MOUNTED BEARINGS

Figure 18 - Solid and split-type pillow block plain (journal) bearings. Figure 19 - Spherical, A, and rod-end, B, plain bearings. Figure 20 - Hydrodynamic pillow block bearing and internal components. Figure 21 - The setscrew locking collar, left, and the eccentric collar, right, are the most common methods used to secure a mounted ball bearing to a shaft.

Mounted bearing units are available with most types of plain and rolling-element bearings. They are convenient and economical, and reduce the time spent selecting and preparing bearing elements, housings, seals, and mounting methods. Mounted bearing units are available off-the-shelf and the only basic selection information generally necessary is shaft size, radial and thrust load, load characteristics, speed, mounting limitations, and environment.

Mounted plain bearings - Several types of mounting are available for plain bearings. A journal bearing mounting, Figure 18, may simply be a bored housing with bearing liner, or a split housing with solid or split internal bearing.

The outer surface of the bearing may have a spherical shape to accommodate a wide range of angular misalignment, Figure 19. These are known as spherical and rod-end bearings. They are commonly used in linkages and low-speed applications.

Hydrodynamic sleeve bearing pillow blocks, Figure 20, operate at low speeds in much the same way as grease-lubricated plain bearings. When speed becomes high enough, however, a full lubricant film is established, eliminating metal-to-metal contact.

Hydrostatic bearings have externally pressurized oil that separates shaft and bearing surfaces. Benefits include no metal-to-metal contact, high load capacity at all speeds, and very low coefficient of friction. The only limit to load and speed is the ability of the external pump to supply pressure for overcoming load and to supply a flow rate sufficient to carry away generated heat. Because an external pump, motor, and other support devices are required, these bearings are impractical in many applications.

Besides providing a pre-engineered support housing for the bearing, mounted bearing units also provide a reservoir for lubricant storage. This reservoir extends relubrication intervals, or may serve as a lubricant supply for the life of the bearing. Mounted plain bearings are available in plain-bored, cast iron, solid pillow blocks; babbitt-lined, solid and split pillow blocks; flange bearings and take ups; bronze-bushed split pillow blocks and take ups; solid-film and self-lubricating bushed pillow blocks; and hydrodynamic and hydrostatic pillow blocks.

Plain-bored iron units, babbitt and bronze-lined, are the simplest. Rigid units require accurate alignment, but babbitt and bronze-lined units wear in and ultimately distribute load over the bearing area. Most babbitt units conform to the shaft, have good embedability, and do not seriously score or damage the shaft under boundary lubrication. Bronze-lined units are recommended for heavier loads, higher temperatures, and shock loads. Most of these units are grease-lubricated. A lubricant film approaching that of hydrodynamic, full-film lubrication separates shaft from sleeve. Automatic lubrication increases life of mounted plain bearings.

Mounted rolling-element bearings - Most ball bearings used with housing units incorporate a wide inner ring with an integral locking collar device. There are two common types: the setscrew collar and the eccentric or cam locking collar, Figure 21. Either type accommodates a slip fit over commercially ground shafting.

The setscrew locking device has two setscrews threaded into drilled and tapped holes in an extension of the bearing's inner ring. An alternate design has a concentric collar that slips over the ring extension. The setscrews are then threaded into the collar, and contact the shaft through untapped holes in the collar extension.

In either case, when the screws are tightened, theoretically a three-point contact is established. These points are the actual two contact points of the setscrews and the one point on the opposite side of the shaft OD.

The eccentric locking collar uses an extended inner ring of the bearing that contains a channel eccentric to the shaft, and a matching channel in a collar that fits over the inner ring extension. The unit is secured to the shaft by rotating the collar relative to the inner ring. When this is done, the force introduced to the rotation is transmitted as holding force perpendicular to the shaft. A setscrew is supplied to prevent loosening of the collar during reverse rotation. This is a precautionary practice, and does not compensate for frequent reverse rotation. Manufacturers agree that eccentric locking collars should not be used for bidirectional applications.

Most mounted ball bearing units have spherical ODs. This compensates for angular shaft misalignment. The bearing may also have a cylindrical OD with a slip-fit in the mounting to allow axial freedom. Cylindrical, spherical, and tapered roller bearings also are readily available as mounted units. Roller bearings are often used for heavier-duty applications than ball bearings, and therefore may require more rigid locking to the shaft.

Selection - The first step in selecting a mounted bearing is determining shaft size, which considers bending and torsional load. However, overhung loads or loads between large centers may require large shaft diameters, while bearing loads are light. If so, consider light-duty bearings or machining shaft ends for smaller bearings. However, shafting is not always chosen on an optimum engineering basis. Design standardization, available stock sizes, and similar factors may dictate size.

Choice of the type of bearing is the next step. The prime consideration is to match bearing design capability to load operating characteristics and maintenance needs. Operating characteristics and allowable friction should lead to the fundamental choice between plain and rolling-element groups - with additional consideration to economy. Within the rolling-element group, selection often involves evaluating types available vs. requirements, then choice of the design that suits a multipurpose application.

The method of securing the bearing to the shaft is determined by considering performance vs. cost. Ease of installation is important to the builder and the maintainer. Maintenance of other portions of a machine may require bearing disassembly. Thus, bearings and bearing assemblies that can be easily installed and disassembled should be used when frequent disassembly is expected.

Choose the housing or mounting unit with regard to its support structure and surroundings as well as to its own strength requirements. Also, consider installation factors such as clearance and structural members required for mounting, because the housing is the interface between structure and power transmission system. In general, housings transfer load to the structure by surface support and contact. Mounting bolts simply locate and secure. When housings are applied so mounting bolts supply support, carefully consider bolt size, bolt hole fit, mounting procedure, or any other factor that may affect capacity.

Also consider housing material strength and configuration. A gray iron pillow block with a thick cross section and reinforcing web may be stronger than a thinner, steel housing, though the tensile strength of steel is greater.