A manual gearbox
1. What type of gears are used for reverse in a manual gearbox?
Reverse gears in a gearbox are straight cut gears, meaning the teeth are straight. reverse gears are usually small ratios as a car does not need to travel in reverse at high speed.
A set of reverse straght cut gears (idler, driver and driven.)
a. Give an explanaiton for your choice
Straight cut teeth are used on reverse gears because spur gears are far easier to mesh together than helical gears. In a lot of cases all of the foward gears in a modern gearbox are helical (angle cut teeth), Because reverse gears have to engage with an idler, how the gears mesh is critical. The disadvantage's to spur gears are: Spur gears wear more as only one tooth is in contact while the gear is spining, and they make more noise than a helical gear.
2. What type of gears are used for all foward gears in a manual gearbox?
Helical gears are used in the foward gears of a gearbox. These gears have angled teeth which means that more surface area and more teeth are in contact at one time while the gear is spinning. Herring bone type gears are used in large applications like trucks or earthmoving equipment, these gears can take the very high amounts of load required in a large application however they are more complex and more expensive to manufacture.
A herringbone gear
A. Give an explanation
Helical gears are stronger and quieter than spur gears. Because of the amount of teeth in contact with the meshing gear a helical gear does not wear as agressively as a spur gear, giving them a longer lifespan. As a gearbox is usually in a foward gear lifespan is critical.
3. What is the purpose of a synchromesh unit.
A synchromesh unit is a part of the geartrain, its job is to make the changing of gears smoother, this is done by sychronising the teeth of the two gears that are about to be engaged. This synchronation of the gear teeth ensures the gears are travelling at the right speed and the gear teeth are in the right position. A synchromesh unit can save the gear teeth from damage and extend the life of the gearbox because it is saved from violent gear changes. If the synchromesh unit is damaged then gear changes may be clunky or may not change at all.
A simple Synchromesh diagram
A. Explain the operation of a synchromesh unit?
A synchromesh unit has to smooth the changing of gears by lining up the gear teeth and to ensure the gears are running at the same speed. This is achieved by a unit called a baulk ring. This unit is positioned on the geartrain between gears, when there is a gear change the baulk ring engages the next gear with a tapered face that grips to another tapered face on the gear and makes them travel at the same speed. There is a small clearance on the baulk ring which allows it to rotate and let the teeth of the rings line up. Gears can now be changed with ease.
Position of the baulk ring on the geartrain.
Synchro or baulk ring
C. How do we check a baulk ring for serviceability?
Because a Baulk ring is under a high amount of stress it can be damaged. The baulk ring has to deal with stress of engaging a gear with a tapered face especially at high speeds. and over time the stress could warp and chip the teeth of the ring. A damaged baulk ring may not engage properly or could damage other gearbox components.
Wear to the tapered face of the baulk ring
4. What type of bearings do we find in gearboxes?
A gearbox needs many types of bearings to run. Caged roller bearings are used on the main and counter shaft. These bearings allow smooth rotation movement of the geartrain. Thrust bearings are used to keep the shafts from 'floating'. lateral movement of these shafts can damage the gears. Needle bearings are used on the inside of shafts, and flat caged needle bearings are used in the final drive assembaly.
Gearbox thrust bearing and tapered roller bearings.
5. Give an explanation of a gear ratio.
A gear ratio is the difference in speed of two connected shafts or gears. The gears travel at different speeds because of the amount of teeth on the driver and driven gear and the size of the gear. A gear ratio is measured in the amount of revolutions one shaft does in one revolution of its connected shaft. A ratio of 2:1 means that the shaft does two revolutions in one revolution of its connected shaft. Small gear ratios allow high speeds, while high gear ratios multiply the torque produced. Gear ratios are important in controlling an engine and keeping the engine in its optimum operating speed.