Introduction to Gears & Dynamometers (Theory of Machines)
Chirag Chauhan #303
Ankit Shah #317
Ishan Parekh #315
Power transmission is the movement of energy from its
place of generation to a location where it is applied to
performing useful work
A gear is a component within a transmission device that
transmits rotational force to another gear or device
TYPES OF GEARS
1. According to the position of axes of the shafts.
3.Rack and Pinion
c. Non-intersecting and Non-parallel
worm and worm gears
Teeth is parallel to axis of
Transmit power from one
shaft to another parallel shaft
Used in Electric screwdriver,
oscillating sprinkler, windup
alarm clock, washing machine
and clothes dryer
The teeth on helical gears are cut at an angle to the face of
This gradual engagement makes helical gears operate
much more smoothly and quietly than spur gears
One interesting thing about helical gears is that if the
angles of the gear teeth are correct, they can be mounted
on perpendicular shafts, adjusting the rotation angle by 90
To avoid axial thrust, two
helical gears of opposite
hand can be mounted side by
side, to cancel resulting
Herringbone gears are
mostly used on heavy
Rack and pinion
Rack and pinion gears are
used to convert rotation (From
the pinion) into linear motion
(of the rack)
A perfect example of this is the
steering system on many cars
Bevel gears are useful when the direction of a shaft's
rotation needs to be changed
They are usually mounted on shafts that are 90
degrees apart, but can be designed to work at other
angles as well
The teeth on bevel gears can be straight or spiral
locomotives, marine applications, automobiles,
printing presses, cooling towers, power plants, steel
plants, railway track inspection machines, etc.
WORM AND WORM GEAR
Worm gears are used when large gear reductions are
needed. It is common for worm gears to have
reductions of 20:1, and even up to 300:1 or greater
Many worm gears have an interesting property that
no other gear set has: the worm can easily turn the
gear, but the gear cannot turn the worm
Worm gears are used widely in material handling and
transportation machinery, machine tools, automobiles
Pitch surface: The surface of the imaginary rolling cylinder
(cone, etc.) that the toothed gear may be considered to replace.
Pitch circle: A right section of the pitch surface.
Addendum circle: A circle bounding the ends of the teeth, in a
right section of the gear.
Root (or dedendum) circle: The circle bounding the spaces
between the teeth, in a right section of the gear.
Addendum: The radial distance between the pitch circle and
the addendum circle.
Dedendum: The radial distance between the pitch circle and
the root circle.
Clearance: The difference between the dedendum of one gear
and the addendum of the mating gear.
NOMENCLATURE…. Face of a tooth: That part of the tooth surface lying outside the
Flank of a tooth: The part of the tooth surface lying inside the
Circular thickness (also called the tooth thickness): The
thickness of the tooth measured on the pitch circle. It is the
length of an arc and not the length of a straight line.
Tooth space: pitch diameter The distance between adjacent
teeth measured on the pitch circle.
Backlash: The difference between the circle thickness of one
gear and the tooth space of the mating gear.
Circular pitch (Pc) : The width of a tooth and a space,
measured on the pitch circle.
Diametral pitch (Pd): The number of teeth of a gear unit pitch
diameter. The diametral pitch is, by definition, the number of
teeth divided by the pitch diameter. That is,
Pd = diametral pitch
N = number of teeth
D = pitch diameter
Module (m): Pitch diameter divided by number of teeth. The
pitch diameter is usually specified in inches or millimeters; in
the former case the module is the inverse of diametral pitch.
m = D/N
VELOCITY RATIO OF GEAR
d = Diameter of the wheel
N =Speed of the wheel
ω = Angular speed
velocity ratio (n) =
ADVANTAGES OF GEAR DRIVES
It transmits exact velocity ratio.
Transmits large power.
DISADVANTAGES OF GEAR DRIVES
The manufacture of gears require special tools and
The error in cutting teeth may cause vibrations and noise
Selection: Gear Drive System
The reliability of a Gear Drive system depends upon its quality of
components, accurate assembly of these components and solid
1. Load demand
2. Duty cycle
3. External loads
4. Input power
5. System accessories
6. Facility needs and environment
A Service factor is another factor on the basic of which the
selection of the drive is determined. This service factor accounts
for the varying of torque by the driven machines and the driving
Application of Gear Drive in Cars
In a car, first, the motor turns the shaft which is attached to the
gears. So, the gears also start to turn one after another.
Gradually, this turning process passes from one gear to another.
When, the last gear a starts to turn which is attached to the car's
axle, the axle starts to move.
As a result, the wheels of the car
start to move. This is how a gear
system moves a car.
Measurement of brake
horsepower is the most
important measurements in the
test schedule of an engine. It
involves the determination of
the torque and the angular
speed of the engine output shaft.
This torque measuring device is
•This dynamometer measure and absorb the power out put of
the engine to which they are coupled. The power absorbed is
usually dissipated as heat by some means.
•Examples of such dynamometers are
Prony brake dynamometer.
Rope brake dynamometer.
PRONY BRAKE DYNAMOMETER.
•It works on the principle of converting power into heat by dry
•In this method of measuring horsepower is to attempt to stop the
engine by means of a brake on the flywheel and measure the
weight which an arm attached to the brake will support, as it tries
to rotate with the flywheel.
ROPE BRAKE DYNAMOMETER.
•It consists of a number of turns of rope wound around the
rotating drum attached to the output shaft. One side of rope is
connected to a spring balance and the other to a loading device.
• The power is absorbed in friction between the rope and the
drum. The drum therefore require cooling.
•Rope brake is cheap and easily constructed.
•It is not very accurate because of changes in the friction
coefficient of the rope with temperature.
•It works on the principle of dissipating the power in fluid friction
rather than in dry friction.
•It consists of an inner rotating member or impeller coupled to the
out put shaft of engine. This impeller rotates in a casing filled with
•The heat developed due to dissipation of power is carried away by
a continuous supply of working fluid, usually water.
•The output can be controlled by regulating the sluice gates which
can be moved in and out to partial or wholly obstructive flow of
water between impeller and the casing.