Calculation note: wall thickness of the hydropneumatic accumulator (1953)

A technical calculation sheet establishing the minimum wall thickness of the Citroën hydropneumatic accumulator and the strength of its threading, based on the burst pressure and the metal used.

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Understanding this document

In this document

Calculation of wall thickness

The sheet presents a technical formula to establish the minimum thickness (e) of the accumulator wall, based on the burst pressure (P), the internal diameter (Di), the tensile strength of the metal (R) and the mean diameter (Dm) p. 4.

The metal selected is designated X C 42, with a tensile strength R = 68 kg/mm² and an elongation A ≥ 15 % p. 4.

For an internal diameter Di = 116 mm and a pressure P = 560 kg/cm² (obtained by applying a safety factor of 3.5 to a pressure of 160 kg/cm²), the calculation results in a minimum thickness of 2.5 mm p. 4.

Thread strength

A second part calculates the pull-out force between the two hemispheres of the accumulator, based on the diameter and pressure, then the working surface of the threads under shear p. 4.

The parameters selected are an engaged thread length h = 10 mm and a root diameter of the upper hemisphere thread Df = 117 mm p. 4.

The working rate of the thread at the minimum wall burst pressure is calculated at 16.1 kg/mm², which the document describes as a "very wide safety factor for the thread" p. 4.

Quantity	Value	Page
Resistance R (metal X C 42)	68 kg/mm²	p. 4
Elongation A	≥ 15 %	p. 4
Internal diameter Di	116 mm	p. 4
Pressure P (at burst, with coeff. 3.5)	560 kg/cm²	p. 4
Calculated min. thickness	2.5 mm	p. 4
Engaged thread length h	10 mm	p. 4
Root diameter Df	117 mm	p. 4
Thread working rate	16.1 kg/mm²	p. 4

Context

The hydropneumatic accumulator is one of the key components of the suspension invented by Paul Magès for Citroën, which originated the systems equipping the DS in particular from 1955. It combines a metal sphere and a membrane separating gas and liquid under pressure, which requires precise dimensioning of the wall and fixing elements to ensure safety at high pressure.

This type of calculation note is part of classic mechanical engineering work aimed at validating, before industrialisation, the resistance of a part subjected to high stresses — a standard procedure in automotive design offices of the time.

Archivist's perspective

This document illustrates the meticulous preparatory work necessary for the development of the hydropneumatic accumulator, before its regulatory validation. It seems to belong to the same period as the Letter from Citroën to the Mines (12 March 1953) and the Regulatory descriptive statement of the Citroën hydropneumatic sphere, both dated 1953 and concerning the same pressure vessel.

The rigour of the safety margins adopted (coefficient 3.5 on pressure, thread working rate deemed "very wide") probably testifies to the concern for compliance with the regulatory requirements for "pressure vessels" of the time, in a context where any failure could have serious consequences for vehicle safety.