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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).
Technical discussion on tank gearbox design, comparing existing systems with a proposed 5-speed synchromesh gearbox for the Merlin engine.

Identifier  ExFiles\Box 144\4\  scan0184
Date  10th December 1940 guessed
  
2.

in that the bulk of its resistance to motion is in the form of tractive resistance, which, unlike windage loss, does not vary with speed. The tractive resistance does however, vary greatly with the nature of the ground. Thus a tank which is geared for maximum speed on the road in top gear may be unable to proceed at all in this gear on grassland with a tractive resistance only 25% greater than the road, but could maintain 80% of the road speed in a transmission ratio 25% lower than top gear.

It follows that in service, gear changing is frequent and it is desirable to have a large number of ratios in the gearbox with small steps between the upper ones. It also follows that the tank may spend a large proportion of its life in any one of the three upper ratios in the gearbox.

Existing cruiser tank designs have straightforward 4 forward speed crash type gearboxes. They differ from the conventional car gearbox in that one of the two shafts in the box is driven by the engine and the other drives the two track sprockets. Thus there is no direct gear. This is because silence is of no importance and it cannot be said that the direct ratio will be used for 90% of the time as in the case of the road vehicle. The shaft driven by the engine is arranged to run at about 1/3 engine speed with the result that the gearbox has to be proportioned to deal with large torques, and is correspondingly large and heavy for the power transmitted.

The gearbox we are projecting for use with the Merlin is identical in general layout with the existing types, but differs in having 5 forward speeds instead of 4, Phantom III type syncromesh on the upper 4 ratios, and an input shaft speed equal to engine speed instead of one third engine speed. It is characteristic of gearbox design that when syncromesh is incorporated on all but 1st. and Reverse, only a small number of additional parts are required to give one additional ratio over an even number of forward ratios. There is therefore, little point in our designing a four forward speed box and the additional ratio is bound to improve the performance of the tank.

The existing gearbox and steering mechanism for use with the 320 h.p. Liberty engine weighs 1400 lbs with alum castings. We anticipate that our gearbox and steering mechanism for 450 h.p. would weigh 900 lbs. if made in the same materials. We attach a table showing the gear ratios we have selected against those of the existing Mark VI tanks. This table is compiled on the basis of the following figures :-
  
  


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