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Hydraulic Power Steering: What it is and How it Works

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Just like how Apple iPhone changed the mobile industry, hydraulic power steering rack is the equivalent in the automotive industry.

Cars are extremely heavy object and can weigh up to 1500 kilograms. Imagine trying to steer this monstrosity with your bare hands – just impossible! That’s exactly why we have steering rack to start with, it made it possible.

But, it wasn’t perfect. If you have driven a Perodua Kancil, you know how stiff it was to steer your car. We knew we had to take steering rack to the next level. The answer, was power steering rack. Steering rack made it possible to steer a car, but power steering rack was the one who made it easy.

For those who have tried both types of steering racks will surely appreciate the impact it made in our daily commuting lives. And because of that, this technology became so widely spread to the point that all cars manufactured in the 21st century adopts some form of power steering.

As of 2018, we have three types of power steering: (i) hydraulic power steering, (ii) electro-hydraulic power steering, and (iii) electric power steering. Today, we will explore and gain a deeper understanding of the grandfather of all power steering system – the hydraulic power steering rack: what it is and how it works.

What is Hydraulic Power Steering?

You know, engineers can be quite literal and practical when it comes to naming things and it really helps make it easy to understand things! Let’s break it down word by word.

The word – “Hydraulic” is just a fancy word for using liquid, which could mean water, oil, and etc. In this case, we are using a bright pink color hydraulic steering oil for our cars.

Then, we have the word – “Power Steering”. When we pressurizes the steering fluid and use them in a smart way (more to that later), we get additional power that helps us steer our car easier.

And there we have it!

In essence, hydraulic power steering pressurizes hydraulic fluid to give us extra power that makes steering our car easier. It is an ergonomic aid to improve control and safe maneuverability.

This is a hydraulic power steering rack - one of the major component of our car steering system.
This is a hydraulic power steering rack – one of the major component of our car steering system.

This is how a typical hydraulic power steering rack looks like. Here’s a neat trick. To know if it’s a hydraulic steering rack, all you need to do is pay attention to the centre of the steering rack. If you see two metal pipes protruding out of the steering rack, you know it’s a hydraulic type. Also, you may not be able to tell from the picture, but they are actually very bulky. Each steering racks can span up to 1.5 metre in length.

Rise of Hydraulic Power Steering – Where did all this power come from?

Power steering have been around for a very long time. I’m talking about like hundred years long. The first ever hydraulic power steering was awarded a patent in 1876. It was then improved by Frederick W. Lanchester in 1902.

In 1926,Francis Davis became the first person to successfully fit a hydraulic power steering unit into a automobile. He installed it in his 1921 Pierce-Arrow and traveled from New York to Los Angeles in just 12 days. Unfortunately, nobody really saw the potential and it was never commercialized. At least not until the 1939 when the second world war erupted.

Francis Davis is the father of hydraulic power steering.
Francis Davis is the father of hydraulic power steering.

During the war, people started looking for ways to control their heavily armored war machine better, faster and easier. In search for a competitive edge in the battle, the technology behind hydraulic power steering was quickly adopted into prominence. By the time war ended in 1945, 10,000 military vehicles have been fitted with a power steering unit.

When tested and proven in such a large scale, it’s hard not to see the potential in this relatively “new” piece of technology. And so it happened. In 1951, Chrysler is the first car manufacturer to make power steering unit commercially available. It was available to the public through their passenger car – the Chrysler Imperial.

Chrysler Imperial 1951 is the first commercial passenger car that is equipped with a hydraulic power steering unit.
Chrysler Imperial 1951 is the first commercial passenger car that is equipped with a hydraulic power steering unit. Such a beauty!

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Image courtesy of order_242.

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Soon after, many other cars manufacturers such as General Motors, Toyota and Honda quickly came up with their own variation of power steering and implemented them. And that brings us to where we are now. In 21st century, virtually all cars are now equipped with power steering.

If you really think about it, power steering is a piece of technology that is born out of difficult times and it blossomed into something that changed our life completely for the better. It’s truly remarkable.

How does a hydraulic power steering work?

If you are completely new to car steering system, I highly recommend starting with How Car Steering System Works – in Simple English first. This is because a regular non-power assisted steering rack works very similarly to a hydraulic power steering system.

The only difference is that hydraulic power steering have a few additional parts to supply the extra power. I’m talking about…

  • Hydraulic fluid
  • Steering fluid reservoir
  • Steering pump
  • Rotary Valve
  • Hydraulic Chamber

Steering Fluid Reservoir

Just like how we have a petrol tank for petrol, we have a steering fluid tank for steering fluid. Whenever we are using fluid, we always have a container that holds them when we are not using them.

There is nothing too fancy about this part here, and its purpose is quite self-explanatory too. But, the journey of a hydraulic power steering begins here. When we fill the steering fluid, we put it into this reservoir. It holds the fluid, and supplies them to the steering pump through rubber hoses.

Steering fluid reservoir is usually this yellow looking container with the word “power steering fluid” written on the cap.

Source

Original image by schwartz.mark (CC BY 2.0)

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Steering Pump

You can find the steering pump attached to the car engine, usually right next to the car alternator and A/C compressor. We connect the steering pump to the engine through a belt-pulley mechanisms using an engine belt.

When the your car’s engine is running, the engine belt turns in a loop and that also turns the steering pump. With that, the pump pulls the steering fluid from the steering fluid reservoir and pressurizes them.

How exactly do they do that? Well, I don’t want to overwhelm you with all the tiny details, but if you are interested in knowing more, we have an article on the steering pump coming up very soon. For now, think of the steering pump as a black box. We put low pressure steering fluid in, and high pressure steering fluid comes out of the other end.

Steering pump for hydraulic power steering system.
Steering pump for hydraulic power steering system.

These high pressure steering fluid then leaves the steering pump, through the steering hoses and into the steering rack, specifically into the rotary valve.

Rotary Valve

Inside the steering rack, we have what is known as the rotary valve. A rotary valve is a highly sensitive metal casing with strategically placed holes that redirects the steering fluid either back to the steering pump or into the steering rack.

A schematic of hydraulic power steering system.
Here’s a detailed look at the inner components of a hydraulic power steering rack.

Think of it as the traffic police at a busy road intersection. It tells the steering fluid which way to go depending on where you turn your steering wheel. Here’s how it works…

  • If the steering wheel is in its original position, the rotary valve redirects the steering fluid back to the steering pump and nothing happens. The cycle of steering fluid moving from reservoir to pump and to rotary valve just keeps repeating itself.
  • But when the driver turns the steering wheel, the rotary valve opens up and steering fluid from the steering pump gets redirected. This time, it doesn’t go back to the steering pump but it exits the rotary valve through the fluid lines and into one of the hydraulic chamber of the steering rack.

Hydraulic Chamber

As the steering fluid from the rotary valve gets redirected into the hydraulic chamber, we start to get power assist! But let’s take a step back and see how it all happened.

In the hydraulic chamber, there is a hydraulic piston right down the middle. It separates the hydraulic chamber into two equal portions: the left side, and the right side. The steering fluid gets redirected into these two chambers, but here’s the twist – they don’t get equal amounts of steering fluid!

When there is more steering fluid on one side of the hydraulic chamber, it creates a pressure differential across the chamber. The steering fluid then pushes the hydraulic piston towards the weaker side of the hydraulic chamber and the steering rack moves accordingly.

Now some of you may be wondering, why is there a pressure differential.

Because of fluid dynamic. Or, more specifically the Bernoulli’s Equation. To give you a metaphor that helps you understand it, imagine two rooms of equal sizes with a movable wall that you can push in the middle. One room is filled with 50 people while the other is filled with 100 people. Because it’s so hot and stuffy in the room, I would push the wall so that I can get more space in my room. But hey, the other room want more space as well! Very quickly, this becomes a tug of war where the stronger team pushes the wall to the other side.

Anyway, this “pushing of the wall” is what gives us the extra power. Since both ends of the steering rack is connected to the car wheels, when the steering rack move to the right, so will the car wheels. And… Voilà! The car changes direction and steering fluid flows back to the steering fluid reservoir to repeat the entire process again.

And that my friends, is how a hydraulic power steering works.

Inherent Limits of Hydraulic Power Steering – not good enough?

Hydraulic power steering is amazing. It provides the much needed steering assistance in our daily commuting lives. But, as with everything else in the world… Nothing is perfect. Hydraulic power steering too, have inherent flaws.

Limitation #1: Mismatch of Power Assist.

It’s unfortunate that we cannot control the amount of power assist that we get. The power assist that we are getting might not be what we need.

I’ll explain what I mean.

By now, we all know that car engine drives our steering pump through the engine belt. They’re connected physically and there is no way we can control how much power to give or not to give. Whether you like it or not, the speed of our steering pump is directly related to the car engine RPM. When we drive faster, car engine RPM increases and pump runs faster. Conversely, when we drive slower, car engine RPM decreases and pump slow down. Now consider these 3 scenarios…

  • When driving at low speed / stationary
  • When driving at high speed
  • When driving straight

When driving at low speed or idling, we needed more power assist because it’s harder to steer the car without any momentum. But in reality, car engine RPM is low when idling, and therefore we are only getting the least amount of power assist when for example, moving in and out of a parking.

When driving at high speed, it’s helpful to have a stiffer steering wheel so the steering wheel feels more firm and stable. But when driving fast, the car engine overwork the steering pump and we get way too much power assist. A slight push on the steering wheel can over-steer the car in a direction that we don’t want to.

When driving straight, we don’t need any power assist at all because we are not turning the car. But, the engine still drives the steering pump whether we need it or not. When you are moving unnecessary parts, energy are wasted and that decreases our fuel economy.

Limitation #2: Prone to leakages

There is one other problem.

Due to the very nature of hydraulic, we need steering fluid for the system to work. Fluid are very sneaky, wherever they go, they will find any small opening to escape. That means that whenever we use fluid, you can be sure as hell that there will come a time when it starts to leak.

Fluid leak is one of steering rack most common and easily detected symptoms.
Fluid leak is one of steering rack most common and easily detected symptoms.

In our case, it’s not uncommon to find steering fluid leaks at the steering hose, steering pump or steering rack after just a few years of usage. When that happens, you will start to notice that steering wheel feels harder to turn. Or, you will find yourself having to constantly top up your steering fluid every 2 weeks or so.

This is just one of the many of the problems that we may face in a hydraulic steering system. If you’d like to find out more about, we have listed out the Top 7 Steering Rack Problems you can Identify [with Videos!] just for you.

Give it a read, it will be worth every minute.

The Future of Hydraulic Steering System

Yes, there are flaws. But, hydraulic power steering have come a long way and made driving easier and safer than ever before. I don’t know about you but I can’t imagine driving without power steering anymore!

The technology is great and that is why we continuously try to improve it. Or at least we tried. The bad news is that we are reaching a maturity for a hydraulic steering system because of some of the inherent limitations.

The good news is that precisely because of that, we are beginning to see a shift into electronic power steering system because we can introduce so much more control over the entire power steering system. Just like how ABS sensors improved the braking system, we can do the same for power steering system too. And who knows, it might just be a stepping stone for a driver-less car in the future. But, that would be another story for a another day.

Until then, drive safe and drive smart!

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