Developed at the beginning of The twentieth century, the Therbligs mark the start of the studies of movements and the taking into account of the ergonomics in the development of processes and methods of work.
The Therbligs were developed by Frank and Lilian Gilbreth between 1908 and 1924. In their writings between 1915 and 1920, only 15 to 16 ” “Movement cycles” are identified. It will only be in 1924, shortly after the death of Frank Gilbreth, that the Therbligs system will be developed and presented in two articles of management and administration. They present 17 Therbligs (the term Therblig will be given in honor of Frank whose name is the anagram).
At the time, the Therbligs were not related to the time studies of F. W. Taylor. Frank Gilbreth indicated ” … Taylor has never done any movement studies.
Through various methods (film and cycle graph), the Gilbreth have performed the analysis of micro-movements. By developing a method to categorize the different types of activities, they have developed consistent processes between operators, and identified unnecessary and tiring movements. However, the Gilbreth never assigned time to the Therbligs because they thought that by improving the method of work, the reduction of the cycle would follow naturally.
Now, movement studies are developing through ergonomics. The principles of the Gilbreth are still widely disseminated particularly at the work of the time studies like the MTM or MOST or ergonomics studies of the posts like the RULA.
|Reach||R||Reach an object with your hand. Initially, this movement was called "Empty Transport".|
|Move||M||Move an object from one position to another. Initially, this movement was called "Transport Loaded".|
|Grasp||G||Grasp the object with your fingers or your hand to control it.|
|Hold||H||Hold one object with one hand while another performs an operation.|
|Release Load||RL||Release load of an object typically by opening your fingers.|
|Use||U||Use a tool or object to perform a task. A screwdriver to screw or a pen to write for example.|
|Pre-Position||PP||Position or orient a piece relative to each other. This movement usually follows a "Move".|
|Position||P||Position or orient an object precisely. This movement is generally suitable for "Move".|
|Assemble||A||Join two parts for old set.|
|Disassemble||DA||Separate parts that were previously assembled.|
|Search||Sh||Look for a part with your eyes or hands. The movement is finished when the object is found.|
|Select||St||Choose an object from among others, usually requiring hand / eye coordination.|
|Plan||Pn||Decide during the action what to do. This usually results in short hesitation.|
|Inspect||I||Determine a characteristic (quality ...) of an object with the eyes or the other senses.|
|Unavoidable Delay||UD||Waiting due to factors beyond the control of the operator and included in the cycle. For example, wait for a machine to cycle.|
|Avoidable Delay||AD||Waiting under the control of the operator and not included in the cycle. For example, open a packet of chewing gum.|
|Rest||R||Break to make up for fatigue and which are included either in the work cycle or between two cycles.|
Initially, there was a 18th movement, “Find“, it was associated with the end of the search movement when the object is found and verified. This movement has evolved to be removed because combined with the movement Search2.
Savings related to the human body
Principle 1 : both hands must be used at the same time
Most people’s natural tendency is to use their preferred hand (right or left hand) to do most of the work. The other hand is therefore very little used, as for example only hold one object, while the other one performs a task. The challenge is to use both hands as much as possible to limit stress only on a limb.
Principle 2 : both hands must begin and finish the movements at the same time
This principle is the continuity of the previous. It is necessary to redesign the process by considering both hands capable of doing a job at the same time and making them work on movements simultaneously to save time.
Principle 3 : the movements of the hands and arms must be symmetrical and simultaneous
This helps to minimize eye-to-hand coordination. If the hands perform the same movements at the same time, the level of concentration required is less than if both perform different and independent movements.
Principle 4 : the work is designed to make the operator work with his favorite hand
The favorite hand is faster, stronger and more skillful. If the work cannot be divided between the two hands, in accordance with the first principle, the use of the preferred hand must be advantageous. For example, it is preferable that the first Graps of a part is done with the preferred hand while the output is on the opposite side. The reason is that the first task requires more coordination, and that with the preferred hand, this is simpler. In contrast, the output of the part work area requires less concentration and can be done easily with the other hand.
Principle 5 : both hands must not do anything at the same time
The working method should not be designed to allow moments when both hands are idle. Only times when there is human-machine interaction or only a machine cycle are times when both hands can be inactive. However, if the machine does not require control, other tasks can be assigned.
Principle 6 : the method of work must consist of flexible movements rather than fast with changes of directions dry…
It takes less time to do the work with flexible motion sequences and curved trajectories rather than direct trajectories with many changes in direction. This is even if all the distances of the curves are longer. This is due to the fact that straight line sequences require accelerations and decelerations that consume time and energy.
Principle 7 : using “momentum” to facilitate work
When the carpenter pushes a nail with a hammer, he uses the “momentum” of the hammer, which can be defined as the mass multiplied by the speed. Let’s imagine that he tries to apply this same force without this dynamic…
Not all situations allow to use the momentum as the carpenter does. However, as far as possible, such a principle must be used to facilitate the work.
Principle 8: using gravity
Less time and energy are needed to move a heavy object from a high position to a low position rather than the other way around. This principle asks to design a Position that uses this type of solution.
Principle 9 : the method must follow a rate of natural movements
The rhythm of work must be natural and fluid movements. The sequence must be done without the need to think.
Principle 10 : use the lowest possible hand and arm classification
At the most the classification (see table below) is low at the most the movement is done as quickly and with the least effort. Therefore, the work must be done with the lowest classification, for example, by bringing together the tools and the pieces.
Description of the Movement
Fingers and handles
Fingers, handles and forearms
Fingers, handles, front and upper arm
Fingers, handles, front and upper arm, and shoulder
Principle 11 : minimizing the concentration and movement of the eyes
Minimizing eye work helps to reduce the level of concentration and therefore fatigue and errors. For this, one can work for example on the reduction of the distances between the parts and the tools.
Principle 12 : the method should be designed to use the feet and legs wisely
The legs are stronger than the arms but the hands are more skillful than the feet. The working method must take into account this, for example by using the legs to lift.
The savings associated with the design of the Position
Principle 1 : define fixed locations for tools and hardware
The challenge is to avoid wasting time in the search for tools or parts. We can think of implementing means to facilitate the implementation of tools and parts in their housing.
Principle 2 : arrange the tools and materials as close to their place of use
This helps to reduce distances, and therefore time and effort. It is necessary to arrange the elements in the normal work area.
Principle 3 : arrange the tools in the order of the work sequence
The elements must be arranged in a logical order depending on the sequence of work. The first tool used must be on one side of the work area, the second on the side and so on.
Principle 4 : use gravity boxes to deliver small parts
It is easier for the operator to take a Part that is on hand than to fetch that same piece of Part room at Part the bottom of a box. For this, there are boxes ” bottomless “which allow small parts (screws, washer…) to flow as they are consumed and thus save time.
Principle 5 : use gravity to circulate finished products
To save time and effort, it is better to use a gravitational or conveying system to circulate the finished products. This replaces advantageously the fact that the operator comes to position his product in boxes or on the next Position .
Principle 6 : adapt the light
The challenge of light is above all a matter of ergonomics. This makes the eyes less tiring and ensures comfort in movement and work in general.
Principle 7 : the chair must be adapted to the operator
Usually this means that the chair must be adjustable to fit the person and the Position . These settings must sweep all needs and apply to both the seat and backrest, and in some cases, the feet.
Savings related to the design of tools and equipment
Principle 1 : support for maintenance and Graps can be designed according to the tasks
Specific handles, maintenance or support systems can be designed. This saves time and above all a gain of effort. One can for example be able to seize several pieces at the same time, use magnetic systems…
Principle 2 : the hands can be relieved if possible by a foot pedal
A system via foot pedal can be advantageously implemented instead of hands. Sewing machines are the best example. This allows either to relieve them or to make them work simultaneously.
Principle 3 : combine multiple functions with one tool
If the same tool brings together two functions, it saves time and movement. Thus, they must be conceived in this sense and if possible that the same tooling is used in the same sequence of work one after the other.
Principle 4 : perform simultaneous operations rather than sequentially
The tools and equipment must allow to be able to perform simultaneous operations. The machining machines that come to work the parts on one side and the other at the same time are a perfect example.
Principle 5 : perform the work on several pieces at the same time
Always in the same objective and in the continuity of the previous principle, it is necessary to try to conceive workstations allowing to do a work on several pieces at the same time. For example, you can make a drilling on 2 or more pieces by designing suitable equipment.
Principle 6 : designing equipment to facilitate work and avoid mistakes
This is especially the case of the different control buttons, these must be close to the operator and easy to access. This is to limit movement and movement.
Principle 7 : hand tools must be designed for operator comfort
The grip of the tools must be simple and ergonomic. Specific handles must be put in place to be used comfortably throughout the working day: its position must be designed to guarantee safety and efficiency, and its Shape must provide for use by the hand right and left hand.
Principle 8 : mechanizing or automating manual operations if this is economically and technically feasible
Generally, mechanization or automation of a task will reduce the painfulness, gain in productivity and quality. However, the economic feasibility of the project will depend on the quantity to be produced and its evolution.
1 – D. Fergusson (2000) – Therbligs: The key to simplifying work
2 – M. Lehto, S. J. Landry (2013) – Introduction to human factors and ergonomics for engineers
A. H. Mogensen (1932) – Common Sense applied to motion and time study
R. Barnes (1980) – Motion and Time study
N. J. Manek (2001) – Comprehensive Industrial Engineering
B. Niebel (1992) – Motion and Time Study
M. P. Groover (2007)-Work Systems: The methods, measurement and management to work