But here is the problem.
That interval was probably set for a set of assumptions — a certain speed, a certain temperature, a certain level of cleanliness. If your actual conditions are different — hotter, dustier, wetter, more vibration — that interval could be dangerously too long. And if your conditions are actually milder than what was assumed, you could be wasting grease, wasting labor hours, and in some cases, over-greasing the bearing to death.
I wrote about over-greasing in an earlier article. This article is the other side of that coin: how to calculate the right relubrication interval and the right quantity so that you are not guessing anymore.
First, let us agree on what relubrication is actually doing
When grease sits inside a bearing, it does not last forever. Over time, the base oil bleeds out of the thickener, oxidizes, gets contaminated, or simply gets displaced. The thickener structure itself degrades under continuous shearing.
Relubrication does two things:
One, it replenishes the base oil at the contact surfaces — the rolling elements and the raceways — so that the lubricant film does not break down.
Two, it flushes out a portion of the old, degraded, or contaminated grease and replaces it with fresh grease.
So the question is: how quickly does the grease degrade in your specific operating conditions? That determines how often you need to relubricate.
The speed factor: where everything starts
The starting point for any relubrication calculation is something called the speed factor, written as n × dm.
Where:
- n = rotational speed of the bearing in RPM
- dm = mean bearing diameter in mm = (bore diameter + outside diameter) ÷ 2
For example, take a 6310 bearing. The bore is 50 mm, the outside diameter is 110 mm.
dm = (50 + 110) ÷ 2 = 80 mm
If the bearing runs at 1,500 RPM:
n × dm = 1,500 × 80 = 120,000
This number tells you how hard the grease is working. A higher speed factor means the grease is being sheared more aggressively, generating more heat internally, and degrading faster. A lower speed factor means the grease has an easier life.
Both SKF and FAG (Schaeffler) publish relubrication charts that plot this speed factor against a base relubrication interval in operating hours. You look up your speed factor, read across to your bearing type (deep groove ball bearing, spherical roller bearing, cylindrical roller bearing, etc.), and you get a starting number.
For our 6310 example at n × dm = 120,000, a typical base interval for a deep groove ball bearing with a good lithium complex grease might be somewhere around 10,000 to 12,000 hours.
But — and this is the critical part — that number assumes ideal conditions. Moderate temperature, clean environment, horizontal shaft, no vibration, normal load. In a real plant, when do you ever have ideal conditions?
That is where the correction factors come in.
The correction factors: where the real answer lives
Think of the base interval as the starting point. Now you need to adjust it — almost always downward — based on what is actually happening around the bearing.
1. Temperature
This is the single biggest factor.
Grease life roughly halves for every 15°C increase in bearing operating temperature above 70°C. This is not a rough estimate — it is a well-established empirical rule that both SKF and Schaeffler use.
Let me show you what that means in practice:
At 70°C or below, you can use the full base interval — no reduction needed. Once you cross 70°C, the degradation accelerates sharply. At 85°C, the interval drops to half. At 100°C, it is down to a quarter. At 115°C, you are at one-eighth of the original interval. And at 130°C, the grease lasts only about one-sixteenth of the base interval.
To put real numbers to this — if your base interval was 10,000 hours but the bearing runs at 100°C, your adjusted interval is already down to about 2,500 hours.
And this is the bearing temperature, not the ambient temperature. A bearing in a 40°C ambient that is running at high speed or under heavy load could easily have an operating temperature of 80–90°C. If you can, measure it directly. If you cannot, estimate conservatively.
Practical tip: If you are in a steel plant, cement plant, paper machine dryer section, or anywhere near a heat source, assume the temperature factor will cut your interval by at least half. Do not use the OEM’s default interval without checking this.
2. Contamination (dust, moisture, process debris)
A clean, sealed bearing in an air-conditioned room is very different from an open bearing next to a conveyor belt in a cement plant.
Contamination accelerates grease degradation in two ways — abrasive particles physically damage the thickener structure, and moisture triggers oxidation and corrosion.
The correction depends on how severe the contamination is. In a light, clean, indoor environment — no reduction needed. With moderate contamination — some dust, occasional moisture — multiply by 0.5 to 0.7. In heavily contaminated conditions — constant dust, water spray, process debris — multiply by 0.2 to 0.3. And in severe conditions — submerged bearings, heavy washdown, abrasive slurry — the factor can drop to 0.1 or lower.
In a feed mill with grain dust everywhere, or a paper mill wet section with constant water, or a TMT bar rolling mill with scale and water, you are looking at a factor of 0.2 to 0.3. That 10,000-hour base interval is now down to 2,000 to 3,000 hours just from contamination alone — before you even apply the temperature factor.
3. Vibration and shock loads
Bearings subject to continuous vibration or intermittent shock loads — think crushers, vibrating screens, hammer mills, press machines — need shorter intervals. The mechanical pounding works the grease harder, accelerates oil separation, and can physically displace grease from the contact zone.
As a general guide: normal, smooth-running equipment needs no reduction. Moderate vibration calls for a factor of 0.5 to 0.7. Heavy vibration or shock loads — think crushers, vibrating screens — bring it down to 0.3 to 0.5.
4. Shaft orientation
This one surprises many engineers.
A vertical shaft bearing needs to be relubricated more frequently than a horizontal one — typically reduce the interval by 50% (multiply by 0.5). Why? Gravity. In a vertical arrangement, grease tends to settle downward and away from the upper rolling elements. The bearing is not uniformly lubricated.
If you have ever seen a vertical pump bearing fail while the horizontal bearings on similar duty last much longer — this is often the reason.
5. Bearing type
Roller bearings — spherical roller bearings, cylindrical roller bearings, tapered roller bearings — generally need shorter intervals than ball bearings at the same speed factor. The contact area is larger, the shearing action is more intense, and the grease works harder.
As a rough guide, if the base interval chart gives you a number for a deep groove ball bearing, reduce it by 30–50% for an equivalent spherical or cylindrical roller bearing.
Putting it all together: a real example
Let us take that same 6310 bearing and put it in two different plants.
Scenario A — Packaging line in a clean, air-conditioned facility
- Speed: 1,500 RPM, dm = 80 mm → n × dm = 120,000
- Base interval: ~10,000 hours
- Bearing temperature: 65°C → temperature factor: ×1.0
- Contamination: light, clean room → factor: ×1.0
- Vibration: none → factor: ×1.0
- Shaft: horizontal → factor: ×1.0
Adjusted interval = 10,000 × 1.0 × 1.0 × 1.0 × 1.0 = 10,000 hours
At 8,000 operating hours per year, that is roughly once every 15 months.
Scenario B — Conveyor drive in a steel/paper mill
- Same bearing, same speed, same grease
- Bearing temperature: 95°C → temperature factor: ×0.3
- Contamination: heavy dust and scale, occasional water → factor: ×0.3
- Vibration: moderate → factor: ×0.6
- Shaft: horizontal → factor: ×1.0
Adjusted interval = 10,000 × 0.3 × 0.3 × 0.6 × 1.0 = 540 hours
At 8,000 operating hours per year, that is roughly every 3 to 4 weeks.
Same bearing. Same grease. One needs relubrication once a year. The other needs it every month. That is the difference the operating environment makes.
If the steel mill maintenance team is following the OEM’s generic “relubricate every 6 months” instruction — they are running that bearing on degraded grease for five months out of every six. No wonder it fails.
How much grease? The quantity calculation
Getting the interval right is only half the job. Putting in the wrong amount of grease is just as harmful — too little and the film breaks down between re-lubrications, too much and you get churning, overheating, and seal damage.
The standard formula for relubrication quantity is:
G = 0.005 × D × B
Where:
- G = grease quantity in grams
- D = bearing outside diameter in mm
- B = bearing width in mm
For our 6310 bearing: D = 110 mm, B = 27 mm.
G = 0.005 × 110 × 27 = 14.85 grams ≈ 15 grams
That is per relubrication event — not the total housing fill.
To put it in practical terms, 15 grams is roughly one and a half shots from a standard grease gun (most manual grease guns deliver about 1 to 1.5 grams per stroke with a standard lever, and roughly 10 grams per shot with a full pump stroke — but this varies by gun, so calibrate yours).
Practical tip: Most over-greasing happens because the maintenance technician has no idea how much grease one pump of the gun actually delivers. Take your grease gun, pump 10 full strokes onto a kitchen scale, divide by 10. Now you know your per-stroke quantity. This simple exercise saves bearings.
A few things I have learnt from the field
- Do not relubricate a hot bearing immediately. If the bearing is running hot due to a problem — misalignment, overload, contamination — pumping in fresh grease will not fix it. It might mask the symptom for a day or two, but the root cause is still there. Diagnose first, fix the problem, then relubricate.
- More frequent, smaller quantities beat less frequent, large quantities. If your calculation says 15 grams every 500 hours, do not skip two cycles and pump in 45 grams at 1,500 hours. The grease was degraded for a thousand hours, and now you are over-greasing. Stick to the interval. If you struggle with discipline, explore automatic lubricators — single-point or centralized systems.
- Let the old grease purge out. When you relubricate, the fresh grease should push a small amount of old grease out through the drain or relief port. If there is no drain, the old grease stays inside, accumulates, and eventually the housing is packed solid. Make sure the bearing housing has a proper drain plug — and that it is open during relubrication.
- Watch the color and consistency of the purged grease. This is free condition monitoring. If the grease coming out is black, hard, or smells burnt, your interval is too long. If it comes out looking almost fresh, you might be re-lubricating too often (or the right amount — take it as a good sign). If it comes out watery or separated, you have a contamination problem. Train your technicians to observe this.
- Document and review. Keep a simple log: bearing location, date, quantity, observations about the purged grease. After 6 to 12 months, review the data. You will spot patterns — which bearings consistently purge black grease (shorten the interval), which ones are always clean (maybe extend it). This is how you evolve from a schedule-based approach to a condition-based one.
The bottom line
Relubrication is not a one-size-fits-all activity. The same bearing in a clean, cool environment might need grease once a year, while the same bearing in a hot, dirty, vibrating application might need it every few weeks.
The calculation is not complicated. You need three things: the speed factor (n × dm), the correction factors for your environment, and the quantity formula (0.005 × D × B). With these, you can build a plant-specific relubrication schedule that is based on engineering — not habit, not guesswork, and not a generic OEM recommendation that was written for conditions that may not match yours.
If you take one thing from this article, let it be this: the conditions around the bearing matter as much as the bearing itself. Every relubrication schedule should reflect that.
A note on resources: Both SKF and Schaeffler (FAG) offer free online tools and mobile apps — SKF LubeSelect and Schaeffler Medias — that automate these calculations. You enter the bearing designation, speed, temperature, and operating conditions, and the tool gives you the interval and quantity. I would strongly encourage every maintenance engineer to use these as a starting point and then validate against your field observations.