Principles of motion economy

Why it matters

Most manual cycles on an assembly bench last a few seconds, and a large part of that time is spent reaching, searching and holding rather than doing useful work. The principles of motion economy are a checklist for removing those wasted motions, which raises output and reduces fatigue and strain injuries at almost no capital cost.

Key ideas

Origin. Frank and Lillian Gilbreth broke manual work into basic elements called therbligs (their name spelt nearly backwards) and showed that the same job could be done with far fewer and shorter motions. Ralph Barnes later grouped the rules into the principles of motion economy, usually taught under three headings.

1. Use of the human body

• Both hands should begin and complete their motions at the same time.
• The two hands should not be idle at the same time, except during rest.
• Arm motions should be simultaneous, symmetrical and in opposite directions.
• Use the lowest classification of movement that will do the job (see below).
• Use momentum to help the worker, and reduce it where it must be overcome by muscular effort.
• Smooth, continuous curved motions are better than zig-zag motions or straight lines with sudden, sharp changes in direction.
• Ballistic (free-swinging) movements are faster, easier and more accurate than restricted or controlled ones.
• Arrange work for an easy, natural rhythm; reduce eye fixations and keep them close together.

2. Arrangement of the workplace

• A definite, fixed place for all tools and materials, so that habits form and searching is eliminated.
• Tools, materials and controls close to and directly in front of the operator, within the normal working area.
• Gravity-feed bins and containers to bring material near the point of use; drop deliveries (let gravity carry the finished part away).
• Materials and tools arranged in the sequence of use.
• Good lighting, and a chair and work height that allow alternate sitting and standing with good posture.

3. Design of tools and equipment

• Relieve the hands of holding work: use jigs, fixtures and foot-operated devices.
• Combine two or more tools where possible (e.g. a double-ended spanner).
• Pre-position tools and materials (tool hangers, holders that present the tool ready for grasp).
• Distribute the load among the fingers according to their strength.
• Levers, crossbars and handwheels placed so they can be operated with the least change of body position and the greatest mechanical advantage.

Classes of movement (lowest is best): 1 fingers only; 2 fingers and wrist; 3 fingers, wrist and forearm; 4 fingers, wrist, forearm and upper arm; 5 all of these plus the shoulder or trunk. Each higher class takes more time and effort.

Working areas. The normal working area is the region swept by each forearm with the upper arm hanging naturally at the side; the maximum working area is swept by the fully extended arm pivoting at the shoulder. Frequently used items go in the overlap of the two normal areas; occasional items within the maximum area. The actual dimensions come from anthropometric data for the user population (see the ergonomics topic).

Therbligs. Effective (useful) ones: reach (transport empty), move (transport loaded), grasp, release load, use, assemble, disassemble, and pre-position. Ineffective ones, to be reduced or eliminated: search, select, position, inspect, hold, plan, unavoidable delay, avoidable delay, and rest for overcoming fatigue. A SIMO chart records a cycle therblig by therblig.

Formulas

Motion economy is mostly qualitative; the numbers come from the cycle time.

Output per hour = 3600 × k / t_c

• t_c = cycle time (s); k = units completed per cycle (k = 2 when both hands each complete one unit).

% gain in output = (Output_new / Output_old − 1) × 100

Standard time ST = NT × (1 + A)

• NT = normal time per unit (min); A = allowance fraction of normal time. Used to convert a method improvement into output per shift and labour cost per unit (see the time study topic).

Worked examples

Example 1 (standard). At present the left hand holds a housing while the right hand fits a spring and cover: 9 s per unit. A two-station fixture is made so that each hand assembles one unit at the same time with symmetric, opposite motions; the new cycle is 11 s and produces 2 units. Find the % increase in output.

1. Present output: 3600 × 1 / 9 = 400 units/h.
2. New output: 3600 × 2 / 11 = 654.5 units/h.
3. Gain = (654.5 / 400 − 1) × 100 = 63.6 %.

The principles used: relieve the hands of holding (fixture), both hands working simultaneously and symmetrically.

Example 2 (GATE level). Moving bins into the normal working area and adding a gravity chute cuts the normal time of a job from 0.30 min to 0.24 min. Allowances are 15 % of normal time, the shift has 480 productive minutes and labour costs ₹300/h. Find the standard times, output per shift and labour cost per unit before and after.

1. ST_old = 0.30 × 1.15 = 0.345 min; ST_new = 0.24 × 1.15 = 0.276 min.
2. Output per shift: 480 / 0.345 = 1,391 units; 480 / 0.276 = 1,739 units.
3. Labour cost per unit: (300/60) × 0.345 = ₹1.73; (300/60) × 0.276 = ₹1.38.
4. Standard time 0.345 → 0.276 min; output 1,391 → 1,739 units per shift (+25 %); labour cost ₹1.73 → ₹1.38 per unit.

Note the output gain equals 0.30/0.24 − 1 = 25 %: with allowances proportional to normal time, the percentage gain is set by the normal times alone.

Common mistakes

• Placing a rule in the wrong group: "a definite place for all tools" is workplace arrangement, not use of the body; "use jigs to hold work" is tool design.
• Thinking straight-line motions are best; curved, continuous motions without sharp direction changes are.
• Treating "hold" as productive: a hand holding a part is an ineffective therblig that a fixture should replace.
• Using a higher class of movement (shoulder, trunk) where fingers or wrist would do.
• Forgetting that the second hand doubling output only helps if the cycle time does not double too.

For GATE PI

Expect one-mark questions classifying a rule into one of the three groups, identifying effective vs ineffective therbligs, or choosing the right improvement for a described workplace; occasionally a short numerical converting a cycle-time reduction into output or cost. Learn the principle lists and the therblig groups well enough to recognise reworded versions.

Quick check

1. Name the three groups of motion-economy principles.
2. Is "hold" an effective or ineffective therblig?
3. Which class of movement involves fingers, wrist and forearm only?
4. A cycle drops from 12 s (1 unit) to 15 s (2 units). What is the % gain in output?

Answers: 1. Use of the human body, arrangement of the workplace, design of tools and equipment; 2. Ineffective; 3. Class 3; 4. 3600/12 = 300 units/h to 3600 × 2/15 = 480 units/h, a 60 % gain.