SKF & FAG Methods, Simplified
Walk through any industrial facility and look at the grease lubrication tags on motors, pumps, and fans. Most will say something like: “Regrease every 3 months.” Ask the maintenance team where that interval came from, and the answer is usually: the original PM schedule, carried forward from commissioning, never reviewed.
That number may be right for some of those bearings. It’s almost certainly wrong for others. A bearing running at 3,600 rpm in a 90°C environment needs regreasing far more frequently than the same bearing at 1,800 rpm in a clean, cool room.
The most common relubrication schedules in industrial maintenance are habits, not calculations. The SKF and FAG/Schaeffler methods provide the calculation framework that replaces guesswork with engineering — and this article explains both methods in terms that translate directly to a maintenance planning spreadsheet.
Why Calendar-Based Intervals Are Almost Always Wrong
The failure mode that results from wrong relubrication intervals is predictable. Too long an interval and grease degrades between applications: base oil oxidizes, thickener structure breaks down, and the lubricant film at the contact zone depletes. The bearing runs in boundary lubrication — metal touching metal through an inadequate film — and fatigue accelerates. Too short an interval and excess grease accumulates: churning generates heat, elevated temperature accelerates degradation, and bearing operating temperature climbs in a self-reinforcing cycle that can rupture seals and eventually destroy the bearing it was supposed to protect.
Both outcomes — under-regreasing and over-regreasing — are caused by the same root problem: intervals set without reference to the actual operating conditions of the bearing.
The Variables That Drive Relubrication Frequency
Before getting into the calculation methods, it’s worth mapping the variables that the formulas account for — because understanding why each factor matters is what allows you to apply judgment when field conditions don’t match catalog assumptions.
The SKF Method: A Step-by-Step Calculation
The SKF relubrication interval method calculates a base interval from bearing speed factor and bearing type, then applies correction factors for temperature and contamination to produce an adjusted interval in operating hours.
Step 1 — Calculate the Speed Factor (ndm)
| n | Rotational speed (rpm) |
| dm | Mean bearing diameter (mm) = (d + D) / 2 |
| d | Bore diameter (mm) |
| D | Outside diameter (mm) |
| ndm | Speed factor (mm·rpm) — primary input to the base interval lookup |
Step 2 — Determine the Base Interval (tf)
The base relubrication interval tf is read from the SKF relubrication interval diagram, calibrated for standard lithium/mineral oil greases at 70°C on horizontal shafts under normal load (C/P ≥ 15). Approximate tf values for deep groove ball bearings:
| ndm (mm·rpm) | Base Interval tf (DGBB, 70°C) | Notes |
|---|---|---|
| 100,000 | ~15,000 hours | Slow-speed equipment; very long intervals typical |
| 200,000 | ~10,000 hours | General industrial — moderate duty |
| 300,000 | ~6,000 hours | Typical motor range; verify with temperature correction |
| 400,000 | ~3,500 hours | Higher speed; temperature correction often significant |
| 500,000 | ~1,500 hours | High-speed; grease life becomes limiting factor |
Approximate values for DGBB only. Use the published SKF diagram or bearing calculator for precise calculations.
Step 3 — Apply the Temperature Correction Factor (fT)
Grease life halves for every 15°C increase above 70°C:
| Operating Temp | fT Factor | Effect | Practical Implication |
|---|---|---|---|
| ≤ 70°C (158°F) | 1.0 | Baseline | Full calculated interval applies |
| 80°C (176°F) | 0.5 | Halved | Reduce interval by 50% |
| 90°C (194°F) | 0.25 | ¼ of baseline | Interval = 25% of baseline |
| 100°C (212°F) | 0.125 | ⅛ of baseline | Very short intervals; consider high-temp grease |
| > 120°C (248°F) | Consult mfr. | — | Standard greases at or above rated limit |
Step 4 — Apply the Contamination Correction Factor (fC)
Step 5 — Calculate the Adjusted Interval
| t_adj | Adjusted relubrication interval (operating hours) |
| tf | Base interval from SKF diagram (hours) — based on ndm and bearing type |
| fT | Temperature correction factor (see table above) |
| fC | Contamination correction factor (0.2 to 1.0) |
Step 6 — Convert to Calendar Interval
| Days | Calendar interval between regreasing events |
| t_adj | Adjusted interval in operating hours |
| h_per_day | Average operating hours per calendar day (e.g., 24 for continuous, 8 for single-shift) |
Bearing: 6310 deep groove ball bearing — bore d = 50 mm, OD D = 110 mm, width B = 27 mm.
Conditions: n = 1,500 rpm | Housing temp = 90°C | Light contamination (fC = 0.7) | Horizontal shaft | 24/7 continuous.
| Step 1 — ndm | dm = (50+110)/2 = 80 mm → ndm = 1,500 × 80 = 120,000 |
| Step 2 — tf | ≈ 13,500 hours (from base interval for ndm 120,000, DGBB) |
| Step 3 — fT | 90°C = two halving steps above 70°C → fT = 0.5 × 0.5 = 0.25 |
| Step 4 — fC | Light contamination → fC = 0.7 |
| Step 5 — t_adj | 13,500 × 0.25 × 0.7 = 2,363 hours |
| Step 6 | 2,363 ÷ 24 = approximately 98 days → Round to 100 days |
Bearing Type Adjustments
The base interval values apply to deep groove ball bearings. For other bearing types, reduction factors apply:
| Bearing Type | Relative Interval | Adjustment vs DGBB | Notes |
|---|---|---|---|
| Deep groove ball bearing | 100% | Baseline | Standard reference bearing type |
| Angular contact ball bearing | 100% | Same as DGBB | Treat as ball bearing in standard calculations |
| Cylindrical roller bearing | ~50% | Halve ball bearing interval | Higher friction; more grease consumption per hour |
| Thrust ball bearing | ~50% | ~50% of ball bearing interval | Axial load orientation affects grease retention |
| Needle roller bearing | ~30% | ~30% of ball bearing interval | High contact stress in small envelope |
| Spherical roller bearing | ~20% | 20% of ball bearing interval | Highest internal friction; most demanding of all types |
| Tapered roller bearing | ~20% | 20% of ball bearing interval | Similar to spherical; high sliding contact in raceways |
Source: Based on SKF General Catalogue relubrication interval diagram; Schaeffler/FAG Rolling Bearing Lubrication technical publication.
The FAG/Schaeffler Method: How It Differs
Key Similarities
Both methods derive a base interval from bearing speed (expressed as nDm or similar speed factor) and bearing type, then apply temperature corrections based on the same underlying thermal degradation physics. Both methods treat 70°C as the baseline temperature. Both produce results in operating hours and both require conversion to calendar time based on equipment duty cycle.
For most practical industrial applications — medium-sized bearings at moderate speeds and temperatures — the SKF and FAG methods produce results within 10–15% of each other.
Key Differences in Practice
The FAG method explicitly treats the relubrication interval as a grease service life calculation, incorporating a grease performance factor alongside the speed and temperature inputs. This allows the FAG method to differentiate between different grease types — standard lithium soap greases versus high-performance synthetic-base greases — within the same calculation framework.
A high-performance lithium complex or polyurea synthetic grease may qualify for a longer calculated interval under the FAG method than under the SKF method’s standard curve. When using premium synthetic greases, the FAG calculation method may yield longer defensible intervals than the SKF baseline provides.
Which Method to Use
For most industrial maintenance programs, the SKF method’s clarity and the wide availability of its calculation tools make it the practical starting point. If your facility predominantly uses premium greases specified by Schaeffler/FAG, or if you’re working with very large or high-performance roller bearings, the FAG method provides more bearing-type-specific precision.
The most important principle is consistency: apply one method systematically across the facility, document the inputs, and update calculated intervals when operating conditions change. A calculation that is slightly less precise but consistently applied and regularly updated is vastly superior to a precise calculation done once and never revisited.
Calculating the Correct Regreasing Quantity
Interval calculation is only half the equation. The quantity of grease applied at each regreasing event is equally critical. Too little fails to displace degraded grease and contamination from the contact zone. Too much causes churning, heat generation, and potential seal damage.
| G | Grease quantity per regreasing event (grams) |
| D | Bearing outer diameter (mm) |
| B | Bearing width (mm) — or bearing height for thrust bearings |
| 0.005 | Constant (metric). Use 0.114 with inches for result in ounces. |
This formula produces the replenishment quantity — the amount added at each scheduled regreasing. It does not represent the initial fill for a new or repacked bearing. Initial fill typically occupies 30 to 50 percent of the bearing’s internal free space.
A standard grease gun delivers approximately 1 to 2 grams per pump stroke — the variation is significant enough that specifying pump strokes as a quantity standard is unreliable. The correct field specification is weight in grams — using a grease dispenser with a built-in meter, or pre-loading syringes to the calculated quantity.
A lubrication route card for a pump station bearing position specifies:
| Bearing | 6312 |
| Interval | 85 calendar days (based on 1,800 rpm, 85°C measured housing temperature, fC 0.7 for light dust, horizontal shaft, 22 hrs/day) |
| Quantity | 18 grams |
| Grease | Mobilith SHC 220 (lithium complex, ISO VG 220 base oil) |
| Note | Verify housing temperature at regreasing; if above 95°C, notify reliability engineer for interval recalculation |
That specification took 15 minutes to calculate and document. It replaces a 90-day calendar interval that was producing early bearing failures on the same equipment.
Converting Calculated Hours to a Practical PM Schedule
~8,760 operating hours per year. Divide the calculated interval by 24 to get calendar days. Round down to a convenient maintenance cycle.
~2,080 operating hours per year. An interval of 1,000 calculated hours = 125 operating days = approximately 25 five-day work weeks.
Calculate at the most demanding condition — highest speed and temperature the bearing regularly experiences. Use that conservative interval as the schedule basis.
When the Calculated Interval Is Very Short
An interval of less than a week typically indicates that scheduled periodic regreasing is inadequate as the primary lubrication strategy. Options include:
- Purge-type regreasing: each event intentionally purges the entire bearing of old grease, rather than replenishing incrementally
- Automatic lubrication devices: single-point or multi-point lubricators that deliver small, consistent quantities continuously
- Sealed-for-life bearing substitution: replacing the bearing with a pre-lubricated sealed bearing that is replaced rather than regreased
- Environmental improvement: better sealing, contamination exclusion, or heat reduction to push the calculated interval to a manageable frequency
Building Relubrication Calculations Into Your Maintenance Program
A calculated relubrication schedule is only as good as the data it’s built on and the process that maintains it. Three practices make the difference between a calculation exercise and a program that actually prevents failures.
Temperature is the dominant correction factor and it is the variable most often assumed rather than measured. Bearing housing temperature should be measured with a calibrated infrared thermometer or thermocouple, under normal operating conditions, before the interval calculation is finalized. A temperature assumption of 70°C on a bearing that actually runs at 95°C produces an interval that is four times too long.
A lubrication specification that says “regrease every 85 days — based on: 1,800 rpm, 85°C, fC 0.7, horizontal, 22 hrs/day” is the specification that can be reviewed, challenged, and updated when conditions change. “Regrease every 90 days” cannot.
Calculated intervals should be reviewed when:
- Operating speed changes (motor rewind, VFD setpoint change)
- Measured housing temperature changes by more than 10°C from the calculation basis
- The application environment changes (new heat source, change in contamination level)
- A bearing of the same type fails before the calculated end of interval
The “regrease every 90 days” tag on a motor bearing is almost never the right answer for that bearing in its actual operating conditions. It might be too frequent. It’s often not frequent enough. And for spherical roller or tapered roller bearings running hot in contaminated environments, it can be off by an order of magnitude.
Get those inputs right, apply the correction factors consistently, document what you calculated and why, and update when conditions change. That process — not the formula itself — is what turns relubrication from a maintenance habit into a reliability practice.
