MMH – How is FREQUENCY determined in a Manual Materials Handling?

 

Frequency is the number of handlings per minute that shows the pace of work.

How this data is obtained depends on the type of task under analysis.

 

Single MMH

In a single task of Manual Materials Handling, the evaluator can use different approaches to obtain the frequency:

  • Counting the number of handlings per minute during a period of time of at least 15 minutes. For instance, if a worker lifts 40 boxes in a 20-minute period, we obtain:

Frequency = 40 / 20 = 2 lift/min

  • If the frequency changes throughout the day, a sampling should be performed during a workday in order to obtain a cycle representative sample that allows you to get the number of handlings per minute. For example, three 15-minute working periods are observed, which are representative of the worker activity during the workday. If the worker lifts 18 boxes in the first period, 14 boxes in the second, and 13 in the third, we obtain:

Frequency = (18 + 14 + 13) liftings/45 minutes = 1 lift/min

  • In practice, the frequency can be calculated from the production data, by knowing the number of “packages” handled by a certain number of workers over a period of time known (within the workday, in a week, a month, etc.). For example, in a production line with two workers, which operates continuously during 6 hours a day, 1440 pieces are obtained. Each piece involves one load lifting operation, from a conveyor belt to a control table. Here we get:

Frequency = 1440 lift/ (360 minutes x 2 workers) = 2 lift/min.

Multiple MMH

In the case of a multiple task, the total work frequency is determined in a similar way to what was described for single tasks but, additionally, the evaluator must distribute this frequency (as appropriate in each case) among the different subtasks that make up the multiple task.

  • For instance, a worker lifts boxes from a conveyor belt and carries them along a distance of 5 meters, to a shelf with two levels. The worker lifts a box from the conveyor belt, carries it to the shelf and leaves it on the lower shelf, comes back to the conveyor belt and lifts another box, carries it to the shelf and leaves i ton the upper shelf, comes back to the conveyor belt to lift another box and so on. This multiple task consists of 3 subtasks:
    • Lifting 1 (origin on the conveyor belt and destination on the lower shelf)
    • Lifting 2 (origin on the conveyor belt and destination on the upper shelf)
    • Carrying (carrying the box from the conveyor belt to any shelf)

In this case, the worker carries 60 boxes from the conveyor to the shelf in 20 minutes (30 boxes to the bottom shelf and 30 to the upper one) and, therefore, 3 liftings/minute and 3 carryings/minute are performed, which the evaluator should distribute among each subtask as follows:

    • Lifting 1. Frequency = 30 / 20 = 1,5 lift/min.
    • Lifting 2. Frequency = 30 / 20 = 1,5 lift/min.
    • Carrying. Frequency = 60 / 20 = 3 carry/min.

 

Variable MMH

In the case of variable tasks (with great variability in weight and handling conditions), the evaluator must enter the following data:

  • Loads, specifying the Weight and Number of liftings for each load handled
  • Total working time (expressed in minutes) during which those loads are handled (*).

In practice, this information about the pace of work (loads lifted and corresponding total working time) can be obtained in different ways.

For example, if a worker completes 10 order sheets during 4 hours of the workday, and the sheets contain the products that are takem from the shelves of a warehouse in order to place them on pallets:

  • Loads are the products (weight and number of times they are lifted) included in the 10 order sheets
  • Total working time = 240 minutes

For example, if 3 workers complete 45 order sheets during 5 hours:

  • Loads are the products (weight and number of times they are lifted) in the 45 order sheets
  • Total working time = (300 minutes x 3 workers) = 900 minutes

From these data entered by the evaluator, the software will automatically calculate the following:

  • Total number of liftings, as the sum of all the liftings entered for the loads.
  • Total frequency (expressed in lift/min), as the ratio between the total number of liftings and total working time.

 

(*) NOTE. It is important to highlight that, when using the Variable MMH module, the total working time has nothing to do with the duration of the task:

  • Duration (short, medium or long) is the length of time that the Manual Materials Handling task under analyzed lasts. To determine the duration, you must consider the classic criteria of the nonstop or uninterrupted “working period” and the “recovery period” that takes place between two working periods, as described in the user manual and in the course on the application
  • Total working time is the time during which the loads under analysis are handled. As it was previously explained, this data is later use by the software to automatically calculate the frequency or pace of work of the variable task.