Patent No. 659 310 (1958): an elastic ring servo-control for Citroën steering

In this document

The problem posed

The memorandum opens with a critical observation on existing power steering systems: they require "special precautions" to operate at full speed, and above all, the driver "never feels the resistance due to the wheels on the steering wheel" p. 1. Another defect noted: in known systems, it is the mechanical transmission components themselves that control the distribution valves, which generates high friction and impairs sensitivity p. 2.

The principle of the invention

Citroën proposes a device that only intervenes in "moderate speed zones" p. 2. Its central feature: an elastic organ is inserted between two parts of the steering transmission — here a rubber ring placed between the pinion and the steering shaft p. 2-4. As long as the resistance of the wheels remains moderate, the column acts directly; beyond a certain threshold, the elastic organ flexes and this slight angular offset is transmitted to the hydraulic distributor valve p. 3.

The text also specifies that a valve placed on the circuit can isolate the motor device (servo) from the rest of the circuit at high speeds, with manual or automatic control (p. 2 and 7).

Detailed technical description

The pinion rod (6) and the steering shaft (7) each carry two diametrically opposed millings, slightly offset from each other p. 4. An axle equipped with four rollers passes through these millings and moves laterally according to the direction of rotation imposed, driving the valve (14) of the distributor (15) via a rod (13) p. 4-5.

The hydraulic circuit combines a reservoir (17), a pump (20), an accumulator (19) and a piston cylinder (2-3) connected to the rack (5) p. 4. The valve (24), operated by a lever (28), can either isolate the two chambers of the cylinder or connect them directly, thus cutting off the assistance p. 4.

The text specifies that the torsion of the elastic element is limited to "2 or 3 degrees" by stops (p. 5, manuscript precision "degrees" added to the typed text), and that the forces required to move the valve are on the order of "500 gr." p. 5.

The final summary p. 6-7 reformulates these points as claims: elastic element between two mechanical organs, notches and rollers, complete hydraulic circuit, valve with automatic or manual control.

Context

This patent, filed on 3 December 1958 by the Société Anonyme André Citroën, is part of the research into steering assistance that accompanied, at the same time, the development of the hydropneumatic suspension of the DS, of which Paul Magès is the main architect at Citroën: having joined Citroën in August 1925, he led the first work on brake limiters there from 1939-1940, then, from 1944, the plans for the hydropneumatic suspension that would lead to the DS presented on 6 October 1955 (autobiographical notes ; 1944 plans). This 1958 patent is therefore located in the period when, at the design office, he was working on the hydraulic components associated with the DS.

Historically, hydraulic power steering would become a major feature of Citroëns from the 1950s-1960s (the DS would offer a power steering known as "DIRAVI" a few years later). The use of rubber as an elastic calibration element also recalls a design philosophy very present in Citroën patents of this period, where flexible components replace classic metal springs.

Archivist's perspective

This patent memorandum illustrates an engineering research focused on sensitivity and lightness of control, two concerns that seem to run through several works of the Citroën house at the end of the 1950s. It is not explicitly signed with the name of Paul Magès in the transcribed text (the applicant mentioned being the "Société Anonyme André Citroën", p. 1); its inclusion in the collection that bears his name is explained by the 49 years he spent at Citroën, where he was a technical writer and then deputy head of the Super-Control at the time of the DS developments (autobiographical notes), which suggests that he may have contributed to it or had custody of it.

The use of an elastic rubber ring as a force detection organ, rather than a classic metal spring, seems an ingenious solution to obtain a system with low friction, consistent with the concern for sensitivity stated from the introduction p. 1-2. The curious automotive reader will see it as a probable milestone in the genealogy of Citroën power steering, the best-known outcome of which remains the DIRAVI steering of the DS.