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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).
Analysis of marks, inclusions, and failure in a bearing race using photomicrographs.

Identifier  ExFiles\Box 114\2\  scan0055
Date  13th April 1934 guessed
  
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of one of these marks is shown on the left side of photomicrograph No. Al.009, X150. The shape of the pits is clearly indicative of the removal of slag, note also the thinner string of inclusions next to it, and the other particles in the neighbourhood.

On the left side of photomicrograph No. Al.010, is shown one end of this same transverse mark, which further confirms its origin. From this and photomicrograph No. Al.011. it will be seen that the bar from which this race was made must be classed as dirty material, and compares badly with the Ransome and Marles bearing, which is shown in photomicrograph No. Al.012. The "comet" marks would probably be caused by the dragging out of the larger solitary inclusions, or by bruising by particles, already dragged out, being rolled into the surface.

Further to confirm the nature of the transverse mark in the Hoffmann race, the specimen was polished until all signs of cavities disappeared when the line was clearly shown as slag. To do this the thickness of the race was reduced by only 0.0015".

Magnetic crack tests were also made, but showed no cracks. It may be noted also that when the fracture tests were made there was no particular tendency for the pieces to fracture through the transverse marks.

With regard to the failure of the ball-race, this is probably due to the same cause, but the piece you left with us carried only the unloaded part of the ball track, and the other pieces carrying the loaded track were not necessarily made from the same bar of steel. To enable us to confirm this point we have enquired in your office for the remainder of the race, but this has not been found.

Hl. H

Photomicrographs Nos. Al.009/10/11/12 att.
  
  


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