In part 1 of this series, we introduced the concept of mechanism design and explained why it is useful and how it can be applied. We also mentioned that there are several types of mechanisms and we took a look at levers as one of the simplest, oldest, and indeed one of the most useful mechanisms ever invented. In part 2 of this series, we shall go further and touch upon two more types of mechanisms: springs and cams.
A spring is a mechanism that is able to absorb, store, and release energy as a result of its elastic properties. This elastic property allows it to deform, either as a result of compression or extension, and then return to its original dimensions – provided that a limit called the elastic limit is not passed. The elastic limit is when the extension or compression becomes permanent and failure occurs. Before this limit is reached, the extension or contraction of most springs is proportional to the force that is being exerted by the spring, which is articulated by Hooke’s law:
F = -kx
In this equation, F is the force exerted by the spring, x is the distance extended or compressed from the natural relaxed position of the spring or simply the displacement vector, and k is a constant known as the “spring constant.” The negative in the equation indicates that the displacement vector is always in the opposite direction of the force exerted by the spring. Hooke’s law applies generally to elastic materials or mechanisms that behave in an elastic manner; for springs specifically, the spring constant is derived by the formula:
In this equation, G is the shear modulus of the material, d is the wire diameter, D is the average spring diameter, and na is the number of turns.
Springs can be classified based on a number of criteria – geometry, how a load is applied to them, the type of force that the spring exerts when it releases stored energy, etc. For the purpose of conciseness, we will look at the most common types of springs and outline where they fall based on these criteria.
Extension springs are designed to resist a tensional load. It is made in such a way that the coils of the springs are tightly wound together so that there are no spaces between the coils when the spring is relaxed. It stretches when a load is applied to it.
Compression springs are essentially the opposite of extension springs, as they are designed to resist compressive forces and shrink upon being loaded. The coils are spaced apart; the distance between the coils determines how far it can be compressed from its relaxed position.
Torsion springs are different from both extension and compression springs in that the load they bear is not linear. The force is, instead, a twisting force, or torque. A torsion spring is designed to resist rotational force and is usually compact with both ends extended, which is where the load is applied.
Springs Constant force springs are unusual because they do not obey Hooke’s law. Instead, they deliver an almost constant force as they unwind from their coiled position. The spring is like a ribbon made of sheet metal that has been wound around a drum. They store and conserve energy extremely well and are usually used for delicate and precise applications. There are many other types of springs, but they are usually a combination of two or more of the aforementioned springs or a variation of the basic principles governing them. Examples are volute springs, negator springs, and mainsprings.
Cams are mechanisms that convert rotary, oscillatory, or sliding motions into reciprocating linear motions or vice versa. Cam mechanisms typically consist of two components – the cam and the follower, with the follower usually being fixed. Cams are known for their versatility and virtually any reciprocating motion can be achieved using them. Since cams produce reproducible and predictable movement, they are very useful in applications where automation and precision are important. Cams can be classified using a number of criteria, including the cam shape, the follower configuration, and the relationship between the input motion of the cam and the output motion of the follower.
Examples of cam mechanisms based on the shapes of the cams:
- Plate cam: the plane of motion of the follower is perpendicular to the axis of rotation of the cam. It is the most commonly used cam.
- Cylindrical cam: the motion of the follower is governed by a groove along the periphery of the cam. This results in a translating or oscillatory motion. Sometimes the cylinder is replaced with a cone, which adds more range of motion to the follower.
- Linear cam: this cam moves linearly or with a sliding motion instead of rotating. A good example is a key for a pin tumbler lock – as the key enters the lock, it displaces the pins (which serve as the followers) just the right distance and in the right direction for the lock to be turned.
Examples of cam mechanisms based on the follower configuration:
- Knife-edge follower: the follower tip is pointed, like the edge of a blade. This follower generates some friction.
- Roller follower: the follower is a roller that rolls around the cam as it moves. The friction is minimal.
- Flat-faced follower: the follower uses a plate-like structure, which the cam pushes as it rotates.
- Spherical-faced follower: this follower is round but is not a roller. It is commonly half of a sphere, which allows the cam to impact on it with little wear.
Examples of cam mechanisms based on the relationship between input and output motion:
- Rotating cam/translating follower: this is the most common type of cam mechanism, where a rotational motion of the cam is converted into a translating motion of the follower.
- Rotating follower: the motion of the cam causes the follower to rotate or oscillate around an axis.
- Translating cam/translating follower: both the cam and follower motions are translating or sliding. As you can see, cams are very versatile and can be used to produce very complex and irregular motions that are difficult to design using other mechanism types.
That concludes part 2 of this series. In part 3, we will take a look at some more mechanisms and how we can exploit them to our advantage.
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