Line Balance Optimisation
Why are automated Production lines set up to have accumulation?
Block production lines operate with most machines running at the same speed. Typically block lines run with very little conveyor space between the machines (often single track). Therefore a stop on any of the blocked machines quickly or instantly causes a direct loss of OEE / performance for the line. Machines are typically controlled through linking the PLC controls to change speeds/stop automatically for the entire line.
Accumulation production lines operate with individual machines linked with large amounts of conveyor space and generally take up a lot of factory space. Typically the machines in an accumulation line have the capability of running at variable speeds, and will normally have one machine (or one block of machines) which runs the slowest; we call this the bottleneck or ‘critical’ machine. The OEE / Performance of this line is determined by the amount of time that we’re able to keep the critical machine running at its rated speed. Machines are typically controlled by conveyor-based sensors with little direct interaction between the machines.
The accumulation production line set up goes against lean principles whereby the machines are far apart and there is a lot of buffer between the machines.
Therefore why are automated production lines designed this way?
The answer to this question is related to typical efficiencies the individual machines are capable of achieving. Let’s consider this example: If there are 5 main machines on the line and each of them are typically running at 96% efficiency. In the block setup the maximum efficiency of the line will be 96% x 96% x 96% x 96% x 96% = 81%. If there are conveyors between the machines that allow some buffering, the minor stops can be absorbed and the impact of the stops does not cause the other machines to stop as well. In a lot of cases the ineffective set up and control of the lines reduces this benefit. To better understand how to get the most benefit from your conveyors on an accumulation line, please refer to our article on Optimising Line Control.
Ultimately there is always a financial impact and cost benefit analysis to determine the set-up of a production line. When the individual machines will be capable of achieving close to 100% efficiencies, there will be only one type of set-up considered. Until then, we have to consider accumulation in an automated production line set-up.
How should the accumulation space (typically conveyor size) be decided for accumulation production lines?
Traditional approach of deciding accumulation size is the bigger the better and the limitations to this are typically cost or space. However this can be counterproductive.
To consider the relevant size of conveyor space, take into account the Mean Time Between Touch (MTBF) and Mean Time to Repair (MTTR) of the machines.
The empty space required on the conveyors to buffer the containers should the downstream machine stop should be greater than the MTTR of the downstream machine. This will ensure that during small stops, the upstream machine does not get affected.
By calculating how long it will take the conveyor to return to nominal state and comparing this with the MTBF of the downstream machine will provide the information if the line set up is correct. The time the conveyor will take to get to nominal state (i.e. the condition before the stop), is calculated by using the speed differential between the machines and the length of time taken to process the extra containers on the conveyor. If this value is lower than the MTBF of the downstream machine, the conveyor accumulation space and speed of machines during recovery is correct.
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Line Balance Optimisation is:Correct
In nominal state all variable speed machines should:Correct
Fill the gap:
- A structured methodology to achieve perfect flow is called (5 Levels of Control, Five levels of control)