Three-way valve with adjustable overlap — Magès patent application (20 May 1961)

In this document

Two distribution principles

The text, written by Paul Magès to support a patent application, begins by explaining the general operation of hydraulic distribution spools. Two systems are compared:

• The "open centre" system: "continuous liquid flow" and "only one user circuit per pump" as disadvantages, but great ease of pressure adjustment p. 1.
• The "closed centre" system: a single source can supply several circuits without energy loss, but pressure adjustment is much more difficult, making precise servo-control "practically impossible" p. 1.

Magès notes that, in both cases, varying the pressure from zero to maximum requires the spool to travel a distance equal to l + l', a travel deemed "always detrimental" in servo-control p. 2.

The adjustable overlap device

The stated objective is to combine the advantages of both systems by reducing this travel. Magès 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. 2.

The experimental setup

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

The test setup includes an adjustment screw (V) controlled by a comparator (C), an angular adjustment rod (T), and three ports: intake (3, high pressure P), user (5, pressure gauge U) and exhaust (4, towards a test tube E) p. 3.

Magès concludes that the "drip" condition is "absolutely not achievable in an economical way" through machining precision alone, which justifies the adjustable overlap device described in the patent application, with three examples of implementation p. 4. The document is signed "Magès", dated 20.5.61 p. 4, a date consistent with the handwritten note "20-05-61" on page 1.

Context

This text is part of Paul Magès' research into servocontrol and hydraulic servo-systems developed at Citroën, notably for the DS and its power steering or hydropneumatic suspension. Adjustable overlap spool valves fall under the technology of servovalves, a key field for the precision of automotive servo-controls of the time. This general knowledge is, however, not set out in the document itself.

Archivist's perspective

This technical text, written in support of a patent application, illustrates Magès' working method: starting from a theoretical principle, then validating it through rigorous and quantified experimentation (clearance of a few microns, travel of a few hundredths of a millimetre). It appears to be an explanatory document intended to clarify, for internal or legal interlocutors, the value of an invention already filed.

The diagram cited ("figure 3 of the patent") suggests that this text accompanies a more complete patent file, of which the Technical diagram sheet – Test 43204 could constitute an annex or preparatory piece, although the link between the two documents is not explicitly established here.