Rolling element bearings

Because of the low viscosities of cryogenic Hquids, rolling element bearings seem better suited than hydrodynamic bearings for turbo pumps. AISI 440C stainless balls and rings generally are preferred for their corrosion resistance over the more commonly used AISI 52100 steel. 

Ball and roUer bearings represent the largest business segment with worldwide production estimated at 14 biUion in 1988 (1). U.S. production, forecast for 3.6 biUion in 1991, has fallen 5% aimuaHy for several years (2). This decrease is attributed largely to the slump in the automotive industry which represents 31% of the market for rolling-element bearings (3). 

Table 13. Representative Steels for Rolling Element Bearings...

Table 14. Rolling-Element Bearing Retainer Materials...

Fault to be detected Unbalance Misalignment Bent shaft Oil whirl Eccentricity Wear Faults In gears Faults In rolling element bearings... 

Three types of bearings will be discussed oil-film bearings, rolling-element bearings (also known as antifriction bearings), and magnetic bearings. 

Rolling-element bearings require lubrication for minimizing the friction between the rolling elements and their cage and for dissipat-... 

The selection of rolling-element bearings is based on the radial and axial loads they must support, on the operating speed, and on the expected life. All bearing catalogs provide comprehensive selection data. 

The rotordynamic study of a machine with magnetic bearings is quite different from the one of either oil-film or rolling-element bearings. The stiffness and damping properties of magnetic bearings can... 

The motor carries and supports any axial thrust by the pump (hydraulic or mechanical). The motor on these pumps can be fitting with precision rolling element bearings, either angular contact, spherical rollers, or pillow blocks inclined depending on the thrust load and velocit of the shaft. 

Figure 5-45. Schematic illustration of magnetic bearings and auxiliary rolling element bearings.

Characterized by high frequency response, accelerometers are compact and rugged, ideal for mounting on machinery cases, foundations, piping, etc. Applications to gear trains and rolling element bearings are typical. 

Rotating-machine components, such as rolling-element bearings, impellers, and other rotors, that turn with a shaft. 

Electric motors may incorporate either sleeve or rolling-element bearings. A narrowband window should be established to monitor both the normal rotational and defect frequencies associated with the type of bearing used for each application. 

Screw compressors use both Babbitt and rolling-element bearings. Because of the thrust created by process instability and the normal dynamics of the two rotors, all screw compressors use heavy-duty thmst bearings. In most cases, they are located on the outboard end of the two rotors, but some designs place them on the inboard end. The actual location of the thmst bearings must be known and used as a primary measurement-point location. 

Rolling-element bearings The ability to monitor rolling-element or anti-friction bearing defects requires the inclusion of multiples of their rotating frequency. For example, with ball-pass inner-race bearings, the bandwidth should include the second harmonic (2x). 

Variable-speed machinery Variable-speed machine-train narrowband windows should be converted to their relationship to the running speed (lx). For example, if the frequency of the ball-pass inner-race rolling-element bearing is calculated to be 5.9 times the primary shaft running speed, then the narrowband window should be set as 5.3x to 6.2x. This allows the microprocessor to track the actual bearing rotational frequency regardless of the variation in running speed. 

Rolling-element bearing 0.03 ips-peak 0.05 ips-peak 0.10 ips-peak Rolling-element bearings 0.02 ips-peak 0.03 ips-peak 0.05 ips-peak... 

Damaged rolling element bearings (ball, roller, etc.)... 

With sleeve or Babbitt bearings, looseness is displayed as an increase in sub-harmonic frequencies (i.e., less than the actual shaft speed, such as 0.5x). Rolling-element bearings display elevated frequencies at one or more of their rotational frequencies. Excessive gear clearance increases the amplitude at the gear-mesh frequency and its sidebands. 

Natural frequencies are generated by impacts of the internal parts of a rolling-element bearing. These impacts are normally the result of slight variations in load and imperfections in the machined bearing surfaces. As their name implies, these are natural frequencies and are present in a new bearing that is in perfect operating condition. 

There are four normal rotational frequencies associated with rolling-element bearings fundamental train frequency (FTP), ball/roller spin, ball-pass outer-race, and ball-pass inner-race. The following are definitions of abbreviations that are used in the discussion to follow. 

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