Motion System Design

Basics of sliding metallic-bearing materials: Part 1

This is the first of a series on principles and characteristics of metallic plain bearing materials and how to optimize them in given uses.

A sliding bearing is a machine element that transmits reaction forces to a shaft. A journal bearing is cylindrical (either continuous or segmented) and serves when the force on it is essentially radial. A thrust bearing is washer-shaped (either continuous or segmented) and serves where the force is axial relative to the shaft. Sliding bearings are also called plain bearings. (Some sources prefer to call them “plane” bearings.) This article deals mostly with metallic sliding bearings, though parts apply also to the growing class of nonmetallic bearings such as nylon, PTFE, and polymer composites.

A flange bearing can accommodate both radial and axial force. It is a journal bearing constructed with one or two integral thrust-bearing surfaces. Sliding of the shaft or thrust-collar surface relative to the bearing surface is characteristic of all plain bearings. In many applications, plain bearings offer advantages over rolling-element bearings, such as lower cost, smaller space needs, ability to run with marginal lubricants, corrosion resistance, and capacity for high specific loads.

The first use of a metal alloy for its special properties was probably Isaac Babbitt’s adaptation of a pewter composition in 1839. Since the 1930s, developments have proliferated on all counts for metallic plain bearings. Single-metal, cast-inplace bearings have been mostly supplanted by replaceable-insert bearings of multilayer laminated metals. Bearing mechanical design is now a much more refined engineering specialty. And manufacturing technology has evolved into a complex matrix of metallurgical, chemical, and mechanical processes.

Size, configuration, manufacturing method

Sliding bearings are often classified as thin-wall (less than about 0.2-in. wall thickness) or heavy-wall. In general, bearings of diameter greater than about 6 in. are in the heavy-wall class.

Configurations are also described as half round, full round, flanged, or washer. Society of Automotive Engineers (SAE) standards classify thin-wall bearings as sleeve-type half bearings, split-type bushings, and thrust washers. Most such bearings are made by high-speed forming and machining processes from flat strip. You may sometimes hear them called “striptype” or “sheet-metal” bearings.

Heavy-wall bearings are produced in small lots, say, no more than a few thousand pieces, by more conventional machine- shop processes. Starting materials may be in flat slab or tubular form. They are sometimes called “slab” or “shellcast” bearings. Structural characteristics Sliding bearings are also often classified according to material construction. They can be solid (single-metal), bimetal (two-layer), or trimetal (three-layer) bearings. The terms refer to the number of principal layers used. Each construction is in wide use. Separate layers allow combinations of properties unobtainable with single metals.

Environments & material requirements

In bearing materials design and selection, at least some of these wear damage mechanisms must be considered:
• Surface fatigue wear.
• Abrasive wear.
• Adhesive wear.
• Erosive wear.
• Corrosion.

Figure 1 shows examples of damage these processes cause.

Wear-damage potentials are linked intimately to interactions among system characteristics that include:
• Magnitude of bearing loads.
• Nature of loads (cyclic or steady, unidirectional or reversing).
• Speeds.
• Lubricants and lubrication system characteristics.
• Operating temperature.
• Counterface (shaft) material and finish.
• Alignment and rigidity.
• Life expectancy.

Continue on page 2

Bearing-material properties

Surface and bulk properties. The conditions at which plain bearings must operate and the wide ranges over which these conditions can vary bring up material- properties concerns of two kinds:
• Those of the bearing surface and immediate subsurface.
• Those of bulk properties.

Here are some of the more important relationships between bearing material properties and wear-damage mechanisms (material properties in italic type; damage mechanisms, normal type):
Compatibility: Adhesive wear.
Conformability: Adhesive wear; surface fatigue wear.
Embeddability: Abrasive wear; adhesive wear.
Fatigue strength: Surface fatigue wear.
Hardness: Extrusion; erosive wear.
Corrosion resistance: Corrosive wear.

You can think of compatibility as a purely surface characteristic. Conformability and embeddability involve the surface and immediate subsurface, and relate strongly to the bulk properties of strength and hardness. The other characteristics relate mostly to bulk properties.

Of those six characteristics, only hardness can be measured satisfactorily by standard laboratory methods. Many dynamic test rigs and methods have been developed in the plain-bearing industry to evaluate the other characteristics and their interactions. Most are designed to subject specimen bearings to conditions qualitatively similar to those of intended service, but quantitatively more severe with respect to one or more key parameters, such as:
• Load magnitude.
• Surface speed.
• Running temperature.
• Lubricant supply.
• Lubricant cleanliness.

Such tests are costly and time-consuming and should be preceded by consideration of what is really needed. Then, an appreciation of relative importance of various material characteristics in the given application and an understanding of possible trade-offs will limit practical choices to a manageable number. Fullscale tests then, can do what they should do: confirm the validity of design and material choices.

*Material in this series is condensed from the chapter “Friction and Wear of Sliding Bearing Materials,” by George R. Kingsbury, ASM HANDBOOK, Friction, Lubrication and Wear Technology, ASM International, Materials Park, Ohio, 1992, pages 741-757. For ordering information about the entire book, contact ASM International, Materials Park, OH 44073-0002, ph. (216)-338-4634.

George R. Kingsbury, P.E., recently retired Senior Engineer from Glacier Vandervell Inc., a major producer of metal plain bearings, is principal of his own metallurgical engineering consulting practice in Lyndhurst (Cleveland), Ohio. He is well known in the bearing materials field as an author, lecturer, inventor, and consultant.

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