Ideally, a lubricant’s viscosity should be sufficient to separate two frictional surfaces and prevent wear. However, due to the higher temperatures (or temperature fluctuations) and increased loads of modern machineries, viscosity alone cannot get the job done.
Additives that can increase the ‘load-carrying capability’ of a lubricant, preventing gradual wear or catastrophic welding of surfaces, are therefore introduced to most lubricants. Anti-wear (AW) additives are used where loads are lighter and speeds are higher, forming a chemically reacted hard layer on the base metal, preventing wearing. Anti-wear additives used are based on Fats, Zinc and Phosphates. Such additives are common ingredients in hydraulic oils.
Extreme Pressure (EP) additives on the other hand, based on Sulfur and/or Chlorine, are applicable to applications where temperatures and loads are very high, such as in gearbox applications. They also form a chemically reacted film on the base metal, albeit softer than of anti-wear additives. These layers are sacrificial, preventing the wearing of the base metal.
Care should be taken however, in selecting lubricants with extreme pressure additives, or in selecting the type of extreme pressure additives in a lubricant, as some can be corrosive to non-ferrous metals, such as bronze that can be found in worm gears. In such cases, seek a lubricant with a non-corrosive EP additive, or consult your lubricant supplier.
The addition of these ‘load-carrying’ additives greatly increase the performance of lubricants in mechanical load tests such as 4-ball weld, 4-ball scar, FZG gear and Timken.