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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Laboratory report on a broken front road spring mainplate from a 25/30 HP chassis.

Identifier  ExFiles\Box 107\3\  scan0112
Date  31th January 1938
  
To BY.{R.W. Bailey - Chief Engineer}
c. Rm.{William Robotham - Chief Engineer}
c. Ds.

H1/EJW.5/GS.31.1.38.

21.

LABORATORY.

[Signature]
[Stamped: KENNINGTON]

BROKEN FRONT ROAD SPRING MAINPLATE -
25/30 HP. CHASSIS. GUN-34.

Your Ref. BY/NS.{Norman Scott}15/G.14.1.38.

This spring was broken through the front eye, as shown in photo. M.1,521/1 having run 8741 miles.

The fracture - photo. M.1,521/2 - started from the surface inside the eye at "X" and developed as a fatigue failure.

The surface at "X" was not visibly decarburised, but its hardness was 376 Brinell as against 394 Brinell in the interior of the plate, indicating partial decarburisation.

The outer surface of the eye was visibly decarburised to a depth of 0.005", but the flat part of the spring examined a few inches away from the eye was not decarburised on either side.

The inner surface near the fracture contained a number of small transverse cracks - photo. M.1,521/3.

A longitudinal section of the spring through these cracks is shown in photomicrograph M.1,521/4.

Similar cracks were found inside the eye at the unbroken end of the spring - photo. M.1,521/6.

Some of the cracks at the broken end are shown, opened up, in photo. M.1,521/5. Owing to the shape of the fracture only those in the centre of the photo. can be clearly seen - they are of half-moon shape, developed in stages.

The analysis and micro-structure confirm that the material is normal.

There was no oxide visible in the cracks.

Definite evidence was found of corrosion of the steel inside the eyes. This did not extend to the ends of the eyes, and the cracks were all within the corroded area, and associated with corrosion pits - see M.1,521/7.

We have considered the possibility of the cracks having been formed at an early stage in manufacture, but consider that there is little evidence to support this theory, and we have therefore reached the following :-

CONCLUSIONS.

(a). That the spring has failed at the point of maximum stress within the eye, as a result of corrosion fatigue.

(b). That the corrosion is due to ingress of moisture to parts of the eye from the open side, and to the electrolytic couple existing between the bronze bush and the steel.

H1/EJW [Signature]
  
  


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