Introduction to Gear/Gearing/Gear box/Gear Ratio
Before going to the system we will first understand some basic definitions about the system components.
Gear is a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion.
Gearing is wheelwork consisting of a connected set of rotating gears by which force is transmitted or motion or torque is changed.
Gearbox is an automotive assembly of gears and associated parts by which power is transmitted from the engine to a driving axle. The term gearbox is sometimes also refers as transmission.
Gear Ratio is the numeric indication of the relationship of number of teeth on two gears. In other terms it can also define as, the ratio of the speeds of rotation of the initial and final gears in a gear train
Introduction to Transmission system
When two or more gears working in tandem, it is called a transmission. In automotive system, transmission generally refers to the whole drive train, including gearbox, clutch, prop shaft (for rear-wheel drive), differential and final drive shafts. As the name suggests it is a system to transfer the motive power from the engine to the wheels. The most common use is in the motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels.
Need of gears and gearboxes
A gearbox provides speed and torque conversions from a rotating power source to another device using gear ratios. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel.
This is where the transmission comes in; the transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process.
Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox will rotate at slower rate than the input shaft, and this reduction in speed will produce a mechanical advantage, causing an increase in torque. A gearbox can be setup to do the opposite and provide an increase in shaft speed with a reduction of torque. Some of the simplest gearboxes merely change the physical direction in which power is transmitted.
Many typical automobile transmissions include the ability to select one of several different gear ratios. In this case, most of the gear ratios (often simply called "gears") are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types that increase the output speed.
Working of gears and gear systems
The following is a simplified working of gears and gear systems.
The number of teeth cut into the edge of a gear determines its scalar relative to other gears in a mechanical system. For example, if you mesh together a 20-tooth gear and a 10-tooth gear, then drive the 20-tooth gear for one rotation; it will cause the 10-tooth gear to turn twice.
Gear ratios are calculated by dividing the number of teeth on the output gear by the number of teeth on the input gear. So the gear ratio here is output/input, 10/20 = 1/2 = 1:2. Gear ratios are often simplified to represent the number of times the output gear has to turn once. In this example, 1:2 is 0.5:1 - "point five to one". Meaning the input gear has to spin half a revolution to drive the output gear once. This is known as gearing up.
Gearing down is exactly the same only the input gear is now the one with the least number of teeth. In this case, driving the 10-tooth gear as the input gear gives us output/input of 20/10 = 2/1 = 2:1 - "two to one". Meaning the input gear has to spin twice to drive the output gear once.
By meshing many gears together of different sizes, you can create a mechanical system to gear up or gear down the number of rotations very quickly. As a final example, imagine an input gear with 10 teeth, a secondary gear with 20 teeth and a final gear with 30 teeth. From the input gear to the secondary gear, the ratio is 20/10 = 2:1. From the second gear to the final gear, the ratio is 30/20 = 1.5:1. The total gear ratio for this system is (2 * 1.5):1, or 3:1. i.e. to turn the output gear once, the input gear has to turn three times.
Collections of helical gears in a gearbox are what give the gearing down of the speed of the engine crank to the final speed of the output shaft from the gearbox. The table below shows some example gear ratios for a 5-speed manual gearbox.
| Gear | Ratio | RPM of gearbox output shaft |
| 1st | 3.166:1 | 947 |
| 2nd | 1.882:1 | 1594 |
| 3rd | 1.296:1 | 2314 |
| 4th | 0.972:1 | 3086 |
| 5th | 0.738:1 | 4065 |
Here you will see that the 4th and 5th gear are overdriven, i.e. the gearbox output shaft speed is higher than the engine speed. From the table we see how we can effectively obtain 5 different output speeds even when the input speed remains constant.
In the next part of the series we shall look at the working of a gearbox and other components of a transmission.
Disclaimer
The article doesn't include any confidential information. Nor does it include any references from the sources which doesn't allow taking excerpts from articles. The images have been taken from internet not necessarily from direct source. The article is only for reference and educational purpose only.
No comments:
Post a Comment