Brake Design and Servicing
Page 50
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Girling Engineer Covers 20 Years of Brake Development, Indicates Modern Tendencies and Emphasizes Importance of Maintenance
A; S stated by Mr. J. W. Kinchin„ director and chief engineer, Girling, Ltd., his paper, "The Trend of Brake Design as it Affects Service," read before the institute of Road Transport Engineers in London, last week, lid not lay down specific methods of service, because ser. 'Acing as such was so. involved with the type of vehicle, the ierrain and the duties carried out. Instead, it. reviewed braking equipment, pointed out faults, traced the .changes in equipment and servicing problems, and forecast what braking equipment would be in five to 10 years.' .. Until about 1932; the majority of vehicles used simple eam brakes mechanically operated. From about 6 tons gross Upwards they had vacuum servos, hydraulic actuation was hardly used; almost all employing rods, but a few had Bendix self-energizing shoes with cables. The servo systems were either the single, master vacuum cylinder or the triple
• ;ervo—one cylinder for the rear brakes and one for each a: he front.
From about 1937-3,the two-leading-shoe brake • was introduced, and by 1939 a number of lorries ,up to 91-ton gross was using some proprietary make. Until then nearly all brakes had been made by vehicle or axle manufacturers. Then came •brake, specialization by prOprietary companies, and by 1939 most medium vehicles were using the twoleading-shoe, with hydraulic operation. This added complication and involved more service. With this brake and the pedal effort aCceptableiv.Was possible, with '91, ton's, 'to delete the servo, whiet.;•Was idVantageous onoil engines by dispensing pith the exhauster.
Two-leading-shoes were introduced On a few vehicle's tip : io 12-ton gross, with, hydraulic actuation and servo, usually :. • vacuum. One or two makers produced 19-ton six-wheelers 3r 22-ton eight-wheelers with this system, but such vehicles mostly used fixed carn.brakes.. After the war, similar conditions prevailed for a year or so, but brake makers introduced a new two-leading-shoe type for commercial vehicles. In this the front brake was operated by two separate internal wheel cylinders instead of an external pusher hydraulic cylinder. The internal design reduced the parts and heat did not prove detrimental to the seals or cause fluid vaporization, although this problem had not been solved with rear brakes. Development could be assisted by a transmission hand brake. .
Bendix and Girling Designs
The Bendix two-leading-shoe had an expander operating one shoe by tappet, the other tappet controlling bell-cranks on the trailing shoe, the bell-cranks connecting the trailing end of one shoe to the leading end of the other by a strut. Strut adjustment for machining tolerances was by eccentric at the bell-cranks. The adjuster unit was of the wedge type. held in clearance holes on the hack plate, permitting the brake to be centralized.
The Girling design differed, using a standard-type shoe for leading, the special shoe being a carrier in which the faced shoe was mounted. Load was applied via the carrier through four rollers pivoting on. pins in the carrier, reaction of the faced shoe being taken by a torque plate on the back plate. Both shoes had fixed pivots. Operation was by wedge-androller, slidably mounted on the back plate for centralization, the adjuster setholts being fitted into clearance holes in the hack plate. Shoe alignment in both Bendix and Girling designs was by adjustable, screwed steady posts. With the Bendix and -Girling merger, came a unified design, the expander unit being changed to free-floating roller, but the unit was spigoted to the back plate and centralized by mounting the operating wedge sliclably in the brake-operating plunger. The shoes were slidable, and the special shoe strut-operated, with adjustment of strut and bell-cranks altered to screwed adjustment only for the strut.
The new expander unit .was a step forward, not open to abuse by leaving the castellated nuts so loose that the assembly could drop into the drum, or so tight that' no float was available.
B16 Largely as the result of overseas experience, designers were forced to concentrate on overcoming certain troubles. The most important were, facing wear and period between adjustment Another was brake fade, particularly in . Australia and South Africa, where long descents and heat were experienced., It was a function of facing area and heat dissipation. As the latter could be only slightly improved, the major contribution was increased facing area. Overseas users also demanded reduced pedal effort, involving greater servo. effect.
." Generally, the trend up to about 12 tons gross was to retain two-leading-shoes at front and rear, introduce front . operation by internal cylinders, increase facing width to reduce fade, raise the available volume of facing, and increase the shoe centre lift for the full stroke of the pedal.
Present practice Was to increase the shoe centre movement to between 0.08. and 0.115 in. about doubling the period between adjustments. it appeared that harder, moulded facings were more severe on drums, and drum life might not increase with decreased loading of facings through using larger areas. Drum cooling had improved by some makers adopting 15i-in. drums in place of 161-in., giving more air space and stiffer drums if required, which also provided greater heat storage.
New Model for Heavies ,There was available a heavy-duty, two-leading-shoe brake speciaBy" designed to give strength, rigidity and long life.
It -_had a single, master vacuum servo. Actuation was hydraulic, ,by air pressure direct on the brakes, or by hydraulic constant-flow system. The last would find increasing use because of smaller size, lighter weight, and potentially lower cost than' an air system. Compared with the vacuum, its use should increase, for, with the high shoe centre movement and low pedal effort now required, a much larger vacuum servo was needed but difficult to locate. The constant-flow pump was relatively small.
Coach and bus makers remained faithful to the fixed cam, often with air operation, but the author believed that the hydraulic constant-flow system would spread to these.
On medium-weight goods vehicles, the internal cylinder two-leading-shoe front brake gave two trailing shoes in reverse, increasing the pedal effort required to prevent running backwards, so Most makers had fitted two-leadingshoe rear brakes in both directions to give lighter effort. This caused additional complication at the rear, although simplifying the front.
The latest systems were, in many cases, more complicated, and the emphasis would be on a proper educational programme to train maintenance personnel.
It seemed that for the next few years brakes requiring servicing would be covered by the following:—Lockheed Phases II and III, Girling single web, Girling twin web. both wedge-operated and cam-operated. The author described these and gave the methods of adjustment.
As to hand brakes, he had found that many complaints were due to wrong setting of the linkage. The transmission brake was widely ;Ised in America, and its introduction in drum or disc form would have definite advantages. It must, however, be used only as a parking brake or in case of failure of the main type, because it could not be arranged with sufficient area to deal with the heat dissipation involved in stopping from speed—it would take torque, but not energy.
Interesting observations were made on bonded facings cemented to the shoes. In these, when requiring renewal, the facings were burnt off at a temperature of 450 degrees C. After refacing, proof tests showed a safety factor of 3 to I, whilst the destruction test was on a factor of safety of 12-17 to 1, but the belief that every facing can be bonded satisfactorily was a fallacy.
He concluded the paper by summarizing the brake service which should be necessary on different classes of vehicle.