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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).
With a diagram explaining the operation and advantages of a two-speed supercharger.

Identifier  ExFiles\Box 173\3\  img005
Date  10th March 1934
  
To: Mr. Hives
From: Mr. Robotham

THE BENDIX AVIATION CORPORATION

Two Speed Superchargers

[Diagram annotations]
Free wheel clutch
Low Speed Drive
High Speed Drive
Centrifugal Clutch fitted as in our drive
Centrifugal clutch normally held out by manual control up to 5000 ft.

[Handwritten text]
SGWOA R G E By HDY{William Hardy}
LR{Mr Ellor}/DR/HS{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}

Above is a diagrammatic sketch of drive employed. The low speed drive is operated through a free wheel clutch. As soon as the high speed clutch is engaged this gear runs in free wheel. The high speed drive shaft is actually duplicated, the two shafts being at 180 degrees and the gears fitted with centrifugal clutches so that a balanced load is obtained as in our case, and the bearings are saved.

The claim of the two-speed supercharger is that, by using a low speed drive up to 5000 ft. about 50 more H.P. are available than if a high speed drive were used and the blower throttled to give the same maximum permissible boost as with the low speed blower. It has been demanded by the high speed commercial transport machines to enable them to get above storms, icing conditions and contrary winds without sacrificing take-off. Also they can get a higher cruising speed for the same B.H.P. and petrol consumption at 15,000 ft. with the V.P. propellor than they can at 5000 ft. The two-speed supercharger is used in conjunction with boost control.
  
  


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