|
Last time we learned that the geometric shape specific to spur gear teeth is known as an involute profile. Today we’ll look at the geometry behind this profile and the very specific place at which gear teeth meet, known as the point of contact. The transmission of mechanical energy between meshed gears may seem on its face to be straightforward, after all their gears are interlaced and interact with one another. But their interaction involves some rather complex geometry, because forces are directed in a peculiar fashion between the teeth of the driving and driven gears. Let’s consider the following illustration to get a better understanding.
 Meshed Gear Tooth Geometry
As we learned previously in this series, the pitch circle of a gear is an imaginary arc passing through each tooth between their top and bottom lands. The pitch circles of the driving and driven gears are represented by heavy red dashed lines in the illustration. To ensure proper alignment and smooth action between gear teeth during rotation, the gears are spaced so that their pitch circles just meet but never intersect. This specific point is known as the point of contact. It is the only point at which gears will come into actual physical contact with one another, and it provides just enough contact so that when the driving gear turns in one direction, say clockwise, its teeth exert pressure upon the driven gear teeth, forcing it to move in the opposite direction, counterclockwise. The forces which come into play at the point of contact are represented in the illustration by a black dot with oppositional blue arrows extending from it. These arrows represent the opposing mechanical forces, F1 and F2 , which act upon the teeth when they make contact. We’ll learn more about the effect of those forces next time when we follow a locomotive from a stationary position into one of movement.
_______________________________________ |
Posts Tagged ‘gear technology’
Meshed Gear Teeth and Their Point of Contact
Monday, January 27th, 2014The History of Gears
Monday, December 23rd, 2013|
Could it be that after cave men invented the wheel they moved on to invent another circular object, the gear? Gear assemblies are found in a wide variety of applications, from tiny ones used inside wrist watches to massive ones found in aircraft carriers. No one knows for certain when gear technology was first employed, but we do know that gear driven machinery has been around since before the Industrial Revolution. As far back as the Renaissance we’ve documented their use within flour milling equipment and the first primitive clocks. Going even further back in time, Roman engineers are known to have developed a primitive gear driven odometer. It was attached to horse drawn cart wheels and the number of revolutions performed allowed the distance traveled to be calculated. In fact gears have been used far longer than scientists originally thought. In October of the year 1900 sponge divers stumbled upon an ancient Roman shipwreck at the bottom of the Aegean Sea near the Greek island of Antikythera. Inside this wreck they found mineral encrusted fragments of an artifact composed of a bronze alloy. This amazing discovery appeared to be a remarkably modern looking gear assembly which would come to be known as the Antikythera Mechanism.
 The Antikythera Mechanism Analysis of the Mechanism conducted over the last 100 years has revealed it to be a highly complex device. Still visible engraved inscriptions disclose it to be of Greek origin, dating back to about 100 BC. As such it’s the oldest known complex gear driven mechanism in the world. Prior to its discovery it was thought that mechanisms of its kind were not made until 1400 AD. As to the purpose it served, that remains a subject of controversy, since many of its parts are missing.
 X-Ray View of the Antikythera Mechanism The X-ray image reveals some of the Mechanism’s hidden complexity. Based on detailed examination of these images coupled with engineering analysis, it’s theorized by scientists that the mechanism may have been configured as illustrated below.
 Possible Configuration of the Antikythera Mechanism Since there’s no evidence that ancient Greeks possessed motors, such as those used in modern clocks, some scientists believe that the gears in the Mechanism were set into motion by simply turning a hand crank. Others believe that the arrangement and size of the gears indicate that the Mechanism’s movement is analogous to planetary motion within our solar system. They theorize further that it may have been used to calculate the positions of the Sun, Moon, and other celestial bodies. Next time we’ll fast forward to present day to familiarize ourselves with the basic terminology of gears and then later see how they’re used in modern devices.
________________________________________ |
