In the previous installment, we explored the nature of radial spherical plain bearings and rod ends, and the vital role they play in a variety of segments connected to the off-highway industry, including mining, construction, drilling, and more.
Here we will examine the varieties of radial spherical plain bearings that exist, how they’re individually applied within the off-highway category, and essential key qualities. The following is Part 2 of a three-part post. You can read Part 1 here.
Varieties and use cases
Maintenance (steel-on-steel) spherical plain bearings are more common in off-highway applications. These bearings have a hardened sliding contact surface on both rings. The surfaces are treated with molybdenum disulfide (MoS2) lubrication and a phosphate coating. They have both wear and corrosion-resistant characteristics. Bearings with this sliding contact surface combination require initial, and regular lubrication. The high strength of the sliding contact surfaces makes these bearings especially suitable for bearing arrangements where heavy loads of alternating direction, shock loads or heavy static loads have to be accommodated. Unless properly protected, maintenance free bearings with liners (PTFE composite or PTFE fabric) are subject to deterioration from contamination, and they are not well suited for heavy alternating loads.
Since bearings with steel-on-steel sliding contact surface combinations require regular relubrication, they typically have lubrication holes and grooves in the inner and outer ring to allow grease to flow into the areas of sliding contact. It is important these features line up with the grooves and grease fittings in the mating components when mounted.
When mounting bearings with a fractured outer ring, it is critical that the joint be positioned at 90⁰ to the main load direction, otherwise the service life will be shortened, particularly under heavy loads. The angle of tilt, or misalignment, is usually noted with the symbol α in most catalogs and can vary from 3⁰ to 15⁰. Bearing tolerances, internal clearances, and recommended fitting practices are listed in the Engineering sections of most catalogs. In the US, ANSI/AFBMA Standards 22.1 and 22.2, Series 3, covers these types of bearings.
One of the most common uses of spherical plain bearings in off-highway equipment is with hydraulic cylinders, linear actuators designed to deliver force in a single direction, because they’re effective in dealing with misalignment -- a common cause of hydraulic cylinder failure. Spherical bearings are often used in the clevis style mounts of hydraulic cylinders. Deviations from the centerline of the load, introduce what is known as “side loading”. This places lateral stress on cylinder pistons, seals, bushings and rods -- these components are not designed to withstand these forces, so they will fail quickly. Spherical bearings allow for some degree of misalignment due to side loading, improving cylinder longevity.
A typical excavator or backhoe has a half dozen cylinders, or more, for the bucket, boom, dipper, outriggers, steering, and other equipment and accessories on the vehicle where these bearings are used,
in addition to steering linkages, shock absorbers, and pivot points. Spherical plain bearings are available in various types and configurations to address specific loading and operating conditions, and
Spherical plain bearings are available with seals to protect the sliding surfaces in severe environments. The seals are affixed to the outer ring and make positive contact with the inner ring keeping dirt and debris out, and the lubricant in. Variations in seal materials and design vary from manufacturer, but most seals are either a polyester elastomer material or a buna nitrile rubber bonded to a sheet steel insert. These plastic and rubber seals are best suited for applications where the bearings see frequent tilting or circumferential movement. This tends to keep the contact surfaces clean.
Extended Inner Ring
To ease assembly, and eliminate the need for spacers, these bearings are also available with an extended inner ring design. These have a slightly higher cost.
Depending on their size, most spherical plain bearings accommodate misalignment in the 5 to 15 degree range. However, higher angles of mis-alignment can be accommodated with a special series. This series has an increased cross section inner ring which allows for the higher angle of tilt, up to 20 degrees.
In some off highway applications shock or impact loading is common and can have a devastating effect on bearings. Most spherical plain bearings are manufactured from thru hardened 52100 chrome steel. These bearings have good wear resistance and fatigue strength. However, with their high hardness, they are susceptible to chipping, cracking, and breaking under these conditions. Spherical plain bearings manufactured from low carbon, carburizing steels, such as 8620, and processed with a special heat treatment known as case hardening are recommended in these applications. The case hardening provides an outer layer of high hardness for wear resistance and fatigue and the core of the bearing is “softer”, or more ductile, which allows the bearing to absorb impact. These bearings carry a price premium.
Stay tuned for more on loading considerations and bearing maintenance as advised by AST engineers in Simple, but Tough – Part 3. Coming September 17.