Page 97 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Best Practice 2 .13

Fig 2.13.11 Journal bearing types

thousandth of an inch. There are various types available e refer Fig 2.13.12 Hydrodynamic bearing performance
to Figure 2.13.11.

Sleeve bearings

Sleeve bearings are commonly used as radial bearings in cen-
trifugal pumps. Lubrication is usually supplied by an external
pressurized system, although some slow speed sleeve bearing
applications (dN less than 400,000) can utilize ring oil
lubrication.

Tilt pad bearing
Tilt pad journal bearings are sometimes used for high horse-
power, high speed, centrifugal pumps where rotor stability may
be a concern. They are always oil-pressurized bearings.

Multilobe bearing
This type of bearing has found some limited use in centrifugal
pumps, but it can also be applied on high speed pumps where
increased damping and stiffness is desired for lightly loaded
bearings (most vendors will select tilt pad bearings).

Hydrodynamic bearing performance
When at rest, the journal settles down and rests at the bottom
of the bearing (refer to Figure 2.13.12a). As the journal begins Fig 2.13.13 Bearing pressure distribution
to rotate, it rolls up the left side of the bearing, moving the
point of contact to the left. There is then a thin film of oil
between the contact surfaces, and fluid friction takes over for
metal to metal contact (refer to Figure 2.13.12b). The journal from the load line. The pressure distribution across the bearing
slides and begins to rotate, dragging more oil between the varies and the maximum unit pressure reaches a value about
surfaces, forming a thicker film and raising the journal. As the twice the average pressure on the projected area of the bearing
speed of rotation increases, the oil drawn under the journal (refer to Figure 2.13.13).
builds up pressure that forces the journal up and to the right in The permissible unit pressure (PSI) is a function of static and
the direction of rotation shown, until a condition of equilib- dynamic forces and the dimensions of the bearing (projected
rium is reached resulting in a point of minimum clearance area). Industry guidelines for continuously loaded bearings range
(refer to Figure 2.13.12c). from 50 to 300 PSI. Pressures exceeding 300 PSI can potentially
As the oil film builds up under the journal, the center of the result in breakdown of the oil film and subsequent metal to
journal moves and the location of minimum clearance is away metal contact.

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