Senior Editor
Woodex bearings come in flange, pillow-block, roll-end, spherical, and hanger types. Style 226 hanger bearings and brackets are typically used in conveyors for grain, slurries, and dry chemicals. |
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A wood jackstaff halyard bearing used in submarines. |
Steel, bronze, and plastic likely come to mind when you think of plain bearing materials. How about wood? Bearings and components from Georgetown, Maine-based Woodex Bearing Co. Inc. are fashioned from rock maple impregnated with nontoxic oils and waxes. The grain structure of the wood is such that a complex matrix of tubular cells form pipelines that serve as a sump and metering system for the lubricant. Bores made at right angles to the grain enhance the property. Frictional heat brings lubricant to the load bearing area, while cooling causes it to reabsorb in the bearing surface. The cycle can repeat without intervention making the bearings good for applications where maintenance lubrication is difficult.
Bearing wall thickness is adjusted to provide adequate lubrication over expected life. In fact, the addition of external lubricants may impair the self-lubricating process.
Wood bearings lubricate in three modes: boundary, full film (hydrodynamic), and mixed film. Under fullfilm lubrication, a thick film of lubricant separates a journal and bearing so service life is nearly infinite. Boundary lubrication involves bearing-journal contact so wear is proportional to load and speed. Mixed film, as the name implies, is a combination of the other two modes.
The bearings handle mechanical stresses to 2,000 lb/in. 2 without distortion or cold flow. A coefficient of friction about that of Teflon lowers heat buildup at shaft speeds to 2,000 surface ft/min, or higher in certain wet conditions. Wetness swells the close-grained wood at a slight and predictable rate so bearings can be sized for submerged and washdown service. Wood bearings withstand many caustic and acid compounds that attack seals and metal bearings, though certain solvents should be avoided because they can interfere with bearing lubricants. And they work in moderately abrasive environments because particles entering a journal interface embed themselves in bearing walls where they are covered with lubricant and kept away from shafting. Wood bearings are also particularly good at damping noise levels in machinery.
Design for wood bearings is similar to that for conventional types. Operating clearances must be sufficient to build a proper oil film but not excessive which would encourage radial movement between parts. Inside diameters of sleeve, flange, and roll-end bearings are machined oversize at the factory to accommodate press fits. Press fits for wood bearings are more aggressive than with steel because the former has higher compressibility. About 70% of a press fit reflects as "close-in" of the ID. Designs requiring tighter tolerances can use noncompensated bearings that are then reamed to size after installation. Reaming won't harm the lubrication qualities. In all cases, be sure to eliminate sharp edges and burrs on housings that could cut bearings when they are pressed in.
| WOOD-BEARING SPECS | |
Lubricant content by weight |
41 to 47% |
Tensile strength |
1,150 psi |
Compression permanent deformation (for 0.005-in. set) |
7,500 psi |
Specific gravity |
1.3 |
Coefficient of thermal expansion |
4.3 10-6/°F |
Temperature range |
50 to 160°F |
Coefficient of friction |
0.07 to 0.115 |
Density |
0.039 lb/ 3 |
Wear rate factor |
<0.1 10- 10 |
Max PV factor |
12,000 pia-sfm |
Max P |
2,000 pia |
Max V |
|
| where, F = shaft load, lb; d = shaft diameter, in.; L = bearing length, in.; and n = shaft rotational speed, rpm | |
| COMPARING COMMON PLAIN BEARING MATERIALS | |||
—— Maximum values —— |
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| Material | PV(pia-sfm) | P (pia) | V (sfm) |
Injection-molded nylon 101 |
3,000 |
380 |
475 |
Delrin |
3,000 |
1,000 |
100 |
UHMWPE |
4,000 |
1,200 |
50 |
Woodex |
12,000 |
2,000 |
2,000 |
Babbitt (high tin 10%) |
18,000 |
1,300 |
1,400 |
Oilite bronze |
50,000 |
2,000 |
1,200 |
Cast bronze SAE 660 |
75,000 |
3,000 |
750 |
| PRESS FIT & CLOSE-IN | ||
| NOMINAL OD (in.) | PRESS FIT (in.) | CLOSE-IN (in.) |
to 0.5 |
0.003 to 0.006 | 0.003 |
0.5 to 1.5 |
0.004 to 0.008 | 0.004 |
1.5 to 3.0 |
0.005 to 0.010 | 0.005 |
3.0 & up |
0.006 to 0.012 | 0.006 |
| WALL THICKNESS | |
| SHAFT DIAMETER (in.) | RECOMMENDED MINIMUM WALL THICKNESS (in.) |
to 0.5 |
0.0625 to 0.125 |
0.5 to 1.0 |
0.125 to 0.1875 |
1.0 to 1.5 |
0.1875 to 0.3125 |
1.5 to 2.0 |
0.3125 to 0.375 |
2.0 to 2.5 |
0.375 to 0.5 |
2.5 to 4.0 |
0.5 to 0.625 |
| INSTALLED BEARING CLEARANCES (in.) | ||
| Shaft diameter (in.) | Operating conditions | |
| DRY | WET | |
to 0.5 |
0.002/0.010 |
0.020/0.028 |
0.5 to 1.0 |
0.004/0.016 |
0.024/0.032 |
1.0 to 1.5 |
0.007/0.019 |
0.038/0.046 |
1.5 to 3.0 |
0.010/0.025 |
0.045/0.060 |
3.0 & up |
0.015/0.031 |
0.050/0.066 |
MAKE CONTACT
Woodex Bearing Co. Inc.,
Georgetown, Maine,
(800) 526-8800,
www.woodex-meco.com