Novel Servo System Features Plastics
Page 35
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Service Tests Have Demonstrated the Efficacy of a New Valve-operating Mechanism, and of Plastic Components in the Assembly
f N our issue dated April 23, 1943, Isome details were given regarding a plastic vacuum cylinder embodied in a, servo-brake system by Feen37 and Johnson, Ltd., Ealing Road, Wembley.
The plastic cylinder was; at first, used in conjunction with a metal -piston fitted with the usual leather washer. Further developments, however, have • now resulted in the production of an assembly consisting of not only a plastic cylinder but also a plastic piston, together with a washer fabricated in a soft, wear-resistant rubberlike synthetic; this is shown in an accompanying illustration.
The entire plastic assembly, which is shown outlined in black above, now forms part of the complete unit illustrated. Many novel features are embodied, and have been made the subject of patents; two in particular are worthy of some detailed reference.
It is well known that an early fault of the original vacuum-type brake was the tendency for " snatching " to occur. Little could be done to prevent this, as pedal control through the master cylinder was, in general, insufficiently sensitive to permit " feel." Because of this, various elaborations of valve mechanism were devised, most of them incorporating some type of linkage between • the valve and the
pedal, which, whilst giving, in part, the -sensitivity required, resulted also in considerable lost motion.
The novelty of the present system lies in this, that direct reaction is possible between the vacuum cylinder
and the control valve. There is no lost motion between the pedal control and valve -mechanism, and movement may be fully regulated throughout the whole of the operating cycle. Reference to the drawing will indicate how this object is achieved. Motion through the pedal is taken through the ball end, then through the rocking end -to the sliding shaft A, whence, through an appropriate-lever system, movement is transferred to the valve mechanism as shown by the arrows. As a result of lack of restraint, made possible by the ball end, movement of the whole lever system to equilibrium by reaction becomes possible, and is such as to make precision control of the valve through the pedal readily attainable.
A further, feature of the asseinbiv illustrated lies in the ease with which the valve may be detached for examination or repair without disturbing any other part of the mechanism. The valve unit may he removed on the road in a few -seconds.
The incorporation of plastics into this unit makes possible a comparatively large saving in deadweight. Together with fixing brackets (which are adjustable for height), and the 14-in. Lockheed master cylinder, the assembly with the 8-in: vacttuna cylinder weighs, in all, only 21 lb. A unit of this type his now been in use on a lorry carrying a very heavy load, day in and day out for 12 months; and has given every satisfaction. No wear of the vacuum cylinder has • occurred and the rubber-like plastic washer has proved in no way inferior to leather.
Purely from the engineering standpoint, the use of non-metallic materials in this assembly is of the highest possible interest. Some attempt was made in the previous account to assess the service conditions which would have to be encountered by the plastic parts described, and reference was made to empirical tests, designed to indicate the probable behaviour of those materials under shock loading.
Only by actual field tests, however, was it considered possible to determine wear resistance in the vacuum cylinder and in the washer, and resistance to bend stresses in the case of the plastic piston. Actual running tests have noii shown that the particular plastics selected. for each of these component have proved quite adequate to. meet all demands imposed upon them.