Manual or Automatic Greasing — What Actually Decides It
Neither method is better in general. An automatic lubricator is the right answer for a point that is hard to reach, keeps getting missed, wants small frequent doses, or would be expensive to lose. A grease gun is the right answer for a point on a route that is genuinely walked, a bearing that needs flushing rather than topping up, or a cheap bearing where a failure costs an hour.
What decides it in every case is the same two things: whether the quantity is right, and whether anyone looks at the bearing afterwards. A wrong quantity delivered automatically is still wrong — just reliably so.
The short answer
- Work out the grams first: 0.005 × outside diameter × width, both in millimetres.
- A grease gun has no fixed output. Weigh ten strokes before you trust a stroke count.
- Automate the points the route keeps missing, not the points it already reaches.
The difference is the dose, not the tool
A bearing wants a small amount of grease at regular intervals. A grease gun delivers a large amount at long intervals; an automatic lubricator delivers a small amount all the time. Over a quarter the total can be identical — what differs is how long the bearing spends with too much or too little.
- Manual — a full charge, then nothing
- Automatic — a small, steady feed
- What the bearing wants
Swipe the table sideways to see every column.
| Grease gun on a route | Single-point automatic | |
|---|---|---|
| How the grease arrives | A full charge at each visit, then nothing until the next one. | A small amount continuously, set as a rate over a period. |
| What sets the amount | The fitter, the chart, and the output of that particular gun. | The setting chosen at installation — right or wrong, it repeats. |
| Who notices a problem | The fitter, if the bearing is hot, noisy or purging badly on the day. | Nobody, unless someone checks the unit and the bearing on a schedule. |
| How it usually goes wrong | Too much at once: a pressure spike at the seal and heat from churning. | Set once and never reviewed, or stopped without anyone knowing. |
| Where the cost is | The walk, the access and the permit, not the grease. | The unit and the review, not the grease. |
How much grease a bearing wants
The standard replenishment figure for a rolling bearing is G = 0.005 × D × B grams, where D is the outside diameter and B the width, both in millimetres. The bearing maker’s own figure always wins where you have it; this is what to use where you do not.
Take a 6310 deep-groove ball bearing: outside diameter 110 mm, width 27 mm. 0.005 × 110 × 27 = about 15 g per relubrication.
Now the gun. Noria, who publish Machinery Lubrication, put the average manual gun at about 18 strokes to the ounce — roughly 1.6 g a stroke, so about nine strokes for that bearing. But they also say output varies by a factor of ten between guns. On one gun those 15 g are three strokes; on another they are thirty. “Give it five shots” on a lubrication chart is not a quantity until you know which gun.
To turn grams into a daily rate and a lubricator setting, use the lubrication calculator on our automatic-lubrication page — the same formula, with the interval corrections for speed, heat, shaft orientation and contamination.
If the point stays manual, do it this way
- Calibrate the gun. Pump ten strokes onto a scale, divide by ten, and write the grams per stroke on the gun. Do it again when the gun or the cartridge changes.
- Put grams on the chart, then strokes. Work out the grams for each point from the formula or the bearing maker, and convert to strokes for that gun. A chart that only says “five shots” is written for a gun nobody has measured.
- Clean the nipple and open the relief. Wipe the grease nipple so dirt does not go in with the grease. If the housing has a relief or drain plug, take it out so old grease has somewhere to go.
- Pump slowly. Add the grease slowly. A fast charge builds pressure in the housing faster than old grease can leave, and that pressure is what ruptures seals and pushes shields into the race.
- Run it, then close it. On an electric motor, run it up to operating temperature with the plug still out so the excess can purge, then refit the plug. If fresh grease goes in and no old grease comes out, stop and look for a blocked passage.
- Write down what happened. The quantity, the date, and what the purge looked like — colour, texture, any water. That record is what tells you next time whether the quantity is right.
What over-greasing does
A grease gun’s usual failure is too much, not too little. Noria list what an overfilled bearing housing does:
- Heat and lost power — the rolling elements churn the excess grease instead of rolling through a film, and the bearing runs hotter.
- Faster breakdown of the grease — the heat oxidises it and speeds up oil bleed from the thickener.
- A hard crust — cooked thickener builds up and can block new grease from reaching the bearing at all.
- Burst seals — lip seals rupture under the pressure, and water and dirt get in through the gap.
- Collapsed shields — on shielded bearings, the shield facing the grease supply can be pushed into the race.
- Grease on motor windings — in electric motors the excess can reach the windings, and the insulation fails as well as the bearing.
Which one for which point
The same questions we ask in a lubrication assessment. The reasoning, the three drive types and the sizing are on the automatic lubrication page.
Swipe the table sideways to see every column.
| The point | Use | Why |
|---|---|---|
| A point on a route that is genuinely walked, and the bearing looks right | Grease gun | Calibrated, with grams on the chart. |
| At height, behind a guard, in a classified zone, or needs a permit | Automatic | The cost of greasing it is the access, not the grease. |
| It keeps getting missed | Automatic | The route is telling you it does not reach that point. |
| The bearing wants a few millilitres a week | Automatic | A full gun every quarter is a spike and then months of nothing. |
| The grease in it has degraded and needs flushing out | Grease gun | A lubricator adds grease; it will not clean out the old. |
| Nobody will review it at three months | Grease gun | An unreviewed unit is a guess that has been automated. |
| A cheap bearing, and a failure costs an hour | Grease gun | The unit costs more than the problem. |
Frequently asked questions
Is automatic lubrication better than manual greasing?
Not in general. An automatic lubricator is better for a point that is hard to reach, keeps getting missed, wants small frequent doses or would be expensive to lose. A grease gun is better for a point on a route that is genuinely walked, a bearing that needs flushing, or a cheap bearing. Either one fails if the quantity is wrong or nobody reviews the bearing.
How much grease should I put in a bearing?
Use the bearing maker’s figure if you have it. Otherwise the standard replenishment quantity is 0.005 × outside diameter × width, in grams, with both dimensions in millimetres. A 6310 bearing, 110 mm outside diameter and 27 mm wide, takes about 15 g.
How many grams does one stroke of a grease gun deliver?
There is no fixed answer. Noria put the average manual gun at about 18 strokes to the ounce, roughly 1.6 g a stroke, but say output varies by a factor of ten between guns. Pump ten strokes onto a scale and divide by ten before you trust a stroke count.
Can an automatic lubricator over-grease a bearing?
Yes. A unit set once from a catalogue figure and never reviewed delivers the wrong rate every day without anyone noticing. Review the bearing at three months — temperature, purge at the seal, and the condition of the grease coming out — and correct the setting from what you find.
Sources. Noria / Machinery Lubrication — Electric Motor Bearing Greasing and Lubrication; Noria / Machinery Lubrication — The Dangers of Overgreasing. The replenishment formula is the standard rolling-bearing figure used by the bearing makers.