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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 memorandum discussing the performance and issues with 'SS' brakes and servo mechanisms.

Identifier  ExFiles\Box 16\3\  Scan028
Date  12th December 1928
  
To DA.{Bernard Day - Chassis Design} from Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer}
c. to BY.{R.W. Bailey - Chief Engineer}
c. to Wor.{Arthur Wormald - General Works Manager}

+7270

Hs{Lord Ernest Hives - Chair}/RmL/LG18.12.28.

'SS{S. Smith}' BRAKES. +7420

The two chassis we have so far tried with brake leverages to N.Sch.2539 and servo arrangement to N.Sch.2538 both indicated that the servo was not doing sufficient work.

On N.Sch.2539 the servo torque multiplication was taken as 2.5. the present servo multiplication on the 40/50 is 2.

We do not know what coefficient of friction was assumed in evolving this torque multiplication, but we cannot quite see how the servo shown on N.Sch.2538 can be expected to give 20% greater torque multiplication than the standard Phantom servo to G.72550.

One point where the two servos differ apart from leverage is the radius of the driving pins. Those on the 'SS{S. Smith}' servo are at 1.900" rad; those on the Phantom 2.375".

It will be remembered that the first plate servo wwe made suffered from the fact that the torque was taken on dogs at a small radius and in consequence as the output required was increased the servo became self-locking. The dogs were then altered to pins at 2.375" radius with a very large increase in efficiency, but these pins still have a marked effect on the μ which can be assumed for the servo because they cannot be lubricated.

Evidence of this is that on test the servo shews a higher efficiency under very rapid application than under slow application.

On the road this is confirmed by the fact that sometimes with the old brake distribution it was
contd :-
  
  


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