Match the lubrication system to how fast the shaft turns and how long the machine runs unattended: grease cups and oil holes for slow, intermittent shafts oiled by hand, drip and wick oilers for continuous service, and ring oilers or pumps for fast, heavily loaded shafts. Whatever the system, a bearing’s load and speed limit assumes it keeps oil at the surface, so it must be refilled on a written schedule.
Part of Bearings and Bushings
The fittings and methods that keep oil or grease supplied to a bearing, from a hand-turned grease cup to a pumped oil circuit.
An Oil Supply Does Two Jobs: Keep The Film And Carry The Heat
A lubricant separates the surfaces, reduces friction, carries heat away from the contact, and has to survive the heat, water and air it meets (Lubricant Types). A lubrication system is only the way of getting a supply of it to the bearing. Its two measures are how steadily it feeds, and how much heat it removes. Moving oil carries heat out of the bearing; stagnant grease only conducts it slowly.
That is why a speed limit for a bearing is never a property of the bearing alone. The same bearing on a ring oiler can run faster than on a hand-filled grease cup, because the oil circulates and cools it.
What You Need
- An honest estimate of the bearing’s speed, load and duty (see Friction Fundamentals)
- A lubricant with a temperature limit above the bearing’s working temperature (tallow 40°C, lard oil 50°C, castor oil 80°C, mineral oil 120°C and up, per Bearings and Bushings)
- A written schedule and a person assigned to it (Lubrication and Failures)
Method: Pick The System
| System | How it feeds | Suits |
|---|---|---|
| Grease cup | Screw the cap down to push grease in | Slow, heavy bearings, dust or wet |
| Oil hole | Drilled hole with a hinged cover, oiled daily | Light, intermittent duty |
| Drip oiler | Reservoir with needle valve drips oil at a set rate | Continuous service |
| Wick oiler | Felt or cotton wick draws oil by capillary action | Unattended running |
| Ring oiler | Loose ring on the shaft dips in a bath and lifts oil | Horizontal shafts, self-feeding |
| Forced circulation | A pump moves oil through the bearing | High speed and high load; complex to build |
The first four are manual or gravity systems and are described in the carrier article; ring and pump systems are covered in Bearings and Shafts and Plain Bearings. Grease works at lower speeds where a dynamic film cannot form and is far more forgiving of infrequent maintenance; oil gives a film at speed and removes heat.
Step 1: Decide how often the bearing can be attended. Daily means an oil hole or cup is enough; unattended means a drip or wick oiler.
Step 2: Decide how dirty the place is. Dust and water favour packed grease.
Step 3: Decide the speed. Faster shafts need oil that moves.
Step 4: Cut the oil path to the right place (Grease Cups and Oil Holes, Drip and Wick Oilers).
Step 5: Write the schedule and post it at the machine.
Worked Example
A shaft housing runs 30°C above the air of a 20°C workshop, so 50°C. Against the lubricant limits above: tallow (40°C) is past its limit and runs out; lard oil (50°C) is exactly at its limit with no margin; castor oil (80°C) has 30°C of margin; mineral oil has more. The 30°C rise is assumed for the example, but the method is general: find the working temperature, then pick the lubricant whose limit sits well above it. If only tallow is available, the answer is to cut the speed or the load until the housing runs cooler, not to hope.
Common Mistakes
- Choosing the system and ignoring the lubricant’s temperature limit.
- Believing a wick oiler is maintenance-free. It holds about a day’s worth of oil in the plain-bearings design and has to be refilled.
- Packing grease in a fast bearing. It cannot cool it.
- Not writing the schedule. A bearing fails invisibly until it is scored.
Safety
Oiling is the classic way to lose a hand
Reaching into a running line shaft, belt or gear train with an oil can pulls the arm in. Oil only a stopped machine, or fit extended oil tubes and cups so every point can be reached from outside the danger zone. Name where the stop is and how long the machine coasts before anyone leans in.
What’s Next
- Learn next: Grease Cups and Oil Holes - the simplest hand-fed fittings
- Combine with: Drip and Wick Oilers - continuous feed for unattended running