We've all seen the cartoon: the skin-clad, primitive man with the stone hammer, fashioning a stone wheel. The cartoonist has an endless stream of gags to go with the picture. Remember the guy who made the triangular wheel? It was, according to the Johnny Hart, the cartoonist, a great improvement over the square wheel: it eliminated one bump.
Maybe it's thanks to those hackneyed gags that we share a common image, right or wrong, of early wheels. But we're less united where bearings are concerned. Most people tend to think of bearings as metal assemblies of races and rolling elements. While engineers and mechanics may also envision plain bearings made of soft metals and plastics, not many people are willing to accept bearings made out of wood.
People have been using rotating shafts of one sort or another for thousands of years. While we might not easily think of wood as an effective bearing material, it was, in fact, almost certainly the material used by the inventor of the wheel. Surprisingly, several thousand years haven't done much to limit the appeal of wood as a bearing material.
Wooden bearings, usually lubricated with tallow or other animal fat, were used by the ancients--and everyone since--on carts and wagons of all sorts. Remember those epic Technicolor films with the thundering horses and chariots? Those gory wrecks with the disintegrating wheels were probably all-too realistic: the tallowed wheel bearing probably wouldn't have tolerated sustained high speed without overheating, likely with disastrous results.
Wooden bearings supported a huge variety of stationary machines, as well, from milling machinery, to water wheels and pumps, construction cranes and even siege engines. Wood was plentiful in most locations; it was strong and shaft-kindly, long lasting, and easy to replace. It was the natural choice for most bearing purposes.
When the steering sweeps on ships gave way to vertical rudder shafts, wood was the bearing material of choice. Lignum vitae, an extremely dense, tropical hardwood, was soon found to be best for this and other submerged marine applications. So dense that it will not float, "lig" has a high natural oil content, making it self-lubricating and especially resistant to deterioration in water.
When kept wet, as in most shipboard applications, lig possesses tremendous strength and wear resistance. When steam propulsion arose, lignum vitae became the bearing for propeller shafts, and it was common to see bearings last scores of years--frequently as long as the ships themselves.
Lignum vitae naturally attracted landlubbers' interest, and it became a popular bearing material for waterwheels. Many hydro turbines today still operate on lignum vitae bearings installed fifty or more years ago, despite having received little or no maintenance since their installation.
Lig was common in the most demanding applications. In 1722, John Harrison built a friction-free tower clock in Brocklesby Park, England that still tells time. The bearings were made of lignum vitae. (Harrison went on to complete a successful forty-year quest for the first reliable means to determine longitude at sea.)
Lig was relatively rare anywhere more than a few degrees north or south of the equator, and less demanding applications were addressed with whatever wood was available, with varying degrees of success.
All woods possess structural attributes and can furnish naturally occurring or artificially introduced lubricants via their grain structure. Some are far better than others, but in a pinch, any wood will do for a time. Check out the turbine bearings in any of the surviving old grain mills and you may find apple or pine or several species cobbled together. Often, these were temporary installations that lasted long enough to gain permanent status.
In 1839, Isaac Babbitt developed a revolutionary antifriction alloy with a low melting temperature. Poured into a mold around an iron shaft, it formed a durable bearing surface. The arrival of babbitt metal put a serious crimp in wood's dominance as a bearing material, but wood retains a valid place, even today.
Lignum vitae, (Guaiacum Officinale, Guaiacum Sanctum) is a slow-growing wood. Taking three hundred or more years to achieve significant size, and long prized as a bearing material, lignum vitae has become so scarce as to be no longer viable for the bearing designer, except where stashes of old material can be found and used.
Innovative bearing manufacturers replaced lignum vitae in most applications with rock maple (Acer Saccharatum), impregnated with petroleum wax. While not nearly as durable as lig in wet applications, impregnated maple displays different, but equally attractive, properties in dry service.
When used in outdoor conveyors, particularly those carrying agricultural products, impregnated maple's compressibility becomes a real virtue. When sand or grit makes its way into the journal interface, it can cut both steel shafts and metal bearings. While plastic bearings, such as UHMW polyethylene, will be lacerated by intruding abrasive particles, their "self-healing" properties constantly work to expel the abrasive from the cut, back into the journal interface, where shaft damage can occur. A vicious cycle of cut-expel-cut can ensue, and the shaft can quickly suffer severe abrasive damage.
In contrast, wood's compressible fiber structure often allows intruding abrasives to be driven into the wood and held in place. The contaminant is buried, covered with a film of lubricant, and becomes a benign part of the bearing. Minimal shaft damage occurs.
Maple, itself a hard, dense wood, exhibits impressive capillary action. When completely impregnated with the right wax, maple carries a heavy load of lubricant. At room temperatures, the lubricant remains a solid, stored in the wood's fibers. As the shaft rotates, friction heats the lubricant beyond the melting point, and oil flows into the journal, providing boundary lubrication. As temperature continues to increase, more wax is melted, and lubricant eventually floods the journal interface, supporting the shaft in a hydrodynamic state. When the shaft stops, the journal cools. Liquid lubricant is drawn back into the bearing by capillary action where it solidifies, awaiting the next use of the shaft. Impregnated maple is thus a permanently lubricated material, in most applications requiring no additional lubrication throughout its life.
Impregnated wood is a valid alternative to many common bearing plastics, such as Teflon(r), Nylon(r), and high molecular weight polyethylenes. Although wood lacks the inherent lubricity of Teflon and UHMW-PE, it can tolerate greater loads and speeds than most plastics, and permits less shaft damage in abrasive environments.
For wood to serve best, the grain direction must be considered. If the shaft turns parallel to the grain of the wood bearing, lubricant will flow parallel to the shaft, and will not provide friction reduction where it is needed. Improper grain direction is the most common design error in wood bearings.
Properly applied, the grain of a wood in the bearing lies perpendicular to the axis of the shaft. End-grain should face the moving surface of the shaft. This presents the wood's natural oil-delivery system directly to the journal, providing the best lubrication.
Where large bearings are required, it isn't necessary to find a block of wood large enough to make the whole bearing. Smaller pieces of untreated wood can be glued into larger sections before being impregnated with oil. With very strong woods such as lig, a large bearing can be made up of radial staves, either built into a holder, or assembled with spacers and bolts. As long as the exposed area of the bearing is large enough to keep load beneath the maximum pressure rating, stave bearings can work as well as--or better than--solid pillow blocks.
Comparative P/V ratings
In addition to grain orientation, speed and load must be taken into account. Impregnated maple has a P/V factor of 12,000, with a maximum P or V of 2,000. This means that impregnated maple can be used in applications where the load (P) is less than 2,000 pounds per inch of exposed area, where speed (V) is less than 2,000 surface feet per minute, and where the product of load and speed does not exceed 12,000. These are maximum limits, and must be reduced by safety factors appropriate for the application.
P/V factors for a number of popular plain bearing materials offer a glimpse at wood's place in the modern engineer's bag of tricks. These values place impregnated maple in the transition zone between soft plastics and metal bearings. Table 1 shows P/V ratings for various bearing materials.
|Material||Max. P||Max. V||P/V|
|WOODEX impregnated maple||2,000||2,000||12,000|
Wood bearings are available in a variety of configurations, from screw conveyor hanger bearings to sleeve bearings, roll ends and self-centering spheres. Wood bearing manufacturers offer an almost limitless selection of custom pillow blocks and inserts for odd-shaped holders.
Many wood bearing applications lie in agricultural process and handling machinery. The presence of dust and grit in this environment makes wood an ideally shaft-friendly material. Equipment ranges from harvesters and field implements, belt, roll and screw conveyors, fruit and vegetable washers and other primary agricultural processing machinery. Wood is an indispensable material for processors who bring in produce directly from the field. Wood should also be considered for conveying and processing mineral products and abrasive chemicals.
Wood retains a place in wet applications. Even in a time when large cross sections of lignum vitae are no longer to be had, arrays of small staves can still be used to build rugged, long-lasting bearings for waterwheels and turbines.
Impregnated maple can effectively addressed other wet applications. These begin with bearings for submerged drag chain and sludge conveyors, and range to bearings for snowblower augers and water theme park rides. One Isreali application consists of a wood bearing, supporting a turntable, driven by a tethered cow. The shaft passes through the bearing, down to the impeller of a low-speed irrigation pump.
Wood is a popular material for wear strips in bucket elevators carrying dry materials. It has traditionally been used for small diameter, high-speed bearings in paper folding machinery because of its natural noise-dampening properties. Wood can be just as effective for quieting small low-load, high-speed shafts that "sing" at high pitch when supported in metallic bearings.
While bearing makers have put their stone tools aside in favor of CNC machinery, there remain many applications where an old technology still gives the greatest satisfaction. When addressing thorny design problems, it may not be necessary to reinvent the wheel.