Study of a permanent magnet hydraulic pressure regulator (January-April 1964)

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Theoretical study of the pressure regulator (January 1964)

The file opens with a manuscript note dated 30 January 1964, titled Disjoncteurs Conjoncteurs Hydraulique p. 48. Magès recalls the principle of these components: to limit the maximum pressure of a hydraulic circuit by directing the pump flow either to the reservoir (cut-out) or to the working circuit or an accumulator (cut-in) starting from a minimum pressure. He insists on an essential condition: "to be unable at any moment to obtain an equilibrium position" at the moment of switching, to avoid unnecessary heating of the fluid. The solution studied here relies on "the very rapid variation in the lifting power of permanent magnets," deemed "of great simplicity of construction" p. 48.

A page of calculations and sketches from the same day ("Lunch at Armant", 30-01-64, p. 46) explores part dimensions, spring forces, and a "magnet force" of 5 kg, in connection with a cross-sectional drawing of a valve/cylinder assembly.

Questions posed to the magnet supplier

Several pages (p. 50-51, p. 56) cover, in the form of a draft and then a structured questionnaire, the unknowns to be resolved with the Établissements Giffey-Prêtre: shape of the magnet ("circular or bar?"), dimensions and weight for a lifting power of 4 kg then 8 kg, lifting power curve as a function of the air gap, manufacturing dispersion (± 5%), variation with temperature, durability over time, shock resistance, operation in oil, lifting power on one side versus the other, and price for production rates of 500, 1000, or 1500 pieces per day p. 50-51.

On 3 February 1964, a "Giffey-Prêtre" note calculates the balance of forces: with a 15 kg spring and a 5 kg magnet, the force required to break the equilibrium is 20 kg; if the spring increases to 21 kg, the holding force becomes 16 kg p. 50-51, 56.

Spring and spool calculations

Pages of calculations p. 52, 54, 58 detail the dimensioning of a spring (wire diameter, deflection per coil, number of coils) and a "4 mm diameter spool, 12.5 mm² section." A table p. 58 gives the equilibrium force for two pressures:

The same sheet proposes a table relating the spring stiffness (R, R') and the load increase according to the stroke e, as well as an estimate of thermal variation of "0.2% per degree C" p. 58.

Annexed documents

A provisioning voucher from Ets Giffey-Prêtre, dated 6 April 1964, confirms the free delivery of "2 Suction cups 7 kg" to Monsieur Thaon, "for Mr Magès" p. 47. Two dimensioned plans (tube section, drilled circular part) appear on p. 49, and a structural blueprint ("Motor axis", "top of side member") on p. 57 — without explicit textual link to the magnet study. Finally, two pages extracted from the journal Precision by Lockheed (October 1963) present self-adjusting disc and drum brakes, a tandem master cylinder, brake boosters, and a Purolator fuel filter p. 53, 55.

Context

The hydraulic pressure regulator is a cornerstone of the high-pressure circuits of the Citroën DS, regulating the supply to the suspension, steering, and braking. Paul Magès had already filed the principle as early as 1953 (see Principe du conjoncteur-disjoncteur hydraulique, 1953). The permanent magnets mentioned here — likely ferrite, a material then booming for its robustness and low cost — offer a brutal shift in force at a very short distance, a property sought to avoid the unstable equilibrium positions mentioned in the note of 30 January. The attached Lockheed journals, a British company then a reference in automotive braking, illustrate the industrial context of technical monitoring in which the Citroën design office participated, with no direct link demonstrated here to the magnet under study.

Archivist's perspective

This file documents a precise and dated research path with beautiful continuity (30 January to 6 April 1964): improving a component already invented by Magès in 1953 by exploring a new technology, the permanent magnet. The transition from a still-hesitant questionnaire p. 50-51 to more assured force and spring calculations p. 56, 58 provides a rare glimpse into the engineer's experimental process — it seems the idea was tested up to the stage of ordering real components (the "suction cups" delivered on 6 April, p. 47).

The presence of Lockheed clippings from October 1963 suggests broader technological monitoring, without it being possible to affirm, on the sole basis of the document, a direct link with the magnet study. This file usefully complements the Principe du conjoncteur-disjoncteur hydraulique (7 October 1953), by showing, a decade later, an attempt to evolve its design.