CHAPTER FIVE

Thrust Bearings

Chapter Outline
Antifriction Thrust Bearings 53
Hydrodynamic Thrust Bearings 56

This chapter will focus on monitoring and trending parameters for Thrust
Bearings.
Just like radial bearings, there are only two types of thrust bearings used
among all the equipment types, antifriction and hydrodynamic.

ANTIFRICTION THRUST BEARINGS

Most single-stage pumps do not exhibit a lot of thrust, and are
typically fitted with an antifriction bearing that can accept some thrust
load. Fig. 5.1 shows a centrifugal pump fitted with antifriction ball bearings.
Note the bearing closes to the coupling is wider than the one closest to the
process. The wider bearing is what is called a double row angular contact ball
bearing.
Fig. 5.2 shows a cutaway of this type of bearing to see how it works.
You can see in Fig. 5.2 that there are two rows of rolling elements (balls)
and there is shoulder at the outer diameter for each of the rows of balls
toward the center of the assembly. These shoulders allow the balls to absorb
the thrust force in either direction, which is enough axial support for a single
stage pump. Remember, this bearing is also supporting the rotor in the radial
direction at the same time.
Table 5.1 should look familiar, as it is the same table used in Chapter 4 to
outline the parameters taken on a monthly basis by the condition monitoring
group. Axial vibration is the key parameter used to monitor the thrust com-
ponent absorbed by the angular contact bearings.
Typically, the parameters listed in Table 5.1 are taken on a monthly basis
using a portable data logger (CSI, etc.), but it is highly recommended to have

Forsthoffer’s Component Condition Monitoring © 2019 Elsevier Inc. 53

https://doi.org/10.1016/B978-0-12-809599-7.00005-1 All rights reserved.
54 Forsthoffer's Component Condition Monitoring

Fig. 5.1 Pump with antifriction ball bearings.

Fig. 5.2 Double row angular contact bearing. (Courtesy of www.skf.com).

Thrust Bearings 55

Table 5.1 Antifriction Bearing Monitoring

Pump Bearing System Condition
Monitoring
Item #
Date
Housing temperature (Deg F)
Outboard end
Coupling end
Housing vibration (in/s)
Outboard journal horizontal
Outboard journal vertical
Coupling journal horizontal
Coupling journal vertical
Coupling end axial vibration
Oil condition
Appearance
Water content
Constant level oiler condition
Bottom bracket oil condition monitor? (Y/N)
Action required

critical and bad actor pump casings equipped with permanent accelerome-
ters, so the values can be continuously monitored and trended.
When vibration has increased by more than 20%, the vibration group
should capture a spectrum analysis to analyze at what frequency the majority
of the vibration increase occurred (as discussed in Chapter 4).
Fig. 5.3 summarizes the parameters to monitor angular contact bearings
and their threshold limits.

Angular contact bearings—parameters to

monitor
• Bearing bracket axial vibration—An increase by 20% of baseline
should warrant a vibration frequency check to analyze what is causing
the vibration increase.
• Bearing bracket temperature—If temperature increases by 20%, look
at oil condition and water content in bearing bracket
• Remember to always look at performance on same timeline as axial
vibrations, as thrust force will change depending on where you are
operating on the performance curve!!!
Fig. 5.3 Parameters to monitor for antifriction angular contact bearings.
56 Forsthoffer's Component Condition Monitoring

HYDRODYNAMIC THRUST BEARINGS

Just like hydrodynamic journal bearings, hydrodynamic thrust bear-
ings use oil to support the rotor. The bearing itself has a clearance (depends
on bearing design and equipment manufacturer but usually between 14 and
18 thousandths of an inch total clearance) on the thrust disc, and oil is
injected between the bearing and thrust disc. The oil wedge created (the
oil between the bearing and thrust disc) is only about 20–25 μm tick, about
the thickness of a human hair. Oil can handle a load of approximately 500 psi
before it breaks down, so the equipment OEM selects a bearing design and
size (and size of the balance drum appropriately) to be loaded at a maximum
of 250 psi at normal operating conditions. See Figs. 5.4–5.6 for pictures of a
typical hydrodynamic tilting pad thrust bearing which is used in the majority
of compressors and steam turbines.
For a hydrodynamic thrust bearing, vibration probes (the same as for a
hydrodynamic journal bearing) charged with a DC current measure the axial
position of the shaft in mils or mm. A change in axial position by 20% should
warrant further investigation into the cause.

Thrust collar

Fig. 5.4 Hydrodynamic tilting pad thrust bearing assembly. (Courtesy of Elliott company).
 Thrust Bearings
Shoe and leveling plate

Base ring

Six shoe assembly

Collar

Shoes

Leveling plates

Fig. 5.5 Tilting pad thrust bearing components. (Courtesy of Kingsbury, Inc).

57
 58
Shoe
Collar

Forsthoffer's Component Condition Monitoring

Leveling plates Base ring
Fig. 5.6 Tilting pad thrust bearing—top view. (Courtesy of Kingsbury, Inc).
Thrust Bearings 59

A number of pads, usually two on each side of the thrust disc, are fitted
with RTDs to measure temperature of the pads, which corresponds to load
on the thrust bearing. If you see an axial position shift of 20% or more, you
should see the pad temperature increasing in the direction of the shaft move-
ment. If not, this indicates that the rotor has not exerted more loads on the
thrust pads. If thrust pad temperature has increased by 20%, the cause should
be investigated.
The final, and probably most important, parameter to monitor for a
hydrodynamic thrust bearing is the balance line differential pressure. See
Fig. 5.7 that shows the concept of the balance drum.
The purpose of the balance drum is to limit the load on the thrust bear-
ing, by taking a large differential pressure (many times close to the full dif-
ferential across the machine) across it. The pressure on one side of the
balance drum is total discharge pressure, and the balance drum has a tight
clearance labyrinth seal on the out diameter and a port on the other side that
goes back many times to suction of the machine to equalize the pressure on
the other side very close to suction pressure. This large differential pressure
acts on the cross-sectional area of the balance drum to push the rotor in the
opposite direction of normal thrust in the machine.
Usually there are pressure taps, one close to the casing where the balance
line exits and the other close to where it goes back into the machine (suction

Rotor thrust force

PF

Final impeller

Pi*
PN Bearing Thrust
Seal
bearing
Shaft

Balance drum * Referenced to P inlet, P ATM, or a lower pressure.

Total impeller thrust (LB) = Σ Individual impeller thrust
Balance drum thrust (LB) = (PF–Pi) x (Balance drum area)
Thrust bearing load (LB) = total impeller thrust–balance drum thrust
Examples of rotor thrust
I 6-Stage series II 6-Stage opposed III 6-Stage series
(no balance drum) (no balance drum) (balance drum)
intercooled intercooled intercooled
Thrust load Thrust load = 0 Thrust load = 0

Balance drum

Fig. 5.7 Balance drum.

60 Forsthoffer's Component Condition Monitoring

or a lower pressure stage). The differential pressure is then measured across

the balance line. The key, again, is to have a baseline of the balance Line DP
and if this DP increases over time, it indicates that the labyrinth seal has worn
around the balance drum. If this happens, you will see an increase in axial
position toward the active pads and a corresponding increase in pad temper-
ature in that direction. If the balance drum laby is worn, this requires open-
ing the case to fix, while replacing just the thrust bearing will not do
anything. Therefore, it is essential to monitor the balance line DP to assure
you do not perform any excessive unnecessary maintenance on the thrust
bearing. A balance drum laby replacement can be done at the next planned
shutdown. It is highly recommended to have a DP transmitter installed in
order to have this parameter brought into the DCS and trended in real time.
See Table 5.2 which is a spreadsheet detailing the parameters needed to
be monitored and trended for thrust bearings on a steam turbine-driven
compressor train.
See Fig. 5.8 for a summary of parameters to monitor for a hydrodynamic
thrust bearing and threshold limits.

Table 5.2 Bearing Monitoring Parameters for a Steam Turbine Driven Compressor Train
Component Condition Monitoring
Worksheet
Item #:
Date/Time:
Journ. brgs.
Compressor DE horiz. vibes (micron)
Compressor DE vert. vibes (micron)
Compressor DE pad temp (Deg C)
Compressor DE pad temp (Deg C)
Compressor NDE horiz. vibes(micron)
Compressor NDE vert. vibes (micron)
Compressor NDE pad temp (Deg C)
Compressor NDE pad temp (Deg C)
Steam turbine DE horiz. vibes (micron)
Steam turbine DE vert. vibes (micron)
Steam turbine DE pad temp. (Deg. C)
Steam turbine DE pad temp. (Deg. C)
Steam turbine NDE horiz. vibes (micron)
Steam turbine NDE vert. vibes (micron)
Steam turbine NDE pad temp. (Deg. C)
Steam turbine NDE pad temp. (Deg. C)
Thrust Bearings 61

Table 5.2 Bearing Monitoring Parameters for a Steam Turbine Driven

Compressor Train—cont’d
Component Condition Monitoring
Worksheet
Thrust brgs.
Compressor displ.
Compressor displ.
Compressor active pad temp. (Deg C)
Compressor active pad temp. (Deg C)
Compressor inactive pad temp. (Deg C)
Compressor inactive pad temp. (Deg C)
Balance line diff. P (kg/cm2)
Steam turbine displ.
Steam turbine displ.
Steam turbine active pad temp. (Deg. C)
Steam turbine active pad temp. (Deg. C)
Steam turbine inactive pad temp. (Deg. C)
Steam turbine inactive pad temp. (Deg. C)

Hydrodynamic thrust bearing—parameters to

monitor
• Axial position—A change by 20% should be confirmed by pad
temperature increase. Typical alarm limit is approximately 15—20 mils
(0.4 to 0.5 mm)
• Pad temperature—An increase in 20% indicates a load increase on
thrust bearing. Alarm for thrust pad temperature is around 220 F
(108 C)
• Balance line DP—Should be trended, an increase corresponding with
increased axial displacement towards the active pads and increased
pad temperature indicates the case should be opened to replace the
balance drum labyrinth seal at next planned shutdown.
Fig. 5.8 Hydrodynamic thrust bearing parameters to monitor.

Note the comment in Fig. 5.8 regarding axial displacement typical alarm.
This is assuming that the thrust bearing monitor is set up where 0 is in the
middle of the total thrust bearing clearance. This is the recommended setup
so you can easily see that a positive value is toward the suction side pads and a
negative number is toward the discharge pads. See Fig. 5.9 showing a typical
thrust bearing monitor, illustrating this.
62 Forsthoffer's Component Condition Monitoring

Displacement mils
30 30
25 25 N
o
20 20 r T
T m
15 15
A
10 10 a A
l Key:
5 5
0 0 A = Alarm
–5 –5 c
o T = trip
–10 –10
A u A
–15 –15 n T
T Note: Alarm and
–20 –20 t
e trip settings are
–25 –25 r determined based
–30 –30
on vendor’s
A A recommendations
O O
D D
Normal Counter

Fig. 5.9 Typical thrust bearing monitor.

Note, the alarm and trip settings shown are recommended based on API,
but need to be based on the vendor recommendations for your specific
machine.
Now, we will be discussing the key parameters to monitor for compo-
nent # 4, seals, in all types of rotating machinery.