Surfaces that slide upon one another can be lubricated by one of two mechanisms, (a) heavily loaded surfaces working at low speed depending upon the oiliness of the lubricant and (b) high-speed bearings depending upon its viscosity. In the latter, conditions must be adjusted so that an adequate supply of lubricant will be provided and an opportunity given for it to be trapped between the surfaces and actually to wedge them apart. The bearing must therefore have a certain clearance over the journal. Oil-grooves must supply means for the oil to enter the bearing and the assurance that it cannot escape without doing its work. In general, they should not be placed on the loaded side of the bearing. The tendency is to draw the oil from the point of minimum pressure through that at which the pressure is the maximum, and for the oil to spread out and travel in a spiral along the bearing toward the ends.
With a view to making an investigation of the behavior of the oil after it has reached the bearing, a visual study was made by means of a glass bearing and the results were reproduced by a film, the action of the lubricant being made visible by introducing into the oil a small quantity of dyed glycerine solution of about the same viscosity as the oil. A description is given of the results obtained.
It is a well-recognized fact, at least in literature, that two sliding surfaces can be lubricated by one or the other of two mechanisms. One, occurring primarily in bearings that are heavily loaded and working at slow speed, is due to oiliness. The other is due to the viscosity of the lubricant and is called fluid-film lubrication. This consists of a film of fluid that is maintained between the two rubbing surfaces in spite of the load that may be carried and is the type that must be maintained in high-speed bearings. By high-speed bearings are meant bearings such as those of automobile engines and, of course, all bearings operating at higher speeds.
The fluid film is formed by the action of the lubricant in adhering to the journal, and, in the case of a revolving journal, in dragging the lubricant along with it between the rubbing surfaces. Conditions must therefore be adjusted so that an adequate supply of lubricant will be provided and an opportunity be given for it to be trapped between the rubbing surfaces and actually to wedge them apart. Consequently, the bearing must have a certain clearance over the journal. In addition to the proper clearance, a certain flow of oil through the bearing must be present from some point at which the journal can pick it up, through the portion that is carrying the load and out at some other point.
A bearing fitted with the proper clearance normally has all the exit space for the used oil that is required and at times the end leakage from bearings is entirely too high. Another item that is important in designing bearings is the providing of a proper means for the entrance of the oil into the bearing and the assurance that the oil cannot escape without doing its work. This requires that oil-grooves be in the proper location for the oil-supply, and making sure that no oil-grooves are located where a surface for carrying the load should be located. In general, no oil-grooves should be placed in the loaded side of the bearing.