One of the issues we are often asked about is the balance of CHRA’s and why the balance graph varies between different VSR machines.

This is often a cause of concern to our customers because if they receive a balanced CHRA from Melett and try to confirm the balance of the CHRA on their own machine there are often different results from the graph supplied with the CHRA.

It is also common that the CHRA appears to be out of balance on their own machine and is therefore not acceptable.

Unlike low speed rotor balancing, the high speed CHRA balancing process is extremely complex. There is a large number of factors which influence the residual imbalance of the assembly at high speed – the most important point to understand is that when a CHRA is run up to speed on a VSR machine, the VSR machine frame and mechanism itself will also resonate giving a certain vibration reading.

One of the critical processes during the manufacture of a VSR machine is to find the exact resonance of the machine and using the software, cancel out this vibration profile for each CHRA run up. This then leaves only the vibration of the CHRA which is displayed on the screen.

Put very simply:

The problem the manufacturers have is that the machine vibration will vary slightly as a result of many uncontrollable factors from machine to machine. By understanding this, it makes these machine to machine variations much more understandable.

Two of the more significant variations are as follows:

1 The Adapter – From manufacturer to manufacturer, the adapter design is different. Also from adapter to adapter of the same turbo part number, each adapter has slightly different properties (casting wall thickness, plate thicknesses, exact material properties). As a result, each adapter will give a slightly different vibration during the run up in comparison to the original vibration, which was cancelled out by the machine manufacturer. The design of the adapter and the resulting vibration level will therefore vary between adapters.

2 Clamping Force – It is possible to get the same CHRA to produce slightly different graphs by clamping the CHRA tighter into the housing, as this affects the transfer of vibrations from the CHRA to the machine. The clamping forces will vary from machine to machine and operator to operator:

• As adapters generally use a taper-lock style fixing, slight machining variations in the taper components will produce very different clamping forces and will vary from adapter to adapter;

• Different operators will clamp the CHRA with different forces;

• The different taper design between machine manufacturers will produce different clamping forces.

Taking these into account, it would actually be very difficult to get two different balancing machines to give the same balancing graph for the same CHRA.

There is no easy answer to the concern that a previously balanced CHRA gives different readings on different machines. For consistency, Melett uses Turbo Technics balancing machines for all our CHRA production and to investigate this problem further, we have also purchased both CIMAT and Schenk machines and have them set up in our technical department so that any claims relating to balancing can be investigated on the appropriate machine.

One thing to note – generally, the variations between machines should not be that great as the machines are designed to achieve the same result.

One thing we are confident about – balancing failure is obvious to spot when analysing failures – the out of balance usually creates a taper shape to the journal bearings. Through years of production experience and thorough analysis of any returned warranties, it is extremely rare to receive anything back which has signs of balance failure – and where we see this, it is usually as a secondary failure rather than a primary. Nothing goes into a Melett box unless we are 100% happy with the quality – this gives you confidence to produce quality repairs.

Melett Technical Support. [email protected]