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.

Turbo repair > Turbo balancing machines > Why do you need to balance a core?

The variable nozzle (also referred to as a variable geometry), is designed to change the exhaust gas inlet area with the engine speed to closely match the desired boost requirements of the engine. For low speed response, the nozzle vanes move to the ‘closed vane’ position to reduce the nozzle area – this increases gas speed through the turbo giving improved response at low engine speeds – similar to squeezing the end of a hose pipe to make the jet of water more powerful. As the engine speed increases, the actuator moves the nozzle vanes to the fully open position to maximize the exhaust gas flow

Turbo repair > Air flow rig

During the design review of the GT15 nozzle assembly, Melett Engineers analysed failure modes of many units. They concluded, the preferred route for the turbo aftermarket was the solid vane design for the 753420-* GT1544V nozzle ring, rather than the original welded strip metal design. This provides a stronger repair solution and allows you to offer your customers an upgraded repair.

Design Improvements 

Trim is the area ratio used to define the turbine and compressor wheels. To calculate the trim you use the inducer and exducer diameters.

For Example: Inducer²/Exducer² = Trim

When a compressor wheel has been weakened, it will begin to show signs of fatigue.
In terms of the compressor wheel fatigue, the blades are exposed to a continuous cycle of positive and negative stress caused by the wheel spinning fast and then slow. As the compressor wheel reaches full speed, the blades bend back and then as it slows down they bend back into position, repeat this over many repetitions suction is created which creates negative stress pulling the blades even further in.
Eventually the continuous stress will become too much and the blades will reach their endurance limit and break causing the turbocharger to fail.

Why do turbos fail? > Hidden Dangers

Flatback: Earliest design of compressor wheel and still used by some manufacturers. flatback
Superback: This design was introduced due to the increased speeds which the turbochargers rotate, because of the speed increase the force on the compressor wheel increases significantly, in particular the exducer diameter of the compressor wheel suffered the most. This is the point which rotates the fastest and is therefore under the most stress. The Superback reinforces the back face of the compressor wheel preventing the compressor wheel tearing from the bottom up. superback
Deep Superback: An exaggerated design of the Superback, generally used on more recent applications. Again, one theory is due to the increasing rotation speeds of the turbo. deep-superback
Deep Superback – Extended tip: This design promotes greater airflow providing a faster boost response at lower engine speeds. The extended tip design increases the efficiency of the Superback compressor wheel at higher boost pressures. MFS – Machined from Solid compressor wheels are becoming increasingly popular as these new developments from the OE’s continue to arrive in the aftermarket. The wheels are fully machined and balanced on leading 5-axis machining equipment and precision balanced on fully automated balancing stations with auto-correction.

 Turbo components > Compressor wheel > Compressor wheel variations PDF

Machined from solid compressor wheels are becoming increasingly popular as these new developments from the OE’s continue to arrive in the aftermarket. The wheels are fully machined and balanced on leading 5-axis machining equipment and precision balanced on fully automated balancing stations with auto-correction. Melett BV40 Machined from Solid (MFS) compressor wheels replace the original BV40 compressor wheels and feature the new extended tip design.

Turbo components > Compressor wheel

Traditionally, the GT15 shaft and wheel (1102-015-439) had a slender shaft and open back. The more recent GT15-15 shaft & wheels (1102-015-442), have a straight shaft and and full back, which helps reduce the risk of fatigue on the inducer diameter.

Design Improvements > GT15-25 Slender shafts

Air Cooled: Ambient air and oil act as a cooling mechanism.
Water Cooled: High temperature applications need extra cooling to perform efficiently – more commonly used in gasoline applications.

Turbo Components  > RHF4 Air Cooled & Water Cooled Bearing Housings

Twin feed bearing housing incorporate two oil feeds to the journal bearing and use a twin grooved journal bearing/ Single feeds bearing housings have just one oil feed and use a single oil hole journal bearing.

Turbo Components > K03-4 Bearing Housings

Melett Minor Repair Kits – All repair kit parts excluding thrust flinger/collar/washer/spacer;

Melett Major Repair Kits – All repair kit components including thrust flinger/collar/washer/spacer;

Melett Universal Repair Kits – Similar to the major repair kit, however the universal kit includes each variation to accommodate for where there are several variations of a model i.e. flat or superback compressors wheels, straight or slender shafts etc.

Parts Production > Product Range > Repair Kits

The original design was prone to oil leaks at the compressor end as the piston ring seal wasn’t good enough. The upgraded twin ring is available on new complete OE turbos for most newer applications, Melett produce the twin ring flat back flinger to upgrade all old designs as well as new ones.

Design Improvements > BV/KP31 Thrust Flinger 

1st Generation slender shaft bearing with smaller lands, provides less resistance meaning increased run-up speeds. 2nd Generation straight shaft bearing with large lands, is used predominantly on larger applications.

Design Improvements > GT15 Journal bearings

Melett invests heavily in our own tooling to ensure each part is precision manufactured to the correct tolerances and specification.
• The tool for the RHF4 Bi-metallic thrust washer is a very intricate tool;
• Made in Japan, the multi-stage press tool – starts with a strip of bi-metallic material (copper and steel), which then goes through 9 different stages to create the finish product;
• Producing the correct tooling requires significant investment and usually cost tens of thousands of pounds.

Devil is in the Detail

If a standard screw is used, it is possible for oil to work its way up the thread and leak into the compressor cover. To prevent this, the screw has a sharp feature under the head which digs into the seal plate creating the necessary oil seal;
Melett accurate replacement screws are made on special tooling produced to ensure our parts contain this critical feature

Devil is in the Detail

When using the 1102-015-324 large pad thrust bearing, it is important that this is used in conjunction with the 1102-015-220 thrust spacer and the 1102-015-242 thrust washer, rather than the 1102-015-240 thrust collar.
Although these may appear dimensionally the same at first glance, this is not the case as the thrust washer and thrust spacer offer a different clearance to the thrust collar. If the thrust collar is used this can create premature failure of the bearing system.

Devil is in the Detail

1102-012-100 – Replaces the original GT12 Z-Bearing used on the Smart Car and Ford GT1544Z turbos and is identified with a single ring on the OD. The full repair kit is 1102-012-755.
1102-012-102 – Geometrically the same as the 1102-012-100, but is now produced using an improved high strength material specification and is identified with two rings around the OD. (OD 11mm)
1102-012-103 – Larger OD than the original GT12 journal and is identified with two rings on the OD. High strength material (OD 12mm)

Turbo Components

The design of the Z-bearing is popular on the smaller turbos because it simplifies the assembly by incorporating the thrust bearing faces onto the ends of the journal bearing. The design means that there are fewer parts, and hence less room for accumulated tolerance and balancing errors. The detail of the oil ramping on the thrust face also includes some complex geometry rather than the standard linear ramping found on standard thrust bearings.

 Turbo components > Z-Bearings

Seal plates are prone to failing around the compressor wheel diameter, to improve the strength the Melett 1102-015-300 seal plate has been designed to incorporate extra strengthening ribs to help prevent failures.

Design improvements > GT15 Seal plate

If a standard screw is used, it is possible for oil to work its way up the thread and leak into the compressor cover. To prevent this, the screw has a sharp feature under the head which digs into the seal plate creating the necessary oil seal. Melett accurate replacement screws are made on special tooling produced to ensure our parts contain this critical feature

Devil is in the Detail > Seal plate screw

Critical Points – highlight the areas of the Thrust Bearing where if the material is too thin, material specification is not strong enough, or the incorrect manufacturing process is used, it will crack.

Why do turbos fail? > Hidden Dangers

Often, the use of poor quality replacement parts, which are produced using the wrong manufacturing processes and incorrect materials, creates turbo failures – but these are misdiagnosed as being a typical oil related turbo failure creating unnecessary warranty issues.

Technical articles > Did the turbo fail due to oil contamination

Oil ramping is designed to allow the thrust bearing to resist thrust forces put on it. It is essential to understand the importance of oil ramping and the part this plays in reducing wear and prolonging the turbos life. The ramp sizing is barely visible but is an extremely important feature on the thrust bearing. It creates a wedge of oil as the thrust component rotates and helps to force the parts away from the surface, reducing wear and therefore increasing the turbos life.

Devil is in the Detail > What is thrust bearing ramping

The original BV35, BV39, KP35 & KP39 thrust bearing was the 3 pad, 180 degree design, due to the open thrust pad area oil retention was an issue.

In newer OE turbo applications, the 180 degree design has now been phased out and replaced with the 360 degree design, this has resulted in better oil pressure and lubrication.

Melett Design Improvement > KP31/36/39 - 360ᵒ thrust bearings in place of 180ᵒ

Stamped Strip – Involves stamping the initial blanks out of a strip of brass material and is used for smaller thrust bearings. This method limits the strength of the material as this has to be weighed against the life of the stamping tool, an increase in strength in the material causes a massive reduction in tool life.

Design Improvements > K03/04 Thrust Bearing

Lower Quality Sample (RHF4)
To the untrained eye, initial impressions are they look ok, however looking at the parts in more detail;
• The surface is very pitted – pitting can score the thrust parts and lead to premature failure
• Badly de-burred
• The edges are not clean and sharp
• Over polished surface (trying too hard to look nice)
• No scratches to create surface tension
Melett Sample (RHF4)
• Clean, sharp edges
• Machined surface
• No de-burring required – when manufactured correctly this is not required
• Performs in the way it is designed to
• Scratches to create surface tension

Devil is in the Detail > IHI RHF4/5 Thrust Bearings

In newer OE turbo applications, the 180 degree design has now been phased out and replaced with the 360 degree design. This has resulted in better oil pressure and lubrication. Melett only use the 360 degree thrust bearing design in all BV35, BV39, KP35 & KP39 CHRA / Core Assemblies, giving the turbo longer life. A breakdown of the oil film will cause premature failure of the bearing system, often with no obvious signs of lack of lubrication or oil contamination.

Design improvements > KP31/35/39

Seal Plate is also known as a back plate or insert
Turbine Wheel – Shaft and Wheel
Compressor Wheel – Impeller
Bearing Housing – Centre Housing
CHRA – Core Assembly – Cartridge
Nozzle Ring Assembly – VNT – VGT – VVT -VTG
Compressor Housing – Compressor Cover
Heat Shield – Heat Shroud
Turbine Housing – Exhaust Housing

BV43 is a VNT version of K03. BV50 is a VNT version of K04.

Due to Homologation, once a design is approved by a vehicle manufacturer, the turbo OEM’s cannot deviate from this original approved design without seeking a revised approval, something the vehicle manufacturer will not test due to associated costs.
Melett is not restricted by homologation and where possible incorporates improvements in the original design and uses upgraded parts.

Design Improvements