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Software optimised seal design

Original article date: June 1999

A new design of seal lip has been developed by SKF for bearings used in automotive alternators. Alan Quinn investigates.

A car’s alternator may have 20 or 30 electric motors to support from windscreen washers electric windows and CD players to seat adjusters fuel injectors power-assisted steering and engine management systems. In addition a host of other circuits demand power – such as lights and information systems – while the battery has to be maintained at full charge.

While alternator manufacturers have to shoe-horn this critical component into an ever tighter space they are also being pressurised to make the unit at ever lower prices.

Based on these challenges the design goals for bearings and seals include a service life in excess of 2000 hours under variable loads up to 1800N of side loading created by the tension of the latest poly V drives speeds up to 20 0 and angular accelerations up to 2000rad/s/s.

The ability to withstand vibration an ambient temperature of 120degC under both dry and wet conditions and a starting temperature as low as -40degC also has to be considered while quiet running is vital. High performance automotive alternators can supply electrical current up to 150A. However most modern car installations are between 50 and 110A.

To meet such performance criteria it is desirable for the bearing components – seal cage lubrication and steel ring – to be optimised with trials carried out using Failure Mode Effect Analysis (FMEA) supported by laboratory verification tests.

The most common bearing arrangements used in alternators are two deep groove ball bearings mounted on the rotor shaft while the bearings mounted on the pulley drive locate the rotor. Two high performance fans located inside the alternator body at each end of the rotor coil help to keep the unit cooler than previous generations which had a single external fan positioned behind the pulley.

In some cases to provide better servicing and other functions the slip ring and its two brushes are located at the end of the rotor instead of between the two bearings.

The key requirement has been determined to be the ability of the bearing seal to survive a linear speed of 27m/sagainst the bearing inner ring counterface. While this speed is double the normal requirement the seal has to continue to maintain a sealing efficiency against contaminants such as dust and water. In addition the lubricant has to be retained inside the bearing.

A finite element computer program developed by the SKF Engineering and Research Centre in the Netherlands has been used to optimise seal design. The program determines the sealing lip contact force friction and stress with the objective of avoiding significant changes to the contact force of the lip. This contact force has to be maintained regardless of the lip interference with its counterface which can vary due to the tolerance range built up from the axial clearance of the bearing components.

As a result a new design of hammer seal lip the HSL has been developed by SKF. Its designation reflects the overall shape. The seal is made from a polyacrylic rubber a low contact low heat-generating seal which introducing a number of important features principally a long thin yet flexible radial lip that acts on the bearing inner rung counterface and resists any tendency to tear.

Any contaminant is excluded by a special seal grove and the shape of the internal seal acts as a labyrinth so the inner ring shoulder followed by a tapered rubber shape allows the correct grease flow while ensuring the ideal level of grease retention.

The HSL seal has a metal insert that is able to increase the internal volume of the bearing. It also provides ideal lubrication conditions while a tight fit and good seal position are achieved through optimising the design of the seal anchorage area.

Case Study: New steel for diesel taxis

Following experiences of diesel fleet taxis in particular where some alternator bearings displayed spalling of the outer ring raceway SKF decided to investigate the cause. The engineer found that a stress field was being set up by engine vibration and deformation of the alternator’s aluminium drive and covers. The solution was to improve the quality of the steel in the bearing.

The new steel composition designated SKF 3M includes molybdenum and improves the structural strength wear resistance and dimensional stability of the bearing rings. In addition the optimisation of carbon content has increased toughness which has resulted in fatigue life increase of some 10 times that of the previous steel.

Improvements to bearing lubrication was also achieved by SKF by introducing a polyurea grease containing synthetic oil. The new grease was extensively tested against the high and low temperature performance requirements high speed capability load-carrying capacity and rust inhibition. The grease also has to be compatible with the different materials used in the bearing.

June 1999

Added on 30 March 2009 at 2:42 pm | Read more Power Transmission articles