Wednesday, December 31, 2014

Hydraulic Steering Principles

Principles of Hydraulic Steering

It's a wonder that we haven't seen more hydraulically driven and controlled motorcycles. Clearly weight is a concern but still... Maybe soon.

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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?