Using the TT VSR™
The BALANCING PROCESS
The purpose of the ‘balancing’ process is to control a turbo’s vibration level, both to reduce noise and to prolong bearing life. Turbo noise results from vibration generated within the turbo being transmitted to the engine through the turbine housing and exhaust mounting, causing parts of the engine and pipework to vibrate and emit sound to the surrounding environment – typically characterised as a ‘wolf howl’. This vibration force is initially transmitted through the turbo bearings, and high levels can cause excessive loading and premature bearing failure.
As such, vibration is the key parameter to control, and the primary purpose of the process is to reduce vibration to a level which results in a CHRA which functions quietly and does not over-load its bearings.
Some companies do use grm-mm, but this cannot be measured directly and can only be deduced from the vibration reading using a transfer function. To do this, it is necessary to characterise each turbo type separately, either across the speed range or at selected speed points. Turbo Technics view is that this is an unnecessary process and only serves to add complication.
Introduction to the principles of
turbocharger balancing using the Turbo Technics VSR™
Turbochargers are assembled from component parts, which are separately balanced using conventional low-speed, component balancing machines, and both the turbine wheel, and compressor wheel are normally balanced separately 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 shaft, the compressor wheel, the thrust collar, and the nose nut can cause an accumulation of balance error.
The rotor system 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 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 250,000 rev/min, depending on wheel size. Conventional practice is to remove material from the nose nut to achieve an acceptable vibration level.
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 mounting flange measures the vibration of the complete assembly.
The admission of air to the turbine is controlled, allowing the CHRA to be accelerated slowly across the speed range.
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 within the speed sensor converts the rotation of the magnetic field into a voltage signal, which is processed internally as a speed signal.
For balancing purposes, only the vibration signal at the rotational speed is of interest. The accelerometer and speed signals are therefore 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 (the ‘clock’ position) to indicate the imbalance position for correction.
The typical response of a turbocharger rotating assembly will normally exhibit two peaks, or resonances, as shown and the apparent ‘clock’ position will change with speed. By moving the cursor to a selected speed point, the imbalance position at that speed can be displayed, and material removed to achieve the desired vibration level.
Advanced Balancing Method
A refinement of the technique, which is applicable to some turbo types, allows the compressor wheel to be balanced in two planes on the VSR™. The technique is to balance the rotating assembly, at the higher speed peak only, by removing metal from the nose nut in the normal manner until a satisfactory balance is achieved, ignoring the lower speed peak. The assembly is then balanced at lower speeds by removing metal from the back disc of the compressor wheel between the blades, a technique that has been employed by TT since 1999.
As a further point, any VSR™ will show the vibration signature of the complete assembly – both CHRA and mounting combined – and these cannot be separated to show only the CHRA. The design of the housing is therefore important and must be included when setting vibration limits.