Why do you need to Balance a CHRA / Core Assembly?
Natural demand in the turbo repair industry over the past decade has led to there being several high speed VSR machines on the market. All reputable repair workshops now have high speed balancing equipment as standard and repairing all types of turbos is standard practice in most countries.
Balancing Machine Variations
At Melett, 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 are 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:
- 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.
- 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.
Why do you need to balance a CHRA?
The turbo repair industry is a well-established global market, which goes back to the 1960’s. Up until the late 1990’s, the turbo repair market was predominantly involved in the repair of larger commercial vehicle turbos. Since the explosion of common rail turbo diesels for passenger cars and the more recent trend of downsizing gasoline/petrol engines through turbocharging, the repair industry has also significantly increased to deal with the large percentage of much smaller turbos. This in turn has significantly increased the demand for high speed balancing.
A complete turbocharger is essentially a Centre Housing Rotating Assembly (CHRA or core assembly) with housings fitted to each side. The turbine housing directs exhaust gas to power the turbine wheel, the compressor housing directs air into the engine. The turbine housing will have a wastegate to control the turbine speed, or as is more common on diesel engines, there is a variable vane mechanism giving much better control of the turbo, reducing lag and improving performance. The wastegate or vane mechanism is controlled by an actuator – which could be controlled either electronically or by a vacuum system.
The CHRA is a rotor (turbine wheel one side, compressor wheel on the other) which is held in a bearing system allowing it to rotate at high speed on a pressurised oil film similar to the engine. Turbo remanufacturer’s traditionally strip the complete turbocharger and CHRA, examine all the individual components, replacing these where necessary, and install brand new bearings and seals.
In exactly the same way in which a wheel needs balancing after changing the tyre, the CHRA also needs balancing after it has been dismantled and rebuilt.
It is important to understand, all objects which rotate around a centre axis have a certain amount of unbalance and this unbalance will create a resonant frequency – or vibration – at specific rpm. The speed and strength of the vibration is directly related to the amount of unbalance within the CHRA. To compare this to the car wheel, the vibration would be felt in the steering wheel. However, if the wheel is balanced more accurately, the resonant frequency is both reduced and moves to a higher rpm – which is above the fastest speed of the vehicle therefore solving the problem.
The turbo CHRA is a complex piece of machinery which has been designed to operate at temperatures up to 800°C whilst reaching rotational speeds in excess of 230,000rpm (that’s over 3,800 revolutions per second!).
To operate under such extreme conditions each component in the turbocharger, which make up the main rotor in the CHRA, is manufactured to some of the tightest tolerances in the automotive industry (as low as 0.002mm). The turbine and compressor wheels are also precision engineered and balanced on specialist equipment as part of the manufacturing process. However, when the rotor is assembled, the accumulation of the remaining tiny imbalance in all the parts can create a rotor, which is out of balance, and will vibrate at critical speeds within the CHRA operating range.
In the same way a car wheel is balanced at low speeds, traditionally, the larger commercial turbo rotors were balanced at lower speeds. By achieving a certain level of balance precision at low speed, the resonant frequency of the rotor would be well above the operating speed of the turbo. However, as turbos became smaller, the rotational speed increased significantly so that the small turbo CHRA now passes through resonant frequencies within its operating range. To ensure these smaller CHRA do not vibrate excessively during operation, they must be balanced on a high speed balancing machine which is capable of accelerating the CHRA through its full operating speed and measuring the vibration as the rotor passes through resonant frequencies.
In over 99% of tests, the unbalanced CHRA has vibrations outside acceptable limits and must be further balanced to ensure the turbo will operate correctly.
Failure to balance the CHRA on specialist equipment can cause excessive vibrations as the turbo speeds up, resulting in noise (whistling) and a breakdown of the oil film in the bearings. This in turn will cause premature failure of the bearing system, often with no obvious signs of lack of lubrication or oil contamination.
Importantly, the life of the turbo will be significantly reduced if the CHRA is not properly balanced – in worst cases the turbo will make unacceptable noise during operation and fail within days.
As a result of the high rotational speeds, the level of precision in the manufacturing of replacement turbo parts is extremely high. Quite simply, using lower quality repair parts makes the CHRA harder to balance. This in turn means that low quality CHRA are often not balanced to the correct level and will fail much earlier than expected.
Using parts which are not precision manufactured to the specified flatness, tolerances or dimensions, can lead to an accumulation of component imbalance issues, again causing premature failure of the turbo.
CHRA, which have a turbine wheel diameter less than around 50mm, will pass through a resonant frequency during normal operation. As a guide, this will cover all turbos fitted to diesel engines up to about a 4L capacity.
All CHRA must be rebalanced – even if all the same components are used – the act of loosening and retightening the shaft nut will change the precise balance of the rotor – so it is not possible to carefully dismantle a turbo, fit a new seal and rebuild. ALL CHRA must be high speed balanced.
The key to maximising the lifespan of a repaired turbocharger is to use quality, precision manufactured and balanced components, use the appropriate CHRA high speed balancing equipment and ensure the careful assembly and calibration of the turbo.
VSR Balancing Machine Suppliers
At Melett, we are often asked to recommend VSR machine suppliers for turbo repair.
There are several machine manufacturers who are currently supplying the turbocharger aftermarket. The machines on offer vary considerably in price and also operate slightly differently. We would recommend looking at the different machines to see which fits your expectations and budget. You should also consider the availability and cost of adapters for the different turbo models as this will be a significant part of the on-going investment. Also review the availability and cost of training for machine operators, the flexibility to create or modify run up templates and any options available like automatic run-up, which we consider to be an important feature.
There are several other machines on the market so please note that this is not an exhaustive list. The three companies below are well known to Melett and we consider them partners in the industry. As such, this is intended to be an unbiased review.
SCHENCK RoTec Gmbh
Schenck is by far the largest rotor balancing machine manufacturer in the World. With a head office is Germany, the company has a large global presence supplying all sizes of machines to many different industries including automotive, aerospace and power generation. As part of their organisation, Schenck have a Turbocharger Division which is focussed mainly on OEM turbocharger manufacturers, supplying fully automated turbine and compressor wheel rotor balancers as well as fully automated, pallet loadable CHRA balancers for high volume OEM production.For the turbocharger aftermarket, Schenck offers a rotor balancer, which is very popular with turbo repairers for re-balancing turbine and compressor wheels. Schenck also offer a workshop VSR balancing machine. The range of adapters tends to be limited although this may have improved. As World leaders, their machine is one of the higher priced in the market.
|Turbo Technics are a UK company who have been offering their VSR machines since the 1990’s. Many long established turbo repair companies recognise Turbo Technics as the founder of the VSR machine and the brand is well established in the market. The company owner – Geoff Kershaw – originally developed the machine when working for Garrett and developing the turbocharger for Saab back in the 1980’s. Turbo Technics also supply some of the turbo OEM companies with special machines developed for mass production.Melett operate a range of Mark 4, Mark 4P and Mark 4SA Turbo Technics VSR machines in our CHRA production department, covering volume production runs and flexibility with adapters. Turbo Technics have developed a good range of adapters for the various machines. The machine prices tend to be in the middle range of machines available.
|CIMAT is a well-established Polish company who has been producing balancing machines for over 30 years. Their product range covers many industries from balancing small rotors through to large rotors for generators weighing several tonnes. As a specialised offering, CIMAT has developed a range of balancing machines specifically for the turbocharger aftermarket, offering everything from rotor balancers to high speed VSR machines and twin machines, incorporating both balancers in one unit.The company is well known throughout Europe and has a large customer base in the industry. Since 2007, Melett has had a good relationship with CIMAT as the company has acted as the Melett distributor in Poland, supplying both machines and Melett repair parts. CIMAT has a good reputation for customer service and has a good range of adapters, which give coverage to many turbo models. CIMAT machines are priced competitively and are usually one of the lower priced machines on the market.