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TILTING PAD THRUST BEARINGS

Heinz P. Bloch


Tilting pad thrust bearings are designed to transfer high axial loads from
rotating shafts with minimum power loss, while simplifying installation and
maintenance. The shaft diameters for which the bearings are designed range from
20 mm to more than 1,000 mm. The maximum loads for the various bearing types
range from 0.5 to 500 tons. Bearings of larger size and load capacity are
considered nonstandard but can be made to special order.

Each bearing consists of a series of pads supported in a carrier ring; each pad
is free to tilt so that it creates a self-sustaining hydrodynamic film. The
carrier ring may be in one piece or in halves with various location
arrangements.


MULTIPLE OPTIONS

Two options exist for lubrication. The first is to fully flood the bearing
housing. The second, which is more suitable for higher speed applications,
directs oil to the thrust face. This oil is then allowed to drain freely from
the bearing housing.

Similarly, two geometric options exist. The first option does not use equalizing
or leveling links (Figure 1). This option is used in many gear units and other
shaft systems where perpendicularity between shaft centerline and bearing faces
is assured.


Figure 1. Flooded Lubrication:
Typical Double-thrust Arrangement

Bearings for both flooded and directed lubrication are intended for machines
where an equalized thrust bearing is specified by API requirements, or where the
bearing may be required for other reasons.


FLOODED VS. DIRECTED LUBRICATION

The conventional method of lubricating tilting pad thrust bearings is to flood
the housing with oil, using an orifice on the outlet to regulate the flow and
maintain pressure. A housing pressure of 0.7 to 1.0 bar (10.1 to 14.5 PSI) is
typical, and to minimize leakage, seal rings are required where the shaft passes
through the housing.

Although flooded lubrication is simple, it results in high parasitic power loss
due to turbulence at high speed. Where mean sliding speeds in excess of 50
meters per second (m/s) are expected, these losses may be largely eliminated by
employing the system of directed lubrication. As well as reducing power loss by
typically 50 percent, directed lubrication reduces the bearing temperature, and
in most cases, oil flow.

Some typical double-thrust bearing arrangements using directed lubrication are
shown in Figure 2.


Figure 2. Directed Lubrication: Typical Double-thrust
Arrangements Designed to Prevent Bulk
Oil from Contacting the Collar

It should be noted that:

 * Directed and flooded bearings have the same basic sizes and use identical
   thrust pads.

 * Preferred oil supply pressure for directed lubrication is 1.4 bar (20.3 PSI).

 * Oil velocity in the supply passages should not exceed three meters per second
   (m/s) to ensure full pressure at the bearing.

 * The bearing housing must be kept free of bulk oil through an ample drain area
   around the collar periphery.

 * No seal rings are required on the shaft.

Manufacturers offer a variety of pad materials. Some polymeric materials are
capable of operating at temperatures up to 120°C (248°F) higher than
conventional white metal or babbitt. Also, pad pivot position can have an effect
on thrust pad temperature.



All pads can be supplied with offset pivots, but center-pivoted pads are
preferred for bidirectional running, foolproof assembly and minimum stocks. At
moderate speeds, the pivot position does not affect load capacity; however,
where mean sliding speeds exceed 70 m/s, offset pivots can reduce bearing
surface temperatures and thus increase load capacity under running conditions.

Thrust bearings can be fitted with temperature sensors, proximity probes and
load cells.

In hydraulic thrust metering systems, a hydraulic piston is located behind each
thrust pad and is connected to a high-pressure oil supply. The pressure in the
system then gives a measure of the applied thrust load. Figure 3 shows a typical
installation of this system complete with control panel, which incorporates the
high-pressure oil pump and system pressure gauge calibrated to read thrust load.


Figure 3. Hydraulic Thrust Metering
Arrangement

For systems incorporating load cells or hydraulic pistons, it is typically
necessary to increase the overall axial thickness of the thrust ring.

Finally, thrust bearings incorporate hydraulic jacking provisions. These
provisions ensure that an appropriate oil film exists between thrust runner and
bearing pads while operating at low speeds.

At startup, the load-carrying capacity of tilting pad thrust bearings is
restricted to approximately 60 percent of the maximum permissible operating
load. If the startup load on a bearing exceeds this figure and a larger bearing
is not an option, the manufacturer can supply thrust bearings fitted with a
hydrostatic jacking system to allow the bearing to operate with heavy loads at
low speeds. This system introduces oil at high pressure (typically 100 to 150
bar (1,450 to 2,175 PSI) between the bearing surfaces to form a hydrostatic oil
film.

It should be noted that a similar approach is taken when making hydraulic
jacking provisions for radial bearings. A hybrid thrust bearing is offered by
Kingsbury and Colherne Company (based in the United Kingdom) under the name
KingCole.

The bearing housing requirements for the KingCole LEG bearing are similar to
those of standard thrust bearings. Oil seals at the back of the carrier rings
are not required because the inlet oil is confined to passages within the base
ring assembly. Fresh oil enters the bearing through an annulus located at the
bottom of the base ring. The discharge space should be large enough to minimize
contact between the discharged oil and the rotating collar. The discharge oil
outlet should be sized so that oil can flow freely from the bearing cavity.

The manufacturer recommends a tangential discharge opening, equal in diameter to
80 percent of the recommended collar thickness. If possible, the discharge
outlet should be located in the bottom of the bearing housing. Alternatively, it
should be located tangential to the collar rotation. The bearing pads and
carrier ring are constructed so that cool undiluted inlet oil flows from the
leading edge groove in the bearing pad directly into the oil film. The cool oil
in the oil film wedge insulates the white metal face from the hot oil carryover
that adheres to the rotating collar.

In contrast to the LEG bearing, the oil for spray-fed bearings is injected
between the bearing surfaces, not directly on them. This can result in uneven
bearing lubrication and the need to supply nonpractical high pressure to achieve
true effective scouring of the hot oil carryover adhering to the thrust collar.
There is also a possibility for the small jet holes to clog with foreign
material.

Friction power loss is claimed to be lower than both flooded and spray-fed
bearings due to the reduced oil flow. The flow of cool oil over the leading edge
lowers pad surface temperatures and increases the KingCole’s capacity.

The resulting performance improvements are shown in Figure 4.

  Figure 4. LEG Bearings vs. Standard Flooded Bearings and Spray-fed Bearings

 

Assuming an oil inlet temperature of 50°C (122.4°F), it is possible to estimate
the white metal temperature of KingCole leading edge bearings from Figure 5.
These temperatures are a function of surface speed and contact pressure.

  Figure 5. LEG White Metal Temperatures at 75/75 Position (6- and 8-pad series,
steel pads)

 


BEARING SELECTION

Thrust load, shaft RPM, oil viscosity and shaft diameter through the bearing
determine the bearing size to be selected.

Leading edge bearings are sized for normal load and speed when transient load
and speed are within 20 percent of normal conditions.

All curves are based on an oil viscosity of ISO VG32, with an inlet oil
temperature of 50°C (122.4°F). The manufacturer recommends ISO VG32 oil
viscosity for moderate- through high-speed applications.


Table 1.
Thrust Bearing Designation
Numbers and Bearing Area
(KingCole 8-pad thrust bearings)


TILTING PAD RADIAL BEARINGS

The basic principles of tilting pad journal bearing operation are explained in
the selection guides and related literature of many competent manufacturers. One
of these is Waukesha Bearings of Waukesha, Wisconsin.

Sources
The Glacier Metal Company in London, England and Mystic, Connecticut; Kingsbury
Inc. in Philadelphia, Pennsylvania and Waukesha Bearings in Waukesha, Wisconsin.

Editor’s Note:
This article was published in Heinz Bloch’s book, Practical Lubrication for
Industrial Facilities. This and other lubrication-related books are available
through Noria's Online Bookstore.


About the Author
Heinz P. Bloch

Heinz Bloch works as a consultant for Process Machinery Consulting. He is the
author of more than 400 technical papers and similar publications. He has
written 17 books on practical machinery ma... Read More


Machinery Lubrication (7/2006)


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