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CHRA Q & A: All About Balance

Posted by Scott Goldfarb on

If you have questions about a turbocharger CHRA, why it is balanced, and what the balancing process is like, you have come to the right spot. Find out all you need to know about the importance of CHRA balancing here.

Q: What is the CHRA?

A: A Center Housing Rotating Assembly, also known as a CHRA or a core assembly or turbo CHRA, is a major component of a turbocharger. It refers to the shaft wheel assembly installed in the center housing but without the compressor and turbine housings. It directs exhaust gas to power the turbocharger.

The CHRA is a rotor that rotates at high speeds on a pressurized oil film. It is held in a bearing system. It is typically made of gray cast iron or aluminum.

Q: Why does the CHRA need to be balanced?

A: Any object that rotates around a central axis will inevitably have the potential to become unbalanced. The unbalance causes vibration at higher RPMs. When an object is balanced more accurately, the vibration, also known as resonant frequency, is reduced and not experienced until a higher RPM is reached.

Designed to withstand temperatures of nearly 1500 degrees Fahrenheit while rotating at over 3,800 rotations per second, turbochargers function under extreme conditions. Because of these extreme conditions, the more balanced and precise the CHRA is, the better it will be able to perform.

Q: What are the tolerances for CHRA?

A: CHRAs are manufactured to some of the smallest tolerances in the automotive industry, as low as 0.002 mm. During the manufacturing process, the turbine and compressor wheels are also balanced on specialist equipment after being precision engineered.

Q: What damage can be caused by not balancing the CHRA?

A: An improperly balanced CHRA will create vibrations in the turbocharger. This will cause a whistling sound and can break down the oil film in the bearings. This oil film breakdown will cause the bearing system to fail early. The life of the turbocharger will be cut significantly shorter. If used while the CHRA is not balanced, it could cause the entire turbocharger to fail within days.

Minor imbalances can cause lower horsepower from the engine and less boost from the engine. Both of these, however, if left unattended, will lead to further damage and eventual bearing failure (and turbocharger system failure).

Q: What happens if you use lower-quality components to balance a CHRA?

A: Lower quality components can be used in a turbocharger and CHRA, but they are much more difficult to balance. They can lead to premature failure of the turbocharger.

Q: Are the different components of a turbocharger balanced?

A: Ideally, each component of the turbocharger would be balanced before being assembled to reach optimal balance once the entire turbocharger is put together.

Q: Which CHRAs need to be balanced?

A: Any time a turbocharger is tinkered with, the CHRA needs to be balanced. The act of loosening and retightening the shaft nut can affect the balance of the CHRA, even if no parts were changed.

Turbochargers also need to be balanced when they experience foreign object damage. Foreign object damage could be a rock that was sucked into the compressor wheel or part of a spark plug that has broken off and fallen into the turbocharger.

Q: How are CHRAs balanced?

A: CHRAs are balanced on high-speed balancing machines that are capable of having the CHRA accelerate to its top operating speed. This machine will measure any unsatisfactory vibrations as the CHRA passes through resonant frequency to help it be calibrated correctly.

Before they are put on the high-speed balancing machines, though, they are tested at lower speeds to gather imbalance readings. Once those readings are adequate, the CHRA is tested on the high-speed machine.

Q: Does the amount of unbalance change at different speeds?

A: The amount of unbalance does not change, but rather the force created by the unbalance increases exponentially.

Q: When a turbocharger is rebuilt, at what point in the rebuilding process are the components balanced?

Often rebuilt turbochargers are balanced multiple times during the rebuilding process. Examples of when the CHRA would be balanced include:

  • Balancing the turbine outboard and inboard sides.
  • Balancing the compressor wheel outboard and inboard sides.
  • Conduct a balance check of the rotor assembly.
  • Perform a final balance in the high-speed balancing machine.

The final balance might find things such as an imperfect bore on the compressor wheel, imperfect thrust parts, or the mating surfaces on either side of the compressor wheel are not perfect.

If you are in need of a new or rebuilt turbocharger for your diesel engine, check our online inventory.

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