Grease is the lifeblood of a bearing. Industry data shows that more than 40% of premature bearing failures can be traced back to lubrication issues – but rarely is the grease itself defective. The real culprits are often the wrong choice of product, the wrong quantity, or a poor re‑greasing schedule.
Good lubrication reduces friction, carries away heat, and keeps contaminants out. Yet many maintenance crews treat it as a trivial chore – just pump in some grease and call it done. They overlook the compatibility of the base oil, the required fill volume, and the proper method of application, which sets the stage for trouble down the line.
A systematic approach to grease management not only gets the best out of the bearing but also saves money on grease consumption and man‑hours.
The factors that really matter
Lubrication performance depends on more than just the brand name on the cartridge. These are the critical parameters:
1. Base oil viscosity
Viscosity is the single most important property. Higher speeds and lighter loads call for lower viscosity; heavier loads and slower speeds require higher viscosity.
If the viscosity is too low, the oil film breaks down and metal touches metal. If it is too high, the bearing churns the grease and overheats. Calculate the required viscosity based on the bearing bore and rotational speed (e.g., using ISO VG grades).
2. Thickener type and compatibility
Common thickeners include lithium, lithium complex, and polyurea. They are generally incompatible with each other. Mixing them can destroy the thickener structure, causing the oil to separate from the soap and leak out.
When changing to a different grease brand or type, purge the old grease completely – do not just top it up.
3. Operating temperature versus dropping point
The dropping point of a grease gives an upper temperature limit, but you should stay at least 20–30°C below that figure in practice. High temperatures accelerate oil oxidation and cause the thickener to break down, turning the grease hard and crumbly.
For hot environments, choose a synthetic‑based or complex‑soap grease, and pay attention to evaporation loss at high temperatures.
4. Fill quantity and fill rate
More grease does not equal better protection. Overfilling leaves no room for the grease to move, so it churns and heats up, which actually shortens its useful life.
As a general guide, fill 30% to 60% of the bearing's internal free volume. Use the lower end for high‑speed applications and the upper end for slow, heavy‑duty ones.
5. Re‑greasing interval
Grease does not last forever. The base oil gradually evaporates or bleeds out, and the thickener oxidises with time and heat.
The interval should be set according to running hours, temperature, and exposure to dust or moisture. Shorten the interval in harsh conditions rather than blindly sticking to a fixed calendar schedule.
Best practices for the re‑greasing process
1. Clean the grease fitting
Before you connect the grease gun, wipe the fitting and the area around it. Dirt on the fitting will be pushed straight into the bearing. Use a pump with a filter if possible.
2. Use the "small and frequent" approach
Add grease slowly while watching the relief port. When you see old grease being displaced, stop immediately – pumping too hard can damage the seal.
3. Avoid mixing different greases
If you are not certain what is already inside the bearing, the safest option is to clean out the bearing completely and start fresh. Keep a lubrication log that records the brand and grade used on each machine.
4. Measure the amount you put in
Do not guess. Use a metering grease gun or weigh the cartridge before and after. This ensures you add only the calculated volume, not more.
5. Keep a lubrication history
Maintain a simple card or digital record for each unit, noting the date, grease type, quantity added, and any observed temperature changes. This helps you fine‑tune the interval over time.
Common myths about grease lubrication
| Myth | Reality |
|---|---|
| Packing more grease gives better protection | Over‑greasing causes heat and leakage, which accelerates failure |
| You can mix brands if they look similar | Unless compatibility is confirmed, clean out the old grease first |
| Same colour means same formulation | Colour is not a reliable indicator – check the thickener and base oil |
| Topping up regularly is enough | Old grease must be expelled or removed, otherwise fresh grease cannot work properly |
| Grease never goes bad | It has a shelf life (typically 3–5 years) – after that, oxidation and hardening set in |
Quick lubrication checklist
| Item | Check point |
|---|---|
| Grease grade | Matches the machine manual; confirm thickener type |
| Fill quantity | Within 30%–60% of free space |
| Tool cleanliness | No dirt on the grease gun nozzle or fitting |
| Re‑greasing interval | Set based on actual temperature, speed, and environment |
| Discharge | Old grease comes out freely when replenishing |
| Grease appearance | Uniform colour, no separation, hardening, or emulsification |
| Record‑keeping | Date, quantity, and grade logged for each session |
Wrapping up
Grease management is the most basic and most undervalued part of bearing care. Move away from the old idea that "a little extra won't hurt" and adopt a precise, measured, and scheduled approach. When you match the viscosity, limit the fill, and keep records, you will see lower running temperatures and longer bearing life – all with minimal investment in time and materials.
