Getting engine power to the ground in a vehicle or farm machine usually means routing torque from the engine flywheel through the transmission, then turning it 90 degrees to spin an axle connected to the wheels. Simple bevel-cut gears allow for that change in direction, but there is another problem that has to be overcome: unless the machine always drives in a perfectly straight line, there are times when a driven wheel on one side of the axle has to travel a longer or shorter distance than the other.
Here’s why. As a machine goes around a curve or makes a turn, the wheel on the outside travels farther, because it’s following an arc with a larger radius than its partner on the inside of the turn. To cover the extra distance in the same amount of time, that wheel has to travel faster than its partner, even though the machine’s overall speed remains constant. At the same time, the wheel on the inside has to slow down an equal amount. If power flowed to both wheels at the same rate, each one would have to skid on the road surface to allow the machine to turn, making steering control difficult or impossible.
The job of the differential, therefore, is to allow engine power to flow continuously to the drive axle, keep the vehicle travelling at a constant rate but still allow the two wheels to vary their rate of rotation. And it has to permit the difference in actual wheel speeds to be constantly variable as the trip continues.
The easiest way to understand how a differential does that is to first talk about what each component inside it does. The differential is located in the centre of the axle housing — or machine chassis. The wheels on each side of the machine are attached to individual axle shafts that fit into a part called a differential case, which turns inside the axle housing. The axles are driven at their ends by bevel-cut gears called differential side gears. These two gears mate with two pinion gears mounted on a shaft inside the differential case. The pinion gears transfer the rotation of the differential case to the differential side gears and, therefore, the axle shafts. The differential case, in turn, is driven by a ring gear which is turned by the pinion drive gear connected to the driveshaft from the transmission.
If all of that sounds a little confusing, the attached images below will help you visualize how all these parts mate together. The whole thing is actually brilliantly simple.
The two pinion gears inside the differential case are able to turn freely on their shaft. Because their job is to transfer drive to the differential side gears on the axle shafts as the differential case turns, their ability to rotate on their shaft is important in allowing each axle shaft to turn at a different rate when necessary.
During a turn, tire friction with the road surface causes the axle on the inside of the curve to have more resistance, it begins to turn slower than the axle on the outer side. This causes the pinion gears to “walk” (turn) on their shaft and increase the drive speed delivered to the outer axle shaft (and wheel) in an amount equal to the speed decrease of the inner axle. The machine’s overall speed remains constant while one wheel speeds up and the other slows down.
The trouble with differentials is they always allow power to flow to the axle shaft offering the least resistance, which can cause trouble in poor traction conditions. For example, if one wheel is on ice and the other has good traction, drive power will likely be directed to the wheel on ice because it has less resistance to torque.
To overcome that defect, limited slip or “Posi-traction” differentials, which use clutch packs to limit rotational variation between the two axle shafts, are used. Of course many farm machines, particularly tractors, are available with manual differential lock which forces even torque distribution to both axles. But that’s a topic for another issue.