Gear drives

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.

Spur gears are used in lathes, milling machines, vehicle transmission, clocks and washing machines.

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.

Advantages of helical gears:

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.

Advantages of bevel gears:

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.

Advantages of hypoid gears:

They are smooth and quiet in operation

The hypoid gear drive has a compact design

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

Advantages of rack and pinion gear:

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

They can transmit torque at right angles

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

External vs internal gears:- External gears are gears where the teeth is situated on the outside of the gear wheel. All gears illustrated so far are external gears.

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

When two gears are in mesh one will always turn clockwise while the other will turn anti-clockwise. See Figure 1

              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.