Do you replace your car headlights at regular intervals of six months? Do you wait to replace your tires until they wear through? Do you check your car engine oil with some sort of oil analysis before replacing it? Each component and system in your car has a function that is prone to failure. Each of those has consequences – some with little importance and others with great importance.
Your ability to make decision on what to do (or not do) will be based on a few factors:
- Do you understand that it can fail and how it can fail?
- Do you understand the implications (consequences) of its failure?
- Do you understand the various technologies or actions you can utilize to deal with it?
- Are you sufficiently motivated to do something about it?
This article is the first in a series covering the basics of the various types of maintenance you can use.
If you run headlights to failure, for example, you can replace them. You cannot tell when they are going to fail; they give you no warning of imminent failure and the consequences of their failure are usually not severe. Running tires to failure, on the other hand, is a different matter altogether. Wear on tires is more obvious and failure can be more easily predicted. The consequences of not replacing worn tires can be fatal. Allowing your spare tire to deflate has no consequence at all, until something goes wrong with one of the four that you rely on as rubber on the road.
Knowledge of the various maintenance options available to you is imperative. Clearly you can’t take action unless you know the options. Like managing risks, your range of options includes:
- Accepting the risk (doing nothing)
- Avoiding it (taking some physical actions)
- Transferring it (get someone else to action)
- Reducing it to a tolerable level (possibly a combination of the above).
The actions fall into a few groups:
- Reactive (accept the risks),
- Proactive (avoid, minimize, or transfer the risks),
- Engineering to design out failures (avoiding or transferring the risks) or consequence reduction.
The decision to act at all will be made on the basis of whether or not it is “worth it” to you to act. Once you decide to act and what to do, you also need to determine when to do it.
Timing
- Scheduled (rigid adherence to some time or usage-based frequency or fixed interval)
- In response to some condition or sign that work is needed.
- Unscheduled (no fixed interval used at all. This only applies to the reactive approach)
- Flexible scheduling (which entails the shifting of scheduled or unscheduled work to convenient “windows of opportunity” when the work can be done with minimal disruption to production. This, of course, implies that the work is something that can be deferred or advanced with minimal consequences)
Your Options
Run–To-Failure
Wait until the equipment fails. This is typical for electronic circuit boards and light bulbs. Generally, the equipment that is allowed to run to failure is noncritical to operations, and its failure is tolerable from the perspective of safety, environmental, and business loss consequences. By default, most reactive maintenance will be done on an unscheduled basis, although some repairs can be deferred to a suitable window of opportunity. If you find that more than half your work is repair work and in response to failures, then you are probably over-using this option.
Preventive Maintenance
This is based on time or use factors, such as kilometers, cycles, throughput, fuel consumption, and running hours. It consists of cleaning activities (removes contaminants), lubrication, minor adjustments, replacements, periodic restorations and other failure prevention actions. The actions are taken before the failure occurs and they prevent the failure – hence the name. Scheduled replacements and overhauls are preventive.
- Scheduled component replacement. At a predetermined point, based on either elapsed time or use, a particular assembly, component, or part is replaced, regardless of its condition. Electric wheel motors in large diesel electric haul trucks are usually replaced on an hours- operated basis because the repair expense skyrockets if they are run to failure.
- Scheduled overhaul. As with scheduled replacement, the equipment is stripped and selected components are replaced or restored. A plant shutdown is essentially an overhaul of a plant with focus on specific equipment or systems. The timing of the shutdown is driven by a few dominant-failure mechanisms that require periodic action (for example, cleaning or replacement of filters or replacement of a major wearing element like a centrifuge bowl or replacement of balls in a mill).
Predictive Maintenance
Here there are two distinct activities, the first of which is recognizing that the equipment is failing and the second of which is correcting the defects before they progress to an unacceptable level. The former is “condition monitoring;” the latter is “condition-based maintenance.” Often both are lumped under the heading of “condition-based maintenance” or “on-condition maintenance”. Maintaining equipment is based on measured conditions. When the condition being measured falls outside acceptable limits, corrective repairs are made. If it remains within acceptable limits, then nothing beyond the inspection is done.
Detective Maintenance
When you check tire pressures on your car you probably check 5 tires – 4 on the wheels and 1 spare in the trunk. That check of the spare helps to ensure it will be ready to use in the event that you lose the use of one of the others. You are “detecting” whether or not it is ready and fit for use. Like the spare tire in the car, your plant probably has a number of protective, backup or stand-by devices. These can be alarms, shutdowns, redundant equipment, warning lights, warning signs, first aid kits, defibrillator kits and other things that are only used when needed and that is generally only when something else goes wrong. These are all providing protection in one form or another. Do they all work? Are they ready to operate when needed? To answer, you carry out a functional check or test. If they pass, you do nothing more until the time for the next test arrives. If they fail, you correct them.
Testing identifies the protective devices that are already failed. It does not prevent the failures nor does it predict them. It finds them after the fact but hopefully before you need it to operate. By testing regularly and correcting for any defects found, you are minimizing the consequences to your operation if the device were needed to act in its protective function.
Redundancy may be built into a system and sometimes into the design of equipment. A great deal of electronic equipment has redundant circuitry to ensure its high level of reliability. If the primary unit fails, the secondary unit is available. All maintenance managers know that having a spare is an excellent way to guard against loss of service. Unfortunately, this is also an expensive option that is (or should be) limited to situations where failure is absolutely unacceptable. Note that redundancy does not eliminate the failures or maintenance action; it merely allows service to continue even when failures occur. Adding redundancy adds assets and cost as well as the potential for additional failure modes and maintenance. If you have redundant equipment or systems you also need to carry out detective maintenance on them to make sure they’ll be useful when needed.
Redesign
Designing out maintenance is used for critical equipment where it is difficult to measure the condition or detect imminent failure. Redesign entails the identification of the cause of recurring or otherwise unacceptable problems that could result from defects and then changing the design to eliminate the problems.
Redesign is often an outcome of the modern quality approach known as Six Sigma, but it is important to keep in mind that it should really be limited to a “fallback” option dealing with a physical root cause of failures. Unless you design your systems right from the outset, the engineering approach to maintenance improvement is almost always the most expensive.
Understanding where to use preventive, predictive, detective and run-to-failure tactics is key to setting up an effective proactive maintenance program. You will be able to choose tactics that are suitable to the failures you might reasonably expect to see occurring. The best way to do this to ensure both an effective and efficient proactive program is to use RCM. The next article will describe RCM and its application. The third article will present an approach to rapidly putting a proactive program in place that might be helpful to get an “out of control” situation back under control.
Some useful info here:
Condition Based Maintenance basics – course
Basic Reliability Management – course
Reliability Centered Maintenance (RCM-R)
