THE NIOSH SHIELD HYDRAULICS INSPECTION AND EVALUATION OF

LEG DATA (SHIELD) COMPUTER PROGRAM

Thomas M. Barczak, Research Physicist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

David P. Conover, Senior Mine Engineer

NSA Engineering, Inc.

Golden, Colorado USA

ABSTRACT

Longwall shields provide essential ground control in longwall

mining, yet a high percentage of shields are operating at less than

peak capacity and many at well below the rated support capacity due

to defective hydraulic cylinders or malfunctions in other hydraulic

components. Leg pressure data are currently collected on state-ofthe-art longwall shields, but typically are not analyzed to evaluate

shield performance. The National Institute for Occupational Safety

and Health (NIOSH) Shield Hydraulics Inspection and Evaluation of

Leg Data (SHIELD) program is a Visual Basic software system that

is designed to analyze leg pressure data and identify shields that are

not performing to rated specifications. The program analysis is

configured to detect the following conditions: (1) loss of leg pressure,

(2) imbalance in leg pressure, (3) low set pressure, (4) low yield

pressure, and (5) full extension of the bottom stage. Other

performance assessment measures include the percentage of the

support capacity utilized (ratio of peak load to yield load), the percent

of time that a shield operates at yield load, the ratio of the set load to

the yield load, and the amount of support capacity that is lost due to

leaking cylinders. Historical record keeping will allow the user to

select a particular shield and review the performance record as

developed by the program for that shield. In addition to these

performance assessment measures, the software will include an

animated description of shield hydraulic systems that will describe the

operation and significance of each hydraulic component and the

impact of component failures on the shield’s capability to provide the

required roof support. A general overview of the SHIELD program

is provided in the paper together with an example analysis of a

2.5-year-old Australian longwall face. 

INTRODUCTION

Longwall shields have grown in size, capacity, and operational

capability since their introduction over 25 years ago. The quest for

more powerful, more reliable, and longer lasting shields continues to

this day. Despite the increased cost of the supports, the increased

cost of downtime, and significantly greater difficulty in repairing the

much larger and heavier shields fabricated from high-strength steel

alloys, there has been relatively little effort to optimize either

preventive maintenance, capacity requirements, or utilization of the

available support capacity.

For the past decade, longwall shields have been equipped with

pressure sensors that monitor leg pressures on every longwall shield.

Modern shields may have control systems that allow these data to be

collected and stored for analysis. However, with as many as 200

shields on a longwall face, the amount of data collected can become

voluminous very quickly, making it impractical to organize and

evaluate the data without the assistance of a computer and some sort

of analysis program. The National Institute for Occupational Safety

and Health (NIOSH) Shield Hydraulic Inspection and Evaluation of

Leg Data or SHIELD program has been developed to provide this

capability. 

SHIELD is a software program developed to NIOSH specifications

by NSA Engineering, Inc. in Golden, Colorado. This stand-alone

program can take data from almost any modern longwall shield

monitoring system, convert it into a standard format, analyze the data

to detect leaking leg cylinders and several other performance

assessment measures, and provide documented results in both tabular

and graphical formats that makes assessing the operational status and

support capability of the longwall shields easy and efficient for even

the novice computer user. In addition to providing assessment of the

shield data, the program provides practical information of how the

hydraulic system and its various components function to provide the

support capability, and what happens to this capability when the

components fail. 

Hydraulic failures that degrade the performance of longwall shields

are more prevalent than most people would think. A recent survey of

longwall operations in the USA indicated that over 60% of the

longwall operations with shields in service for 4-6 years were having

noticeable hydraulic problems (1). Many hydraulic problems go

undetected since the leaks are internal and there are no visible signs

of fluid loss associated with these events. In addition, there are

misconceptions about the effect of these internal leakages (2), which

adds to the poor conditions which exist on some longwall faces. Full

extension of the bottom stage, which can occur naturally due to the

operating height of the shield, or due to internal leakages, can degrade

the setting force by as much as 50% and very little attention is paid to

this condition by anyone. SHIELD will detect this condition and

include it as part of the analysis of the shield performance. It is

believed that mines which institute a good preventive maintenance

program get the most use of their longwall shields. Failure to address

problems as they occur reduces the useful life of the shield. SHIELD
 

Figure 1. Main window showing various tabs that control the overall operation of the SHIELD program. In this

example, the Read Data Files tab is activated.

can provide the necessary information to institute a good preventive

maintenance plan by identifying and tracking problems as they occur.

A shield responds to roof activity by developing additional loading

after it is initially set against the mine roof and floor, and as such can

provide valuable information about its effectiveness in controlling the

ground on a longwall face (3, 4). Despite 25 years of use, many

questions remain unanswered in terms of optimizing the design

requirements and capacity utilization of a longwall shield. Is bigger

really better? Does a higher setting pressure result in a higher final

pressure regardless of the roof condition or geology? Is there an

optimum setting pressure? What impact does the increased stiffness

of higher capacity shields have on the shield response to the ground

movements? Is there a maintenance advantage to having wider

shields with fewer hydraulic cylinders? These are a few questions

that SHIELD can provide some insight into by providing the

capability to utilize the wealth of data currently not being analyzed to

assess the performance capability of active longwall shields through

their service life.

Finally, in terms of safety, it has become more and more difficult

to repair longwall shields on an active longwall face. The equipment

is bigger and heavier than ever before. Changing out a 400-mm leg

cylinder on a 1,200-ton state-of-the-art longwall shield can be

challenging and hazardous. Again, a good preventive maintenance

plan whereby these failures are corrected during panel changes when

spares parts can be interchanged in a more controlled environment,

either underground or in a maintenance shop, can help to ensure the

safety of those involved in doing this work. 

The purpose of this paper is to provide an overview of the SHIELD

program, describe its architecture and give examples of the outputs

that can be generated to evaluate the condition of longwall shields.

In this regard, an example analysis of an Australian longwall is

included. 

PROGRAM ARCHITECTURE

SHIELD is a Visual Basic program that is designed to operate on

most personal computers. The program is a Windows-based

architecture to facilitate easy use with anyone familiar with Window’s

operating systems. The program opens with a disclaimer window and

a general introduction window that shows the program version

number with a picture of a longwall shield. 

The analysis begins with a window asking, “which analysis method

you want to follow?” There are two options: either process the data

manually, in which case the user must provide input regarding the

configuration of the available raw shield data and the amount of data

to be processed and analyzed, or use an automated option where the

same shields are being monitored and the configuration of the shield

data remains unchanged from the last analysis. In the automated

mode, the program will process all available data since the last

analysis was made. Neither option takes much time, but the Auto

feature is the best option once the data configuration has been

defined, and is ideal for periodic (daily or weekly) updates to the

analysis of a particular set of shields.
 

Figure 2. This window is used to define file parameters for the

raw data acquired from an active longwall face.

Data File Configuration

The first time the program is used, the Manual analysis option

must be chosen to set up the raw data configuration files and several

other parameter thresholds to facilitate the analysis of the shield

performance. This process begins with the main window shown in

figure 1 by pressing the Configure button, which brings up the file

conversion window shown in figure 2. It is expected that the raw

shield data (leg pressures from hydraulic sensors on individual leg

cylinders) collected from the underground shield computer control

system will be stored and/or downloaded to a surface computer for

subsequent processing by the SHIELD program. One of the

requirements then is to define the path (Set Path button) where the

raw data files are stored, so that the SHIELD program can access

them. The program is designed with flexibility to accommodate

different data file structures making the program compatible with

most shield data collection systems. In general, the format of the raw

data files needs to be defined so that SHIELD can convert these raw

data files into a standard format for processing. This is done with the

Field Definitions Setup button located toward the bottom of the

Configure File Conversion window (figure 2). The field definition

for the raw shield data defines what each item refers to in the data

stream so the program will have the necessary information to extract

the date-time, shield number, leg position (headgate or tailgate) and

associated leg pressure data. It is assumed that file configurations are

constant for a given panel. A subwindow is included to designate the

defined configurations, which are typically referenced by panel name.

These configurations are saved to file for later use. The user can

provide starting and ending dates and time to help identify and

control the analysis period.

There are four buttons on the right side of the Configure File

Conversion window to provide information on what shields are being

monitored and the shield specifications. These are labeled: (1) Shield

assignments, (2) Shield types, (3) Scaling, and (4) Pump system.

Shield assignments allows the user to define which shields have data

available for processing, descriptive information including the serial

number, purchase date, and date of the last rebuild for the shields

being examined, and a few setup indices to define the shield

numbering nomenclature. The Shield Type menu, which can also be

edited from the Shield Assignments window, can be used to provide

information on the shield size, capacity, and leg cylinder

specifications that will be used to evaluate the shield performance.

Scaling, which can also be edited from the Shield Assignments

window, designates the unit measurements for the shield leg pressures

in either psi, bar, or MPa. Information on the pumping system can

also be included by activating the Pump System button (see figure 2).

File Conversion

Once the data file and shield assignments are defined, the raw

shield data are converted to a standard file format (daily binary files)

using the designated conversion settings by executing the Go button

on the Read data files tab (figure 1) . The raw data files (*.dat files)

and converted data files (*.dtb) are stored in a folder designated by

the user-defined path selection. The formation of both the raw data

and converted data files are designated on a data map which can be

viewed by executing the Data map button (figure 1). 

Data Analysis

 Data analysis is also executed from the main window using the

Analyze Cycles tab(see figure 1). The initial function of this part of

the program is to evaluate the leg pressure data and define the loading

cycles from this data set. Before the analysis begins, the user can

configure the cycle detection parameters by executing the Configure

button. These parameters set thresholds which help to define data

limitations that are useful in identifying shield loading cycles from the

raw data set. The starting and ending date and time can be set to

include either a complete or a partial set of the available shield data.

The analysis is done by executing the Go command on the Analyze

Cycles tab. The analysis examines the loading pattern of each shield

cycle and determines which shield cycles have characteristics which

would suggest that the shield capability is being degraded by leaking

hydraulic cylinders, low setting pressures, full extension of the bottom

stage, yielding at less than the yield rating, or an imbalance in leg

pressures on the same shield. The loading characteristics for all

shields are also analyzed including measures of setting pressure, peak

pressure, time weighted average pressure, and change in pressure after

setting. Averages, as well as maximum and minimum values, are

computed for most performance assessment parameters. The path for

storing or viewing the listing of analyzed (*.cyc) files is chosen from

the Data map.

Description of analysis parameters and leg cylinder conditions

1. Leaking cycles - Cycles are detected in which the leg pressure

decreases after the support is set against the mine roof and floor.

The amount of time that the pressure is decreasing during a cycle

is calculated and expressed as the percent of total cycle time to

measure the severity of the leak. 

2. Cycles at full extension - Full extension of the bottom stage of the

hydraulic cylinder is determined by the characteristic flat-line leg

pressure curve following the setting pressure, whereby the pressure

in the bottom stage does not increase beyond a nominal amount

until the top stage pressure overcomes the setting force developed

in the bottom stage (2). A check is made to see if the pressure does

increase after 10 minutes, suggesting that there is indeed roof

loading occurring to justify the full extension determination as

opposed to a condition where there is simply no roof loading. It is

possible that full extension would not be detected on a leaking leg
 

Figure 3. Once the data is analyzed, performance and problem reports can be generated from this window.

if the leak caused a drop in pressure immediately after the support

was set against the mine roof and floor. 

3. Set pressure - The start of the cycle normally is the first sample

after the end of the previous cycle (pressure drop more than the

threshold). The set pressure is the first sample taken once the set

pressure threshold is reached. If the pressure does not reach the set

pressure threshold within 10 minutes, the maximum reading is

taken as the set pressure. If the set pressure is attained somewhere

within the first 10 minutes of the cycle, the start time for the cycle

is adjusted accordingly. The minimum, average and maximum set

values are calculated for all cycles within the reporting period.

Other assessments made relative to the setting pressure include:

• Percent time less than set pressure - This is a measure of how

long (what percent of the cycle time) the leg pressure was less

than the intended setting pressure. It is useful in judging

whether a positive set operation was successful in achieving full

setting pressure on non-leaking legs, or the effect of leaks on

shields which have leaking cylinders. 

• Average amount less than set pressure - This is a measure of

how low the achieved setting pressure was relative to the

intended setting pressure. It is calculated as the difference

between the intended set pressure and the average of all

measured leg pressures which were less than the intended setting

pressure. 

4. Time-weighted average pressure (TWAP) - The time-weightedaverage pressure is a measure of the average loading on the shield

during a particular cycle. These are then averaged to produce an

average TWAP for a leg cylinder for all the measured cycles. The

minimum and maximum TWAP averages for all cycles are also

calculated. The TWAP is used in various analyses whereby one

condition is compared to another using the TWAP as the indicator

of the shield loading. 

5. Maximum pressure - The maximum pressure is the peak pressure

recorded during a loading cycle. These are then averaged for all

cycles to produce an average maximum pressure for each leg

cylinder. The minimum and maximum of these maximum

pressures for all cycles are also documented.

6. Yielding - Yielding is determined when the shield loading reaches

a peak near the rated capacity of the support, and the pressure

drops to about 10% of the yield pressure as the yield valve closes

after expelling fluid from the cylinder cavity. Performance analysis

associated with yielding include:

• Percent of time yielding is occurring - This is a measure of

how long the shield was in a state of yielding. It is determined

as the number of pressure samples that were equal or greater

than the yield pressure specified in the Shield types screen,

divided by the cycle length and expressed as a percent. 

• Average amount greater than yield pressure - This is an

average of the leg pressures minus the rated yield pressure for all

leg pressure measurements greater than yield pressure. It is

intended to show when the yield setting is greater than the rated

specifications for the shield. 

7. Average pressure difference between legs - This value is

calculated only for shields that have pressure data for both legs.

The signed difference of pressures of the headgate leg minus the

tailgate leg are summed for each sample and are reported only for

the headgate leg. The signed difference is listed for individual

legs; however most displays (and the sorting order) consider the

absolute value of the difference.
 

Figure 4. Report table showing overall face summary and histogram of leaking cylinders.

Data reports and plots

Reports and plots of the analyzed data are made in the Reports &

Plots tab of the main window (figure 3). Again, the starting and

ending date and time can be entered to allow the user to control the

set of data to be included in the report. A Default button is also

included to allow the user to generate a report of all data since the last

analysis was conducted. The report is generated by activating the

Generate button. Once the report is generated, the information can

be displayed in five different formats as indicated on the right side of

the window (see figure 3): (1) Table, (2) Plots, or (3) X-Y Plots,

(4) Pie charts, and (5) Grid. 

Tables

The Report Table is divided into two basic sections. At the top of

the window several options are available to select the type of

information to be displayed in tabular format in the box immediately

below. At the bottom of the window is a histogram of a particular

performance parameter chosen for analysis. Figure 4 shows an

example of the Report Table window with an overall face summary

displayed in the text box and a histogram showing the distribution of

leaking leg cylinders.

• Action items - This selection groups the data into various problem

categories and sorts each group to show the most numerous

problems first. For example, in the leaks section, the legs having

the greatest percentage of leaks are shown first. Typically, items

in this section will be included in this list only if more than 10% of

the shields experienced the condition being described.

• Overall face summary - This format shows summaries and

averages of the various performance parameters calculated for the

entire face.

• Individual shield summary - All data for each leg are shown and

can be sorted, based on any of the calculated parameters. To

reduce the amount of data shown, the list can be reduced to show

only the shield number, leg, and data for the selected sort

parameter. When a sort parameter is selected, the histogram is

synchronized to show the same data.
 

Figure 5. Plot can be generated which show a selected individual shield performance relative to a selected analysis parameter.

The example here shows the setting pressures over the 3 month reporting period for shield number 55.

• Histogram - For the selected parameter, the data for all shields are

sorted into 20 bins covering the range of data. The height of the

bars corresponds to the number of items (legs or cycles) assigned

to that bin. Moving the mouse over the histogram will display the

bin number, the X-axis range of the bin (bin width), and the

number of items in the bin (see figure 4). The histograms can be

used to assess the variability of the data.

Plots

This window shows graphical plots of various calculated

parameters and can also show leg pressures for specific cycles. The

display can be zoomed to a larger or reduced time scale. Performance

values for individual shields for a selected cycle and averages for

several cycles are also shown. This display is useful to evaluate

variation of shield loading over time, load distributions along the

face, comparison of a specific shield with its neighbors, and detailed

analysis of shields not performing to specifications. The plot colors

can be changed by pressing the Colors button. The minimum and

maximum Y-axis value for each parameter can be changed to adjust

the plot scaling. Three main types of plots can be shown:

Cycles for individual shields (figure 5) – This screen plots the

selected parameter values for a particular shield for the designated

reporting period and documents the parameter values for selected

cycle numbers and averages during the reporting period. The X-axis

initially corresponds to the entire reporting period, but can be zoomed

to display shorter time intervals. The Y-axis shows either pressure or

a percentage (%) value, depending on the parameter chosen to view.

The shield number can be selected by entering the number and

pressing Enter or using the adjacent small scroll control. When a

shield is selected, all cycles for that shield (for both legs) are read and

the nearest cycle to the currently selected time are displayed. The

current time can be changed by entering a cycle number, scrolling the

cycle number, or entering an evaluation time directly (which finds the

nearest cycle). Selecting a cycle for one leg will automatically adjust

the other leg to show the nearest cycle. Selecting the display

parameter from the combo box will update the plot and show the

values for the currently selected cycles as well as the average values

for each leg over all cycles.

Cycle distribution for entire face (figure 6) – Utilizes the same basic

format as the individual cycle screen except now the data is shown for

all shields. The X-axis initially corresponds to the shield number for

all shields installed on the face, but can be zoomed to show groups of

fewer shields. For the selected time period, the nearest cycle for all

legs is determined and the values of the selected parameter are

plotted. This plot shows the difference between shields and

generalized loading characteristics along the face (areas of greatest

and least loading, etc.). The vertical cursor represents the currently

selected shield and can be moved by clicking on the plot or entering

a different shield number. The selected time can be entered directly,

in the text box, or indirectly by changing the cycle number, for either

the headgate or tailgate leg.
 

Figure 6. The distribution of a particular parameter for all shields across the longwall face can also be shown. The

example shows the distribution of Time Weighted Average Pressure (TWAP) for all 156 shields. The values for a

selected shield (shield 55) are also shown in the window.

Figure 7. Leg pressure profiles can be plotted for any shield with shearer positions if this data is available. The example

shows leg pressure cycles for shield 28 on 12/24/99. The performance for a specific cycle ( in this case cycle No. 816)

relative to a designated performance parameter (in this case setting pressure) can also be shown in the window and

on the graph if desired.
 

Figure 8. A scatter diagram for several of the loading parameters can also be displayed to show the parameter

correlations. Example here shows change in support loading after setting as a function of setting pressure.

Leg pressure plots (figure 7) – In this screen, the full pressure profile

is shown for selected shields. The X-axis initially corresponds to a

time period of 1 day - 1⁄2-day before to 1⁄2-day after the selected

evaluation time (currently selected cycle). The X-axis can also be

zoomed to show a shorter time period. This plot is similar to the

above Cycle Plot but has leg pressures superimposed on the cycle

data and optionally, the shearer position. 

X-Y Plots (Scatter Diagrams )

This window produces scatter plots that show correlations between

the following performance parameters: (1) set pressure (SP), (2) timeweighted average pressure (TWAP), (3) maximum pressure (MP), (4)

average change in pressure after setting (TWAP - SP), and, (5) total

change in pressure after set (MP - SP). Figure 8 provides an example

where the average change in pressure (TWAP - SP) as a function of

setting pressure is displayed. Filters are provided to remove leaking

legs and miscalibrated legs from the data set.

Pie Charts

Pie charts can also be displayed to provide an overview of the

performance of the shields. Parameters included in the pie charts are:

(1) setting force showing the achieved setting force and deficiency in

setting force relative to the full setting force based on the available

pump capacity, (2) leaking legs showing the percentage of leg

cylinders that are leaking compared to those that are not, and (3) an

assessment of the shield capacity utilization showing the amount of

the available capacity that was used, how much is lost due to leaking

cylinders, and the remaining or unused available support capacity.

Examples of the pie charts are shown in the example analysis of the

Australian longwall following this section. 
 

Figure 10. Component diagrams are also included which describe how a particular component works and

what impact failure of the component will have on the supporting capability of the shield.

Figure 9. A grid can be displayed which also provides an

overall summary of the complete face. In this case, shields with

and without leaking cylinders are identified.

Grid Display

An overview of the performance of entire longwall system of

shields can be displayed in a color coded grid (figure 9). The grid

shows each shield employed on the longwall face in groups of 10,

with a color to indicate legs that are performing properly and legs that

are leaking. Legs for which there are no data are also indicated.

Several other performance measures can be displayed in grid format

as well.

Action Items

When an analysis is complete, this tab accessed from the main

window will list the major findings, problems and statistics for the

last report generated. These data are grouped into various problem

categories and each group is sorted to show the most serious

problems first. For example, in the leaks section, the legs having the

greatest percentage of leaks are shown first. 

Information

The Information module accessed from the main window is

designed to provide information on how the shield hydraulic system

and leg cylinders in particular function to provide the roof supporting

capability. Photos, diagrams, and graphic animations are included to

help explain each component and the impact of failure of that

component in terms of degrading the capacity of the shield.

Figure 10 is one example, showing information pertaining to the

staging valve, and figure 11 is an example of an animation showing

extension of the leg cylinder and subsequent roof loading that causes

pressure changes in both stages of the cylinder.
 

Figure 13. Illustration of how much setting force is lost due to

leaking cylinders and low setting pressures.

Figure 12c. The pie chart shows that 13.8% of all loading cycles

had leaking cylinders.

Figure 12b. As seen in this pie chart, 67.3 pct of the shields had

one or more leaking cylinders.

9

10

11

12

13

14

15

Feb Mar Apr May Jun Jul Aug

MONTH (1999)

PCT OF CYCLES WITH LEAKING LEGS

Figure 14. Trend of increasing leg cylinder leakage occurring

during the extraction of a single longwall panel.

SHIELD - SHIELD HYDRAULICS INSPECTION AND

EVALUATION OF LEG DATA

Table 1. Overall face summary.

Start date: 12/3/1999 12:00:00 AM

End date: 2/29/2000 11:59:00 PM

Number of legs with data = 296

Number of legs with no data = 0

Number of unselected legs = 16

Total number of cycles = 228105

Average number of cycles per leg = 770.6

Number of legs with leaks = 168 (56.8%)

Number of cycles with leaks = 31513 (13.8%)

Number of legs with full extension = 294 (99.3%)

Number of cycles with full extension = 8327 (3.7%)

Average cycle length = 79.8 min

Percent of time operating at less than pump pressure = 34.4%

Average pressure difference when less than pump pressure = 1455 psi

Percent of time operating at more than yield pressure = 15.1%

Average pressure difference when more than yield pressure = 411 psi

Average pressure difference between HG & TG legs = 1585 psi

Average Set pressure = 3993 psi

Average Twap pressure = 4862 psi

Average Maximum pressure = 5717 psi

Minimum pressure = 202 psi

Maximum pressure = 8953 psi

Average SP/MP = 69.8%

Average SP/TWAP = 82.1%

Average TWAP/MP = 85.1%

Average (TWAP-SP)/(MP-SP) = 50.4%

Average TWAP-SP = 869 psi

Another approach in analyzing the performance of a longwall panel

might be to conduct an analysis periodically to see what the trend in

performance is as a function of increased time. For example, the

shield data can be analyzed monthly and the output data plotted to

observe trends. The SHIELD program facilitates this effort by

allowing the user to conduct automatic analysis using new data since

the last analysis was made. Figure 14 is an example using the

Australian mine data to show the increasing trend in leaking cylinders

on a monthly basis over a full panel extraction period of 7 months. In

this figure, it is seen that the percentage of leaking cylinders increases

from 9.6 to 14.1 during the panel extraction period. 
 

Figure 13. Illustration of how much setting force is lost due to

leaking cylinders and low setting pressures.

Figure 12c. The pie chart shows that 13.8% of all loading cycles

had leaking cylinders.

Figure 12b. As seen in this pie chart, 67.3 pct of the shields had

one or more leaking cylinders.

9

10

11

12

13

14

15

Feb Mar Apr May Jun Jul Aug

MONTH (1999)

PCT OF CYCLES WITH LEAKING LEGS

Figure 14. Trend of increasing leg cylinder leakage occurring

during the extraction of a single longwall panel.

SHIELD - SHIELD HYDRAULICS INSPECTION AND

EVALUATION OF LEG DATA

Table 1. Overall face summary.

Start date: 12/3/1999 12:00:00 AM

End date: 2/29/2000 11:59:00 PM

Number of legs with data = 296

Number of legs with no data = 0

Number of unselected legs = 16

Total number of cycles = 228105

Average number of cycles per leg = 770.6

Number of legs with leaks = 168 (56.8%)

Number of cycles with leaks = 31513 (13.8%)

Number of legs with full extension = 294 (99.3%)

Number of cycles with full extension = 8327 (3.7%)

Average cycle length = 79.8 min

Percent of time operating at less than pump pressure = 34.4%

Average pressure difference when less than pump pressure = 1455 psi

Percent of time operating at more than yield pressure = 15.1%

Average pressure difference when more than yield pressure = 411 psi

Average pressure difference between HG & TG legs = 1585 psi

Average Set pressure = 3993 psi

Average Twap pressure = 4862 psi

Average Maximum pressure = 5717 psi

Minimum pressure = 202 psi

Maximum pressure = 8953 psi

Average SP/MP = 69.8%

Average SP/TWAP = 82.1%

Average TWAP/MP = 85.1%

Average (TWAP-SP)/(MP-SP) = 50.4%

Average TWAP-SP = 869 psi

Another approach in analyzing the performance of a longwall panel

might be to conduct an analysis periodically to see what the trend in

performance is as a function of increased time. For example, the

shield data can be analyzed monthly and the output data plotted to

observe trends. The SHIELD program facilitates this effort by

allowing the user to conduct automatic analysis using new data since

the last analysis was made. Figure 14 is an example using the

Australian mine data to show the increasing trend in leaking cylinders

on a monthly basis over a full panel extraction period of 7 months. In

this figure, it is seen that the percentage of leaking cylinders increases

from 9.6 to 14.1 during the panel extraction period. 
 

SUMMARY

Shields play a vital role in achieving successful, safe, and

productive longwall mining. Although shield design has improved

considerably through the years, hydraulic leakages and other

problems that degrade the support performance are still prevalent,

particularly on aging supports that have been in service for several

years. The purpose of the NIOSH Shield Hydraulics Inspection and

Evaluation of Leg Data (SHIELD) Computer Program is to help

operators detect and track these failures and institute a good

maintenance program that will help to preserve the full capability and

life expectancy of the supports while achieving optimum ground

control. By having a historical record of failures in terms of

operating hours and loading, a reasonable preventive maintenance

plan could be executed to allow repairs or rebuilds to be made to the

hydraulic system before they become problematic.

In addition to providing useful maintenance information, the

SHIELD program can monitor the use of the available support

capacity on every loading cycle. This can provide insight into the

utilization of the available support capacity and help to measure a

ground reaction curve for the face area that may lead to more accurate

requirements for support capacity, more efficient use of the available

support capacity, how significant setting pressures are on the ground

response, and in general how well the shield is controlling the

ground. For the first time, this type of information can be collected

and analyzed in a systematic and scientific method. Ultimately, such

analyses may lead to improved shield design or utilization. 

SHIELD is a stand-alone program that is designed for use with any

support system provided that leg pressure data from the active

longwall shields can be stored in a file that can be accessed by a

personal computer. The program is designed to be flexible in

handling various data formats to ensure that it is compatible with the

different support types and computer control systems that are

currently in use. As such, the program should provide a means to

utilize the vast amounts of data that are available from modern shield

installations, in a format that is easy to use, with the capability to

maintain a continual record of the performance of every shield on a

longwall face. It’s modular architecture will allow the program to be

expanded as needed to include other types of analyses. 

Finally, a goal of the program is to help educate and inform people

involved in various degrees with longwall shields on how the support,

and its hydraulic system in particular, functions to provide the desired

roof support. For example, learning and understanding how the

setting force is reduced by as much as 50% when the bottom stage is

fully extended, and when and why the bottom stage reaches full

extension, can heighten awareness of how shields work, and what

steps can be taken to improve ground control and shield performance.

REFERENCES

1. Barczak, T.M. Examining Longwall Shield Failures from an

Engineering Design and Operational Perspective. Proceedings,

New Technology for Coal Mine Roof Support, NIOSH, IC 9453,

2000, pp. 223-245.

2. Barczak, T.M. and Gearhart, D.F. Safety and Performance

Considerations in Hydraulic Roof Support Systems. Proceedings,

17th International Conference on Ground Control in Mining,

Morgantown, WV, 1998, pp. 176-187.

3. Peng, S.S. What Can a Shield Leg Pressure Tell Us? Coal Age,

March 1998, pp 54-57.

4. Stafford, J., Mahoney, S., Conover, D.P. and DeMarco, M.J.

Shield Monitoring to Forecast Severe Face Weighting at the South

Bulga Colliery, NSW, Australia. Proceedings, 18th International

Conference on Ground Control in Mining, Morgantown, WV,

1999, pp. 164-175.