Transcript Slide 1

UNIVERSITY OF JORDAN
FACULTY OF ENGUNEERING AND TECHNOLOGY
ELECTRICAL ENGINEERING DEPARTMENT
ELECTRIC DRIVES
GEARS
• Done by:
1- Saleh Lutfi Ahmad Hussein.
0077067
2- Mohammad Mazen Al-Ahdab.
0087136
3- Mohammad-Saif Baha-Eddin Saad. 0087336
GEARS
• Power transmission is an assembly of parts including the speed-changing gears
and the propeller shaft by which the power is transmitted from an engine to
another device.
• A gear is a component within a transmission system that transmits or
transforms mechanical energy to match the requirement of an application, as
the operating point of motors is generally at higher speeds than the
application requires.
• It can also be defined as:
A wheel with teeth along its rim
Gear Ratio
OutputTeeth speedinput orqueoutput
GearRatio


InputTeeth
orqueinput
speedoutput
Types of Gears
“According to the position of axes of the shafts”
A. Parallel
Spur Gears
Helical Gears
B. Perpendicular Axis
Intersecting
Bevel Gears
Non Intersecting
Worm Gears
Herringbone Gears
C. Planetary
Planetary Gears
SPUR GEAR
• Teeth is parallel to axis of rotation
• Transmit power from one shaft to
another parallel shaft
• Used in Electric screwdriver,
oscillating sprinkler, windup alarm
clock, washing machine and clothes
dryer
External and Internal spur Gear…
Helical Gears
“drive shaft and driven shaft are aligned in parallel”
•
The teeth on helical gears are cut at
an angle to the face of the gear
•
The teeth of helical gears are generally
designed for infinite service life.
•
This gradual engagement makes helical gears operate
much more smoothly and quietly than spur gears
 Evolvent Tooth Advantages:
• Not sensitive to deviations in the distance
between axes.
• smooth movement transmission, i.e. lowvibration running.
• Ease of manufacture, which leads to low costs.
 Evolvent Tooth Disadvantages:
• In the case of external gearing, convex edge parts run against convex
edge parts, which limits the load capacity.
• When using a small number of teeth, the teeth are undercut due to the
manufacturing process.
Meshing of two teeth with Evolvent tooth profile
•
One tooth each of the driving
wheel and the driven wheel
contact each other at a point on
the mesh line.
•
Pure rolling of the teeth only
takes place at the point when
the pitch circles (dw1 and dw2)
form an intersecting point with
the mesh line.
Herringbone Gears
“Double Helical Gears”
•
To avoid axial thrust, two helical gears of
opposite hand can be mounted side by side,
to cancel resulting thrust forces.
• Axial Thrust is a force that is generated
in an axial direction which is along the
shaft.
• Herringbone gears are mostly used on
heavy machinery.
Planetary Gears
“suited for highly dynamic drives”
•
With their weight advantage, planetary
gearboxes are also well established for
extremely high output torque and for
mobile applications.
•
The maximum ratio that can be achieved in a
single stage is approximately i = 12. Larger
ratios are then achieved by adding further
downstream stages.
•
Alongside the increased load capacity, it is also reduce
noise emissions.
Bevel gears
“converting torque between intersecting axes ”
•
Useful when the direction of a shaft's rotation
needs to be changed .
•
Used to transmit and convert torque and speed
between axes that intersect and cross one another.
•
Usually mounted on shafts that are 90 degrees
apart, but can be designed to work at other angles
as well.
•
The design is much more complex.
•
Bevel gear sets typically have a maximum ratio of i = 6, that’s why
bevel gear are used as pre-stage for planetary or helical gearboxes.
 Advantage:
• higher load capacity.
 Disadvantage:
slightly poorer efficiency with this type of tooth system.
•
Types:
Straight
Spiral
•
Examples:
locomotives, marine applications, automobiles, power plants, steel plants,etc
Worm Gears
“globoid wheel is used with a cylindrical worm”
•
the opposite configuration with a globoid
worm is expensive so it is only used for high
performance gearboxes.
•
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. This is because the
angle on the worm is so shallow that when
the gear tries to spin it, the friction
between the gear and the worm holds the
worm in place.
•
Worm gears are used widely in material handling and transportation
machinery, machine tools, automobiles etc.
•
The efficiency of a worm gearbox is based on the
ratio and drops sharply as the ratio increases.
•
Depending on the ratio, the start-up efficiency
can be some 20 to 30% below the efficiency in
operation, due to the lack of a lubricating film
between the worm and the wheel.
•
Worm gearboxes can implement ratios up to 50 or even more.
•
Adding rips on the gearbox housing offer a bigger surface area to improve heat
dissipation.
•
If the efficiency is below 50%, which is possible with high ratios, selflocking kicks in when reversing the direction of force. This occurs when the
worm lead angle ( γ ) equals tan-1 (μ). (µ is the friction coefficient)
•
Self-locking is that the gear does not allow the interchangeability between the
driving and the driven gear.
• Most of the worm gear trains used in industry are of the self-locking type.
Combining gearboxes with motors
•
These geared motors can be created by
connecting the motor to the gearbox
using a coupling.
•
The pinion z1 of the first gearbox stage
is connected to the shaft of the motor via
a suitable shaft/hub connection.
This eliminates the need for bearing
mounting the input shaft of the gearbox.
•
Geared motors cannot only be formed together with standard three-phase AC
motors, but also with servo motors
This then creates geared servo motors.
The various gearbox types can be combined
with different motor types.
Definition OF SPUR GEARS
Definitions
• Pitch surface: The surface of the imaginary rolling cylinder
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 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.
• Flank of a tooth: The part of the tooth surface lying inside the
pitch surface.
Manufacturers must take the following criteria
into account
 Tooth root strength:
• If the permissible loads are exceeded, the teeth tend to break off at their base.
• The tension that occurs at the root of the teeth primarily depends on the length of
the tooth and shape of the tooth in the root area.
Tooth flank load capacity:
• If the maximum tolerable pressure
of tooth flanks in mesh is exceeded,
parts of the tooth flank break off,
leaving behind recesses that resemble
pitting.
 Scuffing load capacity and wear load capacity:
• Scuffing of the tooth system describes
situations when tooth flanks are briefly
and repeatedly welded together and
then separated again as a result of the
lubricant film failing.
• Wear occurs in the form of abrasion on
the tooth flanks when slip takes place
with mixed or dry friction.
Lubricant
• The oil dissipates the high temperatures
in the mesh area.
• Allows heat exchange with the gearbox
housing.
Power loss
•
The figure clearly shows that the
transferable power of a gearbox rises more
sharply as gearbox size increases.
•
With high output torque and high
output speeds, the steady state
temperature can rise above the
permissible temperature range.
•
Once power output exceeds 50kW
gearboxes are generally fitted with an active
cooling system to dissipate the power loss.
GEAR TRAINS
• A gear train is two or more gear working together by meshing their teeth
and turning each other in a system to generate power and speed
Types of Gear Trains:
1.
Simple gear train
2.
Compound gear train
3.
Planetary gear train
 Simple Gear Train
•
The most common of the gear train is the gear pair
connecting parallel shafts. The teeth of this type can be spur,
helical or herringbone.
•
Only one gear may rotate about a single axis
 Simple Gear Train
• In this arrangement we want both gears(yellow
and blue to rotate in the same direction.
• Without the red gear: Vr= N3/N1
N1
With the red gear: Vr= (N2/N1)*(N3/N2)
= N3/N1
• So, the size of the red gear is not important
since it is just there to reverse the direction of
rotation (it doesn`t change the ratio).
 Compound Gear Train
•
•
•
For large velocities, compound
arrangement is preferred.
In this arrangement to avoid using large
gear sizes to achieve certain ratio(large
gearboxes introduce higher power loss)
gears 2 and 3 are connected to the same
shaft and then 3 is coupled with 4 .
Gear 4 doesn`t have to be very large to
achieve the ratio,
N3
N2
 Planetary Gear Train
• In this gear system, the yellow gear (the sun)
engages all three red gears (the planets)
simultaneously.
• All three are attached to a plate (the planet
carrier), and they engage the inside of the blue
gear (the ring) instead of the outside.
 Advantages
•
•
Planetary gear sets can produce different gear ratios depending on which gear you use as
the input, which gear you use as the output, and which one you hold still.
They have higher gear ratios.
 Applications
•
•
Automatic transmissions in automobiles.
Used in bicycles for controlling power of pedaling automatically or manually.
• Gear train example:
For the set of gears shown below, find output speed, output torque, and
horsepower for both input and output conditions and overall velocity
ratio.
Solution:
Vr = (N2/N1)(N4/N3)
= (60/20)(60/20) = 9
n4= n1/Vr = 3600/9
= 400 rpm
T4 = T1Vr = 200 * 9
= 1800 in-lb
hpin= T*n/63000
= 200*3600/63000= 11.4
hpout= 1800*400/63000
= 11.4
Selecting Gear Drives
• Gears can be selected, rated, installed, and maintained by most
users through common standards and practices developed by
the American Gear Manufacturers Association(AGMA).
• The major selection factors include: shaft orientation, speed
ratio, design style, nature of load, service factor, environment,
mounting position, ratio, lubrication, and installation.
1.
Shaft Orientation
“ Input to output shaft position”
Orientation Types:
• parallel Shafts
• Shafts at right angles with intersecting axes
• Shafts at right angles with nonintersecting axes
• Skewed Shafts
2. Speed Ratio
“The ratio of input speed to that of the output”
• determining if a single high-ratio stage is sufficient or if
multistage gearing is required.
• Geared systems can be driven at constant or varying
speeds, depending on the requirements of the application.
3.
Nature of load
“determining the nature of the load is important for long life and reliable service.”

•
•
•
Load considerations:
Maximum horsepower.
Drive inertia.
Overhung load(the radial load on the output shaft extension produced
by a gear).
• Speed limit of the gear.
4.
Environment
“ The type of gear selected must compensate for many unwanted environments ”
• Dust may contaminate the lubricant.
• Heat may accelerate lubricant breakdown and lower gear capacity by lowering material
properties and distorting the gear.
• Wide temp. variation can cause improper lubrication, thereby shortening the life of the
unit.
• Moisture infiltration may accelerate lubricant breakdown and wear of teeth.
5.
Gear Rating
There are two types of ratings for a geared unit :
• Mechanical rating is based on the strength of the gears, shafts, load pressure, or the
resistance of the gears to pitting or abrasion.
• Thermal rating specifies the power that can be transmitted without exceeding a specified
limit above the operating temp.
In high power units additional cooling methods are used:
• Air cooling the housing.
• Circulating water around the unit.
• Using a separate oil sump for greater heat dissipation.
• Using cooler mounted inside the gear housing.
6. Design Style
̒ ̒ In any design shafts should not introduce external load to the gear ̕ ̕
• Enclosed speed reducer with oil lubrication is the preferred design.
• Grease-lubricated open gears can be used in relatively clean environments.
• If the gearbox type only permits a solid output shaft, a coupling is needed to
connect the drive shaft to the application.
• A gearbox design with a hollow shaft allows it to be integrated into the
machine`s drive shaft directly.
(Hollow shaft gearboxes are significantly more expensive.)
7-Lubrication
“Failure to supply lubricant to the bearings and gears will result in their damage ”
• The type of lubricating system should be chosen carefully
• If the unit is operating where temperatures vary widely, oil viscosity should be
changed to suit the conditions.
• For low temperature operation the oil should have a pour-point lower than that of the
extreme minimum temperature encountered.
(pour-point is the lowest temperature at which it will flow under prescribed
conditions).
Lubrication systems:
- Splash systems in which one of the gears dips into an oil path and transfers
the lubricant to the contacting teeth as it rotates.
- Forced-spray lubrication reduces oil churning in which lubricant is pumped
to the gear train, then it`s returned to the reservoir to be recirculated.
( Churning is an undesirable friction between fluids due to continuous
excitation of the lubricant)
Losses in gears
•
Power losses in gear systems are associated primarily with
tooth friction and lubrication churning losses.
• Churning losses are relatively independent of the nature of the
gears and the gear ratios - they are primarily related to the
peripheral speed of the gears passing through the
fluid. Churning losses are difficult to calculate and estimated
based on experience are often used in initial gear design.
• The frictional losses are related to the gear design, the velocity
ratio, the pressure angle, gear size, and the coefficient of
friction.
Summary
• Speed of mating gears is inversely
proportional to the number of teeth.
• Mating gears should have the same diametral
pitch( The ratio of the teeth to the pitch
diameter) for a reliable operation.
• Good gear design should take care of power,
speed, life and material properties.
• A number of gear manufacturing methods are
available such as:
Type
Normal
Ratio Range
Efficiency
Range
Spur
1:1 to 6:1
98-99%
Helical
1:1 to 10:1
98-99%
Double Helical
1:1 to 15:1
98-99%
Bevel
1:1 to 4:1
98-99%
Worm
5:1 to 75:1
20-98%