Theory of Constraints Handbook - James Cox Iii [578]
TOC for Services solves the latter problem by combining replenishment with critical chain. That is, Critical Chain for Services (CCS) uses RS to provide resources to multiple projects being managed with critical chain. For example, resources on the bench waiting for project assignments—the resource buffer—should be sufficient to meet most demand for resources from multiple projects, even when that demand is unpredictable. Whenever the resource buffer drops below the target size, RS automatically replenishes it because the resource buffer is there to protect on-time project delivery and the revenue it produces.
By mitigating, if not breaking, the resource constraint, multiple services projects can be scheduled concurrently with CCS in order to meet the needs of various clients. A stair-step pattern between the projects is therefore not required.
Drum-Buffer-Rope for Services
Drum-Buffer-Rope for Goods (DBRG) is the traditional TOC application for operations management. Briefly, DBRG wrings maximum productivity out of manufacturing operations with an internal constraint by ensuring that the constraint, and only the constraint, sets the pace. Indeed, the “drum” in DBRG refers specifically to this pace setting.
The “buffer” in DBRG refers to WIP deliberately queued ahead of the constraint. This buffer ensures that the constraint has work to do even when there are disruptions upstream.
As noted earlier, keeping the constraint supplied with work is vital because utilization of the constraint governs what the factory produces overall. Non-constraints therefore must do whatever is required to keep the constraint fully utilized—and no more. That means that non-constraints upstream from the constraint typically have less than full utilization so that they won’t overwhelm the constraint with excess WIP. Likewise, non-constraints downstream from the constraint typically have less than full utilization because they can only do as much work as is passed to them through the constraint.
Late delivery of raw materials, equipment breakdowns, worker absences, unexpected scrap rates, change orders—even the weather—may disrupt the production schedule. Therefore, upstream non-constraints sometimes have to sprint in order to keep the constraint busy when holes appear in the buffer. Likewise, downstream non-constraints sometimes have to sprint in order to complete late jobs on time. Nevertheless, contrary to conventional wisdom, it’s normal for non-constraints to be idle occasionally. Indeed, it’s necessary for them to be idle at times.
Rather than pushing work into the factory for utilization, DBRG starts jobs at just the right time and relies on due dates to pull those jobs through the factory in the right order. Thus, the “rope” in DBRG refers to the information systems used to start jobs at the right time and subsequently ensure that the constraint is working on the right jobs based on current due dates.
Having previously established that PSTS are highly customized and rely little on inventory, how DBR might apply can be a mystery. Nevertheless, it’s a mystery that is easily solved.
When PSTS services are highly customized, the customized processes may nonetheless be highly repeatable. That is, when a service provider uses a shared service center to perform processes for multiple clients, each client’s customized process may be performed millions of times. Think of paychecks. Due to organizational structures and compensation plans, no clients’ payroll processes are identical, but the same process is performed for every client’s employees every