An important question — Did the turbo fail due to oil contamination – or did the thrust bearing fail causing oil contamination..?

This is a question turbo repairers need to ask when analysing warranty returns.

Over the years turbocharger designs have evolved as new technology becomes available. Materials and manufacturing processes have changed as the turbo OEM’s have had to reduce weight and learn to work with changing oil viscosities, as well as take advantage of new materials.  The important issue with thrust bearings is that different materials and manufacturing processes require different designs to achieve the same result.

In the aftermarket, we often see thrust bearings, which were originally designed by the OEM as high strength silicon brass alloys using a hot forging process, being replaced by sintered powder metal with low cost tooling as it can result in lower piece part manufacturing costs.

Key Points:

• Important issue – different materials and manufacturing processes require different designs to achieve the same result;
• Powder metal (sintered) is not as strong as the original designed material;
• Thrust bearings can crack and fail in service under normal operating loads;
• The component failure causes the turbo to fail;
• Turbo repairers often diagnose this as oil contamination – poor quality part is not noticed.

Effects on the Industry

This is a significant issue to the industry – but the worst part is that it is silent. When a thrust fails in this way, the resulting failure is usually misdiagnosed as oil contamination. In reality it is a failure through inappropriate use of lower strength materials.  This is causing many turbo failures and warranty issues, which should not happen and this has an effect on the whole industry.

Critical Points — highlighted are the areas of the thrust bearing where it will crack if the material is too thin, the material specification is insufficient or the incorrect manufacturing processes are used.

Sintering is a perfectly good manufacturing process for thrust bearings and is preferred by some turbo OEM’s, as the resulting porosity in the material gives it excellent oil retention properties reducing wear at cold engine start. Other manufacturers choose high strength materials to ensure long life and apply various specialist surface finishes to increase surface oil retention.

Importantly, each thrust bearing is designed accordingly to the physical parameters of the material. Sintered material has a lower tensile strength than stamped or hot forged materials.  As a result, the wall thickness of a sintered thrust bearing is designed to account for the material strength and is usually thicker. This is to ensure that when the oil feed hole is drilled, there is enough material remaining between the wall of the oil hole and thrust face, to withstand the thrust loading and ensure the fatigue life is appropriate for the application.

Traditionally, compressor wheels are produced from aluminium, which is naturally a very weak cast material. Aluminium is the preferred material for compressor wheels as it is a relatively simple and inexpensive process to cast the compressor wheels, however, to create a stronger wheel post process treatments are essential.

The post production processes are defined by the OEM compressor wheel manufacturers and include heat and solution treatments, to create a more robust wheel.  It is these post processes which increase the cost of a compressor wheel, however they are necessary to produce a strong wheel which will withstand the operating conditions of a turbocharger.

Effects of Weak Cast Material

• If the compressor wheel has been produced from weak cast materials, the blade will start to bend as the pressure of the air and the load on each blade increases;
• As the wheel continues to spin at high speeds the blades will continuously bend backwards and forwards;
• This completely changes the compressor map and the compressor efficiency and it means the wheels do not perform like they are designed to.

Aluminium is very flexible, so whilst it might bend at top speed, as the wheel slows down the blades will return to the original position.  The wheel may appear to look fine, however if you compare the performance of a lower quality compressor wheel against a high quality wheel, you will find as it hits maximum speed the lower quality compressor wheel will lose efficiency and eventually fail. This is all down to the strength of the casting. It is very difficult to visually identify which post casting processes have been used and the strength of the compressor wheel.

If the compressor wheel material is not as strong as it should be, the centrifugal force will cause the material to flow away from the centre. At these high speeds the natural grains of the surface material are unable to cope. This can cause what is known as the ‘orange peel’ effect which is often associated with overspeeding.

Fatigue – Understanding Endurance Limit

When a compressor wheel has been weakened, it will begin to show signs of fatigue. For example, if you bend a piece of Aluminium backwards and forwards enough times eventually it will break.

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 backwards and then as it slows down they bend back into position. Repeat this over many repetitions and 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. Therefore, if the strength of the material is a lot lower than it should be the compressor wheel will break from fatigue a lot earlier than if a stronger wheel had been used. Using a stronger compressor wheel will ensure the blades do not move as far, reducing the risk of fatigue and failure.

As engine technology improves to meet the stricter emission regulations, turbochargers face more adverse operating conditions than ever before. To accommodate this, an advancement in turbocharger technology is essential to prevent premature failure.

A good example of such advanced technology is the Borg Warner turbo no. 5303-970-0208, which fits the 2.0 TDI Volkswagen applications. This turbo has been designed with a nickel coated machined from solid (MFS) compressor wheel to avoid corrosion, caused by the EGR system. Without this coating, the compressor wheels would fail well within the warranty period.