JEFF MOORE
Precision Products Div.
NSK Corp.
Bloomingdale, Ill.
A recently developed solid composite comprised of plastic impregnated with large quantities of oil offers advantages over traditional lubrication methods, particularly when it is difficult to keep moving parts lubricated. Such problem applications include equipment subject to frequent washdowns, steam cleaning, or constant exposure to water or dust.
Called molded oil, the composite consists of lubricating oil and a polyolefin resin with an affinity for oil. Oil content is 70% by weight. In this regard it differs significantly from typical oil-impregnated plastics, whose oil content is usually only a few percent by weight.
Typical properties include tensile yield strength of 4.2 to 4.7 MPa and tensile yield elongation of 13 to 14%. Hardness, at 91 HDA, lies between that of rubber and plastics. The material liquefies when sufficiently heated so it can be processed into almost any form or shape.
The structure releases oil as temperature rises, typically from the frictional-heat generated in a linear bearing. The discharge mechanism is thought to be due to the manner in which lubricating oil is held among polyolefin molecules. Molded oil is formed by melting the polyolefin and oil mixture, then cooling and solidifying the material. Accordingly, polyolefin molecules freeze in an elongated state, but with a residual-stress tendency to shrink. Molecules move more actively when temperature rises. This alleviates stress, and the polyolefin molecules tend to shrink and force oil to the outside. In addition, oil is supplied from inside the structure, not just the edges, ensuring a stable lubrication supply for a long time.
Molded-oil seals, such as the NSK K1, have been installed on linear guides with excellent results in a number of areas. They can be shaped for close contact with rail surfaces as well as provide a constant supply of lubricating oil. Several recent tests run with K1 seals on linear bearings show its performance capabilities.
One case involves linear guide bearings running without external lubrication. Immediately after degreasing the end cap, bearing, and rail, frictional forces rapidly increase on the linear guide with a corresponding decrease in operating performance. After several cycles, however, lubricating oil supplied by the K1 seal reduced friction and restored operating performance to predegreasing levels. This confirms that linear guides can perform well without external lubrication or when subject to periodic degreasing.
Tests in other severe conditions show similar results. In fine-dust environments, particulates that accumulate on the rail tend to absorb grease. Or when water or cutting oil splashes over the guide, grease tends to leak out from the bearing, possibly degrading performance. Under such lean-grease conditions, molded-oil seals continuously supply lubricant as well as help keep dust and water out of the bearing.
Durability tests involving linear guides operating around wood chips show a standard seal combined with a K1 seal offers more than double the life versus standard double seals.
When alternating underwater and dry conditions, results were much the same. Visible signs of wear were evident on the double seal about three times faster than on the K1 seals.
Finally, linear guides were tested at high speeds without lubrication. Standard linear guides became inoperative within a short period, while those with molded-oil seals performed well for more than 25,000 km.
Molded-oil seals are suited for many difficult applications. These include maintenance-free line machinery, and medical and other equipment that requires extreme cleanliness and permits only slight lubrication. Other practical uses are on food-processing equipment subject to frequent washdowns and on wood processing machinery where dust tends to absorb lubrication oil.