There have been two previous posts on mechanism design, which can be found (here) and (here). In this third and final part, we are looking at several more mechanisms that are useful in the plastic molding world: the gear and the ratchet mechanisms.

Gears

Gears are mechanisms that are used to transmit rotary motion or torque from one shaft to another. They are also capable of or converting rotary motion into translating motion as can be found in a rack. Gears are one of the most widely used mechanisms and therefore also one of the most useful ones. Two or more gears working in sync become a gear train and it can be considered a machine because there is work being done. The interaction between two gears creates a mechanical advantage that is less than, equal to, or greater than 1 as based on the ratio of the sizes of the meshing gears. The torque follows a proportional relationship between the gears, as do the relative gear sizes and the rotational speeds. The primary gear, which turns other gears, is known as the driver gear; the gears being turned are referred to as the driven gears. Other gears within the gear train between the input and output gear are called idler gears, as their role is just to transmit the torque or rotation.

There are a great many types of gears and gear configurations but we shall classify them according to the relative positions of the axes of revolution. By this classification, there are basically three types of gears: parallel, intersecting, and neither parallel nor intersecting.

Gears for connecting parallel shafts

These gear configurations are such that both the shafts are aligned parallel to each other and are perpendicular to the axes of rotation.

• Spur gears: These are the most basic types of gears and can be of two types – internal and external. An internal gear, like most gears, has its teeth on the outer surface of the cylinder while an external gear has its teeth on the inside surface of the cylinder. This results in an internal and external meshing of the gears, respectively.

• Helical gears: Unlike spur gears, these gears have teeth that are cut at an angle and are not parallel to the axes of rotation. This is an advantage because the teeth between opposite gears make contact more gradually and thus generate far less noise.

• Herringbone (double-helical) gears: This gear is created by joining two helical gears that are symmetrical but mirrored, which results in a V-shaped set of teeth. Herringbone gears are useful because they balance out the axial thrust that can be found in helical gears.

• Rack and pinion gears: This gear configuration consists of a regular gear and a straight rod – the rack – that can be considered to have an infinite radius of curvature. It is especially useful for converting rotary motion into translation.

Gears for connecting intersecting shafts

These gear configurations feature shafts that are not parallel to each other but are so arranged that if they were to be extended, they would intersect at a single point.

• Straight bevel gears: The gear interaction of a straight bevel gear is as of two cones meshing together but with the most of their tips cut off, leaving only the base left. As the gears mesh, the imaginary tips of both cones must meet at a single instantaneous point. The teeth of bevel gears are straight.

• Spiral bevel gears: These gears are virtually the same as the straight bevel gears but differ in one main thing; the teeth are cut in such a way that they would form a spiral if extended to a common center. The advantages and disadvantages between spiral and straight bevel gears are similar to those between spur and helical gears.

Gears for connecting neither parallel nor intersecting shafts

These gear configurations have shafts that are neither parallel nor intersecting.

• Crossed-helical gears: Also known as skew gears, the shaft’s axes are not parallel and neither is its axis of rotation. They are basically two helical gears meshing perpendicularly, resulting in a change in the direction and axis of rotation between the shafts. 

• Hypoid gears: These gears are very similar to the spiral bevel gears, with the major difference being that the shaft axes do not intersect. These gears are mostly designed for shafts that are perpendicular to each other.

• Worm gears: Like bevel gears, worm gears can alter the axis of rotation between two shafts but are limited to one orientation: the shafts are perpendicular to each other. A worm gear set looks like a screw and a typical spur or helical gear mating together to achieve perpendicular rotation. Its major advantage is its capacity to produce gear ratios of up to 500:1 and its major drawback is its low efficiency due to the high amount of friction incurred.

Gears are very useful mechanisms and are not limited to the ones mentioned above. Some other types include: crown gears, magnetic gears, cage gears, harmonic gears, and others; each and every one of them have applications that are adaptable for plastic parts.

Ratchets

This mechanism is comprised of a wheel or rack with teeth. A rack with teeth is much like a gear but without interaction with another gear. They interlock with a driving pawl in such a way that the rotation of the wheel or translation of the rack is restricted to one direction. The driving pawl is attached to an oscillating lever, which governs the movement of the wheel or rack. Then, a supplementary pawl ensures that the motion is not reversed by wedging in place after every fraction of a turn or linear motion but otherwise will only slide over the teeth of the ratchet wheel or rack if it is moving in the correct direction.

Ratchet mechanisms have applications in simple forms, such as seat belt locks and fasteners as well as unusual forms like turnstiles.

This concludes our series of posts on mechanism design. The field of mechanism design is a really interesting one and there are virtually no limits to the ways each and every one of these mechanisms can be merged and made to interact with one another in order to create extremely complex and clever motions. The ingenuity and improvisation involved creating some mechanisms can be really breathtaking.

To learn more about mechanisms, click here

Click on picture to download our eBook on Mechanism Design