All people involved in maintenance activities know that some equipment are really easy to maintain while others can make maintenance work a real nightmare. This attribute is referred as maintainability and I’m going to discuss it and relate it to maintenance, reliability and availability concepts.
I will start by simply defining the different terms, you can skip this section if you are already familiar with them.
Maintenance: is the activity oriented to keep the equipment running. It is divided in two main types, corrective and preventive.
Corrective maintenance: the one that repairs something that is not working according to standards. It is mainly reactive and unplanned. Because this type of maintenance always takes place when the equipment is running, it has a production loss associated which negatively impacts on equipment’s availability.
Preventive maintenance: is the performed to prevent equipment from failing. It is mainly proactive and scheduled and usually doesn’t affect availability.
Maintainability: is a characteristic of an equipment that makes it easy to repair and maintain.
Reliability: the usual engineering definition for reliability is “the probability that an item will perform a required function without failure under stated conditions for a stated period of time”(*). Generally speaking, we expect that an equipment will work for a reasonable amount of time without failure. As everybody knows, there are equipment that are more reliable than others and their reliability depends on their complexity, quality, design, etc.
Reliability and maintainability are characteristics defined during the design stage. They are also affected by the manufacturing process and quality control. However, they can be further improved during their productive life using failure information and field experience to implement modifications, although this is more difficult and less cost effective.
Reliability, maintainability, maintenance and availability are all related. Their relation can be seen in the graph above.
Broadly speaking, reliability will determine how often the equipment fails. Although it is stated during the design stage, this will be an estimated value since reliability can decrease during the equipment life due to several factors like poor maintenance, heavy work environments or incorrect operation procedures.
Corrective maintenance, due to its critical nature, it is heavily affected by maintainability. If the equipment is easy to test, access and repair, we will finish the maintenance work sooner and we will produce less impact on the equipment’s availability.
Preventive maintenance is also affected by maintainability but in a different way. In equipment with poor maintainability, preventive maintenance work will take more time so it would be difficult to have the plant in good condition, especially in 24/7 or 24/5 production environments when there isn’t much available time.
As I mentioned before, maintainability is defined mainly during design phase. The main factors that contribute to it are:
- Ease of access: covers, panels and components are easy to remove and the distribution of the different components inside the equipment facilitates maintenance work.
- Standard tools: when a machine needs special tools to be maintained, this adds more complications to maintenance people, we need to have that tool available on site to do our job and we need space to store it. All components, nuts, bolts and screws should be standard so we can use regular tools to disassemble them.
- Standard components: sometimes manufacturers use proprietary components for common elements (relays, thermomagnetic switches, etc.). This, as well as tools, add more complications than if simple commercially available components were used in the equipment.
- Easy to calibrate or no calibration needed: calibration requirements should be avoided when possible or, if not possible, should be as easy as possible to perform.
- Easy to test: it’s important to know what is wrong with the equipment with as much precision as possible. If the equipment complexity justifies it, built in test capabilities should be included. Otherwise the equipment should offer test points and clear procedures to perform an accurate diagnosis.
- Interchangeability: equipment that has an extreme variety of similar components are very hard to maintain and cause logistic complications. Similar components should be standardized to a single model to allow interchangeability and make spare part planning easier.
- Modular design: for complex equipment, modular design allows to quickly replace the failing module with another one and then repair it with the production line back in service.
The best option is to acquire equipment with high reliability and maintainability, but sometimes we already have the equipment installed and we have to deal with it. In those cases, some ways to improve maintainability are:
- Improve access to the equipment: we can modify aspects of the equipment that will allow us to get access faster and easier. For example, sometimes quick access doors can be added or when we have panels with too many screws, we can replace them with quick release fasteners.
- Standardize components: every time we replace a component we can install an equivalent brand or model that we frequently use in our plant (this can also be done all at once with higher costs). This can be applied to thermomagnetic switches, fuses, relays, electric motors and VFDs. In this last two cases, we can even use a model of higher capacity to diminish the number of models in use. For example, if we have many VFDs of 110 kW and one of 90 kW, we can analyse the possibility to replace this last one for one of 110 kW so we use only one model. This has to be considered carefully to see the available space and other compatibility issues.
- Improve fault indication: this is especially useful for PLC controlled equipment. In some occasions, the equipment has failed and there is no fault indication or it is vague or erroneous. Sometimes, the failure wasn’t anticipated during the design phase so we can program a new alarm or indication to clearly communicate the problem in the future.
In this post we can appreciate the importance of maintainability and its effects on maintenance work and availability. This characteristic should be taken into consideration at the time of buying new equipment since poor maintainability could lead to further costs during the life of the equipment.
How good is your equipment in terms of maintainability? Have you experienced serious problems due to poor maintainability?
Share your experiences in the comment section!
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