Rod End & Spherical Bearings
Bearing Selection Factors
Below are general recommendations and guidelines for selecting metal-to-metal, and self-lubricating sleeve, spherical, and rod end bearings. Utilize the helpful checklist at the bottom to identify the many factors you need to consider while designing a bearing into your application.
Rolling Element Bearings / Lubricated Sleeve Bearings
Low-load/high-speed bearings should usually be antifriction rolling element bearings, except for lubricated sleeve bearings under very low load and constant rotation rather than oscillating motion.
Metal-To-Metal Sphericals and Rod Ends
- These are recommended for most joints that are primarily static, need only periodic lubrication and require a minimum of permanent set under high loads.
- They are also recommended for some moving applications such as landing gears, where most of the motion occurs under low loading, but where the bearing is nearly static under the high impact loads when the gear is locked.
- Hardened 52100 or 440C balls with heat-treated outer races of either chrome-moly, alloy steel, or precipitation hardened stainless steel are recommended when loads are very high in relation to the available envelope.
- Aluminum bronze races are less apt to seize or gall under vibratory conditions or if lubrication conditions are minimal, providing the required maximum load capacity is not too great (the load capacity is usually about 1/2 that of the heat treated steel race bearings). In general, materials containing an appreciable amount of copper are good bearing materials.
- A beryllium-copper ball operating against a heat treated stainless steel race is an excellent combination for dynamic oscillating conditions under very high loads, providing adequate lubrication is present. This requires either an automatic lube system or frequent maintenance provisions.
- Metal-to-metal spherical bearings and rod ends are often fitted with aluminum-nickel-bronze sleeves in the ball bore, with lubrication provisions, so that the relative motion and resulting wear take place between the shaft and the sleeve, with the only misalignment taking place at the ball spherical surface. This allows replacement of the sleeves without replacement of the expensive portion of the bearing.
- For extremely high load carrying capacity in a limited envelope, spherical bearings with both ball and swaged outer race made of heat-treated maraging steel of 300ksi tensile strength are sometimes used, and can be formed by a special processing procedure.
- Special types of metal-to-metal sphericals such as loader slot bearings, fractured outer race bearings, or snap-assembled bearings are used for some applications where very hard inner and outer races are desirable for wear and strength reasons, but require special geometry (a relatively narrow ball).
- Spherical and rod end bearings for both high temperature and cryogenic applications are available using special materials such as the Inconels®, Stellite® and other cobalt alloys, A-286, Rene 41® and others. Special dry film lubricants or silver plating in the race I.D. are sometimes used in these bearings.
- Two-piece swage-coined rod ends should be used primarily for applications that require high load carrying capacity in a basically static condition with some misalignment capability, This is because the rod end body cross-sectional area available to carry tension is greater than with a 3-piece rod end, the insert outer race area having been replaced by body area. However, ball-to-race conformity is usually poor, hence rapid wear and/or fretting and galling can occur under dynamic or oscillating loading.
- Two-piece Mohawk rod ends for commercial use or non-critical applications are available. The Mohawk design has better ball-to-race conformity than the 2-piece swage-coined design and can be used in dynamic applications but only at relatively low loads.
- Most metal-to-metal bearings are designed with a small radial clearance to facilitate assembly with the mating part and assure that the bearing does not bind up if assembled into its housing with an interference fit. However, they may be made with a preload, providing there is a fairly large tolerance on this preload, for applications where absolutely no play can be tolerated.
Self-Lubricating (PTFE-Lined) Sleeve-, Spherical-, and Rod End Bearings
- These consist of a relatively thin composite liner containing PTFE (polytetrafluoroethylene) as a lubricant and bonded to a metallic backing material.
- They are recommended for applications requiring considerable oscillation and misalignment under very heavy loads and where frequent lubrication is undesirable or impossible. To gain full life from these bearings, a wear of about .005 from the liner surface must be tolerable.
- This type is especially suited for hydraulic actuators, many aircraft landing gear door applications, vibration damping devices, hinge and actuation link bearings for control surfaces, sliding guide bearings for flaps and leading edge slats, and power control system drive linkage bearings, along with many others not mentioned.
Spherical Bearing Selection/Design Checklist
- Bearing envelope requirements and/or restrictions
- Weight limitations
- Whether used in a static or dynamic application
- For sleeve bearings, whether the shaft is oscillating or rotating continuously in one direction or both directions
(A) Maximum static radial or axial
(B) Maximum and normal dynamic
(C) Reversing or uni-directional
(D) Shock or vibratory conditions
- Relative movement
(A) Angle of oscillation
(B) Velocity in terms of rpm or cycles per minute
(C) Required angle of misalignment
(D) Load-velocity phase relationship
- Allowable wear
- Life requirement, preferably in number of cycles
- Operating temperature range
- Preload or clearance requirements
- Lubrication methods, accessibility, and frequency of maintenance available
- Environmental conditions including exposure to dirt, moisture and other contaminants
- Installation requirements, including staking methods, housing and shaft fits, etc.
For additional considerations, please consult NHBB Applications Engineering staff.