Wood bearings in the new millennium

Print page

Email page

By J.R. Steurnagle, Woodex Bearing Co., Inc.,

PlantServices.com

They have been supporting rotating shafts quite successfully for thousands of years and will continue to do so in the future.

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.

Historical applications
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.

Materials
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.