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How Power Steering Pump works – in simple terms!

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A high definition picture of a hydraulic power steering pump used in car steering system.
This is how a hydraulic power steering pump looks like.

To many, steering pump is just some magical black box that supposedly somehow makes our driving trips much more enjoyable. But how does a power steering pump works exactly?

If you clicked into this article, I bet you want to know more than just the surface. We can help with that. Today, we’re going to dive deep into the world of mechanics, particularly on the inner workings of a steering pump.

Let’s take it apart, and see how it works.

What is a power steering pump?

In a hydraulic power steering system, we have the steering rack and the steering pump. These two components are like Batman and Robin, Thor and Mjolnir, peanut butter and jelly – basically inseparable. They work together to give us the ability to steer our car, and at the same time, making it easy to do so.

Generally speaking, steering rack gives us the actual trajectory changes on our car tyres so that we can steer our car. On the other hand, steering pump is the one responsible for making the steering wheel feel “light” and easy to turn when we steer.

In formal terminology, power steering pump is a centrifugal vane type hydraulic pump that pressurizes steering fluid through high speed rotations in order to create a pressure differential that translates into “power assist” for your car’s steering system.

That’s way too geeky for my liking. Here’s my take.

It’s essentially just a water pump, except with steering fluid. It spins like crazy, pressurizes steering fluid like crazy and then send them to the rest of the steering system so that we can steer like crazy.

The steering pump has the size of about a coconut, and you can usually find it attached next to your car’s engine. If you want to have a look, just follow along the engine belt and you will eventually see it.

You can always find the power steering pump located close to the engine. Why? Because it runs on the engine belt.
Power steering pump is always placed close to our car engine.

Source

Original image by Sukhwinderteji (CC BY-SA 3.0).

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How power steering pump works?

Although power steering pump is metaphorically known as the “heart” of our power steering system, it is just a drop of water in the ocean. There are a myriad of other mechanisms surrounding and supporting it so that the whole system ticks. With that being said, it can get very complicated very fast, especially if one is unfamiliar with the power steering system as a whole.

In that spirit, we’ve simplified “how power steering pump works” into three sections to ease you through the process. These sections are (i) before steering pump, (ii) within steering pump, and (iii) after steering pump.

Before Steering Pump

Let’s start from somewhere we are all familiar with, the steering fluid reservoir.

If you open up your car hood, you will find a (usually) yellow-ish container that has the word “power steering fluid” written on the cap. It is the container where we pour our steering fluid into.

If you lift your car hood, you should be able to find this yellow container that has the word "Power Steering Fluid", "MIN" and "MAX" embedded on it.
Steering fluid reservoir is usually this yellow looking container with the word “power steering fluid” written on the cap.

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Original image by schwartz.mark (CC BY 2.0)

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It’s nothing fancy. Just like how we have a petrol tank for our petrol, we have a power steering fluid tank for our power steering fluid.

The sole purpose of this tank is just to hold the steering fluid and supplies them to the steering pump through a set of rubber hoses. When we are not using the fluid, it rests in the reservoir. When we need the steering fluid, it gets sucked out of this reservoir into wherever it needs to go.

As the steering fluid flows through the rubber hoses, it reaches the steering pump.

Now, we need to get the steering pump spinning first. In order to crank it, we need a continuous supply of power going to the pump, and that comes from none other than the car engine itself.

Our car engine produces power by igniting petrol-air mixture with electrical sparks. When the petrol-air mixture explodes inside the engine, it produces energy that pushes the engine piston up and down. And because our engine piston is mechanically connected to the crankshaft, the crankshaft harvests these energy and the crankshaft itself starts to rotate.

Finally, we put an engine belt around this rotating crankshaft, and connects it firmly to the pulley on the steering pump. When we start our car, the engine belt pulls onto the steering pump pulley and the steering pump starts spinning.

Lo and behold, the steering pump is now working.

Within Steering Pump

At this stage, we got the steering pump spinning, and the steering fluid readily available to the pump. The next step, is to pressurize the steering fluid.

Here comes the exciting part. We will now take a peek at the core of a steering pump to help understand how this majestic device creates high pressure fluid.

The core of our steering pump comprises of solid metal blocks, machined to precision to form the housing, rotor, and vane.
When we’re looking at the core of the steering pump, these are the three main components that you need to focus on.

Source

Original image by D S AUTO.

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The picture above is what you would typically see inside a power steering pump. Granted, not all steering pump looks like this, and there are other intricacies here and there that we took out, but let’s keep it real simple here. There are only 3 distinct items that you really need to pay attention to, namely the (i) housing, (ii) rotor, and (iii) vane.

The rotor is this solid metal block that has only two distinct features: it has a hollow hole in the middle, and then some cavities on the exterior. Both serve a different purpose.

The center hole connects directly to the steering pump pulley through another cylindrical metal. So if you can imagine, when the steering pump pulley is rotating, the rotor itself will consequently rotates as well, and hence the name “rotor”.

Remember the cavities on the exterior? These tiny pocket of holes are where the vanes will rests. They act as a railway for the vanes to be moving in and out. When the steering pump is not rotating, they rests closer to the center (see left picture below). When the rotor rotates, all the vanes get pushed outward and against the pump housing (see right picture below).

The vanes are movable within the cavity in the rotor.
Picture on the left shows a retracted vane, while picture on the right shows the vane in an outward position.

Source

Original image by D S AUTO.

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Why?

Well, it’s like playing with a carousel at the kid’s playground. You can find a comfortable place to sit on the carousel, and then get a friend to spin you like crazy! The faster he spins, the more you will feel like you are being thrown outward. If he spins hard enough, you may just end up amongst the mud!

A carousel is a circular platform that you can spin around. If you are standing on the platform, you will feel the centrifugal force trying to toss you outward.
Good ol’ days. This used to be cool but we could hardly find these anymore in our current smartphone era.

Source

Original image by schwartz.mark (CC BY 2.0)

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This is the centrifugal force at play.

Similarly in the power steering pump, all the vanes will be pushed outwards against the pump housing. But that’s not all. Imagine the same centrifugal force, but with engine RPM, which is typically measured in the thousands per minute. The vanes are pushed so strongly against the pump housing, that it starts to form tiny chambers which traps the steering fluid. In our example, there are exactly 11 of the tiny chambers.

When the steering pump is spinning, all vanes will be pushed outward against the pump housing. This creates chambers to carry the power steering fluid around.
When all the vanes are pushed outwards, they form tiny pockets to carry the power steering fluid around.

Source

Original image by D S AUTO.

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As the rotor rotates in a clockwise fashion, the tiny chambers follow along (together with the steering fluid inside of it!). Chamber No.1 will move from it’s original position, into Chamber No.2, then Chamber No.3, and so on and so forth at an incredible speed until we switch off the engine.

This is where the magic happens. The steering fluid pressure starts to increase, not without a reason of course.

If you look closely, the pump housing is not geometrically round, they are designed to be oval in shape, purposefully. When the rotor is placed directly at the center, you will notice that the bottom part of the groove is significantly bigger than the top part of the groove.

A picture paint a thousand words. Take a look.

If you squeeze more things into a smaller space, the pressure increases.
This is a single action power steering pump, where we have a low pressure area and a high pressure area.

Source

Original image by D S AUTO.

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The differences in volume is not a design error or manufacturing flaw, it serves a purpose, and a very important one that is.

Here’s why.

When the pump continues to rotate, the steering fluid are carried around the oval groove like a merry-go-round. Because of the eccentric oval shape, the steering fluid moves from a large area, and gets squeezed into an increasingly smaller space. If we look to Physics, we know that when the area decreases, pressure increases. Engineers know what’s up. But hey, don’t take my word for it, I’ll even prove it to you.

Pressure = Force / Area.

Pascal’s Law

Think of it like squeezing a balloon filled with air.

First, you blow a balloon until a decent size, then you tie a knot around the openings to seal the air inside. As you squeeze the balloon, the space inside the balloon reduces. With nowhere to escape, the same amount of air gets crammed into a tighter confinement which increases the air pressure. If you squeeze it hard enough, it will reach a point where the air pressure is greater than the strength of the balloon and it finally bursts.

Air acts just like any other fluid, which includes steering fluid. Given the same amount of fluid, when you reduce the area, the fluid pressure will increase. And that is exactly why they designed the cam ring to be oval instead of a perfect round shape.

Coming back to how power steering pump works.

We talked about how these tiny chambers continually spin the steering fluid into a tighter space to increase the pressure. After that, the high pressure steering fluid eventually exits out of the steering pump through the pressure control valve.

This leaves behind a void / low pressure area in the steering pump, which helps to draw in more steering fluid from the steering fluid reservoir.

Anyway, these high pressure steering fluid leaves the steering pump, and into the two hydraulic chambers of the steering rack.

After Steering Pump

For those who are familiar with Hydraulic Power Steering: What it is and How it Works, you would have a very good idea of how the story ends. For those who don’t, here’s how it basically goes down.

The high pressure steering fluid leaves the steering pump, and into the steering rack. Basically speaking, the steering rack is divided into two hydraulic chambers, the left and the right side.

When the steering fluid enters the two hydraulic chambers, they are distributed in a way that one of the hydraulic chamber in the steering rack gets more steering fluid than the other. If the left chamber gets more fluid, it becomes stronger than the right chamber. And guess what happens? The steering rack pushes to the right thanks to the difference in fluid pressure. The reverse is true when you want to turn to the left.

This pushing motion right here, is the power assist. It is the reason why your steering wheel feels lighter when your car has power steering system. 

After that, the steering fluid exits the steering rack through steering hoses and returns back to the steering fluid reservoir.

The whole process just repeats itself on and on and on and on until we finally switch off the engine.

Finishing the Final Lap

And that my friends… is how a power steering pump works. It’s beautiful, isn’t it?

I sincerely hope that I did justice in dissecting the inner workings of a steering pump and hopefully it helps us better appreciate the marvel of engineering that goes into our car.

If we’ve missed anything in the article, feel free to leave a comment down below and we’ll add it into the article for sure.

Until then, drive safe and drive smart!

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