Hydraulic Power Steering: A Piston That Follows the Driver

Sheet 1 of US 1,790,620 shows the car's layout: steering wheel 12 turning a post 31 down the column 11 into the power cylinder 10, a pump 18 drawing from tank 21 and delivering through a supply pipe, and the cylinder's piston driving the cross shaft to the steering arm. Inside the cylinder: the post's thread drives nut 29, which slides inside piston 26. The nut's collar 38 is trapped between shoulder 380 and ring 39, so it can move relative to the piston only "a few thousandths of an inch". Grooves and ports in the nut and piston form the valve. The pump's delivery passes a spring-loaded relief valve 25 set to a predetermined pressure. The patent's liquid is "oil or a glycerine".

Use a double-acting syringe cylinder and a clear spool valve, built as in this batch's cylinder and spool-valve blueprints. Clamp the cylinder body to a board. Fix the spool valve's **body** to the cylinder's **rod**, so the valve travels with the piston, and pipe its A and B ports to the cylinder's two ends with flexible tube. Drive the **spool** with a hand lever pivoted on the board: that is the driver's nut. Feed P from a garden hose through a ball valve and a pressure gauge, and T to a bucket. Move the lever slowly: the rod, and the valve body on it, follow until the spool is centred again, and stop. Mark lever and rod positions on the board and check they keep the same relation — a follow-up servo. Close the supply and move the lever again: now you feel the lever bear on the valve body and push the whole rod yourself. Load the rod with a spring scale and compare the pull on the lever with and without water.

**Francis W. Davis** of Waltham, Massachusetts, filed his first hydraulic steering application on **14 April 1926**; **US 1,790,620**, filed 11 May 1927 as its continuation, was granted **27 January 1931**. He patented further components of his system through the 1930s. Power steering reached production cars in the early 1950s, and with it the same principle: the driver's small movement opens a valve and oil does the heavy work, which is now often done by an electric motor instead. The patent is explicit about two dangers of making steering light by removing friction: the wheels shimmy, and road shocks kick back through the wheel ("wheel fight"). A follow-up servo avoids both by keeping a stiff mechanical path. **Honest limits.** The notebook's kingpin moment and flows are illustrative, not Davis's; real systems add feel by letting part of the load reach the driver's hands. Pressure fluid lines on a car must never be opened hot or running.