Application
and Function of gear drives
When the teeth of gears are interlocked we say it is in
mesh. Gears in mesh are capable of
transmitting force and motion from one gear to the other. The gear that
generates or start the motion is called the drive gear and the gear that is
connected to the drive gear and receive the force or motion is called the
driven gear.
Where two gears mesh, the smaller of the two is called
the pinion gear. The larger is
simply called “the gear”
Gears are 98% effective in transmitting force. If the difference between input and output
become too far apart, the input and output is limited and if power is to be
transmitted over a long distance we rather use belts or chain drives.
Types
of gears:
Spur
gears:- The axes of spur gears are always parallel and the
teeth are spread evenly around the circumference of an imaginary circle called
the pitch circle.
Advantages of Spur gears:
These gears are relatively inexpensive
They are easy to manufacture
They are easily available if spare
gears are required.
There is no axial force on the
connection.
Disadvantages of the Spur gears:
They only work with a mating gear.
The axis of each gear must be parallel
to the axis of the other gear/s.
Spur gears are noisy at high speeds.
Helical
gears:- The helical gear form developed from the spur gear
with the teeth set at an angle to the gear. It can transmit more power because
before the contact ceases between one pair of teeth, the following teeth is
engaged. Helical gears enable smooth, quiet and continuous action under heavy
loads.
Backlash is reduced
They operate smoothly and quietly due
to gradual tooth engagements
They can be used for speeds in excess
of 25 m/s
More tooth engagement allows for
greater power transmission for given gear size
Disadvantages of helical gears
Because the teeth lie at an angle a
side thrust is created. To take up the side thrust a side thrust bearing have
to be used.
They are more expensive to manufacture
They are subject to axial thrust
Double
helical gears:- Also called herringbone gears. Instead of
using two gears, the two helices can be cut on the same blank gear and called a
double helical gear.
Advantages of double helical gears:
These gears can be used on heavy duty
machinery
They transfer power smoothly because
more than two teeth are in mesh at any given moment.
Unlike helical gears they do not
produce an additional axial load
Disadvantages of double helical gears:
They are high-precision gears and
therefore more expensive to manufacture
They require specialised gear cutting
tools
Bevel
gears:- These gears are conical in shape and allow the shafts
of the pinion and the gear to intersect at 90°. The teeth has the same shape as
the teeth on spur gears but it taper towards the point of the cone (Apex of the
cone). They are used in printing presses, machine tools and other differential
gear units.
Bevel gears provide right angle
drives.
They are relatively easy to
manufacture.
Disadvantages:
There is axial load, which complicates
bearings and housings.
One wheel of a bevel gear set is
designed to work only with its complimentary gear and is therefore not easy to
replace.
The gears must be precisely mounted.
Hypoid
gears:- These gears are used where the shaft axes are offset
and do not intersect. They have the same design as bevel gears. The smaller
gear in this pair of gears is called the pinion gear and the larger gear is
called the ring gear. Hypoid gears are mainly used in the differential drive of
motor vehicles where the drive shaft are at right angles to the wheels.
They are smooth and quiet in operation
The pinion strength is high
Hypoid gear drives produce large speed
reductions
Disadvantages of the hypoid gears:
They are expensive to manufacture
They are supplied in pairs and should
be replaced in pairs
They are subject to axial thrust
They require special lubricants.
Note: in hypoid gears there is a
considerable amount of sliding and rolling action between the teeth. Special
oils, known as hypoid or EP (extreme pressure) oils, have been developed for
hypoid drives. Under no circumstances must any other type of oil be used to
lubricate hypoid gears.
Rack
and pinion gear:- these gears convert rotational motion into
linear motion. A circular gear, called the pinion, mesh teeth on a linear gear,
called a rack. The rack is a bar with teeth that mesh with those of the pinion gear.
The rack can be any length. (Short, in the case of a vehicle steering or long,
used on an electrical gate
Smooth and quiet in operation
Can operate over long distances
Disadvantages of rack and pinion gear:
It cannot transmit large amount of
torque
Long gears are difficult to
manufacture
Worm
gear and worm-wheel gear:- A worm drive consists of a screw-like worm
(comparable to a pinion) that meshes with a larger gear, called a wheel. The
worm act as a screw. Several revolutions of the screw will pull the wheel
through one revolution. Worm drives are used to transmit motion between
non-parallel shafts. They are widely used in conveyer machinery, machine tools,
guitar string tuning mechanisms, rudders, etc.
Advantages of worm drives:
Worm gears provide a smooth and quiet
drive operation
There is no back driving (the wheel
cannot drive the worm)
Good for positioning systems
Compact size
Provide for large reductions in one
step
Disadvantages of worm drives:
Most inefficient gear system due to
friction
Needs regular maintenance
It can only be used on slower speed
applications
Internal gear, is when the teeth are formed on the
inside ring (inner surface) of the cylinder or cone. An internal gear can only
be meshed with an external gear. The pinion and gear will rotate in the same
direction. Internal gears may be spur or helical type gears. This allow for
closer shaft center distances and these gears have stronger teeth than
equivalent external gears.
Advantages
of gear drives compared with V-belt drives:
They are more durable that V-belt
drives
They are more efficient compared to
V-belt drives
They have high speed ratios
When shafts are non-parallel, gears
offer the best options
They are more practical at small shaft
center distances
When timing and synchronization are
required, V-belt drives tend to slip, therefore gears are preferred
Disadvantages
of gear drives compared with V-belt drives:
They are expensive to manufacture
compared to V-belts
If a gear breaks it is difficult to
repair
Gear drives require constant
lubrication
The drive and driven shafts need to be
close together.
Advantages
of gear drives compared with chain drives:
Gear drives are more efficient
compared to chain drives.
They have high speed ratios, whereas
chain drives have not
They are more practical at small shaft
center distances
Disadvantages
of gear drives compared with V-belt drives:
They are expensive to manufacture
compared to V-belts
If a gear breaks it is difficult to
repair
Gear drives require constant
lubrication
The drive and driven shafts need to be
close together.
Velocity ratio and mechanical advantage in gear drives
When two gears are in mesh with each
other, their velocity, measured in m/s, is the same. Their revolution ratio,
also known as gear ratio, is not the same if the number of teeth is different
on each gear.
Two gears in mesh with a gear ratio of
2:1 tel you that for every one revolution of the big gear the small gear wil
complete 2 revolutions. It also indicate that the big gear has two times more
teeth than the smaller gear or pinion.
To work out the gear ratio between two
gears in mesh, you have to divide the number of teeth on the big gear by the
number of teeth on the small gear (pinion gear) If the number of teeth on the
big gear is 36 and the number of teeth on the small gear is 6 you will divide
36 by 6 and end up with a gear ratio of 6:1 So for every one revolution of the
big gear the pinion wil complete 6 revolutions.
Teeth on gears fit into each other and
transfer torque and power from one gear to the other. Gears may change the
direction, speed and power and in the prosess give one gear a mechanical
advantage over the other gear.
Decrease in speed or velocity wil
create an increase in power/torque. The increase in torque depends on the
number of teeth on each gear in mesh. An reduction in speed leads to an
increase in torque and vice versa, in the same proportion as the gear ratio.
This mean that in a ratio of 3:1 the big gear will have a velocity three times
less than the pinion gear but it will have a torque three times more than the
pinion.
The purpose of intermediate gears
Figure 1 Figure 2
If the driver gear needs to turn in the same direction
as the driven gear, then an intermediate gear may be used. See Figure 2. The intermediate gear can also be named the idler
gear as it has no effect on the system. For instance, the gear ration is still
determined by the driven and driver gears no matter what size of the
intermediate gear is.
Pitch
Circle Diameter (PCD)
The diameter of each gear is measured on an imaginary
circle called the pitch circle. The PCD is measured at the point where two
teeth that are fully meshed touch. If the PCD of one gear is divided by the
number of teeth on the same gear, then that ratio is called the module of the
gear.
If the PCD of a pinion is 72 mm and it has 8 teeth,
then its module is 9 mm (
mm)
If the PCD of a gear is 180 mm and it has 20 teeth, then its
module is 9 mm (
mm)
Because the two modules are the same the gears will mesh
perfectly.
Backlash
When gears mesh there must be a small amount of clearance
between the teeth that are meshed. This allow lubricant to penetrate to the
high pressure area between the teeth to reduce friction. It also allow gears to
expand when temperature increase. This clearance is also called the backlash.
Simple
and compound gear trains.
In a simple gear train all the gears mesh with each other in a
straight line or on the same plain. This, however take a lot of space. To
increase the torque without using up to much space we put more than one gear on
the same axil creating a new gear train on a new plain but still working together
with the first gear train. We call this arrangement a compound gear train.
The
application of reduction gearboxes
The purpose of a gearbox is to change the speed and
corresponding torque. It can also be used to change the direction of movement.
(reverse) The gearbox of a motor vehicle for instance, allow the torque of the
engine to put the vehicle in motion and then to maintain a fast speed at
minimum torque. It also allow the vehicle to reverse.
Gearboxes are not just found in motor vehicles but also in
milling machines, lathes and drilling machines.
We distinguish between the following reduction gearboxes.
·
Single
reduction gearboxes: Often a machine only needs a small speed
reduction or increase. A single reduction gearbox consist of two gears in mesh
and are simple and cheap to manufacture.
·
Double
reduction gearboxes: If the reduction or increase in speed is
more than what can be achieved by two gears in mesh, then a double reduction
gearbox can be used.
·
Worm
reduction gearboxes: When a large reduction is required in a
single step, then a worm reduction gearbox can be used. Reductions of 40:1 and
more can easily be achieved.
Main
components of a typical gearbox:
·
Gears
·
Bearings: Thrust, roller, ball, bush,
sleeve and split bearings
·
Shafts: Main shaft, and idler shafts
·
Keys (that fit in shafts to keep gears
from slipping)
·
Thrust washers
·
Circlips
·
Seals
These components are common to all or most gearboxes. In
addition you will also find these components;
·
A lubrication system: Pump, slinger disc
and ring systems
·
Gaskets
·
Covers
·
Couplings
·
Caps
·
Shims
Safety
precautions and care when working with gear drives
NB:
when you are careless you put yourself and those around you in danger.
Revolving machinery: Unless
moving or revolving components of machinery are in such a position or of such a
construction that they are safe as they would be if they were securely fenced
or guarded, the user shall cause
a)
Every shaft, pulley, wheel, gear,
sprocket, coupling, collar, clutch, friction drum or similar object to be
securely fenced or guarded.
b)
Every set screw, key or bolt on
revolving shafts, couplings, collars, friction drums, clutches, wheels,
pulleys, gears and the like to be countersunk, enclosed or otherwise guarded.
Good
housekeeping
“A
place for everything and everything in its place”
It is very important to keep the workplace clean when working on
a gearbox. A gearbox is a sealed unit so all possible dirt and dust must be
prevented from entering it while it is open.
Some gearboxes work in dirty conditions. It is therefore very
important to clean around the inspection plates of the gearbox before you
remove them. Also take care not drop your tools into the gearbox. Ports and
covers should not be left open during breaks because you can’t know what fell
into the gearbox.
Follow these rules to keep your workplace tidy and safe.
·
Return each piece of equipment and all
tools to their rightful place before you go home each day.
·
Stack materials and equipment straight
and neatly in their designated places.
·
Clean and inspect all tools before
returning them to the tool store.
·
Clean the work area once the work is done
·
Keep the work area tidy and neat at all
times
·
When stripping equipment, put all nuts,
bolts and washers in storage bins to prevent safety hazards.
·
If workshop floors are color coded, don’t
leave any tools and equipment in the red area.
·
Do not store anything in front of, on top
of or underneath firefighting equipment. Firefighting equipment must be kept
clear for easy access.
Safety
precaution while working with gear drives:
The following safety rules will keep you and others around you
safe while you work.
·
Before adjusting, replacing, or servicing
any gears or gear drives, make sure that the machine is switched off and
isolated electrically. Make use of a lock-out device and tag out procedure to
ensure power cannot be restored to the machine while you are working.
·
Isolate the machine mechanically and make
sure all moving and rotating parts came to a standstill before opening the
gearbox.
·
Wear appropriate personal protective
equipment for the job at hand.
·
Do not put your finger into the filler
plug opening while the transmission is still working. If the machine was in use
prior to opening the gearbox the fluid inside may still be very hot.
·
Remove the filler plug slowly and
carefully, as the hot fluid may squirt out. Be careful not to burn yourself.
·
Always keep your fingers away from moving
or revolving parts. Use safety guards on driven equipment and machines. Guards
do not always prevent accidents but they greatly lower the risk of accidents.
·
Guards must be kept in place and used
correctly. If you have to remove the guards for maintenance take extra care to
prevent injury.
·
Always use safety stands to make sure the
load is fully supported. Never put any part of your body under a vehicle or
machine that is only supported by one jack.
·
Follow the safety guidelines asset out in
the manufacturer’s guidelines.
·
Report any strange noises or faulty
operation to the supervisor.
Personnel protective equipment and its importance.
You must always use safety equipment appropriate for the work on
hand. When working with gears and gearboxes the following guidelines is
important.
·
Appropriate clothing such as overalls or
a dust coat. No lose hanging clothes.
·
Safety boots. To protect your feet from
falling gearboxes. They are normally very heavy.
·
Earmuffs or other ear protection if the
environment is very noisy.
·
Gloves and barrier cream to protect your
hands from hot/corrosive fluids. Remove the gloves when working close to moving
gears.
·
Face mask and respirator if you are
working in dusty environments or when noxious fumes are present.
·
Remember: “A clean work area is a safe work area”
Interpret
the job card, documentation and engineering drawings.
Make sure you have the correct drawings and check for possible
assembly and disassembly problems.
Get as much information as possible. Manufacturer’s part
numbers, gear and bearing run time (on service schedule), service history (on
service schedule) and lubricant type. (manufacturer’s guide)
Normally the supervisor or maintenance engineer will issue
instructions for any maintenance or repairs to be carried out by the worker.
This instructions will be written on a job card. The job card will include the
following information.
·
The vehicle, machine or piece of
equipment that is to be worked on.
·
The nature of the work that has to be
carried out.
·
The date and sometimes the time on which
the job has to be started.
·
The estimated time that the job will
take.
·
The names of the people responsible for
the job.
·
The tools and equipment responsible for
the job.
·
The materials and consumables that will
be required
·
The safety precautions that must be
followed
·
The name and signature of the supervisor
authorizing the work.
Possible
tools needed for maintenance or overhauling of a gearbox.
·
Feeler gauge
·
Dial test indicator
·
Bearing pullers
·
Telescoping magnet
·
Felt tip markers
·
Circlip pliers
·
Torque wrench
·
Allen keys
·
Spanners
·
Sockets
·
Scrapers
·
Magnifying glass
·
Lifting equipment
Clean
and inspect the assembly
It is important to clean the gearbox before opening it to
prevent dit from entering the box.
Inspecting the gearbox before opening involve all the senses.
·
Hearing:-
listen
while the gearbox is in operation. Everybody working on the machine must be
aware of any change in the sound of the gearbox. If a change in normal sound of
the gearbox occur it must be further investigated.
·
Sight:-
look for obvious signs of wear, corrosion, cracks, pitting or broken gear teeth
or any other signs that is out of the ordinary.
·
Smell:-
any burning smell is an immediate cause of concern. It normally indicate that
the fluid has exceed its running temperature. This means that the fluid has
lost its ability to lubricate and friction will increase as well as increase in
corrosion and subsequent built up of debris inside the gearbox.
Potential
areas of defectiveness and wear
·
Inadequate or incorrect lubrication.
Always refer to the manufacturers manual for the correct lubrication type.
·
Defective bearings. The three types of bearings
used in general are; radial ball bearings, roller bearings and needle bearings.
Thrust bearings are used on worm gear drives.
·
Hostile environments such as dust, heat,
moisture, chemicals and temperature variations will all affect the gear,
bearings, seals and lubricants.
·
Misalignment of axis and gears are the
main cause of wear in gearboxes.
How
do I isolate the machine electrically?
·
Inform all relevant staff that
maintenance is to be carried out on the machine.
·
Switch off the main circuit breaker that
feeds the machine.
·
Switch of the isolator to the machine and
lock it down with a lockout divice. Keep the key with you.
·
Place an information sign in clear view
informing everybody that maintenance are in progress on the machine.
·
Double check the isolation by trying to
switch on the machine.
How
do I isolate the machine mechanically?
Separate the gearbox from the rest of the machine (the input
shaft) if the gearbox drive a pump or another machine it must also be separated
from that end.
Disassemble
the gear drive
Loosen all nuts and bolts that holds the gear drive and remove
the drive. If there are any shims under the base of the unit, measure and mark
them so that they can be replaced in their proper places.
Identify
gear drive assembly components
The gear drive consist out of many different components. As you
remove each component, try and understand its function and mark the part with a
centre punch or permanent marker. Make clear notes of how the part fit into the
gearbox.
Remove
the gear drive assembly
1.
Drain the oil from the unit through the
drain plug.
2.
Inspect the exterior making notes on its
condition with special reference to the condition of the keyways and seals.
3.
Check the alignment of the gears and make
a note of any signs of misalignment.
4.
After the visual inspection, dismantle
the gearbox. Clean all the components and put them in bins or steel trays so
that nothing goes missing.
5.
Gently remove gaskets. Unharmed gaskets
can be reused but as a rule you rather replace the gasket with a new one or one
you have made yourself.
6.
Refer to the manual and remove parts in
the correct order using the correct tools.
7.
Examine the parts as you remove them and
record their conditions. Be on the lookout for cracked or broken gear teeth,
corrosion and small pieces of metal at the base.
8.
Clean all parts in the correct solvent
and inspect all parts again using a magnifying glass.
Take note that the bearings will be the first to fail, and then
the gears. Keep the following in mind:
·
Bearing damage can be caused by corrosion,
contamination, lack of lubrication or electrical discharge on the gearbox.
·
Deformation between rollers or balls
indicate overloads.
·
Bearing wear will cause misalignment of
gears.
Assemble
the gear drive assembly and components
This procedure is basically the reverse of the disassembly.
Before you start, make sure all the parts are clean and free of
any defects.
The bolts must be torqued to the correct amount stated in the
manufacturer’s manual. Make sure the vent hole is clear.
Measure
the backlash on a worm gear
Lock the worm gear so that it cannot turn. Install a key in the
worm shaft. Mount a clock gauge using a magnetic stand. By hand, rock the worm
shaft back and forth. As a rule the backlash should not exceed 0.5 mm
Clean
and replenish gear lubricant
The correct type and amount of oil must be maintained in the
gearbox to ensure al long life. If the gearbox operates with an insufficient
amount of oil, premature gear or bearing failure can occur due to oil
starvation. Over-filling the gearbox will create excessive churning that will
trap excess air. If this occurs, overheating will most likely due to air and
oil mixture that cannot effectively dissipate heat.
Finishing
off
After reassembly, allow the unit to run for a short time so that
the lubricant can warm up. Listen and look for any excessive noise, vibration,
oil leakage or abnormal increase in temperature.
Clean all tools and restore them to their authorised places.
(Toolboxes, tool boards or storeroom.)
Never use compressed air to clean tools because the air will
force dirt in-between moving parts.
If you come across any tool that is defect you must discard and
replace it immediately because defect tool can lead to injuries to yourself and
other people.
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