Lean maintenance tries to put all maintenance activities in relation to expected benefits. Which maintenance strategy is best suited to a particular machine?? For this Bodo Wiegand, Ralf Langmaack and Thomas Baumgarten developed the so called Lean Maintenance System (Aachen 2005).
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Maintenance resp. Maintenance strategies look at the machine as a whole. This includes the mechanics as well as the pneumatics resp. Hydraulics, the machine control And the electric power part. Some machines are particularly important for the production process, because unplanned downtimes inevitably result in delivery difficulties and lost sales for the company. It is therefore only logical that for these machines the maximization of the total plant effectiveness is pursued. In the following you can read how Lean Maintenance can support this process.
Note for the electrician
This applies not only to classic production machines, but also to all electrical power supply systems.
Choice of maintenance strategy
Some machines are rarely required. They may not represent a bottleneck in the process at all: An unplanned shutdown is still annoying, but it is perfectly acceptable for the overall process. So for such machines one will rather find a cost-optimized maintenance strategy select.
The process for selecting the maintenance strategy
Lean maintenance: prioritizing the assets
As already mentioned, plants have a different significance for the plant or. the company. Three criteria are used to more accurately capture and classify this importance:
- Significance for the value stream
- Significance for the production system
- importance for the customer
Significance for the value stream
Value stream refers to the Process from receipt of customer order to delivery of the finished products. In between, the product passes through various stations: Order entry, production control, mechanical prefabrication, coloring, assembly, inspection … In other words, steps that differ depending on the product and the manufacturing technology chosen.
This value stream can now be analyzed with the so-called Value stream analysis record. In the process, one mentally runs through the different steps.
Example of a value stream
Ideally, the product flows directly from one processing step to the next. There are no waiting times. Production begins the moment the part arrives. Then it is forwarded directly to the. In this case the Lead time very low and corresponds approximately to the sum of processing times. Inventories of semi-finished parts are almost non-existent, and inventories of finished goods are also extremely low, as it is possible to react very flexibly to customer requirements. This approach is referred to as Flow principle or as one-piece flow.
Delay due to accumulation stages
However, in practice, this flow principle is only found in extremely excellent companies or in certain types of manufacturing (process industry), and often only in phases. Much more often, the flow principle is opposed in practice by congestion levels. Such Backlog can be:
- (Long) paths between the individual production steps
- Set-up and setup times on machines and systems
- unplanned downtime on machines and equipment
- Different shift models between departments and areas
- quality-critical processes
These accumulation stages ensure Delays and lot formation. It is no longer a product that moves from one production step to the next, but a package. This has the effect of increasing lead times, decreasing flexibility, and increasing inventories of both semi-finished and finished goods.
Just remember the setup times at your machines and plants. Let’s assume that the cycle time for a product on one line is one minute, but 30 minutes are required to change over to another product variant. Then it makes no business sense to retool the machine for just one product. Instead, one determines the optimal batch size, at which the set-up time and total machining time are in an appropriate ratio to each other, and defines a corresponding lot. Already a bottleneck has been overcome.
Similar happens in the case of unplanned downtimes on machines and plants. If the machine only runs with limited stability, the tendency to form larger batches will increase. On the other hand, if it is permanently available, perhaps even a one-piece flow can be realized through this system.
Position of the machine in the value stream
From the position of the machine in the value stream, requirements for maintenance are now derived. If the machine is permanently integrated, a defect leads to the interruption of the entire value stream. In this case, one speaks of Bottleneck. Of course, this is an absolute priority for maintenance: any malfunction that exceeds the so-called maximum fault clearance time inevitably leads to losses of finished products or. to time shifts.
In doing so Interruption time The time understood to elapse from the occurrence of a fault until the system is restarted. The maximum fault clearance time is determined using the following formula:
The order for winding stators for electric motors has a throughput time of four hours. The winding time for each of the 500 stators is 0.4 min. So the following suppression time results:
In our example, the maintenance department has a maximum of 40 minutes to rectify the fault. In the other case, the entire job will be postponed or. quantity losses occur.
Please note, however, that we are always talking about a maximum fault clearance time at this point. If the order has already been moved to the end of the shift, the fault clearance time is reduced accordingly. Of course, previous orders also have a shortening effect on the fault clearance time.
Note for the electrician
The fault clearance time for electrical power supply systems is generally set at zero.
Lean Maintenance: Determining damage effects
By means of plant prioritization, you have now determined which of your machines you need to pay particular attention to, which you can service with less effort, or on which you can even take the risk of an unplanned shutdown. This gives you a rough overview of which plants and machines should be associated with which maintenance strategy.
However, this plant prioritization is still too imprecise. It considers the machine as a unit, as an element. In reality, however, every plant consists of several or even many subsystems.
Note for the electrician
In the following, subsystems are understood as plant components that have their own control system and can thus be operated autonomously.
So, it is now necessary to determine in a second step which components and anomalies you have to pay special attention to: where which faults can occur, as these occur and what influence they in turn have on the entire plant.
The main points of malfunction on the machines and systems are therefore to be identified, so that you are in a position to Use maintenance resources in a much more targeted way. Otherwise, you may spend a lot of effort on non-critical components and not find the time to take care of the actual critical components.
- Gas supply
- Transfer line
- Conveyor belt
- Belt outlet
- Belt infeed
This approach to lean maintenance is also referred to as Determination of damage influences. This is done in a similar way to a Failure Mode and Effects Analysis (FMEA) in the following steps:
Process of determining damage influences
It should also be mentioned that the analysis is of course only useful for those machines that have a high to very high priority. Plants that have been classified with a rather low priority should be excluded for this analysis. Otherwise, you run the risk of investing a lot of time and effort without actually getting closer to the goal of the most trouble-free operation possible.
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