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New Servo System for hydraulic brakes

2nd February 1934
Page 37
Page 37, 2nd February 1934 — New Servo System for hydraulic brakes
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Latest Feeny and Johnson Design Utilizes Vacuum Power, with Mechanical Linkage, Only when Severe Braking is Required

CQNS1DERABLE interest s thaws to a new vacuum-hydraulic brake system evolved by Feeny and Johnson, Ltd., 139-136, Ealing Road, Wembley, the aim of which has been to utilize an hydraulic transmission applied by the physical effort of the driver, and to have available the extra power of a vacuum servo when greater retarding effort is required. At the same time, it was not desired to stress the hydraulic system by subjecting it to the power exerted by the vacuum application.

The system evolved affords all the benefits of smoothness and balanced effort which hydraulic transmission ensures, plus a powerful vacuum application through a mechanical linkage whenever it is necessary to use it. In modern hydraulic brake systems, the practice of employing externally mounted application cylinders acting upon ordinary brake camshafts is being extensively used, and this makes an adaptation to the new Feeny and Johnson layout an easy matter, the mechanical linkage to the camshaft lever from the Marelli vacuum bellows' offering no difficulties. Neither is it necessary to interfere with the master cylinder of the hydraulic transmission system.

All that. is done is to take one more pipe, lead from the _ master cylinder to a vabie controlling the vacuum, and this valve (clearly explained in our inset drawing) is the well-known Feeny and Johiison unit, which, by the principle ofconstant 'leak and variable vacuum, has for long-been so successfully used with the Itlatelli 'Yam= brake.

The valve itself is admirably simple, and nothing could be less .complicated than the incorpbratiOn . with • it of hydrauliC control. The valve is held closed by a coil compression spring, the resistance of which is adjusted by a screw at any desired value. Thus when fluid pressure in the pipe lines to the wheel brakes exceeds this value, the vacuum valve is opened, the bellows exhausted, and the effort on the brake-camshaft levers is augmented by the pull of the bellows acting through • mechanical linkage—not through the hydraulic mechanism.

The power exerted by the bellows varies, of course, according to the degree of pressure (above the predetermined value) exerted by the driver. This is due to the patented design of the Feeny and Johnson valve, and the desirable graduation is obtained just as well when the valve is controlled by hydraulic means as when it is directly linked to the driver's pedal.

The system is intended primarily for incorporation by chassis manufacturers, but in cases where the 'hydraulic-brake system has actuating cylinders within the brake drums, it is still possible to effect a conversion. In this case, the vacuum bellows are Mechanically linked up to the band-brake shoes; the hand-brake linkage is not removed, but a lost-motion device is incorporated. The result is that a moderate pressure on the pedal applies the hydraulic brakes and a further pressure causes vacuum operation of the handbrake shoes through the mechanical linkage.

Whilst the new system, which is patented in the principal countries, is, on the face of it, ingenious, the outstanding feature is its simplicity and conformity to sound principles. The patent No. is 903,668.

We have had an opportunity to make some tests of the layout as applied to the new Dennis 95-cwt. chassis, and were pleased with the positive yet smooth change-over from hydraulic to racuum operation, also with the high maximum power.

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