Retrotechtacular: Principles Of Hydraulic Steering

Have you ever had the pleasure of trying to steer a one-ton pickup from the 1940s or wondered how hard it would be to turn your car without power-assisted steering? As military vehicles grew larger and heavier in WWII, the need arose for some kind of assistance in steering them. This 1955 US Army training film handily explains the principles of operation used in a hydraulically-assisted cam and lever steering system.

The basic steering assembly is described first. The driver turns the steering wheel which is attached to the steering shaft. This shaft terminates in the steering cam, which travels up or down along the camshaft depending on the direction steered. The camshaft connects to the steering shaft through a spline joint, which keeps the travel from extending to the steering wheel. The steering cam is connected to the Pitman arm lever and Pitman arm shaft. Movement is transferred to the Pitman arm, which connects to the steering linkage with a drag link.

The hydraulic system helps the Pitman arm drive the linkage that turns the wheels and changes the vehicle’s direction. The five components that comprise the hydraulic system use the power of differential pressure, which takes place inside the power cylinder. The hydraulic system begins and ends with a reservoir which houses the fluid. A pump driven by the engine sends pressurized fluid through a relief valve to the control valve, which is the heart of this system.

fluid to power cylinderThis control valve contains a spool that directs fluid from the reservoir to the power cylinder and back again through ports and channels. When the driver turns the steering wheel in one direction, the cam causes the control valve to release pressurized fluid into one end of the cylinder. This compresses the piston, which is connected by a lever to the Pitman arm lever. The other end of the Pitman arm lever meshes with the steering cam. It travels a short distance from center and compresses the corresponding end spring. The piston in the power cylinder converts hydraulic pressure into mechanical force which turns the wheels in the desired direction.

Fluid is pumped to the power cylinder as long as the steering wheel is being turned, and the relief valve takes care of any excess pressure buildup in the system. When you do donuts in a vehicle with cam and lever steering, the cam is not rotated during this constant turning. The end spring that corresponds with the direction of your turn stays compressed, and the hydraulic pressure equalizes in the power cylinder. Once you get bored and let go of the wheel, the system returns to neutral, and all of the fluid entering the control valve is immediately channeled back to the reservoir.

Hydraulic steering also helps with the effects of road shock. Without it, every little pebble and crater would send the steering wheel spinning around and you scrambling to right it. The hydraulic steering-equipped vehicle resists these shocks by reversing the action that takes place in the power cylinder. The shock travels through the linkage and Pitman arm to the Pitman arm lever. This actuates the cam, whose movement causes the control valve to equalize the pressure and the wheels to remain straight.

Over the last ten years, manufacturers have been shifting to electric power-assisted steering in the name of fuel consumption. Detractors argue that steering feels rigid and under-powered. Our car turns twelve this year, so we don’t know what EPAS feels like. Which do you prefer?

Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.

27 thoughts on “Retrotechtacular: Principles Of Hydraulic Steering

  1. I like the Retrotechtacular series, especially the old army tech training videos. I find them clear, instructive, and mostly unlike the vast majority of instructional videos I see today. Is anyone still making instructional materials like this? Modern day army tech training videos, maybe?

    1. I completely agree. I don’t know of any modern training films but I would assume they’re just as easy. The military has strict rules that no explanation can be more advanced than an 8th grade reading level. This even includes satellite communications.

    2. I bet today’s military instructional material will end up on the net eventually, and they’ll probably be of similar quality. It wouldn’t surprise me, though, if stuff that used to be done through videos is now handled different ways, like a much shorter video and then a simulator that lets you get hands-on much sooner.

    1. When the engine dies the power steering fails, with enough grunt strength you can still maintain enough control to bring the vehicle to a safe stop. Tough as hell, but doable. I can see the EPAS being safer in this kind of situation unless you have a total electrical failure.

      Is this a correct assessment?

      1. I have EPAS on my Saturn Ion as well. I like it too. Really never noticed a difference when I bought the car until I went to check the power steering reservoir and couldn’t find it. But you are correct, a electrical failure will take down the power steering. Right now there is a recall for the ignition switch where it can shut off unexpectedly and kill the power steering along with engine, air bags…

      2. I drove a car with a broken power steering pump for a few months when I didn’t have enough money to replace it. Low speed turning and parking was tough sometimes, but aside from that it wasn’t too bad.

        This was a Saturn SL, which is a pretty light car. I bet it would be a different story in a heavier car.

  2. The video represents the older, legacy steering mechanism. Many car manufacturers have migrated to rack and pinion steering, and the operation principle of power-assisted rack-and-pinion steering is different. Not much, but different. That makes the video somewhat outdated.
    The video-described mechanism.
    Commonly-used today mechanism.

  3. Having driven for several years a car with no power steer assist, I think the EPAS on my current car is perfectly fine both for feedback and steering assistance. However I had the occasion to drive it with a failed battery, and the feel of the steering suddenly hardening whenever the brake lights turn on is scary.

      1. Actually the alternator was fine, but would not provide enough power to run both the power steering, spark plugs and brake lights at the same time. In normal operation the battery supplies enough extra power for all of these services to work together, while the alternator will still have excess power to recharge the battery when you are not braking and steering at the same time, a situation that really shouldn’t happen very often when driving properly.

        1. Yes, meanwhile in the real world. One of those emergency journeys where you get the sensation something is wrong but its 2am on a freezing stormy night on the autoroute, & you try not to touch the brakes and need the wipers at the same time and when you do the engine cuts briefly! Or you notice the wipers going slower and slower with the heater on for demist and turn them on slower to use the heater a little faster. And you slowly loose services until you realize that you are starting to be dangerous, pull over and resolve the problem *somehow* in some isolated windswept service area from hell…

  4. I do trust all the concepts you’ve offered for your post.
    They are really convincing and can definitely work. Nonetheless, the posts
    are too brief for newbies. May just you please extend them a little from next time?
    Thanks for the post.

  5. Worm and ball is still pretty common on heavy trucks. I haven’t worked on one for about 15 years and probably now they have epas equivalence for the extra fuel economy + similicity it potentially brings. Downtime fixing hoses and changing filters is money and ingress of dirt is always a problem with dirty servicing areas in some shops.
    Both epas and hydraulic automotive steering have mechanical failsafe modes however, its been mandatory for good reason. The hydraulic, your steering gets heavier but still works, the epas the same.
    I have hydraulic steering on my backhoe (JCB 3cIII) from the 80’s and it has no failsafe and runs off the general hydraulic pressure rail which runs the backhoe, loader arms, legs etc, if the hydraulic fails, the steering is gone but the brakes are still 100% mechanical and you can always drop a bucket to stop even with a dead engine because of gravity. There’s no mechanical linkage in the steering though just a ram which moves the steering arm on the hubs. Its designed for road use and legal for such too.
    I’m retrofitting epas to my 50’s streetrod build also because it has heavy steering because of fat stiff sidewall tyres, a opel corsa column unit with a simulator to give the ecu signal that the donor car gave. Its a much neater install and better in a few ways than hydraulic pump and lines everywhere.
    One last thing, you only need power steering to make parking and other low speed manoevers easy. When you rely on the steering you are scrubbing the tyres instead of using the vehicle how it wants to go and are accelerating tyre wear.
    Interesting retrotacular.

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