Why can many plastic bushings run dry?
Not needing grease is one of the real advantages plastics have over metal. Many bearing-grade plastics are self-lubricating, and some are made as a blend of a base plastic, reinforcing fibres and solid lubricants. Microscopic lubricant particles sit in the material and are released in minute quantities as the shaft moves, so some lubricant is always present at the contact. In the first phase the shaft and the bushing mate to each other, the contact area grows, the specific load falls and the friction settles to a mostly constant value (K02-Q1-13).
When must you lubricate?
When the demands of the application exceed the pressure, speed or PV limit of the plastic. All three must be under their maximums at the same time (Q1-02). The reason is heat. A dry plastic bearing makes friction heat at the sliding surface, and the heat has to leave through the shaft and the housing. Plastic conducts heat badly, so when PV is too high the surface warms, the polymer softens, the size changes and wear accelerates. A lubricant either lowers the friction that makes the heat or carries heat away, which is why lubrication can raise the permitted PV. Correction factors for this are specific to each material, and are covered in Q1-02.
Where a plain, unfilled bearing-grade plastic has to run greased, grease grooves and a grease hole can be machined into it. That is common for slow, heavily loaded parts such as kiln wheel bushings and sprockets.
Can you add grease to a self-lubricating plastic?
Sometimes. Some grades accept standard lubricants, and a one-time lubrication at installation improves start-up behaviour and lowers friction heat. Others do not. Ask the maker whether your grade accepts a lubricant, and which.
In dirty places, grease is often the wrong move, and the mechanism explains why. A sticky film at the contact collects dust and grit and keeps it in the clearance. The grit then acts as an abrasive between the shaft and the bushing and scores both. A traditional lubricant can therefore reduce wear resistance in dirty environments compared with running dry. A dry-running bushing in a dusty place tends to shed debris instead of holding it.
Water is a special case. A medium around the bearing can sometimes take on the role of a lubricant, which is why some dry-running grades do well in wet service (Q1-07).
What lubricant is compatible?
The answer comes from the maker of the grade, not from the lubricant label. A lubricant is a chemical that stays in contact with the polymer for the whole life of the part, so judge it like any other chemical. Ask these questions:
- Does the maker allow any lubricant with this grade? If the datasheet says dry running only, adding grease is a departure from the design.
- Is the lubricant compatible with the polymer at your temperature? Resistance to a chemical depends on temperature, time of exposure and mechanical load. A lubricant that is harmless at room temperature may not be harmless hot and under load. Use a chemical resistance chart for the grade, and do not assume (Q1-07).
- What does it hold at the contact in your environment? Dust, sand, process residue and swarf all stay in a greased film.
- Does it have to be food-safe? A lubricant used in a hygiene-sensitive place must itself be approved for that use, and its maker has to state it (Q1-08).
A lubricant that attacks the plastic is worse than none.