K*+k
K,*K, + K-

The stiffness of the individual stages can be determined using
the bulk modulus of the oil as shown in equation 8. The
manufacturer of the oil can provide accurate data on the bulk
modulus. A value of 320,000 psi is recommended in the absence
of manufacturer data. This value can also be used for water-oil
emulsions such as those used in longwall shields.

A*B

KA ®

where K, = stiffness of stage n, Ibs/in
A= area of stage n, in?
B = bulk modulus of the oil, psi
L = stage extension, in

Since the cylinder structure (i.e., casing) is not perfectly rigid,
the stiffness of the cylinder structure should also be considered in
the calculation of the overall leg cylinder stiffness. The cylinder
casing can also be considered to act in series with the hydraulic
stiffness to provide a total stiffness as shown in equation 9. The
stiffness of the cylinder casing will vary depending on the design
of this steel structure. A study of mobile roof support hydraulic
cylinders revealed cylinder structural stiffness of 2,300 kips/in for
a two-stage cylinder and 3,200 kips/in for a three-stage cylinder
(kip = 1,000 Ibs).

Kk. = Katyatic * Keytar )

System
Kyiyarautie + Koyinder

where Keuuem = Overall leg cylinder stiffness, Ibs/in
Kyyanie = Hydraulic (oil component) stiffness, Ibs/in
Kinder = Cylinder structural stiffness, Ibs/in

Key Points in Determining Support Stiffness

A close examination of these stiffness principles reveals
several key points regarding the determination of the stiffness of
hydraulic support systems

1. The stiffness of a hydraulic roof support is not significantly
affected by the setting pressure.

2. The stiffness of a hydraulic roof support is @ nonlinear
function of operating height, decreasing exponentially as the
height increases.

3. The diameter of the hydraulic cylinder primarily controls the
stiffness of the hydraulic support with increased stiffness as
the diameter of the cylinder increases. However, an
important point to remember in multi-stage cylinders is that
the smallest (diameter) stage will have the least stiffiness and
therefore largely control the overall stiffness of the support
This is why a three-stage hydraulic cylinder will always be
less stiff than a two-stage hydraulic cylinder with the same
size (diameter) bottom stage.

4. The stiffness of a multi-stage hydraulic cylinder is also
dependent on the extension of the respective stages. The
stiffness of each stage decreases exponentially as the stage
extension increases. The top stage will produce the most
influence on the support stiffness. The stiffness of the
support increases dramatically when the top stage is not
extended at all or only very slightly extended. In this
condition, the support can be as much as 10 times stiffer than
when the all stages are extended greater than 75 pct of their
stroke, A minimal extension on the top stage of 3 in reduced
the stiffness of a mobile roof support two-stage leg cylinder
by 45 pct compared to no extension of the top stage.

5. In supports using multi-stage hydraulic cylinders, the same
overall support stiffness can occur at two different operating
heights, depending on the combination of stage extensions
In other words, 5 in of bottom stage extension and 15 in of
top stage extension will not produce the same stiffness as
15 in of bottom stage extension and 5 in of top stage
extension. Figure 5 depicts a stiffness envelope for a 150-ton
two-stage hydraulic cylinder used on a 600-ton mobile roof
support. For this particular cylinder, a stiffness of 400 kips/in
‘occurs at operating heights of 64 and 75 in. Likewise, at a
64 in operating height, the stiffness can vary from 400 to
620 kips. A theoretical stiffness envelope can be determined
by calculating the individual stage stiffness (eq. 8) and the
system stiffness (eq. 9) at different stage extensions.

300
00 NOTE: 150 TON LEG CYLINDER FROM
|__ MOBILE ROOF SUPPORT UNTT
5 70
= 600 \
2 x }
400
Z 300
E 200 — ——'
100 a
°
0 6 70 80 1 tg
(OPERATING HEIGHT, inches
Figure 5 Stiness envelope for a two-state 150-on hydrauic mobile roof support
yinser

6. The total displacement in a hydraulic cylinder will equal that
of the roof-to-floor convergence, but this displacement will
be distributed differently among the stages in proportion to
the stiffness of the individual stage. The stiffest stage will
move (displace) less than the softest stage. This means that
most of the convergence will occur in the top stage of multi-
stage hydraulic cylinders.

Impact of Stiffness Issues on Ground Control

A stiffer support will develop more load carrying capacity for
the same roof-to-floor convergence than will a softer support
(fig. 6a). Conversely, a softer support will allow greater roof
movement to occur, assuming that the supporting force is
controlling the roof movement (fig. 6b)

The following recommendations are made regarding design
considerations for hydraulic roof support systems in regards to
the stiffness of the support


6,500

— GOOD LEG CYLINDER

LEG WITH BAD INTERNAL CHECK VALVE

6,000

5.500
5,000
4500 +
4.000

3:50

LEG PRESSURE, pal

| Ff

3,000

2,500
ee
ADDITIONAL SHIELD LOADING AFTER SETTING, tons

Figure 10.-Bottom stage pressure respense with bad stage checking vave,

Safety should also be exercised when removing the hydraulic
cylinders from the roof support. Both longwall shields and
mobile roof supports will become unstable once the hydraulic
cylinders are removed, and it is critical that the structure be
properly supported when removing these supporting elements.
Repairs should be made by a reputable shop with expertise in
hydraulic systems. It is critical that the cylinders be load tested
before and after the repair to make sure that all components are
working properly. Rebuilding of a hydraulic cylinder will
generally entail replacing all seals and check valves, machining
and rechroming the cylinders bores and pistons to bring them
back to design specifications.

When the cylinders are rebuilt, the ports are often plugged to
keep the fluid in the cylinder. One common mistake that is made
is not to remove the retract port before attaching the pressure line
and extending the cylinder. The bottom stage area is typically
greater than the retract annulus area by a factor of 8 to 10. Ifthe
retract port is blocked when extend pressure is applied to the
cylinder, the pressure in the retract annulus will be intensified by
a factor of 8 or 10. For example, a 4,000 psi setting pressure will
produce 40,000 psi of pressure in a blocked retract annulus. This
magnitude of pressure will literally rip the threads off the end cap
of the cylinder casing.

CONCLUSIONS

Hydraulic roof support systems are used extensively in
underground mining for critical roof support in many areas. The
performance characteristics of these hydraulic supports need to
be fully understood to optimize the use of these hydraulic support
systems and to evaluate problems when they occur. A proper
understanding of these high pressure systems is also necessary to
prevent injury during repair operations.

The following conclusions are made regarding the
performance characteristics of hydraulic support systems and
their impact on ground control capabilities,

1, Setting forces are reduced whenever the bottom stage is fully
extended, which occurs on operating cycles which establish

a new maximum operating height. The setting force will

always equal the pump pressure times the area of the largest

stage which is not fully extended, Force reductions of 40 to.
50 pet are common for two-stage hydraulic cylinders
Reductions as large as 70 pct are common for three-stage
cylinders when both the bottom and middle stages are fully
extended,

2. The stiffness of a support controls the development of the
support loading in response to the ground movements that
cause roof and floor convergence. Several conclusions are
made in reference to the stiffness of hydraulic support
systems.

(a) Stiffness is not significantly affected by the setting
pressure.

(b) Generally, stiffness is a nonlinear function of operating
height, decreasing exponentially as the height increases
However, in multi-stage cylinders, the stiffness is also
dependent on the combination of stage extensions. The
same stiffness can occur at different operating heights,
and certain combinations of staging can produce greater
stiffness at higher heights.

(c) Stiffness is primarily controlled by the diameter of the
hydraulic cylinder with increased stiffness as the diameter
of the cylinder increases. In multi-stage cylinders, it is the
smallest (diameter) stage that largely controls the stiffness
of the support. Higher capacity longwall shields tend to
be stiffer due to the use of larger diameter hydraulic
cylinders. Two-stage systems should be used whenever
possible, since two-stage systems are stiffer than three-
stage systems for the same (bottom stage) cylinder size.

3. The supporting force of a longwall shield is reduced by
approximately 10 pct each time it yields due to excessive
hydraulic pressure which opens the yield valve. Yielding
causes an increase in shield stiffness due the reduction in
cylinder length

4. The support loading (both setting forces and a large
percentage of the post setting load development) cannot be
determined from bottom stage pressure measurements
whenever the bottom stage is fully extended.

5. Both internal and external hydraulic leakages will reduce the
support capacity to zero under static loading conditions. The
time required for complete loss of load carrying capacity
depends on the initial loading and the leakage rate, and
thereby ranges from a few minutes to a few hours.

6. Observations of the relative positions of the cylinder staging
can be used to identify cylinder problems and the cause of
hydraulic leakages. One tell-tale sign of internal leakages is
when the bottom stage is consistently fully extended
Another sign is when there are several inches of difference in
staging on leg cylinders on the same support. Leakages
associated with the staging check valve can be isolated by
fully collapsing the shield and monitoring the leg pressures on
the operating cycle after the support is reset against the mine
roof and floor. An immediate increase in pressure in the
bottom stage indicates that the check valve is leaking
sufficiently to not allow the pressure in the top stage to be
intensified.

Finally, safety precautions should be made when working
with these high pressure hydraulic support systems. The fluid
pressure is more than adequate to cause serious bodily harm


A P = Yield pressure - reseating pressure, psi
V = Volume of fluid in bottom stage when yielding, in?
V = Area of bottom stage times extension of bottom

stage, in®
B = bulk modulus of oil, psi (i.e., 320,000 psi)
AV
AL = =
A ay

where A L = reduction in bottom stage extension due to
yielding, in
A V = reduction in volume of fluid in bottom stage due to
yielding, in?
‘A= Area of bottom stage, in?

ERRORS IN ASSESSING SUPPORT LOADING

Support loading is determined from measurement of the
pressure in the bottom stage of the hydraulic cylinder. Since the
pressure increases in direct proportion to the increase in support
loading, this provides an accurate assessment of the roof loading
through the full loading cycle.

However, when the bottom stage is completely extended to
its full stroke, a portion of the increase in roof loading after the
support is set against the mine roof and floor will not be detected
by changes in hydraulic pressure in the bottom stage (see
figure 7), The reason for this period of undetected roof loading
is that when the bottom stage is fully extended, the bottom stage
piston is being held against the mechanical stops with a force
exerted by the pump pressure at the time the support was set.
The pressure in the bottom stage will only increase when this

Initial loading

Additional loading

Figure 7.- Initial roof loading after the support is set will be
undetected by changes in bottom stage pressure whenever the
bottom stage is fully extended.

piston is moved off the stops and begins to compress the
hydraulic fluid in the bottom stage. In order for this to happen,
the pressure in the upper stage, which is at pump pressure when
the support is set, must increase to cause a force in the upper

stage that exceeds the setting force in the bottom stage. The
additional roof loading that is required to produce this additional
pressure in the upper stage is the roof loading that is undetected
by the gages measuring hydraulic pressure in the bottom stage

The mechanics of the pressure development in the various
support stages can be described by examination of equilibrium
requirements for each individual stage, realizing that each stage
must carry the same load (1). The undetected roof loading for a
two- and three-stage hydraulic cylinder with the bottom stage
fully extended can be calculated using equation 12. Equation 13
must be used for a three-stage cylinder where both the bottom
and middle stages are fully extended. An examination of these
principles indicates that the period and magnitude of undetected
roof loading will increase as the setting pressure increases, The
undetected roof loading occurs immediately after the support is
set. The remaining shield capacity (rated capacity minus setting
load and undetected load) will be measured by the pressure

gages.

1) * Py 7 A, * P,

f

URL ro-soge | Three stage 7000 (12)
A, * Py - A, *P,

URL. ee ee ee ee | 13

“Taree-stage 2000 (13)

where URL = undetected roof load, tons
A, = area of first (bottom) stage, in?
ea of second stage, in
A, = area of third stage, in?
P, = P, = P, = setting (pump) pressure, psi

Figure 8 depicts undetected roof loading for the following
state-of-the-art hydraulic support systems: (a) 700-ton (two-
stage) longwall shield, (b) 600-ton (two-stage) mobile roof
support, and (c) 600-ton (three-stage) mobile roof support set at
full pump pressure. It is seen from this figure that the unrecorded

~ HUNDETECTED ROOF LOAD |

MISET FORCE
DETECTED ROOF LOAD.

600 TON MRS-3 stage cylinder (bottom & middle stages fully

600 TON MRS — 3 stage leg cylinder (bottom stage fully extended)

3

600 TON MRS ~ 2 stage leg cylinder

700 TON LONGWALL SHIELD

400 500 600700800
LOAD, tons

100 200-300

Figure 8.-Magnitude of undetected roof loading for various
support systems.


WH dda

ROOF DISPLACEMENT

ZZ YMA

SOFT SUPPORT
50 TONS

STIFF SUPPORT
100 TONS:

WLLL

Figure 6a.-Stiffer support develops more loading due to roof
and floor movement.

a

SOFT STIFF
SUPPORT SUPPORT

Figure 6b,-Softer support will allow more roof movement to
occur.

1. Two-stage systems should be used whenever possible, since
two-stage systems are stiffer than three-stage systems for the
same (bottom stage) cylinder size and yield pressures. If
three-stage cylinders are required to obtain the desired
operating range, consideration should be given to making the
top stage diameter sufficiently large to provide the required
stiffness, even if this requires a larger diameter bottom stage.

2. Since hydraulic support systems are considerably less stiff at
high operating heights, they are less efficient at controlling
roof movement when used at high operating heights.
Therefore, when designing a support, the collapsed height
should be no lower than is necessary. This practice will
minimize the operating range and provide the least reductions
in support stiffness as the operating height increases.

Since the stiffness of the support changes as function of the
leg staging, the interpretation of loading on hydraulic support
systems may require knowledge of the leg staging, particularly if
the support forces are not controlling the main roof movements
which is generally the case in longwall mining, In essence, the

developed support loading due to roof/floor movement will be
greater when the support stiffness is greater. This issue also
complicates the utilization of load rate monitoring devices, such
as those being developed for mobile roof supports to be used to
assess impending strata caving. The rate of loading is likely to be
dependent on the stiffness of the support, hence changes in the
support configuration due to leg staging and operating heights
will cause the loading rates to change for the same roof behavior
(movernent).

YIELDING BEHAVIOR,

Yield valves are normally connected to the bottom stage of
multi-stage hydraulic cylinders to provide overload protection.
These valves open whenever the pressure exceeds the design
threshold and allows the pressurized fluid to escape from the
bottom stage of the cylinder. This loss of fluid causes the
pressure to drop in the bottom stage until the valve reseats. The
reseating pressure is typically about 90 pet of the yield pressure,
hence the support capacity will drop by approximately 10 pct
when the yield valve opens. As the pressure drops in the bottom
stage, a force imbalance occurs between the top and bottom stage
(in a two-stage hydraulic cylinder), which causes the bottom
stage to lower until force equilibrium between the two stages is
attained. The lowering of the bottom stage is associated with
lowering of the mine roof, hence excessive yielding is not
desirable from a ground control perspective. Successive yielding
events will eventually cause the bottom stage to be fully
collapsed, at which time the top stage will begin to collapse
Additional yielding can continue until the top stage is fully
collapsed, at which time the support is considered to be iron-
bound.

How much the bottom stage lowers due to yielding depends
on three factors: (1) the area of the bottom stage, (2) the
extension of the bottom stage, and (3) the yield pressure and
reseating pressure of the yield valve. The reduction in support
height due to yielding which equates to roof lowering will be
more for large extensions of the bottom stage than for small
extensions of the bottom stage. The magnitude of bottom stage
lowering due to yielding can be calculated using equations 10 and
11. Using a 700-ton longwall shield with a bottom stage
diameter of 300 mm and a yield pressure of 6,389 psi as an
example, the bottom stage of each hydraulic cylinder will drop
approximately 0.028 in during yielding if the bottom stage was
extended 14 in when yielding began. A reduction in support
loading of 70 tons would be caused by the yielding. As the
support yields, it also becomes stiffer due to the reduction in
‘operating height (stage extension). Hence, less roof movement
will be required to produce additional yielding

AP
6

AV =

(10)

where A V = change in volume of fluid in bottom stage of
cylinder, in’
A P = change in hydraulic pressure in bottom stage of
cylinder, psi


DETECTION OF COMPONENT FAILURES.

The first point to remember about hydraulic leaks is that in all
cases there is a loss of pressure, and this pressure loss will
continue until there is no pressure in the cylinder (assuming the
leakage rates are greater than the pressure increases caused by
any additional roof loading). A consistent decline in pressure on
each operating cycle after the support is set against the mine roof
and floor is an indication of a hydraulic leak. Significant pressure
losses during idle shifts are also good indications of cylinder
problems.

Observations of the relative positions of the cylinder staging
can be used to identify cylinder problems and the cause of
hydraulic leaks. As previously described, these leaks are often
internal with no visible external fluid leakage. The first indication
of internal leakages is when the bottom stage is consistently fully
extended. Recalling the previous discussion on multi-stage
cylinder operation, the bottom stage should be fully extended
only on operating cycles which establish a new maximum
operating height. Hence, on the majority of operating cycles,
the bottom stage should not be at full extension.

Another indication of problems is when there is a large
difference in stage extensions of leg cylinders on the same
support. For example, on longwall shields the left and right leg
should be at nearly the same extension. Some differences in
extension may result from uneven roof heights across the width
of the canopy, but these differences should be small and will tend
to balance themselves out over the course of many operating
cycles. A large difference, such as those shown in figure 9, is an
indication of a hydraulic leak that can result in no support
capacity. Table 3 depicts the stage movements associated with
each component failure (leakage). For two-stage hydraulic

IMBALANCE IN
BOTTOM STAGE
EXTENSION -

Figure 9.-Imbalance in stage extension is an indication of
hydraulic leakages.

cylinders, it is noticed in this matrix that the bottom stage
displaces upward or downward due to specific problems, while
there is no movement in the top stage. This matrix can be used
to provide insight into the nature of the problem. For example,
when the bottom stages are consistently fully extended, the
matrix indicates that this condition is likely caused by a bad
staging check valve or bad top stage seals. Likewise, bad bottom

stage seals will likely result in the bottom stage being extended
less than the bottom stage on adjacent cylinders.

Table 3. Relative movement and resulting hydraulic pressure of
the bottom and top stages for a two-stage hydraulic cylinder for
specific component failures (source of leakage) for static loading
conditions.

COMPONENT |Top Stage [Bottom Stage| Bottom Stage | Top
FAILURE [Movement] Movement | Pressure | Stage
Pressure

[Staging check None Up 0 0
[valve
First stage seals | None Down 0 0
ISecond stage | None Up 0 0
seals
[Pilot operated | None Down 0 r)
Jcheck valve

ield valve None Down 0 0

Combinations of component failure will produce a resultant
movement of the bottom stage that depends on the pressure
changes in the bottom and top stage. For example, failure
(leakage) of the bottom stage seals causes the bottom stage to
move down, while failure (leakage) of the top stage seals causes
the bottom stage to move up. If the leakage rates are equal, the
bottom stage will move up if the volume of the bottom stage is
greater than the volume of the top stage, and the bottom stage
will move down if the volume of the top stage is larger than the
volume of the bottom stage.

Problems with the staging check valve can be isolated by fully
collapsing the shield and monitoring the leg pressures on the
operating cycle after the support is reset against the mine roof
and floor. The requirement is to have the bottom stage fully
extended when the support is reset. In this configuration with a
staging check valve that is functioning properly, the pressure in
the bottom stage will not change significantly until the force in
the top stage due to additional roof loading overcomes the setting
force developed in the bottom stage (see figure 10). An
immediate increase in pressure in the bottom stage indicates that
the check valve is leaking sufficiently to not allow the pressure in
the top stage to be intensified.

REPAIR AND MAINTENANCE OF HYDRAULIC
CYLINDERS

All hydraulic support systems will require repair and rebuild
sometime during the life of the roof support system. Since these
systems are operating at high pressures, safety should always be
exercised when working with hydraulic cylinders, Only 300 psi
of hydraulic pressure is required to produce a stream of hydraulic
fluid that can puncture the skin and cause serious bodily harm.
In addition, bacteria in hydraulic fluids can be toxic and cause
serious infection and health problems if left untreated


Performance and Safety Considerations of Hydraulic Support Systems

Thomas Barczak, Research Physicist

National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
Pittsburgh, PA

David Gearhart, Project Engineer

SSI Services Inc.
Pittsburgh, PA

ABSTRACT

Hydraulic cylinders are used in several roof support systems,
such as longwall shields and mobile roof supports, to provide
critical ground control. Many state-of-the-art hydraulic support
systems operate at pressures as high as 7,000 psi to provide the
necessary support capacities. Pressure in the upper stages of
multi-stage cylinders can be intensified by a factor of 2 or 3
providing pressures of 20,000 psi or greater. A proper
understanding of the operation of these high pressure systems is
necessary to safely maintain and repair these support systems.
Likewise, the performance characteristics need to be fully
understood to evaluate the impact of design parameters on the
capability of the support to maintain effective ground control.
This paper examines the basic operating principles of state-of-the-
art hydraulic cylinders and discusses relative issues pertaining to
(1) setting loads, (2) support stiffness, (3) yielding behavior,
(4) errors in assessing support loading, and (5) hydraulic failure
mechanisms and how to detect them.

INTRODUCTION

Hydraulic support systems are used frequently in
underground mining to provide critical ground control, Some
‘examples of their applications include: (1) shields which provide
ground support along the working face in longwall mining, (2)
mobile roof supports which protect miners from caving strata
during retreat mining pillar extraction operations, and (3)
temporary support systems such as those used on bolting
machines which are used to support the mine roof while more
permanent support can be installed.

Hydraulic roof support systems have two major advantages
over other types of supports. First, they have the flexibility to
extend and retract, thereby allowing the same support to be
installed in a wide-range of mining heights. Second, hydraulic
supports can be pressurized to provide active roof support. By
actively setting the support against the mine roof and floor,
hydraulic support systems close any gaps within the immediate

roof structure, creating a laminated roof beam that is much
stronger and more stable. In contrast, passive roof supports such
as wood cribs and timber posts, develop load carrying capacity
only through the convergence of the mine roof and floor. The
downward movement of the roof weakens and eventually causes
failure of the roof beam. In addition, an active hydraulic support
creates a stress concentration at the support that acts to break off
a roof fall, whereas there is a much greater probability of a roof
fall riding past the support into the working area with a passive
support system.

On the surface, it appears that the performance of hydraulic
supports is straight forward, and that little consideration needs to
be given to the design, selection, and operation of hydraulic roof
support systems, Upon closer examination, it can be seen that the
performance of hydraulic support systems is dependent upon
several factors that can influence ground control capabilities and
safe operation of these popular support systems. The purpose of
this paper is to examine the characteristics of hydraulic roof
support systems that impact ground control and operator safety.

HYDRAULIC CYLINDER DESIGN AND BASIC
OPERATING PRINCIPLES

The support capacity of all hydraulic roof supports is
provided by one or more hydraulic cylinders. The remaining
structure in these supports is simply to transfer load to the
hydraulic cylinders to provide the necessary work to support the
weight of the mine roof and control the movement of the mine
roof and floor.

‘The basic operation of a hydraulic support can be described
as follows (see figure 1). A hydraulic power supply pumps fluid
into a cylinder(s) on the roof support structure. The fluid acts
against a piston, causing the piston and attached steel rod to
displace outward. This displacement will continue with very little
hydraulic pressure until the support is set against the mine roof
and floor. The hydraulic pressure will then increase rapidly until
the full pump pressure is reached. After the pump supply is


Support structures should be properly stabilized during repair
work. Incorrect connection of hydraulic hoses to the cylinder can.
result in unintentional damage to the hydraulic cylinder and
potential injury to the mine worker. A well trained person should
be conducting the removal, installation, or repair of hydraulic
support systems.

REFERENCES

1. Barezak, T. M. and Gearhart, D. F., 1997, “Full-Scale
Performance Evaluation of Mobile Roof Supports,” published

in proceedings: New Technology for Ground Control in
Retreat Mining, NIOSH IC 9446, pp. 99-126.

2. Merritt, H. E., Hydraulic Control Systems, John Wiley &
Sons, Inc., New York, 1967, p. 21


roof loading is quite large. For example, approximately 200 tons
of undetected roof loading, which represents 46 pct of the
remaining shield capacity, will occur on the 700-ton longwall
after it set against the mine roof and floor. The undetected roof
loading for mobile roof support using three-stage hydraulic
cylinders can consume nearly the entire loading cycle (see
figure 8).

HYDRAULIC FAILURE MECHANISMS AND HOW TO
DETECT THEM

Hydraulic roof support systems depend on hydraulic fluid
being maintained under pressure to provide the necessary
supporting capability. Any condition that results in pressure loss
reduces the capacity of these hydraulic roof supports. In many
cases, the pressure loss will be internal such that the fluid loss is
not noticeable. Sources of pressure loss and ways to detect them
are described below.

SOURCES OF PRESSURE LOSS AND IMPACT ON
SUPPORT CAPACITY

Pressure losses in hydraulic roof support systems are typically
due to leakage of hydraulic fluid. The major components of a
hydraulic support system were identified in figure 1 as follows
(J) bottom stage piston seals, (2) upper stage(s) piston seals, (3)
pilot operated check valve, (4) yield valve, and (5) staging check
valve(s). Each of these components must function properly to
prevent leakage of hydraulic fluid that will result in pressure
losses. Each of these component leaks result in the exponential
decay of pressure with time to zero, meaning the support will
have no load carrying capability after a certain amount of time
(assuming the leakage rates are greater than the pressure
increases caused by any additional roof loading).

The change in volume required to produce pressure losses can
be theoretically determined by equation 14. Assuming a drop of
fluid measures 1/8 x 1/8 x 1/8 in and thereby consumes 0.002 in’,
table 2 shows the time in minutes necessary to produce pressure
losses of 1,000, 2,000, 3,000, and 4,000 psi in the bottom stage
of a longwall shield cylinder for a leakage rate of 10, 100, and
1,000 drops per minute assuming no additional roof movement or
loading occurs. In a recent test of a longwall shield with a bad
staging check valve and worn seals, 2,300 psi of setting pressure
dropped to zero psi in approximately 4 minutes.

AP = us (4)

where A P = change in pressure, psi
B = bulk modulus of oil (i.e., 320,000), psi
‘A V = change in volume, in’
V = original volume, in?

Table 2. Time required to produce leg cylinder pressure losses

Average Time in minutes required to produce
Leakage Rate designated pressure losses

-1,000 psi] -2,000 psi |-3,000 psi|-4,000 psi
10 drops/min | 255.5 | 5109 | 766.4 | 10219

100 drops/min 25.6 51.0 76.6 102.2
1,000 2.6 51 16 10.2
ldrops/min

1 drop = .002 in’ = 1/8 x 1/8 x 1/8 in cube

02 in’ ~ 1/4 x 1/4 x 1/4 in cube

2 in’ ~ 4x %x % in cube

2 in? ~ 1-1/4 x 1-1/4 x 1-1/4 in cube

Note: Data in table 2 assumes a cylinder (bottom stage) area of
110 in?with a 15 in extension and an initial pressure of 4,000 psi

Table 2 describes pressure losses relative to setting pressure of
4,000 psi in terms of average leakage rates. The leakage rate is
governed by the pressure differential where the leak is occurring.

Hence, the rate of leakage will decrease in time in an exponential
fashion as the leakage causes a reduction in pressure differential

The cooling of the hydraulic fluid can also be responsible for
pressure losses in hydraulic support systems. This problem is
much more prevalent on mobile roof supports than longwall
shields, since each mobile roof support contains a hydraulic
power supply, and the fluid which is heated by the pump is not
cooled to ambient temperatures before it is pumped into the
hydraulic cylinders. The steel in the cylinder then acts as a large
heat sink to reduce the temperature of the fluid, which causes a
reduction in volume and an associated pressure loss. The thermal
expansion coefficient for commercial hydraulic oils ranges from
0,0003 to 0.0005 per degree F (2). Hence, a 10 degree change
in temperature will change the volume of the fluid by 0.3 to
0.5 pet. On a mobile roof support with two-stage hydraulic
cylinders, this would translate into a pressure loss of about
1,200 psi. Unlike the pressure losses caused by mechanical
component leakages which result in no load carrying capacity, the
temperature-induced pressure losses will decay to some non-zero
value, meaning the support capacity will be diminished but
significant capacity will remain.

Another source of pressure loss in hydraulic support systems
is an extension of leg cylinders in the absence of additional
roof/floor loading, This can be due to fracturing of immediate
roof or floor strata or more likely the compaction of roof or floor
debris (broken rock or loose coal). On two-leg longwall shields,
the leg pressures can also be reduced if the canopy or base slides
along the roof or floor due to the horizontal component of the leg
forces. An analysis of the 700-ton longwall shield that has been
used in previous examples, indicates that 0.11 in of extension of
the operating height is sufficient to cause a 1,000 psi drop in leg
cylinder (bottom stage) pressure. Like the temperature induced
pressure losses, unintended extension of leg cylinders due to non
rigid roof and floor boundaries tend to reduce the supporting
force, but not completely eliminate it as is the case with the
component leakages previously described.


height beyond the initial mining height. The top stage extension
will never be less than the initial extension unless the support is
lowered to the point where the bottom stage is fully retracted.
The bottom stage extension will fluctuate with changes in mining
height on each cycle, and will be fully stroked whenever a new
‘maximum operating height is attained

ACTIVE (SETTING) LOADS

Hydraulic support systems can be actively set against the mine
roof and floor by pressurizing the hydraulic cylinders. The force
that is exerted against the mine roof and floor is dependent upon
the size of the cylinder and the hydraulic (pump) pressure. In
multi-stage cylinders, the setting force can be determined by
measuring the pressure in any stage and multiplying that pressure
by the (piston) area of that stage. It is important to remember
that the pressure in all but the bottom stage is intensified during
the setting operation, and only the bottom stage will be at pump
pressure once the support is set (assuming none of the stages are
at fully stroked). In most multi-stage hydraulic cylinders, such as
those used on longwall shields and mobile roof supports, the
hydraulic pressure is measured only in the bottom stage, hence
the pump pressure times the bottom stage area should be used to
calculate the setting force. The total setting force for the support
is simply the sum of each individual hydraulic cylinder forces.

If one or more stages is fully stroked, the setting force will be
diminished. The stage that is fully extended no longer transfers
all of its force to the mine roof and floor, instead part of it is
consumed by tensioning the cylinder casing as the piston is forced
against the mechanical stops that limit its travel. And, the
pressure in the stage above the fully extended stage will not be
intensified, but instead will be at pump pressure. The setting
force will always equal the pump pressure times the area of the
largest diameter stage which is not fully extended. Hence, in a
two-stage cylinder, the setting force will equal the pump pressure
times the second stage area whenever the bottom stage is fully
extended. Ina three-stage cylinder, the setting force will equal
the pump pressure times the second stage area whenever the
bottom stage is fully extended, and the setting force will equal the
pump pressure times the third stage area whenever both the
bottom and middle stages are both fully extended.

The reductions in setting force due to full extension of stages
can be very large. Table 1 below documents the actual
reductions in setting force for a state-of-the-art, 700-ton, two-
stage, longwall shield and a 600-ton, three-stage, mobile roof
support.

Table 1. Reduction in setting force due to stage extensions

ACHIEVED SETTING FORCE,
PCT OF FULL SETTING FORCE.
STAGE 700-ton Longwall 600-ton
CONDITION Shield Mobile Roof
Support
No stage fully 100 100
extended
Bottom stage fully 54 55
extended
Bottom and NIA 30
middle stage fully
extended

Recalling the operation of the hydraulic cylinder previously
described, the setting force will be diminished whenever the
bottom stage is fully extended, which occurs on the operational
cycle where a new maximum operating height is attained. The
setting force will be restored to its full capability whenever the
support is operated at a height less than the highest operating
height on any previous operating cycle.

SUPPORT STIFFNESS

Stiffness is a measure of how quickly the support develops its
load carrying capacity in response to roof-to-floor convergence.
The stiffness of hydraulic support systems is examined in terms
of the parameters that affect the support stiffness and the impact
of support stiffness on ground control.

Discussion of General Stiffness Principles

The stiffness of a hydraulic cylinder can be determined
theoretically using the following principles. Full-scale testing of
longwall shields and mobile roof supports in NIOSH's Mine Roof,
Simulator load frame verify that these principles are valid

The various stages of a multiple stage leg cylinder act in
series to provide an equivalent stiffness as described by
equation 5.

tlotid 1
jteteds. ed 5
a KK K, ©

where K,,= equivalent stiffness of the leg cylinder
K, stiffness of stage 1
K, = _ stiffness of stage 2
K, = _ stiffness of stage n

For a two-stage and three-stage leg cylinder this equation can be
transformed as follows
K, +k.

Kiwo sage = KK, (6)


+P, =A, * P, =A, *P, @

where A, = area of bottom stage, sq. in,
P, = pressure in bottom stage, psi;

= area of second stage, sq. in;

pressure in second stage, psi;

A, = area of nTM stage, sq. in,

P, = pressure in nTM stage, psi

Using a two-stage cylinder as an example, if the top stage
area (A,) is one half (50 pct) that of the bottom stage area (A,),
the pressure in the top stage (P,) will be twice that of the
hydraulic fluid in the bottom stage (P,)

Q@)

@)

(4)

ressure in bottom stage, psi,
pressure in middle stage, psi;

ressure in top stage, psi;
‘A, = area of bottom stage, sq. in;
A, = area of middle stage, sq. in;
A, = area of top stage, sq. in.

The extension of specific stages on a hydraulic support
system, such as a longwall shield or mobile roof support, at a
particular point in time (cycles of operation) depends on the
history of the operating heights of the support. When the support
is initially raised from a fully collapsed position, the bottom stage
will extend first. If the mining height is greater than the stroke of
the bottom stage, the second stage will extend until roof contact
is made, or in the case of a three-stage cylinder, the third stage
will extend after the second stage is fully stroked to achieve the
roof contact. On subsequent cycles, the upper stage(s) will
remain at the initial extension(s) until: (a) the mining height is
increased beyond that of the initial cycle, or (b) the support is
sufficiently lowered such that the bottom stage is fully collapsed.
Assuming the support is not lowered to the point where the
bottom stage is fully collapsed, the upper stage will extend
beyond the initial extension only when the mining height is
increased. In essence, the bottom stage will be fully stroked
when: (a) the support is initially raised from a collapsed position,
and (b) on the mining cycle that establishes a new maximum
operating height, whenever the support height is higher than it
has been on all previous mining cycles. On all other cycles, the
bottom stage extension will reflect changes in mining height
relative to the initial mining height and the maximum operating
height at which the support was utilized.

‘A two-stage hydraulic cylinder, as shown in figure 2, is used
as an example to illustrate these concepts. As shown in figure 4,
the initial setting of the support causes the bottom stage to be
fully extended (24-in stroke) and a 12-in (partial) extension of the
top stage. If the roof-to-floor mining height is reduced on second
operating cycle by 3 in, the bottom stage extension now will be
21 in while the top stage extension will remain at 12 in. Now if
the mining height is increased by 6 in from that of the second
cycle, the bottom stage will again be fully extended (24-in stroke)
and the second stage extension will increase to 15 in on the third
cycle. This exercise can continue as shown in figure 4. As
shown in this exercise, the top stage extension on any cycle will
‘equal the initial extension plus the incremental increase in mining

Figure 4.-Stage extensions produced by changes in operating
height


Retract
Annulus
‘
H Retract line
Beer
Piston ___
Seals
Piston - - -F--7 i
. Pilot Operated
Hydraulic TH Check Valve_| Hydraulic
Power
ine |_ Su
Yield Pressure line |_Supply
Valve

Figure 1.-Basic components of a hydraulic roof support.

tumed off or isolated from the support, this pressurized hydraulic
fluid is trapped inside the cylinder by a pilot-operated check
valve. The pressure of the hydraulic fluid inside the cylinder is
then intensified in proportion to any increase in roof loading. A
yield valve limits the maximum pressure inside the cylinder to
prevent excessive loading that would damage the cylinder. Most
state-of-the-art hydraulic cylinders used in modern mine roof
support structures are double acting cylinders, meaning that they
are hydraulically powered to both extend and retract (see
figure 1). This requirement stems primarily from longwall shield
supports, where powered lowering of the shields is required to
minimize the advance cycle time, Powered lowering of the
support is achieved by pumping fluid inside the retract annulus
(see figure 1), while at the same time applying pilot pressure to
open the check valve to allow fluid from inside the main cylinder
cavity that supports the weight of the mine roof to escape back
to the hydraulic power supply tank.

The previous example described the basic operation of a
single-stage cylinder, where extension and retraction is provided
by one stage. Many supports, including longwall shields and
mobile roof supports, utilize multi-stage hydraulic cylinders
Figure 2 depicts a two-stage hydraulic cylinder used on a state-
of-the-art longwall shield, and figure 3 depicts a three-stage
hydraulic cylinder utilized on a state-of-the-art mobile roof
support. For discussion, the stages are referenced starting with
the largest diameter as stage 1, and each successively smaller
diameter as stage 2, stage 3, etc., or in reference to their location
with designations of bottom (stage 1), middle when applicable,
and top stage. Incorporating more stages into a leg cylinder
design generally allows for a lower collapsed height, combined
with an equal or greater maximum working height to provide
operation in a wider range of mining heights than fewer stages
would permit

The basic operating principles of these multi-stage designs are
the same as the single stage hydraulic cylinder previously
described, but there are some noteworthy differences relative to
how the individual stages perform

Y

TOP STAGE

INTERNAL
CHECK
VALVE

BOTTOM STAGE

Figure 2.-Two-stage hydraulic cylinder.

1. In multi-stage cylinders used on most mining support
systems, the largest (diameter) stage extends and retracts
first, followed in succession by the stage with next smallest
diameter. For example, in a three-stage leg cylinder, the
bottom stage would extend first, followed by the middle
stage, and then the top stage. Each stage will extend until it
is fully stroked before the next stage will begin to extend.

2. A check valve is incorporated into the piston(s) of all but the
top stage. For example, in a two-stage leg cylinder, a check
valve would be installed in the bottom stage piston. This
check valve functions to allow hydraulic fluid to flow from
the bottom stage to the top stage to cause the top stage to
extend during the setting operation whenever the bottom
stage is fully extended. It also isolates the bottom stage from
the top stage to allow the hydraulic fluid to be intensified in
the top stage once the support is actively set against the mine
roof and floor. This is necessary to allow the top stage to
carry the same load as the bottom stage. A three-stage leg
cylinder would incorporate check valves in both the bottom
stage and middle stage pistons as illustrated in figure 3

How the staging functions to provide the necessary support
capacity is essential to understanding the operation of multi-stage
leg cylinders. For proper operation of the support cylinder to
occur, force equilibrium must be satisfied for each stage, meaning
each stage must carry the same load. This requires different
hydraulic pressures in each stage, since the area of each stage is
different. Mathematically, this requirement is expressed by
equation 1