Designing for Lightness in
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CON TAIN ER B ODIES THE -lift .van or container is made in various capacities from 15 cwt. to 6 tons, for carrying a wide range of commodities. As with the body which is fixed to the chassis, attention has been directed to the reduction of the weight of the lift van, in order that the pay-load shall be al; large as possible.
The lift van has no wheel-arches, thus simplifying the layout of the bottom framework. This framework, however, although, during transit, carried by the platform and chassis of the lorry, must have sufficient strength to support its own weight and the load when it is lifted. Moreover, it may occasionally be subjected to racking strains during lifting operations.
• Notwithstanding these difficulties, containers are lightly built. This fact is the result of selecting the right materials, and designing the framework and panelling in the most scientific manner, in order that, without loss of strength, there shall be no superfluous weight.
Framework Considerations.
If the lilt van is to have a wood framework, the greater part may be of oak, or this timber may be restricted to the bottom framework, because of its superior resistance to moisture. English, European and American varieties of oak and ash are employed. The average weight in lb. per cubic ft. of the timbers concerned is as follows :—English oak and ash, 50 and 46 respectively, European oak and ash, 49 and 45, and American oak and ash, 42, and 41. In many instances, the oak will be American and the ash English.
Columbian pine is sometimes advocated as a substitute for ash for the framework with the exception of the bottom. It is so called because it is B38 the chief tree of British Columbia, but it also grows in several American States.
Other names for it are Oregon pine and yellow fir, whilst it is cultivated in this country under the name of Douglas fir. This timber weighs about 34 lb. per cubic ft., so that, after allowing fot more substance in the pillars, as compared with ash, it should be possible to effect a saving of 10 per cent. in the weight of the framework.
The difference between the two principal methods of constructing the bottom of a van is that, by the first, the chief sources of strength are the cross-bearers and, in the other, the longitudinals. The first method is that usually adopted for the fixed body, because the side members of the chassis provide muèh of the longitndinal support.
As the lift van will often be deprived of this support, the bottom frame should have longitudinals of extra rigidity, with cross-bars of sufficient strength and number to tie the two sides together and provide means for lifting. The strength of a longitudinal is utilized to the best advantage if its larger dimension be placed vertically.
If the number of bottom cross-ears is to be reduced to a minimum—for instance, one at each end and two intermediate ones for a 2i-ton lift van some 10 ft. long, the ends of the intermediate ones being intended for lifting—it will be advisable to make these of mild-steel channel. These channel bearers are bolted under the longitudinals and the ends are connected to the sling irons, which are fastened to the sides of the body, with lifting shackles at the roof level.
The sling irons are not always necessary, because the lifting eyes may be attached at the end of the channel bearers at the floor level, thereby saving the weight of two pairs of irons. If each iron be 7 it. long and made of 21-in. by f-in, materials, it will weigh about 25 lb., so that the saving in weight on a set of four irons is considerable.
When the van • is lifted from the bottom, there are suitable guides on each side for the steel ropes, whilst the lifting tackle is equipped with spreaders.
The panelling may be attached to the inside or outside of the framing, or it is made double, which is essential if an insulated lining be required. When the panels are of matchboarding, they are often laid diagonally, because, in this position, it is considered that the boards nailed to the pillars act as a brace and strengthen the framework more effectively than when laid vertically or horizontally. Matchboarding is also used with a covering panel of aluminium, but wood and metal are • much better combined as a metal-faced plywood.
A marked saving of weight is effected if the panels be of fabric. For a large van, these are conveniently made up in wood frames and held. by mouldings fixed with metalthread screws.Any panel is easily detachable when renewal is necessary. Felt is inserted between the panel and battens to keep the fabric in shape. The fabric is protected on the inside with wire mesh made up in frames.
If the bodybuilder wishes to save weight in a wood-framed body, he fits aluminium 'panels, or, if it be a metal reinforcement, he makes it of duralumin, instead of mild steel. Duralumin, which is an alloy of aluminium, is of about the same weight
as that metal, but, weight for weight, it is three times as strong as mild steel.
The side framework of a duralumin lift van consists of channels to which the overlapping panels are fastened on the inside. The top rails at the sides and ends are of angle. The side angles are slotted for the channel roof ribs, which are reinforced at intervals by a lattice structure, this being built up between upper and lower roof members.
Roof and Floor Features.
Additional plates and -struts strengthen the roof at the corners. The metal floor is laid on lengthwise channels, under which the side channels are continued to form a member extending from one side of the body to the other.
With this form of construction, the reduction in weight is remarkable. The tare weight of a cluralumin 2-ton lift van is only 3 cwt., of a 3-ton van 4 cwt. 2 qrs. 14 lb., with unladen weights of 8 cwt. and 10 cwt. for the 5-ton and 6-ton sizes respectively. These figures represent a saving in weight of about 50 per cent. over containers of similar capacity which have a wood framework.
It is admitted that the initial expense is high, owing to the cost of the materials, but this is lustified by the saying in freights, low maintenance cost and cleanliness, whilst,. even when worn out after years of serviee, there is a "considerable scrap value.