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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).
Hardness of AC 7 production bearings, comparing them to AC 6 alloy and proposing new acceptance limits.

Identifier  ExFiles\Box 115\3\  scan0158
Date  18th December 1934
  
Swell - I should agree to pass 65 Brinell. what do you say. Rm.{William Robotham - Chief Engineer}
To BY.{R.W. Bailey - Chief Engineer} from H.I.
c. Rm.{William Robotham - Chief Engineer}
Laboratory Odee 18.12.34.
1020.
AC 7 PRODUCTION BEARINGS.
Mr. Talbot reports that several batches of bearings produced from the AC 7 ingot supplied by High Duty Alloys have been giving hardness figures above 55, which was taken to be maximum for this material.
It is to be noted, however, that the experience of the Experimental Department is that AC 6 is a better material for Bentley bearings etc. than AC 7.
The production AC 7 bearings which are coming out hard appear from analysis to be at about top limit in magnesium, i.e. about 0.5% as compared with about 0.4% in normal AC 7 of the past and 0.6 to 0.9% magnesium in AC 6.
AC 6 was, I understand from extensive Experimental Test, less liable to "hammering out" than AC 7.
In consideration of the above I feel that we should be quite safe in allowing AC 7, up to about 65, to pass for production (AC 6 was passed for experimental use from 65 to 78 Brinell). These two alloys are, other than in their magnesium contents, of the same composition.
It is to be noted that the magnesium maximum on ingot supplied was increased from .40 to .50% recently, in view of the tendency to loss in melting; also that the heat treatment temperature has been raised about 15°C, both these changes being made to avoid undue softness.
H.I.
HC
  
  


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