carfix_online: September 2014

The weight of early cars made them difficult to maneuver, and the widespread adoption of pneumatic tyres further hindered drive ability.

Demand from customers for convenience features and the drive to improve safety have made vehicles bigger and heavier. But turning the steering wheel hasn’t become any more of a struggle, and that is owing to hydraulic power steering, developed more than 80 years ago by Francis W Davis.

The American engineer was well aware of the problems drivers faced in the early 20th century. Cars were difficult to drive, and weight was the key factor: “There has been a trend toward larger automobiles equipped with an increasing number of accessories adding still further to the weight,” he said in his 1927 patent continuation. The introduction of pneumatic tyres increased steering resistance yet further.

Many engineers attempted to reduce friction in the steering mechanisms. While this slightly eased the problem, according to Davis it created new challenges: “It gave rise to a greater evil, that is the tendency on the part of the steering wheels to vibrate excessively.” Vacuum, mechanical, electrical and hydraulic power-steering systems were all designed, but all failed. Problems of durability, packaging and cost hindered the development of a solution. Davis was convinced that a hydraulic system was the answer.

Davis had graduated from Harvard University in 1906 after studying mechanical engineering. He worked for Pierce-Arrow after graduating, which gave him an insight into how hydraulic technology worked – the OEM’s press tools were all hydraulically operated. When he left the company in 1922 to become a consulting engineer he began to research the technology more closely.

The challenge for Davis was to make the technology scaleable – hydraulic systems used in industrial presses needed huge storage tanks of pressurised oil, pumps, unloader valves, accumulators and hose lines. After a myriad of failures because of pressure losses, leaks and unworkable packaging, Davis changed direction. Rather than use a pressurised closed valve, he developed an open-valve system that allowed oil to flow continuously, but when power-steering assistance was needed it was closed and pressure built up.

Davis described the system in his 1926 patent: “I supply the gear, particularly suitable for automobiles, which is adapted for direct hand steering where the steering resistance is slight, which instantly and automatically augments the manual steering efforts of the operator by the application of power from fluid pressure when the steering resistance exceeds a predeterminable value.”

Davis was so sure of the system that he installed it in his Pierce-Arrow Roadster in 1925, proving that it not only made the car easier to drive but also removed vibrations through the steering wheel.

Just as modern power-steering systems cause debate about driver feedback, so Davis was also aware of the issues surrounding driveability.

Steering reversibility, as Davis described it – when the car hits an obstacle and the force is transmitted to the steering wheel – can cause the wheel to be wrenched from the driver’s hands. But removing reversibility completely can cause a loss of feeling between the driver, steering inputs and movement of the car. Davis solved this by making his system tuneable, so it could be constructed with a degree of reversibility, where the maximum torque that could be impressed on the steering wheel by road shocks was limited to a predetermined value.

As ingenious as Davis’s design was, the depression of the 1930s meant that few car manufacturers were interested. But military applications opened up, helping Davis to develop the system. When the car market picked up after the Second World War, Chrysler was the first OEM to introduce a hydraulic power-steering system on its Imperial sedan. That was in 1951 and the system was based on design principles that Davis had patented, but by this time his patents had expired. Davis signed an agreement with GM to license his system to the OEM.

By 1956, more than two million vehicles had been sold with power steering in the US. Thanks to Davis’s persistence, the technology is an intrinsic part of today’s vehicles.

Just as modern power-steering systems cause debate about driver feedback, so Davis was also aware of the issues surrounding driveability.

By 1956, more than two million vehicles had been sold with power steering in the US. Thanks to Davis’s persistence, the technology is an intrinsic part of today’s vehicles.