Solid Lubrication (Pallube)
Solid lubricant (Pallube) is a coating that helps in wear resistance and reduces friction on two sliding surfaces without collecting dust. The coating can increase heat resistance and electrical conductivity properties. Even though, the most effective way to reduce friction and wear is the use of fluid lubricants such as oils and greases but it has limitations in some aspects of use.
These solid lubricants can be particularly advantage under the following conditions,
- Loads are high. Extreme pressures squeeze fluid lubricants out from between mating surfaces, causing high friction and wear e.g., heavy machining, wire drawing through dies.
- Operating temperatures are extreme. The oil will have a higher viscosity when the temperature rises and will result in lower performance. In additional, oils and greases suffer thermal decomposition and oxidation at elevated temperatures, oils become extremely viscous and eventually solid. Solid lubricants film has better thermal stability, that they provide a wider service range from -180°C to +300°C e.g., oven chains and sprockets, turbojet bearings.
- Environment may wash away oils and greases. Where parts must operate immersed in liquids or exposed to rain, solid lubricants have the advantage of not being washed away e.g., outdoor drive chains, parts immerse in water all the time.
- Environments are dirty or corrosive. The surfaces of oils and greases collect dust particles, which reduces the efficiency of lubricants. Solid lubricant films are often superior in dusty applications, where complete sealing is impractical e.g., exposed gearing in equipment for mining, road construction, and farming.
- Parts are inaccessible after assembly or operation. In complex equipment where relubrication by oils or greases is not feasible, solid lubricant films can often provide permanent, in place lubrication e.g., locks, flexible shafts, hinges and pivots in aerodynamic linkages.
- Assembly of parts is difficult. Solid lubricants prevent galling and seizing during press-fitting or assembling close fitting parts, and bolts are more easily torqued to the proper tension. Disassembly is also easier e.g., various machine parts.
- Electrical conductivity or insulation is necessary. Graphite type is an excellent conductor of electricity, e.g., electric switch plates, bonded films of PTFE type provide excellent electric insulation.
Pallube or defric coat is solid lubricant that can be separated in 3 types as follows
- Molybdenum Disulfide (MoS2)
- Polytetrafluoroethylene (PTFE)
- Graphite (C)