Power Transmission
Fundamentals
November, 2008
Topics
•Measuring Systems
•Speed and Velocity
•Circumference of a Circle
•Service Factors
•Force
•Ratios
•Torque
•Power Transmission
Terminology
•Work
•Power
•Horsepower
•Efficiency
2
Measuring Systems
A. British Engineering Units (BEU). The British Engineering Units
use the foot (ft), pound (lb.), and second (sec) as the basis of
measurement.
B. International Standard (SI). The International Standard uses the
metric system of measurement, which is based upon the meter (m),
kilogram (kg), newton (N), and second (sec).
3
Circles
A. Circumference or the measurement around the outside
of a circle.
B. The Diameter of a circle is a straight line drawn from a
point on the circumference through the center to another
point on the circumference.
C. The Radius of a circle is ½ the diameter.
4
Force
A. Force is the push or pull of one body on or against another
body, usually measured in ounces, pounds or tons. When the
applied force exceeds the resistance of the object, motion is
produced.
B. Motion is defined as a changing of position or displacement.
When a shaft turns, it has motion.
C. Types of Motion
1. LINEAR (Straight line)
2. ROTARY (circular)
5
Torque
Torque is a twisting or turning force, which “tends” to produce
rotation. Torque is determined by multiplying the applied force
times the distance from the shaft center.
Therefore: Torque = Force x Radius
The force can be in either pounds or ounces, and the
distance is in inches or feet. Torque is generally expressed
in terms of foot-pounds,
6
Work
A. The Engineering Definition: Work occurs when a force moves a
body a measured distance. If the force does not move the object,
no work is being done because no movement takes place.
B. The quantity of work done is the amount of the force multiplied by
the distance moved.
C. FORMULA: Work = F X D
7
Power
A. To calculate Power, we must add one more element: Time
B. Power is the amount of work done in a given amount of time.
Therefore, it can be said that power equals work divided by time.
C. FORMULA: Power = Work (Force x Distance) divided by Time.
8
Horsepower
The most common unit of measure is horsepower.
James Watt developed the steam engine in the 18th century. He
needed a way to compare its power with that of a horse.
Experiments were conducted with horses lifting weights. It was
estimated that a horse could raise 550 lbs. vertically at the rate of
one foot in one second.
Using these figures, the unit of horsepower was established at
550 ft.- lbs. per second or (550 x 60) 33,000 ft.-lbs. per minute.
9
Horsepower
Formulas:
Hp (seconds):
HP = Work (ft.-lbs.)
Time (Sec) x 550
Hp (minutes):
HP = Work (ft.–lbs.)
Time (min) x 33,000
HP
=
Torque(lb.ft) x rpm
5250
Torque (lb.ft.) =
HP x 5250
rpm
10
Horsepower
It is the responsibility of the customer to specify
the required motor Hp, Torque, and accelerating
time for his application.
For estimating purposes only.
Rules Of Thumb (Approximation)
•At 1800 rpm, a motor develops a 3 lb.ft. per hp
•At 460 volts, a 3-phase motor draws 1.25 amp per hp
•At 230 volts a 3-phase motor draws 2.5 amp per hp
11
Efficiency
Friction, created in the moving components of a machine, causes
energy from the power source to be lost as heat; therefore, energy
output is always less than energy input and is usually expressed as a
percent (%).
Efficiency is an important consideration in selecting a motor to drive
an application through a speed reducing mechanism. The efficiency
will vary depending on the type of reducing element and speed.
12
Efficiencies
Typical Efficiencies
•Single Stage Cyclo Reducer 95%
•Helical gear reducer: Single reduction 98%. Double reduction 96%
•Single Worm gear reducer 50% to 90%
•Hyponic reducer: 85%
Helical Gear
Worm Gear
Hypoid Gear
13
Double Helical
Speed and Velocity
The velocity (speed) of an object is the distance it travels in a given unit
of time. More often, we are concerned with rotary motion (RPM or
Revolutions Per Minute).
In many cases (especially in working with conveyors) the customer may
ask you to supply components based upon a driven speed given in feet
per minute.
V (FPM) = P.D. x .262 x RPM
14
Service Factor
The service factor could be called appropriately an “experience”
factor.
The service factor takes into account such things as type of driver
(motor or internal combustion engine), type of application, hours of
operation, and miscellaneous conditions (abrasive dust, excessive
heat or moisture, shock load etc.).
It should always be applied in the selection of any power transmission
component.
Motor Horsepower x Service Factor = Design HP
15
Ratio
Ratio is a fixed relation or proportion between two like objects. In power
transmission it can be used to express size or speed relationships.
Ratio is calculated by dividing the larger number by the smaller number.
Ratios are never added or subtracted. They may only be multiplied or
divided.
16
PT Terminology
A. Mechanical Rating: Maximum HP or Torque a speed
reducer can transmit over a continuous period without damage.
B. Thermal Horsepower Rating: Actual horsepower a speed reducer
will transmit continuously without overheating.
C. Demand (or Brake) Horsepower: The amount of HP
required to keep an application running at design speed, fully
loaded.
D. Design (or Equivalent) HP: Application HP times
required service factor. A drive or coupling must rate at least
design or equivalent HP/kW to be a good selection.
17
Terminology
E. Breakaway Torque: The amount of torque that is required to
overcome the machines inertia load and friction.
F. Overhung Load: A load applied at a right angle to a rotating
shaft. V-belts, pinions, chain and sprockets impart overhung loads.
G. Thrust: A load applied parallel to a shaft’s axis, either inward or
outward.
H. Center Distance: Distance between centerlines of input and
output shafts of parallel shaft and worm gear speed reducers.
I.Bearing L10 Rating: It is calculated life expectancy. Not more than
10% of the bearings will be expected to fail within their design life.
18
Terminology
AMBIENT TEMPERATURE
Ambient temperature is the temperature of the air surrounding the
equipment in question.
AUXILIARY DRIVE
Belt or chain drives used with a speed reducer to provide additional
speed reduction (and torque multiplication) in a drive system.
PRIME MOVER
Driving member of a drive system; can be an electric motor, internal
combustion engine, turbine, etc.
19
Congratulations
20