bottlenecks
OPT defines a 'bottleneck' as a resource whose capacity is equal to or less than the market demand placed upon it. This is the constraint that is preventing increased throughput from the factory. Improving the bottleneck will start to optimise the whole system and directly increase throughput and profit. Bottlenecks are easy to spot in the factory - they are the operations with lots of WIP stacked up in front of them. Conversely, a non-bottleneck is a resource whose capacity is greater than the market demand and improvements here will not increase throughput.
The bottleneck concept is best explained in the hiking analogy from 'The Goal'. The speed of a group of hikers must be maximised to get to the campsite by nightfall - but the speed of the group is limited to the speed of the slowest hiker (the bottleneck). If the slowest hiker is at the front of the group this slows down the whole group and increases the time required, i.e. reduces the throughput. If the slowest hiker is anywhere else in the group they still slow the whole group but also increase the length of the group (the inventory).
The only way to reduce the length (the inventory) and achieve the fastest time (the throughput) is to find a way of moving the slowest hiker faster, i.e. working on the bottleneck. An hour lost at a bottleneck, for any reason is an hour lost to the whole system and cannot be recovered. Don't think you can get it back later because you can't. The cost of the lost hour is the total cost of running the factory for one hour because the bottleneck is governing the throughput.
The location and management of bottlenecks is the heart of OPT. An hour saved at a bottleneck is an hour saved for the whole system but an hour saved on a non-bottleneck machine simply increases inventory and does nothing to improve throughput. It is wasted effort.
OPT and JIT both concentrate on quality, lead times and batch sizes but OPT regards the 'river and rocks' analogy of JIT as flawed. In OPT terms, the river is not a flat and evenly flowing stream but has waves of inventory (WIP) moving through it depending on the order situation in the factory. All will be fine until the inventory is at the bottom of a wave. A problem then it is likely to rip the bottom out of the boat and sink the business! This is closer to reality than the JIT model.
The river and rocks model of OPT
OPT considers that the inventory river is not smooth but consists of waves of inventory going through the factory, depending on the order state at any given time. Rocks that may be acceptable when on an inventory peak are not acceptable when in a trough.
OPT and MRPII share a computer based approach with databases of product and machine information for schedule calculation. OPT also requires information on how the product is made, the production route, set-up times [next page]