Defining Push, Pull, and Flow in Lean Manufacturing
Lean Manufacturing is primarily a "pull system", but it's important to understand the differences between traditional push signaling methods and various pull choices.
What is a Push signal?
Doing work or delivering material based on a schedule or plan, without considering whether the work or material is needed now by the customer.
When you are pushing work or material, you are simply following the plan or schedule, and moving material to the next step without considering whether it is needed or not. Your measure of success in a Push environment is schedule adherence for your department. Whether the product actually gets finished or not, is not your problem.
The classic example of a Push signal is a Work Order, which will specify a start date, a due date, and a quantity for a specific item. The goal for a Work Order recipient is to produce the quantity needed on the due date. If the requirements change, with either the dates or the quantities needed, the Work Order would also need to be changed.
What is a Pull signal?
Doing work or delivering material based on a signal generated by actual downstream usage of the item.
When work or materials are being Pulled, you can't actually move the work or material forward until there is a signal to do so, like an empty space, an empty container, or a physical card.
Types of Pull signals include:
- A simple pull method is to only move an item when the next downstream station is empty and available. The empty station "pulls" the item from the upstream station. This is the minimum inventory method, but balancing the work flow is a challenge if all items need to move at the same time to keep things flowing.
- An empty space, either on the floor or a work bench, can act as a pull signal. It works like this: build a unit and put it in the space. The space may allow more than one unit. Keep working at your station until the space is full. You are limited to the number of units allowed in the space, plus one unit at your station. These spaces are called In Process Kanbans or IPKs, or FIFO Lanes.
- The IPK option discussed previously may not work if the IPK cannot be seen easily. As an option, an empty container that can hold a specified number of units can act as a pull signal. When the empty container is delivered to the station, that is the signal to work.
What is Flow in Lean Manufacturing?
Doing work or delivering material one unit at a time, based on an order upstream, and signal generated by actual downstream usage of the item.
Flow is a further refinement on Pull, where you are moving units one at a time, based on customer orders, and also based on a Pull signal.