Production Control

Time based utilization and stroke based load factor

2017/04/08

時間基準の稼働率とストローク基準の負荷率

Tact time and cycle time in the Toyota production system

Tact time (T/T) is the target value that Toyota's boss instructs subcontractors to process their orders in seconds per piece in order to properly digest them.

For example, if Toyota issues orders for 1,000 pieces per day to subcontractors, and the subcontractor plant operates for 15 hours per day (including 3 hours of inactivity), the subcontractor plant will process the pieces at a tact time of 43.2 seconds per piece, as shown below.

  • (12 hours x 60 minutes x 60 seconds) / 1,000 pieces/day
    = 43,200 seconds/1,000 pieces
    =43.2 seconds/piece

If a subcontracting plant processes in 40 seconds per piece, it is necessary to reduce the operating time and adjust the pace distribution, because if the subcontractor processes in 48 seconds per piece, it is necessary to reduce the amount of time the machine is stopped by using a single set-up (within 10 minutes of the set-up).

  • Cycle time > Tact time: Insufficient capacity (out of stock)
  • Cycle time <If tact time, excess capacity (inventory)

While tact time is a customer (Toyota) oriented approach in terms of how many seconds it takes to complete an order, cycle time is an index that focuses on the company (subcontractor plants) in terms of the standard capacity of the company's production facilities.

The Toyota Production System (TPS) focuses on how to eliminate unreasonableness, waste, and unevenness and bring cycle time (the capacity of subcontractors) closer to tact time (the capacity needed to digest Toyota's orders).

Incidentally, the tact time is clear because it is an upward and downward trend, but the cycle time is calculated based on the scheduled operating hours and the target number of production for the next fiscal year as proposed by the production control department of the subcontracting plant.

  • Tact time = Operating hours / Order quantity per day
  • Cycle time = Target operating hours / Target number of production

Whether this operating time includes setup time or not depends on the plant, but the Toyota Production System is trying to educate subcontractor plants on the use of single setup to achieve cycle time (tact time).

Both tact time and cycle time are a way of thinking that calculates the time required based on the number of pieces produced, which is how many seconds it takes to produce one piece. If the cycle time is 40 seconds per piece and an order of 1,000 pieces is digested, the utilization rate is calculated as follows.

  • Operating rate = Operating hours ÷ Operating hours
    = (40 seconds x 1000 pieces) / 43,200 seconds
    =92.6%

Pressing load factor based on the number of strokes

In the case of press-worked products, however, the load factor is calculated based on the machine-oriented concept of GSPH (Gross Stroke Per Hour), which is the number of strokes per hour, including set-up time and stop time, rather than the number of hours of operation.

  • Load factor = demand ÷ supply capacity
    = number of strokes per day / (GSPH x operating hours)

The load factor is "the ratio of demand to the supply capacity of the machine," and has almost the same meaning as the utilization rate. However, it is a stroke-based concept that indicates whether the number of strokes required to digest the demand (order quantity) falls within the supply capacity (how many strokes can be moved) or not, and if not, how many strokes of overtime should be added to the operating hours, and if the overtime is not enough, how many strokes should be saved on Saturdays and Sundays.

Total Equipment Efficiency(OEE)

Overall Equipment Effectiveness (OEE) is an index calculated from production management systems as an indicator of machine availability, which is a comprehensive indicator of production efficiency calculated from three aspects: availability factor, performance factor, and yield.

  • OEE=Hourly utilization rate x performance utilization rate x good quality rate
  1. Hourly operating rate = Net operating hours / Gross operating hours
    Percentage of time the equipment is actually in operation out of the scheduled hours of operation.
  2. Performance uptime = (cycle time x volume) / Net uptime
    Ratio of actual production speed to production speed (original capacity)
  3. Good product ratio = number of good products ÷ number of production
    Ratio of the number of good products to total production

The point is that the index measures the contribution of a machine to production from three aspects: how well the machine works (in terms of time), how well it performs to its original capacity (in terms of speed), and how accurately it produces (in terms of good quality).