sliding bearings are the simplest type of bearing, consisting of just a bearing surface and no rolling elements. Therefore, the journal slides on the bearing surface. The simplest example of a sliding bearing is a shaft that rotates in a bore. A simple linear bearing may be a pair of planes designed to allow movement. Examples include a drawer and the slide on it or a lathe in a way bed one.
Typically, sliding bearings are the least expensive type of bearing. They are also compact and lightweight, and have a high load carrying capacity.
The sliding bearing design depends on the type of motion the bearing must provide. The three possible types of bearings are
Bearing: This is the most common type of sliding bearings; it is simply a shaft that rotates in a bore. In locomotive and railroad vehicle applications, journal bearings specifically refer to sliding bearings, which were once used at the end of railroad wheelset axles and were surrounded by journal boxes. Today’s journal box bearings are rolling bearings, not sliding bearings.
The bearing provides linear motion; it can take the form of a circular bearing and a shaft or any other two mating surfaces.
One-piece sliding bearings built in the use of objects, as in the bearing surface prepared hole. Industrial integral bearings are usually used in bearings from cast iron or pasteurized and hardened steel shafts.
A bushing, also known as a sleeve, is inserted into the housing to provide a separate bearing surface for rotary applications of plain bearings; this is the most common form of plain bearing.
Common designs include solid (sleeve and flange type), split and clamped bushings. A sleeve, split or clamped bushing is simply a “sleeve” that has an inside diameter (ID), an outside diameter (OD) and a length of material. The difference between the three types is that a solid sleeve bushing is always solid, a split bushing has a cutout in the length direction, and a clamping bearing is similar to a split bushing, but with a cutout for a seated (or seized) connection part. Flange bushing is a sleeve bushing, one end of which has a flange extending radially outward from the OD. The flange is used to reliably position the bushing or provide a thrust bearing surface when installing the bushing.
Inch size sleeve bearings are sized almost exclusively using the SAE numbering system. The numbering system uses the -XXYY-ZZ format, where XX is the ID (in sixteenths of an inch), YY is the OD (in sixteenths of an inch), and ZZ is the length (in eighths of an inch). The system dimensions are also present.
Linear bushings are usually not pressed into the housing, but are held in place by radial features. Two such examples include two retaining rings, or a ring molded on the outside diameter of the bushing that matches a groove in the housing. Typically, this is a more durable method of securing the bushing, as the forces acting on the bushing may press it out.
Two-piece sliding bearings, known as full bearings in industrial machinery, are usually used for larger diameter bearings, such as crankshaft bearings. The two halves are called shells. There are a variety of systems available for holding shells. The most common method is a lug on the edge of the parting line that is associated with a notch in the housing to prevent axial movement after installation. For large, thick housings, button stops or locating pins are used. The button stop is screwed onto the housing, while the locating pin locks the two housings together. Another, less common method is to use pins to lock the housings to the shell through holes or slots in the shell.
♦ Pillow blocks: These are standardized bearing mounts designed to accept sliding bearings. They are designed to be mounted on a flat surface.
♦ Ring oiler: A lubrication device used in the first half of the 20th century for medium speed applications.
♦ Stuffing box: A sealing system used to prevent fluid from leaking out of a pressurized system through a sliding bearing.
