Hardwood bearings with impregnated lubricant could bring solutions to many of your bearing design and maintenance problems. To makers of equipment for agriculture, amusement park rides, beverage production, elevators, exercising, film processing, food processing, foundries, labeling, marine use, material handling, office work, optical devices, packaging, paper, pharmaceuticals, plastics, printing, pumping, refrigeration, sand blasting, saw mills, screw conveying, sewage treatment, shoemaking, solar-energy conversion, textiles, vending, and wire and cable making, they have already provided solutions. In short, they are versatile.

What they are
The basic material is a specially selected, close-grained, dried, dense hardwood. Seasoned, kiln-dried, hard maple is an example. The basic wood is impregnated with a blend of fluid lubricants. The material can then be machined into a variety of shapes and sizes for bearings and other wear-reducing components. The finished "wood" bearing is, in fact, up to 40% lubricant by weight.

    In operation, the lubricant on the bearing surface allows for easy initial start-up. Friction at the rubbing surfaces, though low, generates enough heat to cause the lubricant impregnated in the bearing to flow. The wood is a good heat insulator. The lubricant flows toward the working surfaces and prevents excessive heat of friction. When motion stops, the lubricant is reabsorbed and retained in the bearing.
    A wood bearing builds up no static-electricity charge. Thus, it can simplify systems such as some conveyors that must otherwise electrically ground bearings to prevent static charge.

Tolerances can be as close as ± 0.002 in. and reaming does not affect lubricant flow. Most wood bearings are machined with the grain perpendicular to the bearing axis (cross grain) to take advantage of the wood's strength and to provide best lubrication.

Fits tough environments
In general, where the pressure-velocity range is limited to 12,000 psi-fpm and maximum temperature is 180 F or less, hardwood bearings work best. They are not harmed by water, mild acids and alkalis, and most caustic chemicals. Thus, they are often used in such environments, in contact with the fluid or even immersed in it.
    In fact, early users of wood bearings (bearings with no impregnation of additional lubricant) were ship builders and woodworking shops making underwater shaft bearings for tugs and freighters. The natural resins inhibited water absorption and served as a base for a water film between bearing and shaft. Modern impregnated bearings are still in marine use. (You must account for some swelling in initial bearing design and selection. See "Engineering the Bearings.").
    Wood bearings operate well in abrasive environments. Gritty particles that manage to reach the bearing surface embed harmlessly in it.

What you can get
    Impregnated wood comes in dowels, rods, bars, and blanks for machining into special shapes. However, many standard shapes and sizes are available ready-made, in mounted and unmounted versions.
    Standard unmounted one-piece cylindrical sleeve bearings, for example, come in inside diameters of 3/16 to 4 in.; outside diameters of 5/16 to 51/4 in.; and 1/16 to 3/8-in. wall thickness. They handle radial loads only. Sleeve bearings with integral flanges (flange bearings) are also available.

Both straight sleeve and flange bearings can be supplied in two-piece versions, split in the plane of the bearing axis for simple mounting, if needed. Considered best are perfect halves, made by machining the bearing to its centerline after the workpiece is formed into two halves. You can also get thrust washers.
    Roll-end bearings are also available, primarily for use on conveyor rolls. They insert into the ends of the rolls, and they come in standard insertion lengths or shorter lengths for lighter duty. They are fully bored or blind-bored. V-belt roll-end bearings are available, too. They are grooved to be driven by standard V-belts. Another form of roll-end bearing has equally spaced flats on its circumference to allow free draining of liquids from the roll. And conical rolls are available to form troughing belt conveyors.
    Another type of hardwood bearing is the hanger bearing, used in support stations on many screw conveyors. You can get them as split bearings, flanged on both ends so they can be retained by support straps; as sleeves to insert in cylindrical hanger housings; and as lower halves to mate with metal upper halves that are integral with hangers.
    Readily available metal-mounted self-aligning bearings have self-lubricating hardwood spherical bearings mated with pressed steel housings. Mounting dimensions are the same as similar mounts for many rolling-element and direct journal contact bearings.
Styles include:
    Pillow block · Circular center flange · Triangular center flange
    Two-bolt center flange · Two-bolt side flange · Four-bolt side flange
Pillow block and center flange units allow through-shaft mounting. Side flange units are for shaft end mounts.
    You can also get the unmounted spherical bearings for replacement or for service in custom mounts. In general, sizes run from 1-in. spherical diameter with bores from 1/4 to 5/8 in. up to 3 1/4 in. spherical diameter with bores from 1 1/2 to 2 1/2 in.
    For extremely poor environments with acids and alkalis or dirt and grit, you can get custom mounted bearings in which even the housing is of hardwood. Only the nuts and bolts are of metal.
    You can also get a variety of wear strips of lubricant impregnated hardwood. Other power transmission components include rollers and pulleys, of either impregnated or nonimpregnated hardwood.

Physical properties
    Here are typical physical properties of one manufacturer's line of lubricant-impregnated bearings:

• Density, 0.038 lb/in. ³
• Coefficient of friction, 0.07 to 0.112
• Temperature range, -50 to 180°F
• Lubricant content by weight, 40%
• Continuous-duty load limit, 1,000 psi
• Tensile strength, 1,100 psi
• Permanent deformation
  (7,500-psi compression), 0.005 set
• Coefficient of thermal expansion,
  4.3 X 10^-6 in./in. per °F

Engineering the bearings
    To size and select hardwood bearings, you should first determine that the use is within the acceptable bounds of 12,000 psi-fpm PV factor (continuous duty) and 180°F. Though higher PV factors can be tolerated briefly, bearing life is significantly shortened if the factor is exceeded for long periods. Consult with a manufacturer if you cannot size the bearing for 12,000 psi-fpm maximum.
    Generally, shaft size and speed are already set by other factors before you select the bearing. Therefore, bearing- length is your control for unit load. Figure 1 shows allowable bearing load vs. shaft speed. Bearing load is given in units of lb per inch of bearing length. Thus, knowing shaft speed and total load, you can determine the minimum length of bearing surface you must have to stay below the maximum PV factor.
    Because the lubricant blend is contained in the bearing wall, the wall thickness must be sufficient to hold enough lubricant for the application. Table 1 shows recommended minimum wall thicknesses for various shaft diameters.

Because the lubricant blend is contained in the bearing wall, the wall thickness must be sufficient to hold enough lubricant for the application. Table 1 shows recommended minimum wall thicknesses for various shaft diameters.
    Knowing both the minimum length for satisfactory load handling and minimum wall thickness for proper lubrication, you can select the correct bearing from a manufacturer's catalog.

Normal bearing tolerances on both ID and OD
for a typical sleeve bearing are:

• To 1-in. diameter, ±0.002 in.
• 1 to 1 1/2 in., ± 0.003 in.
• 1 1/2 to 3 in., ± 0.004
• Above 3 in., ± 0.005 in.

Sleeve, flange, and roll-end bearings are commonly furnished with oversized ODs to allow press fitting for sure retention. Be sure that the leading edge of the housing is chamfered to prevent shearing the outer surface of the bearing during pressing. Typical press-fit dimensions based on nominal bearing OD are:

• To 1/2 in., 0.003 to 0.006 in.
• 1/2 to 1 1/2 in., 0.004 to 0.008 in.
• 1 1/2 to 3 in., 0.005 to 0.010 in.
• Above 3 in., 0.006 to 0.012 in.

Lubricant-impregnated hardwood bearings are ordinarily furnished with an installed bearing-to-journal clearance. See Table 2. Note that the installed clearance is generally higher for a bearing that must operate in a wet environment. This allows for the slight swelling the bearing must undergo.