The differential; that part of the automobile that is out of sight but not out of mind

The automobile has been a part of man’s life since time immemorial and it is improving every day. New inventions and improvements are being introduced and we all cannot cease to be amazed (can we?).

The automobile is made of numerous systems and parts that work in sync to give the user a smooth and enjoyable ride. Some of these systems and parts are quite visible and easy to identify their functions, as a result, we are aware of how to take care of them. Their care comes seamlessly even from the owner’s perspective. Systems and parts like the engine, the tyres, the air conditioning and the likes (you would agree with me that almost every car owner knows one or two things about these parts).

But did you know that there are some parts that we do not (or cannot) see but they are equally as important and need a good level of care as the visible parts? well today, I would be talking about one of those parts today. The part that keeps us navigating the turns, bends and corners in the smoothest way possible – THE DIFFERENTIAL

As is my custom (that some of you know about), I would like to make an illustration;

Have you ever been to the Olympics? – No? Me neither. But I have watched it severally on TV and I have been to local sports competition before and I am sure you would also therefore, we are on the same page.

Now lets get to the race track. If you are viewing a 400m race, you would notice that the racers all stick to the inner track all through the race until they do the last 100m in a straight line. Have you ever asked why? Well if not, I am asking you.
If you have also watched a parade or better still been in a parade and you co-incidentally fall in the outer line of the parade while marching along a bend or circular track, you would notice that your pace needs to be a little faster than those in the inner line in order for you to be on the same line. Why?

Well, here is the answer. The outer track is longer than the inner one and thus, if a racer picks the outer track in a 400m race, he would need to run faster because he has a longer distance to cover and is most likely to get tired faster and lose the race (who would want to do that?).

You want proof? Lets do an experiment

Take a few ropes of the same length and coil them into circles. The first circle being the length of the rope and each circle inside the first. You would notice that as you proceed with the circles inward, the length of rope you need reduces and you would need to cut out a part. After the experiment, if you stretch out the ropes, they would not be of the same length. That is to show that the inside of a circular track is shorter than the outside.

How does this relate to the automobile?

When driving around a curve or corner, the wheels on the sides of the vehicle take the path of two circles of different circumferences. The outer side being the longer therefore, the outer wheels would need to turn faster than the inner ones in order to keep the vehicle in a straight path.

A little science? Yes!

Recall that Speed S is Distance travelled divided by time D/T
The wheels would need to travel their individual distance in the same time therefore, the outer wheel would need to have a higher speed so as to cover its distance in the same time as the inner wheel.

How can this be achieved?

If the wheels are connected by one straight shaft, the difference in the speed of the two wheels would tell on the shaft and one of two things are likely to occur – it is either the shaft breaks or the slower wheel drags. Any of the two options are not desirable and thus there is the need for something that would allow for the two wheels to rotate at different speeds at the same time. How is that possible? The differential!

IMAGE BY HUNINI licensed under the Creative Commons Attribution-Share Alike 4.0 International license.
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THE DIFFERENTIAL

To the mathematical folks here, sorry to disappoint but I am not talking about the differential equations (sorry) rather an important part of the vehicle transmission
If you wouldn’t mind getting a little dirty, join me as I slide under a vehicle (an SUV for instance) at the rear axle, you will notice a rather “roundy” shape in the middle. That is where the differential lies.

The differential is an arrangement of gears that functions to allow the two wheels at an axle rotate at different speeds independent of one another while transferring the right amount of torque to the wheels. It also helps to transfer the right amount of torque to the wheel(s) in the event of slipping of a wheel in order to move the vehicle out of that situation

In a two wheel drive, you would find the differential in the driving axle. However, the driven axle is not also one single shaft as the wheels are independent of each other to allow a difference in speed when negotiating a turn or in other similar conditions. The rear wheel drive vehicle gives a more visible view of the differential as you have the drive shaft running from the engine all the way down to the differential which then transfers the power to the wheels. All vehicles have differentials, you would want to ask where that of a small vehicle like the Toyota Camry is, well these front wheel drive vehicles usually have the differential as part of the transmission. They are there but you just can’t see them.

What's does the differential do?

As stated earlier, the differential performs the basic function of enabling the two wheels at an axle rotate at different speeds to prevent slipping of one. It also does a few more functions. Such as

• gear reduction, the differential is the final stage of gear reduction before the energy from the engine finally gets to the wheels. The ratio of gear reduction is sometimes used to name a differential.
• it also helps to change the axis of rotation of the engine output by 90^o i.e. the shaft running from the engine is perpendicular to the axle of the rear wheels.

The job of a differential is not only evident when negotiating a turn, it is also needed when driving on bad terrain and one wheel seems to be on a slippery road or stuck. The other wheel (thanks to the differential) would receive the right amount of torque regardless of the slippery one.

source: wikimedia commons under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Parts of the differential

I would like to use the simplest differential design to explain the parts of a differential. The other design configurations are also similar with just little differences (or additions) so here we go;

Like I stated earlier the differential is an arrangement of gears thus, the parts of the differential are essentially gears.
The shaft from the engine transfers the power via a pinion gear which is meshed with a ring gear the ring gear is larger than the pinion gear and thus, the gear reduction function in achieved with this pair.

Next up is the spider gear which is attached to the ring gear. The spider gear performs two types of rotation – because it is attached to the ring gear, it rotates along with it, it also has the ability to rotate along its axis (quite similar to the two rotations of the earth). The spider gear is a bevel gear and there are more than one (2 or 4) attached accordingly on the ring gear. The number of spider gears is usually a function of vehicle type. Meshed with the spider gears are the side gears which are the gears attached to the shaft of the wheels i.e. the two side gears are for the two wheels on the axle.

How the differential does its work

Power is transmitted from the engine through the pinion gear onto the ring gear then to the spider gear before finally arriving at the side gears. When the vehicle is moving straight, the spider gear(s) is restricted to one type of rotation – along with the ring gear and thus, the spider gear(s) turn the side gears at the same speed. In the event of the vehicle taking a turn (assume a left turn), the spider gear(s) take on the second rotation along its axis therefore, allowing the side gear for the right side rotate faster than that of the left side. This statement explains the reason behind that

When properly meshed, the side gear has to have the same peripheral velocity as the spider gear. Technically speaking, both gears should have the same pitch line velocity. When the spider gear is spinning as well as rotating, peripheral velocity on the left side of spider gear is the sum of the spinning and rotational velocities. But on the right side, it is the difference of the two, since the spin velocity is in the opposite direction on this side. source

And that is how the differential does its job of allowing the wheels spin at different speeds.

Types of differential

As I stated earlier, there are different types of differential but their functions and parts are almost same with the difference being as a result of addition of parts that perform extra functions. Let’s take a peek at the types.

• The open differential.
  The open differential is the simplest form of the differential and it contains the parts I mentioned above. It performs the function of splitting speed and the likes but it has a shortcoming – it transfers the same amount of torque to the both wheels and the amount of torque transferred is a function of the least traction on the wheels.
  Simply put, when one wheel encounters a slip, the torque delivered is reduced therefore, there is difficulty in getting out of the slippery situation.

How much of a short coming is this? Lets find out.

If the vehicle with an open differential is on a slippery terrain and one wheel is slipping while the other has traction or one wheel lifts off the road, the open differential would still transfer the same amount of torque to the two wheels which might not be enough to equip the wheel with traction enough to free the vehicle.

• The limited slip differential
  The limited slip differential is equipped to overcome the challenges encountered by the open differential. There are other types of limited-slip differential but I would explain the commonest – the clutch type.
  The limited slip differential contains the same components as the open differential, only difference is the addition of clutches. These clutches help to transfer the torque to the wheel that is not slipping thereby enabling the wheel have more traction and thus be able to move the vehicle out of the slipping situation.

• The viscous coupling
  This type makes use of a viscous fluid to distribute the transfer of torque in the case of slipping. The fluid is housed between two set of plates which are connected to the two wheels. On normal driving conditions, the whole set up spins as one unit but in the instance of a slip on one wheel, the viscous fluid in the set up helps to transfer more of the torque back to the slower wheels (the one that is not slipping) and thus the vehicle is freed.

• Locking differential
  This type of differential depends on the driver for activation. In addition to the normal differential components, either an electric, pneumatic or hydraulic mechanism is installed. When activated in the event of a slip, the mechanism locks* the wheels together so that they can spin at the same speed and free the vehicle.

From these types of differential mentioned, it is safe to conclude that all are champions at bends and corners but they combat slip differently using different mechanisms.

Point to note.
For a vehicle that has a differential that is independent of the engine-transmission compartment (rear wheel and all-wheel drives), it is important to check the level of lubricant in the differential casing to ensure easy operation of the differential. The changing or topping of differential oil is a rather straight forward task and should not cost much at a mechanic workshop.

Car manufacturers recommend an oil change for the differential at about 30,000 to 50,000 miles depending on the manufacturer specification.

CONCLUSION
The differential is a part of the automobile that many car owners do not know its job but it is as active in the operation of the automobile. If not for it, a lot of drive shafts would have broken off or we would always need to change tyres frequently because of excessive wear due to slipping.
So when next you get a view of the underneath of your vehicle, do take some time to thank the differential for helping you get out of trouble and making your turns and corners smooth.

REFERENCES
1 Differential gear
2 Differential
3 How a differential works
4 How differentials work