The Lubrication Challenge at Low Speed
In rolling element bearings, the lubricating film thickness (h) is described by elastohydrodynamic lubrication (EHL) theory. The film thickness is proportional to (speed × viscosity)^0.7. When speed drops from 1,800 RPM (typical electric motor) to 5 RPM (typical mill trunnion bearing), the film thickness decreases by a factor of approximately (5/1800)^0.7 ≈ 0.015 — a 98.5% reduction. At such thin films, surface asperities on the rolling elements and raceways inevitably contact each other. The lubricant's EP additives and solid lubricants must intervene to prevent microwelding, scuffing, and adhesive wear at these asperity contacts.
Selection Criteria for Slow-Speed Grease
| Parameter | Recommendation | Reason |
| Base oil viscosity | ISO VG 320-1000+ (mineral) or 220-680 (PAO) | Maximizes residual film thickness at low speeds |
| NLGI grade | NLGI 1-2 (1 for cold climates / centralized systems) | Ensures pumpability while resisting slump |
| EP additives | Required — S-P or S-P-Zn type | Essential for boundary lubrication protection |
| Solid lubricants | MoS₂ and/or graphite recommended | Provides backup when fluid film is completely lost |
| Water resistance | High — barium or aluminum complex thickener | Many slow-speed applications are exposed to water |
Common Slow-Speed Bearing Types
Slew Bearings (Slewing Rings)
Large-diameter (0.5-5m) bearings used in cranes, excavators, wind turbine yaw systems, and tunnel boring machines. Typically three-row roller or ball-roller combination designs. Key lubrication challenges: oscillating motion (rather than full rotation) limits lubricant distribution, open gear teeth integrated into the bearing ring require simultaneous gear lubrication, and exposure to weather, dust, and water. Accepting that the slewing ring will operate in boundary lubrication, select a grease with robust EP additives and solid lubricants rather than attempting to achieve full film separation.
Trunnion Bearings (Grinding Mills)
Hydrodynamic journal bearings supporting the ends of ball mills, SAG mills, and rod mills in mining and cement plants. Bearing diameters of 1-4 meters operating at 10-25 RPM. These bearings use oil circulation systems with ISO VG 320-680 oils (often Klubersynth GEM series for synthetic gear oils in adjacent gearboxes using the same oil type). Grease is rarely used here — oil circulation systems provide cooling and contaminant flushing in addition to lubrication.
Pitch and Yaw Bearings (Wind Turbines)
The blade pitch bearings and nacelle yaw bearing of a wind turbine are large-diameter slewing bearings that operate under oscillating motion with highly variable loads from wind gusts and turbulence. False brinelling (fretting wear during small-amplitude oscillation) is a major failure mode. Grease must provide robust EP protection and resist false brinelling through adequate film formation during each oscillation cycle. KLÜBER Klüberplex BEM 41-141 and special wind turbine formulations are commonly specified.
Frequently Asked Questions
Q: How do I calculate if a grease will form a film at very low speed?
Use the specific film thickness (lambda ratio): λ = h_min / √(Ra₁² + Ra₂²), where h_min is the minimum EHL film thickness and Ra is the surface roughness of each contacting surface. For slow-speed bearings, λ is almost always <1 (boundary lubrication). Rather than trying to increase λ (which requires impractical viscosity), focus on selecting a grease with effective EP additives and solid lubricants that function in boundary conditions.
Q: Why are solid lubricants (MoS₂, graphite) important for slow-speed bearings?
Solid lubricants have a lamellar (layered) crystal structure — the layers slide easily over each other with low shear strength. When the fluid oil film is squeezed out of the contact, MoS₂ particles shear within their crystal planes, providing lubrication at the asperity contacts where metal-to-metal contact would otherwise occur. MoS₂ is effective to approximately 400°C in non-oxidizing atmospheres; graphite requires some moisture to be effective and works best to about 450°C.