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Hydraulic Power Steering: What it is and How it Works

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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.

Hydraulic power steering is the unsung hero in our daily lives.

Back then, moving heavy item was hard.

Cars are really heavy too and it makes sense that steering our car is not going to be easy.

If you have driven a Perodua Kancil to get your driving license in Malaysia, you probably know what I’m talking about.

Oh the struggles!

The steering wheel feels stiff and it was really difficult to turn it with one hand, especially when stationary.

That is because we didn’t have power steering.

Power steering is short for power-assisted 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.

In this short article, we will explore one of the variation of power steering – the hydraulic power steering.

What is Hydraulic Power Steering?

You know…

Engineers are very literal and practical when it comes to naming things and I absolutely love it.

Hydraulic power steering pressurize liquid (hydraulic) to provide power that makes steering our car easier. It is an ergonomic aid to improve control and safe maneuverability.

The word – “Hydraulic” is just a fancy word for using liquid (water, oil, etc).

We pressurize the liquid by forcing them through a narrow hole.

When the pressure is greater, we generate a stronger force (provided that the area remains the same).

For all engineers out there, you know what I’m talking about!

Pressure = Force * Area

This can help us do some heavy lifting that we typically couldn’t. It’s the same principle behind a simple car jack.

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

Francis Davis is the father of hydraulic power steering.
Francis Davis is the father of hydraulic power steering. His invention shaped the way we drive today. Huge respect!

Power steering have been around for a very long time.

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 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.

At least not until the 1939.

In 1939, the second world war erupted.

People started looking for ways to control their heavily armored war machine better, faster and easier.

And the answer?

Hydraulic power steering.

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.

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! Image courtesy of order_242.

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.

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 is (almost) the same as 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…

  • Steering pump
  • Rotary Valve
  • Hydraulic Chamber
A schematic of hydraulic power steering system.
A typical hydraulic power steering system.

Steering Pump

Steering pump for hydraulic power steering system.
This is how a steering pump looks like.

The journey of a hydraulic power steering begins with the steering pump.

You can find the steering pump attached to the car engine, most likely next to the car alternator and A/C compressor.

Engine belt connects the steering pump through a belt-pulley mechanisms.

When the you crank the engine, the engine belt starts turning and that also turns the steering pump.

The steering pump draws steering fluid from a reservoir and pressurizes them.

The pressurized steering fluid then travels from the steering pump, through the pressure line and into the rotary valve.

Rotary Valve

The rotary valve connects directly to our steering wheel.

It is a metal casing with strategically placed holes that redirects the steering fluid either back to the steering pump or into the steering rack.

    • If the steering wheel is in its original position, the rotary valve directs the steering fluid back to the steering pump and the cycle repeats.
  • When the driver turns the steering wheels, the rotary valve opens up and steering fluid flows through the fluid lines and into the one of the hydraulic chamber of a steering rack.

Hydraulic Chamber

In the hydraulic chamber, there is a hydraulic piston to separate the chamber into two parts (left and right).

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 to the side that has lesser fluid.

Why do they do that?

Because of fluid dynamic, or more specifically the Bernoulli’s Equation. To give you a metaphor to work with, imagine two rooms: one is so over-crowded with people that you can barely breathe while the other one is empty. My instinct? To move to the empty room before I suffocate!

This difference in pressure 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 pump to repeat the whole thing again.

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

Not good enough? Inherent limitation of hydraulic power steering

Hydraulic power steering is amazing.

It provides the much needed steering assistance to our daily lives.

But, as with technology and everything else in this world, it’s not perfect.

Hydraulic power steering have some inherent flaws to it too.

For example, we have trouble controlling the amount of power assist from the hydraulic system. There is a mismatch of power assist and what we really need at different scenarios.

  • When stationary

At idle, we need greater power assist because it’s harder to move.

We know that car engine drives our steering pump through the engine belt. However, car engine produces lesser torque when stationary, which slows down the steering pump. When the pump is slower, it pumps lesser steering fluid and we get lesser power assist.

  • When driving at high speed

It’s easier to steer at high speed so we don’t need as much power assist. But, the car engine overwork the steering pump and we get too much power assist.

  • When driving straight

We don’t need power assist because we are not turning the car. But, the engine still drives the steering pump, causing energy to be wasted and decreasing our fuel economy.

There is one other problem.

Due to the nature of hydraulic, we need steering fluid for the system to work.

Fluid are very sneaky, wherever they go, they try to find a small opening to escape.

That means that leaks can often happen at steering pump or steering rack.

When that happens, you will notice that steering wheel feels harder to turn.

Or, you will have to constantly top up your steering fluid every 2 weeks or so.

This is just one of the some of the problems that we may face specifically for a hydraulic steering system.

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.

If you’d like to find out more, 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 so much more comfortable than before.

Nobody likes to drive without power steering anymore!

The technology is great and that is why we continually improve it bit by bit.

Now, we are beginning to see a shift into electronic power steering system because we can introduce more control over the entire power steering system.

Just like what ABS does for the braking system, we can do the same for power steering system.

Car manufacturers are looking to introduce more safety feature and fuel economy by going electronic!

But, that would be another story for a different day.

If you are interested, kindly subscribe below and we’ll be sure to keep you posted.

Until then, drive safe and drive smart!

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