Grease and Oil: Re-lubrication Intervals

A re-lubrication interval is not a habit and it is not a guess. It is the output of a short calculation that combines the base oil viscosity at the real operating temperature, the speed factor of the bearing, and derating factors for load and contamination. Get the inputs right and the interval follows within a factor of two; skip them and you either re-grease far too often and waste labour, or far too rarely and lose the bearing long before its calculated life.

This guide shows how the interval is estimated, works a concrete example on a 6205 at 3,000 rpm, and covers NLGI classes, base oil and thickener selection, the temperature rule, grease versus oil, fill quantity and the cases where you should not re-lubricate at all.

The re-lubrication interval is a calculation

The interval depends on four things: how fast the oil in the grease degrades, how hard the bearing works, how hot it runs and how dirty the environment is. Manufacturers publish curves that fold these into a single read-off, but you can reproduce the logic with a few numbers.

Base oil viscosity at operating temperature

The viscosity that matters is the base oil viscosity at the actual operating temperature, not the ISO VG number on the tub. A mineral grease rated ISO VG 100 might present only 12-15 cSt at 70 degC, which is the value that governs film thickness. As a rule of thumb, the base oil should reach at least 13 cSt at operating temperature for a ball bearing running at moderate load.

The speed factor n dm

The speed factor is the mean diameter in millimetres multiplied by the speed in rpm. For a 6205 with a 25 mm bore and a 52 mm outside diameter, the mean diameter is (25 + 52) / 2 = 38.5 mm, so at 3,000 rpm the speed factor is 3000 × 38.5 = 115,500 mm/min. Larger speed factors thin the film, raise temperature and shorten grease life.

Load and contamination derating

Once the base interval is read, apply derating. A load above 10% of the dynamic rating, or a contaminated environment, each reduce the interval. Typical practice is to halve the interval for a heavily contaminated position and to shorten it for load ratios above about 0.15 of C.

A worked example: 6205 at 3,000 rpm

Take a 6205 deep groove ball bearing, dynamic rating about 14 kN, carrying 2 kN - a load ratio near 0.14 - at 3,000 rpm and 70 degC in a clean environment. The speed factor is 115,500 mm/min. On a typical lithium-complex mineral grease at 70 degC, a published curve gives a base interval near 8,000-10,000 h. Apply a light load derating because the ratio is near 0.14, and the practical interval lands around 7,000 h. Raise the temperature to 90 degC and the same position drops to roughly 3,000 h - which is why temperature, not speed, is usually the number that catches maintenance planning out.

NLGI consistency classes and where each suits

NLGI consistency describes how stiff a grease is, from 000 (fluid) to 6 (block). Bearing greases live in the middle of that scale.

Class 2 is the general-purpose choice. A central system that must pump grease through long lines needs class 1 or even 0, because a stiff grease will not travel; a large vertical bearing may need class 3 so the grease stays where it is placed.

Base oil and thickener selection

Base oil and thickener are separate decisions. The base oil sets the film and the temperature range; the thickener sets the structure, the water resistance and the compatibility.

Classification follows ISO 6743-9 for the grease family, base oil viscosity grades follow ISO 3448, and the German specification for greases is DIN 51825. A supplier quoting a DIN 51825 grade and one quoting an ISO 6743-9 grade are describing the same product by different routes, so compare the base oil viscosity and the thickener rather than the label alone.

For a wet position, calcium sulfonate or a well-formulated lithium grease holds up; for a hot electric motor, polyurea or lithium complex. Never mix thickener families - see the grease selection guide and the NLGI class notes for the details.

The temperature rule and the low-temperature limit

Temperature is the strongest single lever on grease life. The accepted rule of thumb is that for every 15-20 degC above 70 degC, the grease life roughly halves; read the same curve backwards and each 15 degC below 70 degC roughly doubles it, until the low-temperature limit is reached.

At a reference temperature of 70 degC, a lithium-complex mineral grease gives the base interval. The following relative figures show how temperature moves it.

• 55 degC. About twice the 70 degC interval, which is why slow fan and conveyor positions are often greased once a year.
• 70 degC. The reference point, 1 times the base interval.
• 90 degC. About half the interval, so a 20 degC rise has already doubled the maintenance load.
• 110 degC. About a quarter of the interval, and at this level the base oil type matters as much as the interval itself.

The lower limit is a hard constraint, not a gradual derating. A standard mineral grease will not flow below roughly -20 degC, so a cold-store or outdoor winter position needs a synthetic base oil with a low pour point. If the bearing cannot be reached for re-greasing, treat the interval as a maintenance budget, not a target.

Grease or oil: choosing the medium

The choice between grease and oil is made on speed, heat removal and sealing, in that order.

Grease suits the majority of electric motors, pumps and conveyors. Oil wins where the speed factor is high, where the housing must shed heat, or where the oil also lubricates gears or a gearbox on the same shaft. The oil lubrication systems reference covers circuits, filters and flow rates.

Fill quantity, re-greasing and condition-based top-up

How much grease goes in matters as much as which grease. Too little starves the raceway; too much churns.

Fill quantity and overfilling

The normal fill is 25-35% of the free space inside the bearing and housing. At high speed, reduce it towards 15-20%, because a full housing makes the rolling elements paddle through grease, and the resulting churning raises temperature and destroys the grease faster than any external heat. When re-greasing, add through the nipple until the correct quantity has been delivered, not until grease appears - a visible bleed is not the target.

Grease for life bearings

Many sealed bearings are filled once at the factory and are intended never to be re-lubricated. Their life is set by the fill, the closure and the operating temperature. If the position has no grease nipple, treat the bearing as grease-for-life and plan replacement at the interval the manufacturer quotes rather than attempting to top it up.

Condition-based topping up

Where condition monitoring is available, top up on evidence rather than on the calendar. Rising temperature, a change in the vibration spectrum or a hardening or discoloured grease sample all point to a shortened interval. The vibration side is covered in the defect frequency guide.

How to Select a Re-lubrication Interval

Follow these steps for any re-lubricated position.

1. Measure the operating temperature at the housing, not the ambient, and get the base oil viscosity at that temperature from the data sheet.
2. Calculate the speed factor as bore plus outside diameter, divided by two, multiplied by rpm.
3. Read a base interval from the grease supplier curve for that speed factor and temperature.
4. Apply the load derating for a load ratio above about 0.15 of the dynamic rating.
5. Apply the contamination derating - halve the interval for a dirty position and quarter it for a wet, abrasive one.
6. Set the calendar interval conservatively, then adjust it with the first two results.
7. Record the grease type and quantity so the next service uses a compatible product.

Common mistakes

• Mixing incompatible thickeners. A lithium grease topped up with a polyurea grease can soften or harden unpredictably and lose its structure; when in doubt, purge and replace.
• Over-greasing. Excess grease churns, raises the running temperature and can blow a seal.
• Ignoring the temperature step. A 20 degC rise can halve the interval, and a position planned at 70 degC often runs nearer 90 degC in service.
• Judging grease by its consistency at room temperature when the base oil viscosity at operating temperature is what controls the film.
• Re-greasing a sealed-for-life bearing through an improvised route and calling it maintenance.

Intervals are also affected by how the bearing was mounted and how it is fitted; see the mounting and dismounting guide for the assembly side and the seal and shield suffix guide for how the closure limits what you can re-grease. The temperature and clearance interaction is covered in the temperature and clearance reference, and life relationships in the L10 life guide. Compare published grease fills on any FAG listing, and see self-aligning roller bearings for the large, slow positions where class 3 grease is usual. Our deep groove ball bearing range records the closure and fill on every orderable item.