VSR Turbocharger Core-Balancing Machine
The high speed balancing process
Turbochargers are assembled from component parts which are separately balanced using conventional low-speed, hard bearing balancing machines, and both the turbine wheel and compressor wheel are normally balanced in two planes. At the turbine end, the balance is not affected by assembly into the cartridge, but at the compressor end, small errors in the wheel, the shaft, the thrust collar and the nose nut can cause an accumulation of balance error. At high speed, this can result in a noisy turbocharger, and in a severe case can cause premature bearing failure.
This imbalance can be corrected by running the assembled cartridge at high speed on a flexible suspension, measuring the vibration response, and either changing the assembly position or removing metal from the nose to achieve an acceptable balance. This operation usually requires the unit to be run at speeds close to the normal service operating speed, typically 100,000 to 300,000 rev/min, depending on wheel size.
In operation, the CHRA is mounted in a slave turbine housing adapter, using quick-release clamps to hold it. The turbine housing is in turn attached to a flexibly-mounted air nozzle assembly which directs air into the housing, rotating the turbine shaft. An accelerometer attached to the flange measures the vibration of the complete assembly.
The compressor wheel is covered by a shroud for safety, and to reduce air ‘windage’. The nose nut or shaft end is magnetised, and a coil in the centre of the shroud converts the rotation of the magnetic field into a voltage signal, which is processed as a speed signal.
The accelerometer and speed signals are then processed electronically to remove unwanted frequencies, giving a display of vibration level (g-level ) against speed as the CHRA is accelerated up to the maximum speed. At the same time, the angular difference between the speed signal and the accelerometer signal is displayed to indicate the imbalance position.