A mechanism designed to generate a large force from a small force is called a boost mechanism. Mechanical components that constitute this boost mechanism include links, levers, screws, wedges, gears, and pulleys. For lightweight arms of robots entered in the Robocon robot contest, the link mechanism is frequently adopted for its boosting effect produced from the simple structure.
Lifting up a heavy load using a lever is also an application example of this boost mechanism. In this case, you will be able to lift up a heavy load with a small force, but you will need to apply the force for a longer distance than when you carry the heavy load itself.
As you already know, using the boost mechanism does not change the workload (the force required for moving an object multiplied by the travel distance).

Now, let's take a look at the link mechanism consisting of four-bar linkages in [Fig.1] - a) and consider how the force vector is applied at Joint A.
[Fig.1] - b) is the schematic layout of a link mechanism generating a high degree of boosting effect.
[Fig.1] - c) is the schematic layout of a link mechanism generating little boosting effect.
Let's compare their force vector components at Joint A.

In [Fig.1] - b), "Crank OA" and "Link AB" form an angle of approximately 180 degrees (close to a linear arrangement) whereas "Link AB" and "Lever BC" are almost perpendicular to each other. In this case, if "Torque T" applies to "Driving Shaft OA" and Force F₀ applies to the tangential direction of "Joint A", Force F₁ applied to "Link AB" on "Joint A" will be 2.7 times greater than the tangential force F'₀ based on the vector components at "Joint A".

In [Fig.1] - c) on the other hand, Force F'₁ applied to "Link AB" on "Joint A" is only 1.1 times greater than the tangential force F'₀.

As seen in this example, using the link mechanism to form the relationship shown in [Fig.1] - b), allows effective construction of the boost mechanism. In addition, when the link mechanism is applied to the boost mechanism, all the rotary couplers (in pairs) and the links must be designed with enforced strength.

In the next volume, we will look at an application example of the boost mechanisms using a crank.