo.

With the increasing popularity of small turbos in automotive applications, stackup unbalance has become more of a factor. Due to the light mass and high rotational speeds of these small units, simply balancing the components to an overly close tolerance may not be enough. The typical symptoms of a slightly unbalanced small turbo are oil leakage from the ends of the bearing housing, and “screaming,” an unbalance induced vibration of the rotating assembly. The fastest, most effective method of eliminating the stackup unbalance that causes these problems is to trim balance the moving parts of the assembled turbo CHRA (center housing rotating assembly).

It is possible to build turbos that are well balanced without CHRA balancing. The turbo builder must be very critical during the inspection and assembly portions of the rebuild to assure the quality of all the components, and their fit with each other. Many of the best “custom” turbo builders do not CHRA balance due to a combination of critical inspection and careful assembly procedures. Higher volume builders of turbos, and shops desiring to have complete knowledge and control of the assembly process, perform some type of CHRA balancing.

There are basically two types of CHRA balancing, high speed (VSR) and low speed (balancing machine). The VSR (vibration sort rig) is a machine that uses compressed air to spin the assembled CHRA to a relatively high speed, while pressure oiling the bearings and sensing vibration of the unit. Small unbalance corrections are then made on the compressor nose or nut to fine or trim balance the unit.

Balancing machine CHRA balancing consists of mounting an assembled turbo CHRA in a conventional two plane dynamic balancing machine. The rotating assembly is then driven at a relatively low speed by belt or air, and unbalance readings are taken on both the compressor and turbine ends of the rotating assembly. Oil is not pressure fed into the turbo, as the shaft is prelubricated before the balancing operation. The low speed and short cycle time preclude the need for pressurized lubrication.

Either type of CHRA balancing will generally eliminate stackup unbalance to an acceptable degree. Machine balancing has a slight advantage, in that the rotating components are dual plane balanced, as opposed to single plane balancing with the VSR. The main advantage of the VSR is that the turbo has actually spun at high speed, so the operator may be able to hear unusual noises from the turbo, and in some cases the oil flow can be checked (though this is not very reliable.)

Another common misconception about balancing is that balancing at higher speeds results in closer balancing. This is not inherently true. A rigid rotor that is out of balance by 10 milligram inches at 1000 RPM will be out of balance by 10 milligram inches at 100,000 RPM. The force created by a given amount of unbalance increases exponentially as speed increases, but the absolute amount of unbalance does not. It is critical that the balancing equipment being used has sufficient sensitivity to balance the rotor to the necessary tolerance at the desired balancing speed, but balancing at operational speed is rarely advantageous. The logistic and safety considerations of very high speed balancing rarely outweigh any accuracy gained.

In conclusion, the key to maximum life out of a turbocharger is proper selection of components, precision balancing of those components, and careful assembly of the turbo. An additional balancing operation performed on the completed turbo is not absolutely necessary in most cases, but it does provide a higher degree of confidence in the final product.