Analysis of work and processes
For 450 units per shift, a line of ten tasks needs a 60 s cycle and five stations, with 92 % efficiency. For 540, the largest-time rule uses seven stations even though the theoretical minimum is six.
01Methods study and work measurement
Work study has two parts. Methods study records how a task is done, examines it critically —what is done, where, when, by whom and how— and proposes a simpler, safer and more productive way. It relies on charts: the flow process chart, with its symbols for operation, inspection, transport, delay and storage; the flow diagram on the plant floor plan; and the two-handed process chart for assembly stations.
Work measurement determines how long the task takes with the chosen method. With stopwatch timing, the observed time is corrected by the performance rating of the worker and allowances for fatigue and personal needs are added:
02Markets, products and processes
What and how much is produced is decided by the market: the analysis of demand, competition and prices defines the volume and the variety. Each product goes through a life cycle —introduction, growth, maturity and decline— that changes what matters at each stage: flexibility at the beginning, cost and efficiency at maturity. Depending on volume and variety, the process is organized in different ways:
| Process type | Volume and variety | Example in electronics |
|---|---|---|
| Project | A single, unique unit | A radar station |
| Job shop (by functions) | Low volume, high variety | Prototypes and made-to-order equipment |
| Batch | Medium volume | Laboratory instruments |
| Line | High volume, low variety | Assembly of phones or televisions |
| Continuous | Homogeneous product without interruption | Manufacture of wafers or cables |
03Balancing a line
On a line, the product passes through successive stations and all of them have the same time to work: the cycle time, which is set by demand. Balancing the line means distributing the tasks among the stations while respecting the precedence order, without any station exceeding the cycle and using the smallest possible number of stations:
Ten tasks to assemble a unit, in seconds: A inspect the board (15), B insert large components (45, after A), C solder connectors (30, after A), D touch up solder joints (25, after B), E mount the heat sink (20, after B), F electrical test (35, after C and D), G mount in the cabinet (30, after E and F), H wire the front panel (25, after G), I program (20, after F) and J final test and packing (30, after H and I). A 450-minute shift. Each bar is a station; the red line, the cycle time.
04In the laboratory
Make the flow process chart for the assembly of a power supply in the laboratory and propose a better method by eliminating transports and delays.
Time a soldering task ten times, rate the pace and calculate the standard time with allowances.
Balance the laboratory line by hand for 540 units and explain why it cannot be brought down to six stations, even though the theoretical minimum suggests it.
05Common mistakes
- Measuring without having improved the method. An inefficient way of working gets standardized.
- Forgetting the allowances. A standard time without rest is impossible to sustain.
- Setting a cycle shorter than the longest task. The task has to be split or the station duplicated.
- Believing the heuristic gives the optimum. It is a good starting point, not a guarantee.
06Self-assessment
What cycle time corresponds to 300 units in a 450-minute shift?
90 s.
How many stations are needed at minimum, if the tasks add up to 275 s?
\( \lceil 275/90 \rceil = 4 \).
What efficiency does the line have with those four stations?
\( 275/(4 \cdot 90) \approx 76 \) %.
What process is suitable for producing varied prototypes?
A job shop organized by functions.
07Further reading
- International Labour Organization (George Kanawaty, ed.). Introduction to Work Study. 4th ed., ILO, 1992. (Spanish edition: “Introducción al estudio del trabajo”, 4th ed., ILO, 1996.) Methods study, flow process charts and work measurement.
- Benjamin W. Niebel and Andris Freivalds. Methods, Standards, and Work Design. McGraw-Hill. (Spanish edition: “Ingeniería industrial: métodos, estándares y diseño del trabajo”.) Methods and times, with many worked cases.
- Jay Heizer and Barry Render. Principles of Operations Management. Pearson. (Spanish edition: “Principios de administración de operaciones”.) Process types and line balancing.