Engineering Reference

Rod End & Spherical Bearings

Grease Lubricants

Selecting the optimal grease in metal-to-metal spherical and rod end bearings involves an analysis of a wide array of factors, such as bearing envelope requirements, materials, loading, allowable wear, and operating temperature range, to name a few. Included below is a brief overview of grease lubricants, a description of related bearing features, and a table of the three most common grease lubricant products used with NHBB bearings.

Overview of Grease Lubricants

Grease is an oil to which a metallic soap, synthetic filler, thickener, or a combination of these compounds, has been added to prevent oil migration from the lubrication site. The operative properties of grease depend almost wholly on the base oil.

Grease-lubricated bearings usually contain lubrication holes and grooves, and, in the case of rod ends, lubrication fittings for periodic relubrication. These bearings tend to gall unless lubrication is very frequent, and loads are reversing so that the grease doesn't squeeze out of the load area.

In applications with uni-directional loading, the grease will quickly squeeze out of the bearing area. In these applications, a dry film is a suitable choice. The use of PTFE also should be investigated. 

NHBB grease lubricants are suitable for most airframe applications. If bearings are required to operate in unusual conditions (for example, high vacuum, radiation, or near chemicals such as phosphate ester fluids or propellants), please consult the NHBB Applications Engineering Department before ordering.

Lubrication Network

The image to the left illustrates a lubrication network that provides for lubricating both the ball/race and the ball/shaft (or pin) interfaces. Further, relubrication can be accomplished via the race housing or the ball shaft or pin. If relubrication is to be done via the race housing, and no lubrication is required in the ball bore, lube holes and I.D. lube groove in the ball may be omitted. Conversely, if relubrication is to be done via the shaft or pin, lube holes and O.D. groove in the race may be omitted.

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Lubrication network.

Transverse Lube Groove

This is a transverse lube groove configuration for use on medium to large size spherical bearings in critical applications where lubrication demands are more extreme. The transverse grooves are machined into the cylindrical race blank prior to swaging. These bearings are often bushed with copper alloy sleeves, which in turn may incorporate transverse or equivalent lube groove patterns to provide for maximum possible lubrication.

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Transverse lube groove (lube groove size and depth exaggerated for clarity).

Table of Grease Lubricants

TypeSpecificationCompositionTemperature RangeUse and Remarks
Grease, aircraft and instrument, gear, and actuator screwMIL-PRF-23827Lithium soap, ester oil, anti-rust and E.P. agents-100° to +250°FGeneral purpose grease. Extreme pressure (E.P.) properties, good water resistance
Grease, MoS2, for high and low temperaturesMIL-G-21164Same as MIL-PRF-23827, except 5% MoS2 added-100° to +250°FSimilar to MIL-PRF-23827 but has added MoS2 for extra E.P. properties and anti-wear action under marginal lubrication conditions
Grease, AircraftMIL-PRF-81322Synthetic oil and thickener-65° to +350°FHigh temperature grease