Patent application file 4.379: the three-way valve with adjustable overlap (20 May 1961)

In this document

The problem: two valve systems, two defects

The text, written to accompany a patent application (file 4.379), begins by pedagogically explaining the operation of existing hydraulic distribution spools.

The "open centre" system has a continuous flow and can only supply one circuit per pump, but it allows for easy pressure adjustment: the pressure in the application U goes from zero to maximum over a stroke equal to l + l' p. 34.

The "closed centre" system, on the other hand, can supply several circuits without energy loss, but its pressure adjustment is much more difficult, making "the use of such a process for precise servo-control practically impossible" p. 34.

In both cases, the total stroke required to vary U from zero to maximum remains l + l', which is considered "always harmful in the case of servo-control" p. 35.

The objective and the principle of adjustable overlap

The stated goal is to combine the advantages of both systems without their defects, by reducing the necessary stroke. A handwritten note specifies that the "overlap" depends not only on the difference L - L', but also on the clearance e and the shape of the edges A and A' p. 35.

A test distributor is machined according to figure 3 of the patent: spool of 6.35 mm in diameter (1/4 inch), with a clearance in the bore between 1 and 2 microns p. 35.

The test setup

The setup includes a screw V adjusting the longitudinal position of the spool, measured by a dial gauge C, and a rod T adjusting its angular position to modify the overlap. Port 3 is connected to high pressure (manometer P), port 5 to the application (manometer U), port 4 to the exhaust towards a test tube E p. 36.

The results

By adjusting the rod T to the "drip" position, the pressure U gradually goes from zero to the maximum pressure — 150 kg/cm² in this experiment — in less than 5/100 of a millimetre, without increased leakage. By further increasing the overlap (total sealing, "closed centre"), the spool must be moved by more than 20/100 mm, with less precision p. 37.

The document concludes that the chosen adjustment solution, described in the patent application with three examples of implementation, allows for obtaining and fine-tuning this "drip" condition, which is impossible to achieve economically through simple machining precision. The note is signed "Magès", dated 20.5.61 p. 37. This work is part of the long career of Paul Magès at the Citroën design office, which he joined in 1925 and where he remained for 49 years, until his retirement in 1974 (autobiographical notes ; photographs of the retirement).

Context

As a general reference, spool valves and fine hydraulic pressure adjustment are key components of the hydropneumatic suspension and steering systems developed by Citroën from the 1950s onwards, notably on the DS — presented on 6 October 1955, a few years before this 1961 patent application. The care taken here for micrometric precision (clearances of the order of a micron, strokes of a few hundredths of a millimetre) illustrates the requirements for reliability and sensitivity sought for the hydraulic servo-controls of the time, without the document itself referring to a specific vehicle.

Archivist's perspective

This document likely has the value of a technical supporting document intended to support the drafting of the patent text: it is not the patent itself but a pedagogical, almost didactic explanation of the problem solved, written and signed by Paul Magès p. 37.

The main interest lies in the quantified experimental demonstration — clearance of a few microns, stroke of a few hundredths of a millimetre — which makes the contribution of the "adjustable overlap" device tangible compared to conventional machining tolerances, deemed insufficient here.

This three-way valve file is part of the same vein of research on hydraulic regulation components found in other pieces in the box, such as the 2nd addition to patent P.685.691 on a flow regulator with perforated discs, and could be compared to the summary of the "Hydraulic components" folder to re-situate its place in the overall file.