Week 4

• A pull system is one in which work is released based on the status of the system and thereby places inherent limit on WIP. A push system is one in which work is released without consideration of system status and hence does no inherently limit WIP

• In the push station, entities (jobs, parts, customers, etc.) enter according to an exogenous arrival process. The key aspect of these arrivals that makes the station “push” is that there is nothing that ties them to the status of the process in a way that limits their number in the system

• Entities are allowed into a pull station specifically to replenish an inventory void created by removal of outbound stock

• A pull system systematically limits work releases to limit the total amount of work in the system.

Kanban is certainly one type of pull system, because it does indeed link releases to system status to limit WIP. But other systems can accomplish this as well. So, defining pull to be equivalent to kanban is too restrictive.

• It has become common in the practitioner literature to associate pull with producing to order, as opposed to producing to forecast.

• However, MRP does not take system status into consideration when generating releases, there is no intrinsic bound on WIP level.

A make-to-stock system replenishes inventories without a customer order.

Because at some point, WIP is too high and the release of orders is stopped

1. Reduced costs—low WIP and less rework.

2. Improved quality—pressure for internal quality and better detection of problems.

3. Better customer service—short cycle times and predictable outputs.

4. Greater flexibility—work is pulled into the system only when it is ready to be worked on.

• A pull system cannot build up excessive amounts of inventory

• Each benefit of pull establishes WIP

• The reduced inventory promoted by a WIP cap puts pressure on the system for good quality, because a low-WIP system cannot function with frequent disruptions from quality problems

• Low WIP also makes detection of quality problems easier because it shortens the time between problem creations and inspection operations

• the stabilization of WIP levels induced by a WIP cap produces more predictable outputs, which in turn allows shorter lead time quotes to the customer

• The WIP cap delays releases into the system until they are imminently needed. By keeping orders on the work backlog as long as possible, the system preserves flexibility to make changes in orders or products.

• releases occur only at completion times, so that the WIP level is held constant

• In CONWIP all stations are not pull. Except for the first station in the flow, for which releases are triggered by completions at the last station in the flow, all other stations operate in push mode.

1. Observability. Pull systems control WIP, which is easily observable, while push systems control releases relative to capacity, which must be estimated rather than observed directly.

2. Efficiency. Pull systems achieve a given level of throughput with a smaller investment in inventory.

   1. During periods when the system runs slower than normal (cold streaks), the pull mechanism draws in less work and therefore keeps WIP under control. During periods when the system runs faster than normal (hot streaks), it draws in more work and therefore facilitates higher throughput

3. Robustness. Pull systems are less sensitive to errors in setting the WIP level than push systems are to errors in setting the release rate.

• A pull system achieves higher throughput for the same average WIP level than an equivalent push system

• Because WIP is lower, less holding costs → more profits

• A pull system is less sensitive to errors in WIP level than a push system is to errors in release rate.

  • With push, you regulate release rates. With pull, you regulate WIP

Because of their stability, pull systems promote a focus on continual improvement, rather than a mode of firefighting to deal with short-term problems.

• Communication

• If more CONWIP loops are used, they must be coupled appropriately

In a multi product line, complexity of work can differ

• ConWIP performs poorly in job shop-like production environments, because it does not provide a means of balancing workloads across resources at release; instead, this has to be achieved prior to release, e.g. by limiting the range or mix of jobs that a company bids for or accepts

• POLCA leads to blocking when there is high routing variability, as is common in job shops. POLCA uses card loops between work centers that authorize the start of an operation at one work center (or cell) based, in part, on capacity availability at the downstream partner work center in the loop.

• Workload Control is designed to achieve the same leveling of workload to capacity that is achieved in repetitive manufacturing using lean tools, but it does so while allowing the customers of make-to-order companies to obtain highly customized products

• It reduces the variability of the incoming workload that results from product customization, rather than limiting variation in the product mix itself. Workload Control typically controls the incoming workload of the shop using continuous workload measures or calculations, rather than through the simple use of cards.

• Difficult to calculate and thus to implement

COntrol of BAlance by CArd-BAsed NAvigation

A card-based approach for implementing the core principles of Workload Control in practice. COBACABANA – where card loops are established between the planner (order pool) and each work center – represents a simple card-based means of feeding back information about production from the shop floor to a central planner, providing a visual control mechanism for the shop floor

• COBACABANA establishes card loops between the planner responsible for order release and each work center; the availability of cards authorizes the planner to release new orders onto the shop floor. Different cards are maintained for each work center; for example, cards can be color coded, with a different color used for each work center

• Under COBACABANA, cards represent a certain amount of workload: each card represents the same amount of workload, but multiple cards can be assigned so that the workload of each operation in the routing of an order is accurately represented.

• To release an order, the planner has to attach the right number of cards for each work center in the routing of an order to an order guidance form that travels with the order. The cards related to a certain work center return to the planner after the completion of the corresponding operation. An order can only be released from the pool if sufficient cards are available for each of the work centers in its routing

By controlling the number of cards in circulation – set equal to 100% of the workload norm, i.e. the upper limit or bound established by management on the workload released to each work center – the workload is controlled. Thus, COBACABANA balances the workload as part of the order release decision making process. This load balancing, or workload smoothing, corresponds to one of the main principles of heijunka in lean operations and prevents surges in work that temporarily deplete the capacity buffer and increase the inventory buffer in the form of work-in-process.

• COBACABANA control remains centralized

• COBACABANA is distinctly different from Kanbans and from POLCA where cards typically operate between work centers and, hence, where control is decentralized. A central release function facilitates a ‘global view’ of the shop floor

• ConWIP restricts the number of cards in operation between the pool and shop floor, thereby controlling the number of orders on the shop floor. Hence, although ConWIP controls the number of orders on the shop floor, it does not balance workloads across work centers

• A work center ‘starves’ because either it has a high indirect load (i.e. a high load still upstream of a work center), which hinders the release of direct load to the work center; or work waiting in the pool is not allowed to be released.

• COBACABANA incorporates both a periodic and a continuous release time element

• A periodic release element

  • At fixed (periodic) time intervals, orders in the pool are sorted according to their planned release dates. A planned release date is determined by subtracting the planned work center throughput time for every operation in the routing of an order from the order due date. Orders are then considered for release, beginning with the first order in the sequence

  • Orders are released when the workload norm is not violated

• A continuous element

  • If the direct load of any work center falls to zero, then the first order in the pool sequence with that work center as the first in its routing is released from the pre-shop pool, irrespective of whether its release would exceed any of the workload norms at other work centers in its routing.

  • Hence, COBACABANA recognizes the need to temporarily violate the 100% workload limit at a certain work center to avoid preliminary idleness at another work center