Why does a steel-on-steel bearing need grease?
Because grease is what keeps the two steel surfaces apart, and in a swinging joint that film cannot be taken for granted. A journal bearing turning steadily drags oil into the contact and builds a thick film. A spherical bearing stops and reverses at the end of every swing, the sliding speed passes through zero, and the thick film cannot form. What protects the steel is a thin boundary layer: base oil bled from the grease, plus the extreme-pressure or molybdenum disulphide additive that sticks to the metal. That layer is used up by every swing, so it has to be renewed. The useful life of all spherical plain bearings with metal sliding surfaces therefore depends greatly on regular lubrication, and a one-off initial fill is adequate only if operating loads are very low. How well the grease works depends on the load and its type (constant, pulsing or alternating), the swing angle and the sliding speed (Q1-04).
What grease should you use?
A lithium-soap grease with extreme-pressure properties. For steel on steel, the usual choice is a corrosion-protective, pressure-resistant lithium-soap grease with a higher base-oil viscosity and a molybdenum disulphide (a black solid lubricant) additive, which carries the load when the oil film is thin. For heavy loads the molybdenum disulphide content matters most. Another common specification is an NLGI No. 2 EP grease, applied immediately on installation. If your maker names a grease, use it. Do not put grease in a maintenance-free bearing unless the maker allows it (Q1-02).
How much grease?
Enough to fill the bearing, shown by grease coming out.
- Open type with holes (ES style). Grease immediately after installation until grease is seen exiting between the ball and the outer ring, that is, between the outer and inner ring.
- Sealed type (ES-2RS). Grease until it begins to purge from the seal area next to the ball. With a central lubrication system, the dose matters for the sealed versions, since too high a pressure or dose could loosen the seals.
- Rod ends with a female thread. Fill the space in the shank thread with grease before installation, which cuts the work of lubricating through the nipple.
- Around the bearing. Also fill the space around the bearing. Bearings may be delivered without grease, so fill after installation.
Purging has a second job: the fresh grease pushes old grease, wear particles and dirt out of the contact. That is why you grease to the point of exit, not by a measured number of strokes.
When do you do the first lubrication?
As supplied, a steel/steel bearing usually carries an anticorrosive coating only, so grease it before commissioning or directly after installation. A further lubrication after about one hour of running-in is also recommended, and the interval can be shortened in the start-up period to ease the initial wear. A factory protectant is not the initial lubricant.
What state should the bearing be in?
Unloaded. Both the first and later lubrication should be done with the bearing or rod end unloaded, so the grease can spread without obstruction.
How often?
There is no single interval, because the grease is being used up at a rate set by the duty. Five things decide it.
| What the duty does | Effect on the interval |
|---|---|
| Heavier load, or a higher pV (Q1-04) | Shorter: the boundary layer is rubbed off faster |
| Load always in one direction, or a very small swing angle | Shorter: the same strip carries the load every swing and grease cannot flow back into it |
| Heat | Shorter: grease oxidises and bleeds oil faster as temperature rises (Q1-12) |
| Dust, water, washdown | Shorter: grease is washed out or contaminated, and the seal only slows that (Q1-11) |
| Alternating load, clean and dry, moderate speed | Longer: fresh grease is carried into the loaded zone with every reversal |
With those in mind, one maker publishes a guide: grease at least as often as the bearing's service life G (the total number of rocking motions) divided by 40 for a load in one direction, or by 180 for a fluctuating load (THK). G itself comes from the maker's life calculation.
Example, with illustrative numbers: if the bearing's life under a one-direction load is 2,000,000 rocking motions, the guide interval is 50,000 motions, about 42 hours at 20 swings per minute. For a fluctuating load the life G is not the same figure, so take the G that the life calculation gives for that load and divide by 180; dividing one number by 40 and by 180 does not compare the two cases. The factors are one maker's for its own bearings, and it says the interval varies with the size of the load, how often the bearing moves and other conditions. Use the maker's own interval where there is one.
To tune an interval in service, start from the guide, look at the grease that purges, and let it speak. Dark, gritty or dry grease means the interval is too long or dirt is getting in; clean, soft, light-coloured grease with no growth in play means you can lengthen it. Treat any growth in play as a reason to shorten it (Q1-14).
How do the holes and grooves work?
They bring the grease to the sliding surface. Steel/steel bearings, except some small sizes, usually have an annular groove and two lubrication holes in both the inner and outer rings. On larger sizes (in some makers' ranges, outside diameter of about 150 mm and over) the outer ring can also carry a multigroove system against lubricant starvation, a common cause of early failure where small alignment movements are made under heavy loads that always act in one direction. The multiple grooves spread grease over the heavily loaded zone. The weakness they answer is the one described above: small swing angles and a load in one direction both make it harder for a lubricant film to form.
Make sure the grease path is complete once installed: the hole in the housing must meet the groove in the ring, and the nipple must be reachable (Q1-02, Q1-10).
How do mud, water and a hard-to-reach pivot change greasing?
The rows for dust and water in the interval table apply, and a vehicle meets them in an extreme form. Water crossings and pressure washing flush grease out of the joint, and the seal only slows that. Tie the greasing to events (after a water crossing, a wash, a rough run) as well as to the calendar, and grease until fresh grease shows at the seal or between the rings, as described above. The actual interval comes from the bearing maker or the vehicle manufacturer.
Three habits that matter more on a vehicle:
- Wipe the nipple before the gun goes on. A nipple caked in mud pushes grit into the joint with the first stroke.
- Grease with the joint unloaded, with the suspension hanging so the load is off the pivot, and wipe away the purged grease afterwards, since it collects dust.
- Make sure the nipple is reachable in the finished vehicle, not only on the bench, and that the hole and groove still line up (Q1-10).
Pick a grease the bearing maker names. Ask the maker how that grease behaves against water washout; do not assume it from the label.