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In years past, underground coal mining equipment consisted of simple but rugged machines
powered by electric motors and hydraulics. They were maintained by personnel who needed
only a basic knowledge of hydraulics, electricity, and mechanics. Today, engineers have
transformed these machines into powerful, complex mining systems, requiring an increase in the
knowledge and skills that are necessary of maintenance personnel. Fortunately, during this
period designers have made significant advances in the field of maintainability engineering,
such as the application of sensors and diagnostics and the modular replacement of components.
System design techniques have also improved. Designers are using many of these innovations
and techniques more and more in underground mining equipment.
The cost of maintaining a machine is a direct function of the maintenance frequency and failure
interval for the machine and major components, the time and labor required to complete
unscheduled maintenance actions, and the time and labor required to complete routine
maintenance tasks. Because of the steadily increasing costs of maintaining underground mining equipment,
mining companies have generally focused on ways to contain these costs. These cost control
efforts have usually centered on optimizing scheduled maintenance operations, reducing maintenance
staffs, better control of spare parts inventories, use of contract maintenance support, and deferring
nonessential maintenance. Improved equipment design for maintenance can positively influence all
these efforts.
Beyond escalating costs, maintenance operations account for a persistently high percentage of
mining injuries. MSHA data for 1978-88 suggest that maintenance accounted for 34% of
all lost-time injuries. Various studies funded by the former USBM relevant to maintenance injuries
have determined:
- Over 25% of underground coal mining accidents occurred during maintenance. (1)
- Thirty-two percent of all machine-maintenance injuries involved the lower back, and 38%
of all machine-maintenance accidents were the result of overexertion. (2)
- Approximately 25% of back injuries (the leading cause of lost time) occurred when
the overall task was maintenance. (3)
- Injury data for independent contractor employees in the mining industry from 1983 through 1990
suggest that approximately 20% of the coal mine injuries occurred during machine
maintenance or while using hand tools. (4)
Most efforts to decrease the frequency and severity of injuries to miners have stressed miner
training and work procedures, improved work environments and safety and environmental control equipment,
improved personal protective equipment, improved equipment control and display design, enhanced
lighting and visibility-related research, and organizational issues. However, the industry has
paid much less attention to the design of the mining machine itself with respect to maintenance
cost or safety for the maintainer.
(Note: Most of this information was gathered as part of a contract with the former USBM on Assessment of the Maintainability Design of
Underground Mobile Mining Equipment. (5)(6))
Maintainability Defined
What is meant by maintainability? Several definitions are useful. A simple one is that
maintainability is the ease with which you can repair equipment safely in the least amount
of time.
We can qualitatively define maintainability of equipment as a designed-in characteristic
that imparts to a machine an inherent ability to be maintained with reduced person-hours and
skill levels, fewer tools and support equipment, and reduced safety risks. We can quantitatively
define it as a measure of the speed with which you can restore a mining machine to operational
status following a failure or removal from operation for servicing. We may also define it as the
probability that a machine can be kept in an operational condition or restored to that condition
within a given time when you design it properly or you do the maintenance according to prescribed
procedures and tools.
Maintainability is often confused with maintenance. Maintenance is a series of specific actions
taken to restore a machine to full operational status. These actions may include servicing,
troubleshooting, inspection, adjustment, removal and replacement, or in-place repair of components or
systems on a machine. Preventive maintenance refers to the actions taken to retain a machine at a specified
level of performance. It includes routine servicing and replacement of parts that are likely to fail
during the next operational cycle. Corrective maintenance represents actions taken to restore a machine
to an operational state after it is disabled due to a part or system failure. Reliability is the
probability that the machine will perform its intended function for a specified interval of time under
stated operating conditions.
First Principles of Maintainability Design
With the above definitions, introducing the first principles of maintainability
design for underground mobile mining machinery is possible. The list is rather long, reflecting the
complexity of the topic:
![First Principle logo](images/fp.gif) |
Maintainability should be a designed-in capability and not an add-on option.
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![First Principle logo](images/fp.gif) |
Great maintenance procedures cannot overcome poor equipment design.
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![First Principle logo](images/fp.gif) |
A complex design solution is often easier than a simple solution - until you have to
maintain it. Given the choice, opt for the simpler design.
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![First Principle logo](images/fp.gif) |
Every point where two or more components come together or where you mount a
component on the chassis represents a maintenance point.
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![First Principle logo](images/fp.gif) |
Every maintenance point should be directly visible and fully
accessible to the maintainer.
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![First Principle logo](images/fp.gif) |
All parts or components are replaced eventually, so design for these eventualities.
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![First Principle logo](images/fp.gif) |
Do not design for the "average" or 50th percentile person. To do so could
exclude up to 60% of the users. Design for the user population, which includes the 10th - 90th
percentile person.
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![First Principle logo](images/fp.gif) |
Troubleshooting is not a form of gambling. Design maintenance and troubleshooting procedures
to reduce the odds in the maintainer's favor. Provide specific indicators of pending or actual
failures for all systems and major components.
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![First Principle logo](images/fp.gif) |
In order for the maintenance person to remember maintenance instructions, write them down
and post them on the machine where he/she will make the decision while maintenance is being done.
Label key components, show flow direction, and provide other decision making information.
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![First Principle logo](images/fp.gif) |
Design interfaces so that the component or connection can only go together correctly.
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![First Principle logo](images/fp.gif) |
Design every interface so that you can install only the correct replacement part or
component, such as by using unique bolting patterns, guide pins, or other features.
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![First Principle logo](images/fp.gif) |
Design each interface so that you can install acceptable alternative components without
modifications. If two different components can serve the same function, design the mounting
interface such that you can mount both units without modification.
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![First Principle logo](images/fp.gif) |
Since the unexpected can occur anytime, ensure that you sufficiently derate all
mechanical, electrical, hydraulic, and pneumatic systems to withstand unexpected overloads without
failures, degradation in performance, or negative safety consequences.
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![First Principle logo](images/fp.gif) |
Design line-of-sight visibility for all maintenance tasks that require visual inspection,
servicing, adjustment, alignment, in-place repair, or removal and replacement of components.
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![First Principle logo](images/fp.gif) |
Because the easiest decision to make is often a go/no-go decision, design all maintenance
decisions to be go/no-go decisions.
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![First Principle logo](images/fp.gif) |
Design all systems and subsystems to fail to a safe mode or state so that a component or subsystem
failure will not result in additional damage or employee injury.
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![First Principle logo](images/fp.gif) |
Because it is sometimes difficult to see what is right in front of you, design all systems
so that failures are obvious.
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![First Principle logo](images/fp.gif) |
Special tools are rarely around when maintainers need them, so design all maintenance
tasks to eliminate the need for special tools.
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![First Principle logo](images/fp.gif) |
Design and locate all components and interfaces so that they are directly and easily accessible or
reached for maintenance.
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![First Principle logo](images/fp.gif) |
Do not force fit standard parts as a substitute for reliability, maintainability,
performance, and design innovation.
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![First Principle logo](images/fp.gif) |
Modularization of components reduces maintenance guess work, which in turn
reduces maintenance downtime.
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![First Principle logo](images/fp.gif) |
Maintenance errors add to the maintenance burden, so reduce the maintenance burden by
eliminating or reducing the opportunity for human error.
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![First Principle logo](images/fp.gif) |
Because equipment operators sometimes cause equipment failure and damage, design the mining
machine to be operator-proof by designing operator-controlled systems with emergency relief valves,
overload safety devices, and other precautionary features.
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![First Principle logo](images/fp.gif) |
Do not design maintenance tasks that rely on personnel to lift or maneuver heavy components.
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![First Principle logo](images/fp.gif) |
To save time, design repair tasks, alignments, and adjustments so that there is no need to tear
down or remove components.
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![First Principle logo](images/fp.gif) |
Because a person's effective work envelope is determined by his or her reach, do not put maintenance
or service points where they are effectively out of arm reach.
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![First Principle logo](images/fp.gif) |
Because a person's visual acuity decreases with age, viewing distance, and task complexity, do not
locate visual inspection points more than 36 in (91.44 cm) away from where the maintainer's head is
going to be while doing the inspection. Do not put visual inspection points behind components,
under protective covers, or at other points that require work to reach them.
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Advantages of Improved Maintainability
The purpose of maintainability engineering is to increase the efficiency and safety and to
reduce the cost of equipment maintenance. To accomplish this, it is evident that the
achievement of significant cost reductions in maintenance begins with improved equipment design.
Although maintainability engineering will not eliminate the need for service and repair on mining
equipment, it provides the following advantages:
- Reduction of the time required to complete scheduled and unscheduled maintenance.
- Minimization of the frequency of unscheduled maintenance by improving accessibility for
inspection and servicing.
- Reduction of maintenance errors and incorrect installations.
- Improvement of post maintenance inspection.
- Reduction of maintenance-related injuries.
- Minimization of maintenance personnel training requirements.
- Improved troubleshooting performance.
- The following list contains typical design problems on underground mobile
mining equipment that are addressed by incorporating maintainability design.
- Accessibility problems--Inability of maintenance personnel to access
failed or suspected components to inspect or remove and replace them. These
problems resulted from:
- Inadequate access opening size,
- Poor layout of components in a compartment, necessitating removal and replacement of
non-affected parts
to access the failed units,
- Inability to access mounting bolts or connectors or to use required tools,
- Installing components in inaccessible interior cavities,
- Running cables inside the frame or chassis where you cannot reach them,
- Locating fasteners and mechanical interfaces where you cannot reach them
physically unless the machine is partially or completely disassembled
- Inadequate component handling capability and component machine interface design.
- Inadequate design for routine maintenance, such as the inability to quickly remove and replace leaking
hydraulic hoses and water lines, remove and replace failed hydraulic valves, do routine lubrication,
and perform visual and physical inspections.
- Inadequate fault isolation capability, such as difficulty determining the precise cause and location
of a failure, reaching components to do visual inspections and to perform checks, limited or no designed-in
fault diagnostic capabilities, lack of effective failure indices.
- Increased maintenance burden resulting from poor design and placement of components, subjecting
them to damage.
- Poor design with respect to resources available, such as the need for maintenance personnel
to build tools, handle 100- to 1,000-lb (45.36- to 453.59-kg) components, or use brute human strength to overcome
poor component interface design or lack of needed tools.
- Equipment complexity resulting from poor layout, such as the crowding of components into compartments
without regard for the need to maintain or replace individual items, overlaying hoses and power cables,
and making removal and replacement difficult.
- Multiplying the number of valves, connectors, and other high-frequency replacement components as a
design convenience.
This list was identified by a research project funded by the former USBM on maintainability of underground
mining machinery. One conclusion was that the successful application of maintainability
design principles to underground coal mining equipment could reduce preventive maintenance and corrective
maintenance time by 40% to 70%, maintenance labor costs by 10% to 25%, and maintenance
risk significantly.
You cannot overestimate the importance of maintainability. As a design engineer, you must ensure that
the total emphasis is not on productivity when mining and transporting the material mined. Make sure that
maintainability is a focal point during initial design.
Maintainability Examples
![Large motor](images/hfg_m2.jpg) |
Large components, such as this motor, should have lift points clearly marked.
Access opening should be large enough to accommodate materials handling equipment. |
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![Hydraulic hoses](images/hfg_m3.jpg) |
The design of this machine makes it difficult to perform routine maintenance, such as
quickly removing and replacing leaking hydraulic hoses and water lines, removing and replacing failed
hydraulic valves, or doing routine lubrication. |
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![Opening to access a maintenance point](images/hfg_m4.jpg) |
Workers at this mine had to cut their own opening to access a maintenance point. |
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![Restricted access](images/hfg_m5.jpg) |
Welding structural members over areas where hydraulic hoses and electrical conduit are run makes
it difficult for the maintainer to gain access. You should not crowd components into compartments without
regard for the need to maintain or replace individual items. |
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![Swing out electrical module](images/hfg_m6.jpg) |
There are many examples of good maintainability design in underground mining equipment.
This module swings out from its cabinet to allow easy access to all maintenance points. |
Maintainability Checklists
The purpose of the
Maintainability Design
Checklists is to provide a summary of design review points for the maintainability
assessment of new or existing underground equipment. They specifically focuses on the identification
of equipment design features, tasks, or procedures that impact equipment downtime, repair costs,
labor hours, and maintainer skill level requirements.
Some checklist points are general in nature. The checklists are designed to be used across all
categories of underground equipment. The intent is to draw attention to design features and
maintenance procedures that will increase maintainability requirements. You are encouraged to
adapt this checklist to site-specific or machine specific requirements by:
- Inserting specific performance criteria for various categories of maintenance tasks. For example,
all hydraulic lines on a shuttle car should be replaceable in 15 minutes or 25 minutes, etc.
- Adding or deleting checklist items for different categories of equipment. You would include
environmental control equipment, for example, on face equipment and not on shuttle cars or man trips.
- Adding additional checklist items based on site- or equipment-specific maintenance histories or
experience, company maintenance standards, or other factors.
References
- Hamilton, D. D., J. E. Hopper, and J. H. Jones. Inherently Safe Mining Systems; Executive Summary (contract H0111670, FMC Corp.). USBM OFR 124-77, 1977, 38 pp.; NTIS PB 271150.
- Conway, E. J., W. A. Elliott, and R. Unger. Mine Maintenance Material Handling: Volume II - Prototype Device Specifications (contract H0113018, Canyon Research, Inc.). USBM OFR 13(2)-89, 1988, 51 pp.; NTIS: PB 89-168975/AS.
- Stobbe, T. J., R. W. Plummer, and M. Jaraiedi. Back Injuries in Underground Coal Mining (contract J0348044-05, WV Univ.). USBM OFR 18-90, 1989, 375 pp.; NTIS PB 90-202938/AS.
- Rethi, L. L., and E. A. Barrett. A Summary of Injury Data for Independent Contractor Employees in the Mining Industry From 1983 Through 1990. USBM IC 9344, 1993, 16 pp.
- Conway, E. J., and R. Unger. Maintainability Design of Underground Mining Equipment: Volume I-Final Technical Report (contract J0145034, Vreuls Research Corp.). USBM OFR 39-91-V1, 1988, 35 pp.; NTIS PB 91-241885.
- Conway, E. J., and R. Unger. Maintainability Design of Underground Mining Equipment: Volume II-Maintainability Design Guidelines (contract J0145034, Vreuls Research Corp.). USBM OFR 39-91-V2, 1988, 181 pp.; NTIS PB 91-241893.
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