A grease cup is a threaded cap over a reservoir above the bearing: screwing it down pushes grease through the oil hole, and an oil hole with a hinged cover is a drilled hole that takes oil daily. Both feed by hand, so how much they deliver depends on how often someone turns or fills them, and both suit slow, intermittent shafts rather than fast ones.
Part of Bearings and Bushings
The simplest lubrication fittings: a screw-down grease cup and a covered oil hole, fed by hand on a schedule.
A Cup Delivers A Known Volume Per Turn
The carrier article gives both fittings briefly. A grease cup is a threaded cap over a reservoir above the bearing; you screw the cap down periodically to push grease in. An oil hole is a simple drilled hole with a hinged cover; you add oil daily. Plain Bearings adds that a cup oiler on a low-speed, intermittently used machine may need filling only weekly, and that the oil hole runs from the housing exterior into the oil groove of the bearing.
The useful fact is that a screw-down cup is a small positive-displacement pump. Each turn of the cap moves the grease surface down by one thread pitch, so the volume pushed per turn is the bore area times the pitch. That turns “give it a bit of grease” into a number you can put on the schedule.
What You Need
- A cup body with an internal thread, and a cap that pushes a flat disc or the reservoir surface down
- Grease the bearing can use: for hand-made grease heat oil to 80°C, stir in 10-15% by weight of calcium soap (lime and tallow), and cool slowly while stirring (Bearings and Bushings; see also Axle Grease)
- An oil groove in the bearing bore to receive the grease (Boring and Finishing)
- A drill to cut the oil hole from the housing into the groove
Method: Fit And Run A Grease Cup
Step 1: Locate the hole on the unloaded side of the bearing, feeding the groove, so grease enters where the film is not working.
Step 2: Drill the hole large enough to resist blocking. A small hole fills with dirt or hard grease. The wiki gives no hole size; choose the largest that your groove and housing allow.
Step 3: Fit the cup above the hole and fill it.
Step 4: Turn the cap a set amount on a schedule: a fraction of a turn each shift, written down.
Step 5: Calibrate by weighing. Weigh the cup full, run a day, weigh again. The difference is the real consumption, whatever the arithmetic says.
Step 6: For an oil hole, use a hinged cover and add oil daily.
Worked Example
A cup with a 20 mm bore and a cap thread of 2 mm pitch (both assumed for the example). One turn displaces 3.1416 x 10 x 10 x 2 = 628 cubic millimetres, about 0.63 mL. A quarter turn is 0.16 mL. If the bearing needed 2 mL a day (an assumed figure), that is 2 / 0.63 = 3.2 turns, about 13 quarter turns, which can be spread across shifts.
For grease made at 10-15% soap, a 1 kg batch is 100-150 g of calcium soap and 850-900 g of oil. Changing the soap content shifts the consistency: more soap, stiffer grease.
Common Mistakes
- Filling the cup once and forgetting it. Grease only moves when someone turns the cap.
- Overpacking a fast bearing. Grease conducts heat slowly; see Lubrication Systems.
- Putting the hole where the load presses. Feed into the groove on the unloaded side.
- Soap grease made hot and cooled without stirring. The carrier says cool slowly while stirring.
Safety
Hot oil, lime and a running shaft
Oil heated to 80°C scalds and can ignite; slaked lime is caustic to eyes and skin, so wear eye protection and gloves when mixing soap. Never reach into a running machine to turn a cup. Fit the cup where it can be turned from outside the danger zone, stop the machine first otherwise, and name where the stop is.
What’s Next
- Learn next: Drip and Wick Oilers - continuous feed that does not need a hand
- Combine with: Lubricant Types - choosing the oil or grease