Grease Lubrication of Slewing Rings: the Raceway and the Open Gear Mesh

2. Two lubrication systems on one component

The raceway and the gear mesh are lubricated for different reasons and fail by different routes. The raceway carries the superstructure through rolling elements under very high contact stress at low speed; its grease is enclosed between the rings, retained by lip seals, and replenished through grease nipples while the ring is turned. The gear mesh transmits swing torque between the pinion and the toothed ring; its grease is applied to the teeth and stays in the open, where it collects whatever the machine works in.

The practical consequence is that the evidence of each points to a different fault. Grease on the teeth that has gone hard, gritty or missing is a gear-mesh problem, and its signature is on the tooth flanks. Grease purged from the raceway seals that is dark, watery or full of fine metal is a raceway problem, and its signature is inside the ring. Metal in the oil of the swing drive's gearbox is neither: it is the drive's. Three components, three lubricants, three different conclusions, and the evidence for each can be collected before anything is dismantled.

3. What a grease is, and how it is specified

A lubricating grease is a base oil held in a thickener, usually a metallic soap or a complex soap, with additives. The thickener is a sponge: it holds the oil in place where a liquid would run out of a slow, open or vertical contact, and it releases oil to the contact under shear and temperature. It is the base oil that lubricates. A grease is therefore specified partly by its oil — viscosity above all — and partly by the behavior of the thickener.

Consistency is measured by cone penetration, by ASTM D217⁶ or its equivalent ISO 2137³, and reported as an NLGI consistency grade; the greases used on slewing rings are commonly in the softer-to-middle grades so that they can be pumped through long grease lines and still stay put. Greases are classified in ISO 6743-9⁴ not by application but by the conditions they must work under: its designation encodes the lowest and highest operating temperatures, water contamination and rust protection, whether extreme-pressure performance is required, and the consistency grade. In European practice they are commonly designated under DIN 51825⁵, which encodes the base oil, the operating temperature range and whether extreme-pressure additives are present. The properties that matter most outdoors each have their own test: the temperature at which the thickener lets go, by the dropping point of ASTM D2265⁷; resistance to being washed out, by ASTM D1264⁸; resistance to water spray, by ASTM D4049⁹; protection of steel against rusting in the presence of water, by ASTM D1743¹⁰; and load-carrying capacity under extreme pressure, by the four-ball method of ASTM D2596¹¹.

None of these tests predicts service life. Each isolates one property under standard conditions so that two greases can be compared, which is exactly what is needed when a machine's documentation calls for a grease by property and the question is whether a substitute meets it.

4. The raceway: slow, oscillating and heavily loaded

4.1 Why a slewing ring is a poor place for a lubricant film

A rolling bearing builds a separating film by entraining lubricant into the contact, and the film grows with speed. A slewing ring turns slowly, stops often, and spends much of its working life reversing through arcs of a few degrees. It runs, for the most part, in the boundary and mixed regimes, where the surfaces are separated partly by the film and partly by the additive layer on the metal. That is why slewing ring greases carry extreme-pressure and anti-wear additives, and why the condition of the grease matters more here than in a fast bearing that carries its own film.

4.2 The oscillation angle below which the raceway is not re-lubricated

When one ring of a rolling bearing turns through an angle θ and the other is held, the rolling elements do not move through θ. They advance through about half of it. Relative to the held ring, the ball set turns at half the speed of the moving ring, adjusted by a small correction for the ratio of ball diameter to pitch diameter: subtracted when the inner ring turns, added when the outer ring does. Whichever ring turns, each element rolls along the outer raceway through

where D_w is the rolling element diameter, α the contact angle and d_m the pitch diameter, and along the inner raceway through the slightly larger (θ / 2) (1 + D_w cos α / d_m). On a slewing ring the correction is a few per cent, and it is neglected below; it would raise the critical angle that follows by the same few per cent. With Z rolling elements equally spaced, neighboring elements are 360°/Z apart, and each one sweeps only the stretch of raceway it rolls over. The whole raceway is overrolled, and grease redistributed over it, only if each element travels at least as far as the gap to its neighbor. That sets a critical oscillation angle:

Below that angle, stretches of raceway between the elements are never rolled over at all. Each element works back and forth over its own short track, pushing grease out of the contact without drawing fresh grease back in. The damage this produces is classified in ISO 15243² as false brinelling, one of its two forms of frictional corrosion: a pattern of shallow depressions at the element spacing, produced by vibration or small oscillation without true rotation, and often discolored by the oxidized debris of that movement. It needs no heavy work to form. A machine that stands with its engine running, is transported on a trailer, or digs with small repetitive corrections of swing can produce it while doing little that would be called work.

The practical remedy follows from the relation. Periodically turning the superstructure through a full revolution, or at least through several times the critical angle, while greasing, carries grease over the whole raceway. That is why bearing manufacturers' instructions commonly specify that the ring be turned during relubrication rather than greased at rest.

5. Water, grit and the case for purging

The raceway runs outdoors, under a machine that is washed, stands in water and works in dust. Its lip seals are the first defense and they are not perfect: they wear, they harden with age and ozone, and a pressure jet aimed at the gap between the rings can drive water past them. Water in the raceway corrodes the hardened surface, and a corrosion pit is a stress raiser from which rolling-contact fatigue starts. Grit that gets past the seals is overrolled and indents the raceway. ISO 281¹ accounts for the grit through the contamination factor in its life modification factor: overrolled particles dent the raceway, so contamination does not merely wear a bearing, it shortens the calculated fatigue life of one that is otherwise within its rating. That factor is defined for solid particles, and the corrosion water causes is not captured by it. A slewing ring is also conventionally rated on its maker's static capacity curves rather than by ISO 281¹, so here the standard explains the mechanism rather than supplying a number.

The grease is the second defense, and the way it is applied decides whether it works. Grease pumped into a raceway already holding contaminated grease displaces some of it outward, towards and past the seals. Enough new grease pushes the old grease, and the water and grit it holds, out of the bearing altogether; a bead of fresh grease appearing around the seal is the evidence that it has happened. A small quantity at the same interval does not purge anything. It tops up the reservoir of contaminated grease the bearing is already running in. Purging, not topping up, is the defense, and it is also the cheapest inspection available: what comes out of the seal is a sample of what the raceway has been running in.

6. The open gear mesh

6.1 Film at swing speed

The same film argument applies to the gear teeth, and more severely. The minimum elastohydrodynamic film thickness in a line contact grows roughly as the entrainment velocity to the power 0.7, the dependence DYCO-TP-102 gives for enclosed gearing:

A swing mesh is slow. A ring gear of 1.2 m pitch diameter turning at 8 rev/min has a pitch-line velocity of about 0.50 m/s, and from the relation above the film it can build with a given lubricant is about an eighth of what the same lubricant would build in an enclosed gear stage running at 10 m/s.

An open gear lubricant compensates in the ways the relation allows and one it does not. It uses a base oil of very high viscosity, which raises the film directly; it carries extreme-pressure additives, which protect the flanks where the film is too thin to separate them; and it commonly carries solid lubricants that remain between the teeth when the film does not. The lubrication of open gearing is treated in ANSI/AGMA 9005¹² alongside enclosed gearing, and the distinction it draws between the two is the distinction this section describes.

6.2 A tacky film is also a trap

An open gear lubricant has to be tacky enough to stay on vertical teeth through rain and swing. That same tackiness holds any grit that lands on it, and the mesh then works a grinding paste of lubricant and abrasive into the flanks on every swing. Grease that has gone dark, stiff and gritty on the teeth is therefore not merely a lubricant past its best; it is actively wearing the mesh. The remedy is removal and renewal, not another layer on top, for the same reason that topping up does not purge a raceway.

7. Implications for inspection and for replacement

Three consequences follow for practice, and none of them is a specification.

The first is that the lubricant is evidence, and it is collected before anything is dismantled. What purges from the raceway seals, what sits on the gear teeth and what drains from the swing drive's gearbox are three samples of three systems, and each points at its own component. Collecting them in that order keeps a raceway fault from being blamed on a drive, or a drive fault on a ring.

The second is that damage at the element spacing on a raceway is a record of how the ring was moved, not only of how it was loaded. Regular shallow depressions at the pitch of the rolling elements, often discolored by the oxidized debris of the movement, point to oscillation or vibration below the critical angle, and the remedy is operational: turn the ring through a full revolution when greasing, and when a stationary machine is left running or transported. A true brinell dent, left when a shock overload such as a boom impact is carried on stationary elements, also sits at the element pitch and can look similar, but its remedy lies in how the machine was loaded rather than how it was greased; the two are told apart on the raceway, in the terms of ISO 15243², before either remedy is chosen.

The third is that a replacement bearing inherits the greasing practice. A new ring greased by topping up, never turned while it is greased, and washed with a jet aimed at its seals will fail by the same route as the one it replaced, and the specification of the replacement has no bearing on that. The distinction between a bearing that wore out and a practice that wears bearings out is established from the evidence above, and it is established before the order is placed.