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2 Selecting materials

5th July 1968, Page 71
5th July 1968
Page 71
Page 72
Page 71, 5th July 1968 — 2 Selecting materials
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and, finishes

THE SELECTION of correct material in construction is largely determined by the information available from the work specification.

Accepting the basic premise that dead weight is unprofitable, it follows that the body weight must be as low as possible, but only in so far that it is related to safe and efficient transport.

Furthermore in selecting materials one must assume that each material will be applied in the best engineering and practical capacity and should not be selected because "it has always been done that way. One example of this approach can be seen in a pantechnicon or van body of 1,000 cu. ft. capacity. The floor, forward bulkhead, tailboard and body-sides will, in general, take a severe pounding. Therefore if the floor and bulkhead are of timber, replacement of damaged floorboards and bulkhead boards will be a simple repair which will not result in loss of use.

Wood is heavy but lightweight sides can be built by using light-alloy extrusions for the actual framing. For stability the pillars are tied securely together by a series of 3in. wide hardwood slats spaced at four to five intervals along the full length of the body. The benefit of such a construction is that it uses the light weight of the alloy, the impact strength of the hardwood, and is easy to repair.

If the work specification indicates the need for a heavy vehicle fitted with platform bodywork and required to make regular longdistance trips along motorways with loads of long-bar steel or pipes, or divided loads, then in selecting the materials for construction, particular attention must be given to the forward bulkhead. Such a load, moving forward in an uncontrolled fashion following violent braking from speed can be disastrous for the driving cab and driver.

This means that the bulkhead construction must be of steel capable of withstanding the forces, and must be so attached to the underframing that it will not tear away. So, what is of greater importance than overall bodywork weight is the use of steel sections. They must be welded into an engineered structure in which the forward part of the load is used to assist in maintaining the bulkhead in an upright position under forward strain. This calls for 6 x 4 timber cross-bearers set on full-length runners, steel angle-floor edging raves and ends welded together and bolted right through the extremes of each crossbearer. They form the basis for the steel bulkhead construction extending beneath the body and secured directly to the chassis with +in. diameter U bolts and timber packing. The floor requires 1 fin. boards, and the headboard similar close boarding with a steel lining over the whole area of the body-side surface.

For hanging-meat vans plastics materials are available for body construction these days. Black marks are left on carcases in contact with aluminium sheet lining, for hard plastics film-faced plywood with the edges well sealed gives a surface which will not mark the carcase and itself cannot easily be marked or damaged, yet allows for frequent cleaning down.

When selecting the material for van roofs translucent reinforced plastics mouldings should be considered as a practical method of introducing natural light. This assists the driver where the selection of labelled goods or packages is an everyday operation. Where the load is of a type likely to suffer from the effect of a long journey in the sun, translucent moulded sheets should not be used as a one-piece overall moulding.

For hanging clothes, or materials affected by the sun's rays, the translucent material should be confined to the centre gangway; the remainder of the roof in light alloy will keep the load in shadow. Such a construction needs a sturdy roof-framing of timber or light alloy sections to ensure that the joints between the different panel materials do not open up under the distortions associated with tall bodywork.

Tipper bodywork also requires consideration of the load /material-construction relationship. Where the load is sticky, difficult to handle and discharge as it cools down, then the use of timber is a practical answer. Hardwood timber has natural insulating and heat retention properties. If, however, the load is hot coke, discharged directly from the coke oven through a cooler, then wood is unsuitable. Light alloy plate as the main body material should be used as it is impervious to the sulphur corrosion, and not affected by heat.

Since reinforced plastics material is not responsive to vibrations and the resulting drumming associated with all steel constructions, it is particularly suitable for driving cabs. The main advantage is in the reduction in driver fatigue. In the event of serious impact, the material splinters and when necessary can be broken away without mechanical flame-cutting equipment.

The method employed in this construction is to mount reinforced plastics panels over light, stiff framing of timber or alloys. In most cases these cabs are designed around a steel floor to provide support and form the basis for the framing.

The cost of either a composite cab or a reinforced plastics cab compared to a standard all-metal cab for the same chassis, is greater, But the ultimate decision must be made on the points of efficiency, appearance and a reduction in the costs of repair work for small damage.

Reinforced plastics tanker construction uses resins which are impervious to corrosive chemicals, and gives a clean delivery of each load, with long life for the tanker body. The mounting of the tank on the chassis is of the utmost importance, and this calls for a degree of flexibility in each mounting so that strains can be passed from end to end of the moulding as the chassis flexes when the vehicle is running light.

Structural adhesives are used in commercial vehicle body construction, and secure sheet metals to similar or dissimilar materials. Basically these adhesives are made in one-component form in which a catalyst is introduced during manufacture, and in twocomponent form in which a separate base resin adhesive is activated by a separate catalyst added just prior to use. These structural adhesives are particularly useful when securing lining panels in a van body.

Construction of drop-side and tailboard units can be built by bonding the outer sheet of metal to the frame sections in order to produce a completely flat side surface.

To obtain the most satisfactory service life for these structures, a sound framing must be used, as any movement of the framing relative to the underframe distortion will transmit stresses through the joint and impose them on the unsupported area of the panel.

It has already been mentioned that the choice of materials for a maximum-capacity van body to keep the vehicle weight under 3; tons plated is of considerable importance. The choice of a light alloy in top-hat form as the main body framing, with light alloy underframing cross-bearers can be secured directly to the chassis. This produces a lightweight-van body shell on to which light alloy sheet is then secured. Structural adhesives in place of spaced rivets, results in a very rigid structure and allows the panels to take some degree of stress and keeps the pillars in correct relationship to each other while under stress.

All extraneous fitments which add unnecessary weight such as outside rubbing rails. below-floor skirt panels and their framing, interior close-spaced slatting and lining boards can he done without—likewise any passenger's seat.

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