How the gear works
Gears are used in a wide range of mechanical devices. They perform a number of important jobs, but most importantly, they provide gear reduction for motorized equipment. This is key because typically, small motors that spin very fast can provide enough power for the device, but not enough torque. For example, the gear reduction of an electric screwdriver is very large because it requires a lot of torque to turn the screw, but the motor produces very little torque at high speed. By reducing gear, the output speed can be reduced while increasing torque.
Another thing a gear does is adjust the direction of rotation. In a differential between the rear wheels of a car, for example, power is transmitted through a shaft that runs along the center of the car, and the differential must rotate the power 90 degrees to apply it to the wheels.
There are many intricacies with different types of gears. In this article, we will learn exactly how the teeth on the gears work, and we will discuss the different types of gears that you find in various mechanical gadgets.
On any gear, the ratio is determined by the distance from the center of the gear to the point of contact. For example, in a device with two gears, if one is twice the diameter of the other, the ratio is 2:1.
One of the most primitive types of gears we can see is a wheel with a wooden spike sticking out.
The problem with these gears is that as they rotate, the distance from the center of each gear to the point of contact changes. This means that the gear ratio changes as the gears turn, which means that the output speed will also change. If you use gears like this in a car, it's impossible to maintain a constant speed -- you're constantly speeding up and slowing down.
Many modern gears use special tooth shapes called involutes. The profile has the very important property of maintaining a constant speed ratio between the two gears. Just like the nail wheel above, the contact point moves; But the shape of the involute teeth compensates for this motion. See this section for more information.
Now let's look at some different types of gears.
Spur gears are the most common type of gear. They have straight teeth and are mounted on parallel axes. Sometimes, many spur gears are used at the same time to produce very large gear reduction.
Spur gears are used in many of the devices found everywhere at HowStuffWorks, such as electric screwdrivers, dancing monsters, swinging sprinklers, wind-up alarm clocks, washing machines and dryers. But you won't find many in your car.
That's because spur gears can be very loud. Every time a gear tooth engages with a tooth on another gear, these teeth will collide, and this collision will produce noise. It also increases the stress on the gear teeth.
Most gears in cars are helical in order to reduce noise and pressure in them.
The teeth on a helical gear are at an Angle to the surface of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually expands as the gear rotates until the two teeth are fully engaged.
This progressive engagement allows helical gears to run more smoothly and quietly than spur gears. Therefore, almost all automobile transmissions use helical gears.
Because of the tooth angles on helical gears, they generate thrust loads on the gears when engaged. Equipment that uses helical gears has bearings that can support this thrust load.
One of the interesting things about helical gears is that if the gear teeth are angled correctly, they can be mounted on the vertical axis to adjust the rotation Angle to 90 degrees.
Bevel gears are useful when it is necessary to change the direction of rotation of the shaft. They are usually mounted on shafts 90 degrees apart, but can also be designed to work at other angles.
The teeth on bevel gears can be straight, helical or hypoid. Spur bevel gear teeth actually have the same problem as spur gear teeth -- when each tooth meshes, it hits the corresponding tooth at the same time.
Just like spur gears, the solution to this problem is to bend the gear teeth. These helical teeth engage just like helical teeth: contact starts at one end of the gear and gradually extends to the entire tooth.
In spur and spiral bevel gears, the shafts must be perpendicular to each other, but they must also be in the same plane. If you extend the two shafts out of the gear, they will intersect. Hypoid gears, on the other hand, can engage shafts in different planes.
This feature is used in many automotive differentials. The gear ring and input pinion of the differential are hypoid. This allows the input pinion to be mounted below the ring axis. Since the car's drive shaft is connected to the input pinion, this also lowers the drive shaft. That means the drive shaft doesn't intrude too much into the passenger compartment of the car, freeing up more room for people and cargo.
The rack and pinion are used to convert rotation into linear motion. The steering system in many cars is a good example. The steering wheel turns the gears that engage the rack. As the gear turns, it slides the rack to the right or left, depending on how you turn the wheel.
Some scales also use a rack and pinion to turn a dial that shows your weight.
Planetary Gearsets & Gear Ratios
Any planetary gearset has three main components:
The sun gear
Planetary gears and planetary gear racks
Ring gear
Each of these three components can be input, output, or can stand still. Which part is chosen to play which role determines the gear ratio of the gear set. Let's look at individual planetary gear sets.
One of the planetary gear sets in our transmission has a 72 tooth ring and a 30 tooth sun wheel. We can get many different gear ratios from this gear set.
In addition, locking any two of the three components together locks the entire device at a 1:1 gear reduction. Note that the first gear ratio listed above is a deceleration -- the output speed is slower than the input speed. The second is overdrive -- the output is faster than the input. And then finally it goes down again, but the output goes in the opposite direction. There are several other ratios for this planetary gear set, but these are the ratios associated with our automatic transmissions. You can try these in the animation below:
Thus, this set of gears can produce all these different gear ratios without engaging or disengaging any other gears. By using two such gear sets in succession, we can obtain the four forward and one reverse gears required for transmission. We will put the two sets of gears together in the next section.
