OPT is a software system that contains a proprietary algorithm for production scheduling. From OPT (optimized production technology) has evolved the managerial concept of theory of constraints lTOC). OPTffOC is a production planning and control (PPC) method that attempts to optimize scheduling by maximizing the utilization of the bottleneck in the process. Traditional production planning and control is considered today to be no more than a production tool. Integrated PPC is a concept that marries an underlying philosophy of planning tools to implement that philosophy to optimize the process. There are three major approaches to integrated PPC: push systems. pull systems, and bottleneck sy terns. The forerunner to the push systems is a tool developed by Joseph Orlicky at IB~t in I974-material requirements planning (MRP). In the late 196Os,Taichi
Ohno at Toyota developed the Kantian system, the first of the pull systems. Eli Goldratt's OPT, developed in the late 1970s, is considered the genesis of bottleneck systems." OPT distinguishes between bottlenecks and capacity constrained resources. A bottleneck applies to the case in which a stage or a number of stages in a system cannot process the good or service quickly enough to prevent backlogs (both in terms of work-in-progress and demand). A capacity-constrained resource (CCR) is a good or service necessary for the creation of the-final product that is exhausted before the final product is delivered. To il-: lustrate the difference between a bottleneck and a capacity-constrained resource, let's consider a retail laundry that specializes in cleaning shirts. Claude's Cleaners is noted for their quality and inexpensive service, so much so that there is always a line of customers waiting for the sole clerk, Clark, to process each transaction. Unfortunately, they are not noted for speedy service; it usually takes a week just to' get a shirt cleaned and pressed. Claude's has three presses, but they are used only three or four days each week because the starch for the shirts usually runs out. (In an effort to minimize costs, Claude's uses the just-in-time concept for materials management and it takes three days for the starch to be delivered after an order is placed.)
In this service process. Clark is a bottleneck. The starch is a capacity-constrained resource (CCR)-as its inventory is increased, the flow through the process component (the presses) is increased. The inventory of starch acts like a temporary bottleneck. The effects of CCRs typically can be reduced in the short term by relatively simple adjustments. Improvements to bottlenecks. on the other hand, are usually expensive and time-consuming. To optimize the flow through a bottleneck in the system, the bottleneck must operate continuously and at full capacity. A planning/control communication mechanism, known as Drum-Buffer-Rope, is used to accomplish this objective. Since the bottleneck is the slowest component of the process, it sets the pace or tempo for the system-much like a
drum beat sets the pace for a marching band. With the output of the process limited to the output of the bottleneck, decreases in output at the bottleneck cannot be recovered. Therefore. an inventory of goods or services, or "buffer inventory," is necessary before the bottleneck so it will always be operating at maximum capacity. To assure that the buffer is maintained at an optimal level (that is, just enough to keep the bottleneck operating), the rate at which the bottleneck is processing (the drum beat) must be communicated to the source of the goods or services that the bottleneck is processing. Since this communication is in one direction-from the bottleneck to the source of the goods or services (input)-and it pulls the input to the buffer, it is referred to as the "rope." Returning to Claude's Cleaners, Clark is the bottleneck or drum. The buffer would be the inventory of cleaned clothing waiting for the customer. What is the rope? Is there a rope?
The quantity of goods or services sent to the buffer is called the transfer batch. Because the purpose of the transfer batch is to maintain the buffer at its optimal level, the quantity will be dependent on the processing rate of the bottleneck. The quantity of goods r services produced by the bottleneck's input source at one time is referred to as the process batch. In bottleneck systems it is critical to recognize that to optimize the entire system transfer batches and process batches may not be of the same quantity. At Claude's Cleaners. Bettie and Bert box the shirts after they have been pressed. If we consider Bettie and Bert together as a single input source for Clark. then the process batch size from boxing to the pick-up area is two. But. since Clark can only process one transaction t a time the ansfer batch quantity pone.