Aluminium Tin Alloys

Oiltech Bearings is developing a process to produce high tin content aluminium alloys as a single shot cast product.
This will enable thick walled hydrodynamic bearings to be manufactured from a single piece.

Introduction to Hydrodynamic Bearings

Hydrodynamic bearings (slide bearings) operate by creating a pressurised film of oil between the stationary bearing and a rotating shaft or collar. It is important to note that the pressure in the oil film is created by the relative movement of the surfaces and not any external power source. The oil film between the surfaces means that there is no contact, the operational forces or weight of the shaft are supporting by “floating” the load on the oil film. The oil film is typically 25 to 75 microns thick.

Whilst, ideal conditions there is no metal to metal contact, there will be some contact at each start/stop cycle. In the real world, conditions will not always be ideal and bearings suffer from misalignment, contamination, loss of lubrication supply, overloads and unbalanced rotors, to name a few.

The selection of materials used in plain bearings is largely based on their ability to withstand these conditions and the operating temperature required for the application.

Whitemetal (Babbitt) Bearings

Whitemetal alloys are very popular and used extensively in industrial applications. Typically, the value of the machinery is very high and the bearing can be considered as a low-cost, sacrificial part of the machine. The soft whitemetal alloy is good at absorbing particles (embeddability) and debris that contaminate system. Misalignment to a certain degree can also be ‘absorbed’ by the soft lining without damaging the shaft (conformability). The most important feature of whitemetal alloy is its ability to creep and soften once a failure starts. This allows time for the machinery to be bought to a stop before more damage is done.

Material Properties for Hydrodynamic Bearings

When considering the use of a hydrodynamic bearing, there are several materials available. In order to understand the material selection process, there are many factors to consider.

Embedability

How well the bearing running surface can absorb debris/particles that contaminate the system.

Conformability

How well the bearing surface can allow misalignment between the bearing and shaft.

Thermal conductivity

How well heat is conducted from the oil film through the bearing body to the housing. About 20% of the power loss or heat generated by a bearing is assumed to be transferred through the bearing with the remainder being carried away by the oil flow.

Coefficient of Thermal Expansion

Bearings generate a lot of heat and as a result their expansion at their operating temperatures can be an important factor.

Compressive Strength

Bearings are designed to carry load and therefore the material used in the bearing much have the mechanical strength to support the peak pressures in the oil film.

Fatigue Limit

Rotating machinery, by its very nature will vibrate as it rotates and often machinery will be operating above their critical speed (resonate frequency). The material used in the bearing should have an appropriate fatigue limit for the application.