Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 2 Indirect Power Transfer Devices 85
GEARS
Gears are the most common form of power transmission for several reasons.
They can be scaled to transmit power from small battery powered
watch motors (or even microscopic), up to the power from thousand
horsepower gas turbine engines. Properly mounted and lubricated, they
transmit power efficiently, smoothly, and quietly. They can transmit
power between shafts that are parallel, intersecting, or even skew. For
all their pluses, there are a few important things to remember about
gears. To be efficient and quiet, they require high precision, both in the
shape of the teeth and the distance between one gear and its mating
gear. They do not tolerate dirt and must be enclosed in a sealed case that
keeps the teeth clean and contains the required lubricating oil or grease.
In general, gears are an excellent choice for the majority of power transmission
applications.
Gears come in many forms and standard sizes, both inch and metric.
They vary in diameter, tooth size, face width (the width of the gear), and
tooth shape. Any two gears with the same tooth size can be used
together, allowing very large ratios in a single stage. Large ratios
between a single pair of gears cause problems with tooth wear and are
usually obtained by using cluster gears to reduce the gearbox’s overall
size. Figure 2-14 shows an example of a cluster gear. Cluster gears
reduce the size of a gearbox by adding an interim stage of gears. They
are ubiquitous in practically every gearbox with a gear ratio of more than
5:1, with the exception of planetary and worm gearboxes.
Gears are available as spur, internal, helical, double helical (herringbone),
bevel, spiral bevel, miter, face, hypoid, rack, straight worm, double
enveloping worm, and harmonic. Each type has its own pros and
cons, including differences in efficiency, allowable ratios, mating shaft
angles, noise, and cost. Figure 2-15 shows the basic tooth profile of a
spur gear.
Gears are versatile mechanical components capable of performing
many different kinds of power transmission or motion control. Examples
of these are
• Changing rotational speed.
• Changing rotational direction.
• Changing the angular orientation of rotational motion.
• Multiplication or division of torque or magnitude of rotation.
• Converting rotational to linear motion and its reverse.
• Offsetting or changing the location of rotating motion.