A pump bearing system consists of bearings to carry radial loads and at least one bearing
to carry axial or thrust loads. For horizontally mounted pumps, there are two basic pump
rotor arrangements: the pumping element overhangs the bearing system (Figure 1) or the
pumping element is between bearings (Figure 2). The function of bearings in centrifugal
pumps is to keep the shaft or rotor in correct alignment with the stationary parts under
the action of radial and transverse loads. Bearings that give radial positioning to the rotor
are known as radial or line bearings, and those that locate the rotor axially are called
thrust bearings. In most applications, the thrust bearings actually carry both axial and
radial loads.
Loads on Bearings Centrifugal pump bearings are subject to radial and axial loads that
result from internal pressures acting on the surface areas of the impeller. The resultant
force acting perpendicular to the shaft is considered radial load and the resultant force
acting along the shaft axis is considered axial load. Loads in a centrifugal pump vary considerably
and are approximately proportional to the developed pressure for a given pump
design. Pressure variations occur because of changes in liquid specific gravity, varying
operating speed, changes in impeller diameter and at various operating points on the performance
curve. Centrifugal pump bearings must be carefully selected to operate reliably
when carrying any of these varying loads.
Pump designers utilize various techniques to balance radial and axial loads. Radial
thrust loads are described in Section 2.2.1, along with the influence of impeller and casing
design on the magnitude of radial unbalance. Axial thrust loads are also described in
Section 2.2.1, along with methods of balancing thrust loads to reduce the residual
unbalanced axial loads that must be carried by thrust bearings.
Types of Bearings Rolling-element bearings are used for the majority of pump applications.
They were originally called “antifriction” bearings because the rolling elements “roll”
(and theoretically don’t slide) such that friction is minimized. These bearings can be used
to carry purely radial loads, both radial and axial loads, or purely axial loads, depending
on their design and how they are applied or mounted (installed). Ball bearings and various
types of roller bearing designs (cylindrical, spherical, and taper roller) are commonly
used. Bearing manufacturers’ catalogues usually provide all information necessary to
properly select and apply these bearings.
Ball bearings come in many configurations, such as single and double row with various
contact angles, to handle purely radial loads, combined radial and axial loads, and purely
axial loads. Sometimes ball bearings are mounted in pairs (duplex) or triples (triplex) to
increase load-handling capability or to provide axial thrust handling capability in both
directions, as in the case of an angular contact bearing. The shape of the rolling element
is that of a ball. Ball bearings are considered to be a point-contact bearing, meaning that
the location where the load is transferred from the bearing race to the rolling element is
a small circular area. Ball bearings have a relatively low load rating because the small
area results in relatively high contact stress for a given load (stress = force/area).
Cylindrical roller bearings are used for purely radial loads for applications that require
a higher load carrying capability than a ball bearing. These bearings are available only in
single rows. The shape of the rolling element is that of a cylinder. Cylindrical roller bearings
are considered to be a line-contact bearing because the contact between the bearing
race and the rolling element is in the shape of a line. This shape provides a larger contact
area that reduces the contact stress for a given load as compared to a ball bearing.
Tapered roller bearings accommodate both a radial and axial load. The shape of the
rolling element is that of a truncated cone. These bearings are available only in a single
row configuration but must be mounted in pairs since the axial setting or preload is critical
to proper operation. They are also a line-contact bearing and, therefore, have a relatively
high load carrying capability.
Spherical roller bearings also accommodate both radial and axial loads, though the
radial load rating is considerably higher than the axial load rating. These bearings are
available only in a double row configuration. The shape of the rolling element is that of a
wooden stave barrel. The contact patch is ovular in shape resulting in high load carrying
capability.
When bearing loading or rotating speeds exceed, capabilities of a rolling-element bearing,
hydrodynamic bearings are generally selected. Hydrodynamic sleeve (journal) bearings
can only accommodate purely radial loads (see Section 6.1). The shaft rides on a film
of oil as it rotates inside of a babbit-lined sleeve. These bearings are a line-contact bearing
and have a very high load carrying capability since the bearing contact area is quite large.
Contact area is a misnomer since, during operation, the shaft is supported by a layer of oil
(oil film) that further increases the “contact area.” The bearings are called hydrodynamic
bearings because, during operation, an oil “wedge” is formed between the shaft and the
sleeve, which lifts the shaft away from the bearing metal. Thus “contact” between the shaft
and the bearing metal does not actually occur during normal operation.
Tilting pad thrust bearings are also hydrodynamic bearings and can only accommodate
purely axial loads. These bearings are normally used in conjunction with hydrodynamic
sleeve bearings to accommodate the radial load. Axial loads from the shaft are transferred
through a disk mounted on the shaft to the bearing. A compliment of pie shaped pads is
mounted in the bearing housing facing each side of the disk to handle axial loads in either
direction. These pads are allowed to pivot individually which allows an oil wedge to form
on each of the pads, transferring the load to the housing.
Vertically suspended, magnetic drive, canned motor, and certain types of multistage
pumps utilize internally mounted product lubricated sleeve bearings. These bearings are
often made from rubber, bronze, carbon, or silicon carbide. The fluid flowing between the
bearing and the shaft creates a dynamic centering effect, known as the Lomakin effect.
Magnetic drive pumps, canned motor pumps, and some multistage designs also utilize
product-lubricated thrust bearings or thrust balancing devices for axial thrust, whereas
vertically suspended and horizontal multistage pumps normally handle axial thrust with
externally mounted rolling-element or hydrodynamic thrust bearings.
A pump bearing system consists of bearings to carry radial loads and at least one bearing
to carry axial or thrust loads. For horizontally mounted pumps, there are two basic pump
rotor arrangements: the pumping element overhangs the bearing system (Figure 1) or the
pumping element is between bearings (Figure 2). The function of bearings in centrifugal
pumps is to keep the shaft or rotor in correct alignment with the stationary parts under
the action of radial and transverse loads. Bearings that give radial positioning to the rotor
are known as radial or line bearings, and those that locate the rotor axially are called
thrust bearings. In most applications, the thrust bearings actually carry both axial and
radial loads.
Loads on Bearings Centrifugal pump bearings are subject to radial and axial loads that
result from internal pressures acting on the surface areas of the impeller. The resultant
force acting perpendicular to the shaft is considered radial load and the resultant force
acting along the shaft axis is considered axial load. Loads in a centrifugal pump...
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