"If the environment is laden with dust like in coal-handling plants, and dust ingression is unavoidable, what type of grease or oil should be used for bearings and gearboxes? What can be a remedy for contamination, as it kills oil or grease?"
The dust in a harsh environment plays little role in the selection of a grease or oil. This role is reserved for attributes such as speed, load, temperature, size, etc. The major role the dust plays is in the selection of accessories and lubrication tasks to be performed on the equipment.
Ingression does not have to be unavoidable. In a harsh environment, one of the most cost-effective measures is to make every attempt possible to seal the equipment. The cost of excluding a gram of dirt is often stated as being one-tenth the cost of removing it later. This can be achieved by the use of several accessories. First and foremost is the headspace management. Every piece of equipment "breathes." You want to make sure that when it does, it is breathing clean, dry air.
There are multiple ways to achieve this. The most popular way to control headspace ingression is the use of desiccating breathers. They not only stop very small particles (often 1 micron or smaller) from entering the headspace but moisture as well.
The next focus should be on seals. Shaft seals must be properly selected and maintained. Some seals do a good job of retaining oil or grease but do a lousy job of excluding contamination. Lip seals are a prime example, particularly the ones that are only directed inward. These types of seals tend to wear after a period of time because they make rubbing contact with the shaft. Eventually, the seals no longer function well from the standpoint of both oil retention and contaminant exclusion.
Lip Seal Labyrinth Seal
On the other hand, a labyrinth seal is non-contacting, so it will not have the wear-out condition. These seals are excellent for excluding particle contamination and moisture, even if there's a spray of water nearby.
Another major source of ingression can be service ports such as dipstick tubes. Unfortunately, the two most common methods for checking oil levels in gearbox applications include using either the supplied dipstick or a level port that must be removed for level confirmation. Both of these methods have the potential to introduce unwanted contamination to the system.
Modifications that may be considered for checking the oil level include the addition of a bull’s-eye-style sight glass into those areas where a level port exists or adding a stand-pipe-style level gauge to the drain or auxiliary side port of the gearbox. Simply adding a stand-pipe level gauge does not fully address the possibility of contamination, as it is possible to experience contaminant ingress through the vent hole of the level gauge. Applications that utilize an external level gauge should also have the gauge vented back to the case or to the breather assembly via a T-style fitting.
There is one last point of ingression that everyone seems to either forget or dismiss – new oil. In most every case where particle counting has been performed on new oil, it has been found to be disgustingly dirty. In fact, it is often many times dirtier than what you want running in your equipment. New oil must be cleaned before it is put into a machine component. Missing this one seemingly simple step can be the difference between a clean, reliable machine and a machine that becomes a bad actor.