Non Circular Gears

Functioning

Types

Application

The Non circular plays a critical role in any type of application they are put into, but often their critical importance gets overlooked. It has been seen that the Non Circular Gears can display numerous kinds of unusual motion or speed characteristics. What makes the non circular gears unique is the fact that through the Cams or linkages can also provide these special types of motion requirements, but with non circular gears a more simpler, compact, accurate solution can easily be worked out.

Functions Performed by Non circular Gears
Some of the very common requirements that are effectively handled by non-circular gears consist the following:

• Variable Speed: Conversion of an uniform input speed into a variable output speed.
• Constant Speed Segments: Providing many types of constant-speed segments on an ongoing operating cycle.
• Combined rotation and translation.
• Stop-and-dwell motion.

Types of Non Circular Gears
Actually the type of Non circular Gears is dependent on the type of speed they deliver. On that premise Gears can be of the types that producing Variable speed. Gears With constant speed segments. Those for combined rotation and translation and finally that with stop-and-dwell motion.

Non Circular Gears with Variable speed
In this category there are generally three types.

Elliptical Gears : Elliptical gears can generate a variable output speed. A set of identical elliptical gears has been seen to run at an uniform center distance, but delivers a changing output speed as they rotate.
Elliptical Gears can come in two basic shapes. The first is the Unilobe Gears. The image of which is shown below. Unilobe elliptical gears generally rotates about in one of the two fixed points on its axis.
(Image showing Elliptical unilobe gears providing a variable output speed)

The second type is the Bilobe Gears. The Bilobe elliptical Gears is found to be rotating about its center.
(Image showing bilobe elliptical gears generating twice the period of variable output speeds)

As is seen from the graph, there is a variation in the speed-reduction ratio of these gears. It is varying from 1/K to K for each cycle of the rotation. The practical value of K ranges up to 3. In the image shown above, with the rotation of the gears the radii of both the driving and driven gears change. Consequently there is first a decrease in the speed for 1/4 revolution, then there is increase for 1/4 revolution, so on. These recurring periods of speed increase or decrease takes place four times per revolution.

Triangular Gears : A pair of triangular gears generally has three lobes, or high points on the perimeter as compared to the two lobes in the elliptical bilobe gears. As a result triangular gears produces a total of six periods increase or decrease of speed per revolution in comparison to the four in bilobe gears. But they have a comparatively smaller range of speed ratio than the elliptical gears.
(As Shown in images below) Square Gears : Square Gears are another way of producing variable speed. Square Gears have four lobes. As a result, they deliver eight periods of increase or decrease in speed per revolution. The Square Gears also have a smaller range of speed ratio than the elliptical gears.
(As Shown in images below) Non circular Gears with constant-speed segments
In this category where the application requires numerous constant-speed segments within a cycle, there is multispeed gears, perfectly suitable for the job.

Multispeed Gears : The Multi speed Gears are designed in a manner to work in cases where there is requirement of several constant speed segments. Multi speed Gears delivers one uniform speed for part of a cycle and a different speed for the second part of the cycle. They can make a perfect transition between the speeds. It does so by unique function segments between constant speed sections of a gearing sequence. Typically, Multispeed gear pairs consists of sinusoidal transitions and are not similar.
(As Shown in images below)

However, similar input and output gearing can be achieved with help of elliptical transitions.

Non circular Gears for with rotation and translation
For application that requires both translational and rotational motion, there are certain gears that serve as a proper cam substitutes.

Cam Gears : Cam Gears are often applied for use in labeling machines Cam gears copies the shape of a part that requires labeling and a cam-following rack generally carries the labeling equipment at a constant surface speed. In the welding applications, where a cam gear is used for simulating the shape of a part that is to be welded and a follower is used to carry a welding torch at an uniform speed for a smooth welding. (Image showing a Cam substitute gear that is meshed with a round gear for simulating speed and displacement features of a cam)

Stop-and-Dwell Motion : For some type of machines there is a requirement for stop-and-dwell motion or a reverse motion. This is not an easy task. However, a combination of noncircular gears with round gears and differential (epicyclic gear train) can achieve Stop-and-Dwell Motion or the reverse motion. Stop-and-dwell motion is a common occurrence in the working of indexing mechanisms. (The graph depicts how Noncircular gears in tandem with round gears and a differential can give a stop and dwell or even reverse motions)

Reverse motion is necessary where a transfer device is made to operate between two sites. Here, a noncircular gear arrangement is a better alternative than a linkage assembly. But on the downside, inspite of flexibility offered, such arrangements are costly.

Application of Noncicular Gears
The examples that are given here explains the importance of noncircular gears and how they solve critical manufacturing problems.

Sealing head : A heat sealing device is applied for sealing items like the tops of containers on a constant-speed conveyor. The challenge here is to maintain touch for a short time (less than 1sec) without any slippage between the container and sealing head. A popular method is to install an indexing device which can stop the conveyor as the sealing takes place. In case the conveyor can’t be stopped, then a cam and linkage equipment or electronic servo is also used.
(Image depicting sealing head crank mechanism)

Now to counter this challenge there is a simple solution. The solution is to mount the sealing head on a crank mechanism, driven by elliptical gears. A sliding action between yoke-and-pin makes the head stay in touch with the container for a moment.

A different output speed being provided by the gears, jointly with the yoke action, allows the sealing head to follow the container without slippage and then return well in time for the next container.

Rotary Cutoff Knife : Functioning of rotary knife requires cutting materials on a conveyor to different lengths. But the conveyor would run at a fixed speed. Actually for cutting materials of different length without altering the speed of conveyor(web) necessitates the use of different knife sizes (diameters) and also varying the speed of the knife. To rise to this challenge there comes the Noncircular Gears. Elliptical or multi speed gears, are now applied to drive the knife.

By altering input gear speed with respect to web speed, is able to change the length of material to be cut. A minor adjustment in the phasing between gears in terms of the cutoff point makes the knife to match with the web speed.