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Rubber• Suspension and In-line Bogie •by Pitt
Page 89
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ALOW-LOADING cranked-frame semi-trailer with independent rubber suspension, and a straight-frame semi-trailer with twin in-line axles, will be the main exhibits of Charles Pitt (Barton Stacey), Ltd., at the Commercial Motor Show at Earls Court.
Pitt are well known for their industrial and agricultural trailers, but have only recently embarked upon the production of semi-trailers. The concern are taking part in a Commercial Motor Show for the
first time. The ingenuity apparent Pitt trailing-arm rubber in the units shown should stand suspension is fitted to a the company in good stead for cranked-frame semi the specialized market which traid;lewr orfr painwteoeitn4iceomrii they intend to serve. springs are housed in the The 6-ton cranked-frame semitubular pillar at the rear.
trailer with rubber suspension is
mined the So-Lo and is primarily designed as the basis of a van. The trailing-arm suspension layout for the single wheels makes the least possible encroachment on well space, whilst the progressive characteristics of the rubber springs provide good roll resistance and a well-sprung unladenride.
_ Welded Stub Axles
Each stub axle is welded to the centre of art 8-in.-deep and 2-in.-wide forgedsteel trailing arm. Ahead of the wheel, which carries an 11.00-20-in. (14-ply) tyre. the arm is pivoted, through a large phosphor-bronze bush, on a bracket attached to an outrigger on the main rolled-steel chassis longitudinal.
At the rear, each trailing arm is turned through 90 degrees to end in a flat tongue which enters a slot in a vertical tubular pillar mounted directly behind the wheel. Inside the pillar, above and below the tongue, are Aeon rubber springs. The top spring, which reacts to upward wheel movement, is a threeconvolution type, whilst the lower Aeon rubber, which accommodates rebound forces, has a lower load capacity, and only two bellows.
The upper spring abuts against a hinged 'flap covering the top of the pillar. The position of this flap can be changed by releasing or taking up two locknuts. This alters the characteristics of the suspension to suit conditions of loading by varying the static stressing of the springs.
Adjustable Suspension
The adjustment can be carried out by the driver, the flap being screwed down for fully laden running and eased off, to soften the suspension action, when the load is removed.
Rear location for the trailing arms is provided by rubber-mounted bobbins attached to brackets on them which slide up and down steel columns.
On the suspension fitted to the semi-trailer to be exhibited, the only components requiring lubrication are the forward pivots for the trailing arms and the wheel hubs. If required. Pitt can fit self-lubricating bearings in the pivots to reduce maintenance still further:
The suspension of the So-Lo semitrailer provides only 23in. overall deflection at the stub axles, but, because of the progressive characteristics of the rubber springs, this is quite adequate. Apart from the suspension, the So-Lo design is of conventional construction with a frame made up of rolled-steel channel-section Iongitudinals and tubular cross-members. It is an all-welded fabrication and is fitted, with a Scam-nen automatic coupling forecarriage and Girling centre-pull brakes working on 31-in.-wide shoes in I51-in.-diameter brake drums.
Although this is the first production application of rubber suspension carried out bY Pitt, its 'success on six similar units already in service has led to further design studies with other axle layouts, all using Aeon rubber springs. .
Projected designs include tandem-axle bogies with independently suspended wheels (with and without inter-axle compensation), detachable rubber-sprung axles for machinery carriers and single, straight-through axle applications.
Life of the rubber springs in the SO-Lo suspension is expected to be greater than that of similar units used only as helper springs, the rubber having a better fatigue life when used constantly than when employed intermittently.
It is estimated that the springs should give some 70,000-80,090 miles before replacement. If they do fail, the vehicle is not immobilized, as the bellows merely collapse to give a harder but not unacceptable ride. Replacement is simple and relatively inexpensive.
Pitt's other design for the Show is a I2-ton straight-frame semi-trailer fitted with their own in-line-axle bogie, Each stub axle is supported on short laminated leaf springs mounted above and below its centre. The forward spring eyes pivot on Metalastik rubber bushes mounted in a fabricated box which, with stub-axle oscillation, swings within a surrounding cradle on two taper-roller bearings.
A similar pivoting block at the rear embraces the flat trailing ends of the springs which slide on detachable steel slipper plates incorporated in it. Each pivot assembly is substantially braced by a box-section beam dropped at an angle from the underside of the chassis frame.
The stub axles carry eight-stud hubs and 9.00-20 in. (12-ply) tyres. Brakedrum diameter is 151 in., and the linings are 5 in. wide. The centre-pull-type Gitlin& brake units (used on all. Pitt models) are actuated by Clayton Dewandre vacuum servos.
Novel Flooring
'The chassis frame is of melded construction and consists of rolled-steel channel-section longitudinals and crossmembers. The channel-section side raves are outrigged to an overall width of.7 ft. • 6 in. and their upper flanges are raised above the main longitudinals to permit a novel method of flooring. The transverse boards are wedged into place by tapered blocks screwed to the raves.
A proprietary fifth-wheel coupling is fitted bin the retractable landing gear is a special Pitt design. The widely spaced legs, each fitted with twin cast wheels, fold rearwards when retracted by a central worm-and-screw mechanism. The screw gear does not receive any of the forces arising from shock loads on the legs when the semi-trailer is picked up and dropped.
Such loads are carried by wheels attached to the angled landing-gear supports, which run in tracks formed by the semi-trailer's normal sub-frame, The landing-gear legs have four alternative mounting points which permit their height to be adjusted so that the .sernitrailer can be matched closely to different tractors.
The semi-trailer shown will be suitable for a platform body 23 ft. 6 in. long and 7 ft. 6 in. wide, but it will be only halfdecked to expose the underfloor Thomp son winch. Pitt will also show the rubber suspension unit, as fitted to the So-Lo trailer, and an in-line bogie as separate exhibits.