Air Springs for Twin In-line Axles
Page 72
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THOUGHT to be the first application
1 in this country of air suspension to a semi-trailer bogie employing twin axles in side-by-side formation, the new layout by Taskers of Andover (1932), Ltd., to be shown on a 15-ton semi-trailer at the Commercial Motor Show is robust and simple.
The short axles are clamped to tubular beams with plain bushed bearings interposed between the two members to allow the axles to oscillate without transferring torsional stresses. Of 51-in. outside diameter, the tubes of the trailing beams have 3i-in. dianleter tubular cross-pieces passed through them and welded in position at their forward ends to form T-shaped structures. A full-width channel-section cross-member is suspended below the main frame of the semi-trailer and braced to it by struts at wheel-centre height at the front of the bogie.
Bolted to this . cross-member are two pairs of brackets, each pair fitted with Silentbloc bushes which carry the T members, thus locating them against forces arising from deceleration, acceleration and side thrust, as well as allowing the beams to pivot as the axles rise and fall.
Canister Air Springs
Single canister-type air springs are mounted at the other ends of the trailing beams. As the axle is mounted in the centre of the beam, deflection of the air spring is exactly twice that of the axle.
The springs used are supplied by the Andre Rubber Co., Ltd., and built to a design originating from the General Tire and Rubber Co., of America. Each spring takes the form of a sleeve made. of Neoprene, reinforced by nylon cord with beaded edges at top and bottom.
At its top each sleeve is joined to a plate welded to the lower flange of the chassis side member, the beaded:edge of the air bag being clamped into a circular. channel. A 'hole in the plate allows for the entry and exit of air while levelling is taking place and a solid annular rubber ring is fitted to the plate as the top half of a bump stop.
The opening at the bottom of the sleeve is clamped to a steel canister attached by two bolts to a platform on the trailing beam. The canister provides additional air capacity for the spring and the rubber sleeve rolls down over it on deflection. A metal gaiter around the main portion of the air bag prevents expansion, maintaining a diameter of 11 in.
Where the bag joins the canister, however, it is not restricted and bulges out to a diameter of 13 in., forming an annular lobe. It is this lobe which rolls down over the canister, thus permitting a change of volume and, in the case of the present Taskers system, giving a frequency of about 100 c.p.m. laden.
Bolted to the bottom of the canister c:36 is a tube carrying a circular platform which, on extreme deflection, contacts the annular ring at the top of the spring as a bump stop.
The springs have a normal set height of 121 in., with a closed height of 8 in, against the bump stop and a fully extended dimension of 17iin. in the maximum rebound condition. They operate at an internal pressure of about 60 p.s.i., giving them a direct load-carrying capacity of about 2f tons each. Their effective capacity is doubled by the geometry of the layout to give the complete bogie a capacity of 11 tons at this
pressure. •
Air for the suspension is provided.from a separate reservoir fed from the tank for the semi-trailer air-pressure braking system through non-return and regulator valves of Bendix-Westinghouse manufacture. The regulator valve is set to allow air to pass only when the pressure available for the brakes has reached 60 p.s.i.
Combined height-control and levelling valves are operated by a linkage taken from each trailing beam forward of the, axle. These valves are also supplied by Westinghouse and are set to give direct action without any delay.
Armstrong telescopic dampers mounted in pairs at the extreme end of each trailing beam control the air springs. They have a 12-in stroke and act also as rebound stops for the suspension.
A notable feature of the straight-frame semi-trailer to which the bogie will be fitted is the incorporation of the new Bendix-Westinghouse magnetic braking system, designed to synchronize tractor and semi-trailer braking and eliminate delay. It was described in The Commercial Motor on August 15.