Magès study: 3-way distribution valve with adjustable overlap (26 June 1961)

In this document

The problem posed

Paul Magès explains that fluid distribution valves (spools) exist in two known types: the "open centre" spool and the "closed centre" spool. He notes that "specialists know the advantages and disadvantages of each of these systems", and that it would be interesting to create a spool located "at the exact limit of the two principles", but that in high pressure this is "practically impossible while remaining with the designs usually used" p. 31.

The proposed solution: the two-piece spool

Magès defines a geometric convention: when L < l, the device is said to be "open centre"; when L > l, it is said to be "closed centre" p. 30-31. His proposal consists of making the spool in two pieces T1 and T2 joined by a screw V, which allows for precise adjustment of l relative to L to obtain "an intermediate system between the open centre and the closed centre" p. 31.

A spring R, described as a "braided spring", has the function of taking up the play of the threads of screw V in spools T1 and T2 and preventing any maladjustment once the adjustment has been made by rotation p. 31-32. The number of threads of screw V is voluntarily reduced to a minimum, with play calculated so that "no stress is transmitted to the cylindrical part" of the spools sliding in the bore p. 32.

A second variant

Another "way to solve the problem" is presented: spring R pushes spools T1 and T2 apart, placing them in contact with yokes P1 and P2 via the adjustment screw V, with two or three rods M ensuring the connection between P1 and P2 p. 32. This variant allows, if T2 is larger than T1, to obtain on the moving assembly "a reaction F proportional to the pressure obtained in V" — a result deemed difficult to achieve with a single-piece spool p. 32-33.

Technical justification

Magès concludes that the major difficulty of a single-piece spool is the concentricity of the two bores, "practically impossible to obtain within the desired tolerances" in high pressure; the two-piece spool makes this constraint "practically" disappear p. 33.

Page 31 bears a handwritten note: "Photocopy given to Mr. Elivart. Seen and accepted Bletry project 2.9.61", as well as the archival reference "P 2895".

Context

This study is part of Paul Magès' broader work on high-pressure hydraulic circuits developed at Citroën for the hydropneumatic suspension and associated components (braking, steering) of the DS. Spool distribution valves are classic components of the hydraulic robotics of the time, and the "open centre / closed centre" dilemma is a generic problem of servovalve design — not specific to Citroën but central to the precision and responsiveness of onboard hydraulic systems.

Archivist's perspective

Dated and signed twice by Magès (p. 30 and p. 33), this study illustrates his working method: posing a theoretical problem known to "specialists", then providing a simple mechanical solution — here the two-piece screwed assembly — which bypasses a machining constraint deemed insurmountable in a single piece.

The note "Seen and accepted Bletry project 2.9.61" p. 31 suggests that this is an internal working note, probably validated with a view to development or a patent filing, even if no patent number is mentioned in these pages.

This document seems to belong to the same vein of fine engineering of hydraulic components found in the folder "Hydraulic components", and can be compared with the principle of the hydraulic pressure regulator, another cornerstone of pressure regulation designed by Magès for the same circuits.