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TYRES and
Page 88
Page 89
Page 90
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TRACKS
for cross-country vehicles
Much Attention is Being Given to Equipment which Facilitates Transport Over Almost Any Class of Surface
0 NE of the greatest problems to be overcome in connection with the efficient operation of transport overseas in districts where difficult terrain has to be traversed is the provision of tyres and other equipment 'which will provide adequate traction, prevent undue vibration and avoid sinking in sand or soft ground. In some instances a variety of surfaces is encountered, and, consequently, the question is rendered more difficult, because special wheel or track equipment which may be entirely suitable for use on one
surface may be ineffective on another, and in such cases a mean has to be struck.
Great progress has, however, been made, which only a few years ago would have been deemed almost impossible. The multi-wheeler has come into its own, but it is being closely followed by four-wheeled machines fitted with special tyres and track chains, whilst track machines are now giving mileages with which types of a few years ago cannot compare.
So far, the best equipment for allc42 round work, particularly where much of the running is on hard surfaces or ordinary roads, seems to be the extralow-pressure pneumatic.
Many of the most successful TransSaharan expeditions and the Nairn regular services run in Mesopotamia partly owe their excellent results to the employment of Michelin extra-low-pressure tyres, which were also exclusively used for the vehicles taking part in the rally of oil-engined transport media which quite recently made a long trip in' the Sahara.
The Michelin Tyre Co., Ltd., Stokeon-Trent, points out that, at one time, the belief was widely held that tyres for use in rock and lava country must be very robust, with thick walls and heavy casings. This idea, however, has been almost abandoned, because such tyres offer a high resistance to flexing, and are inevitably cut, whilst the generation of heat causes premature failure.
Michelin tyres for cross-country work have strong but flexible casings which will give to obstacles, whilst the design reduces to a minimum the internal heat. Some sand drifts are of such a nature that it is almost impossible to prevent wheel sinkage. In such cases it has been found extremely useful to employ. under twin tyres, an aluminium tube of 5 ins. to 6 ins. diameter. The tyre walls pinch this tube and allow sufficient adhesion to give a drive.
A 38-in. by 9-in. tyre loaded to 43 cwt. and at a pressure of 115 lb. has a contact area of 42 sq. ins., so that the pressure is approximately 115 lb. per sq. in. The equivalent extralow-pressure Michelin is inflated to 60 lb., has a contact area of 80 sq. ins., gives a pressure of only 60 lb. per sq. in., and can negotiate yielding surfaces. The advantage in terms of area is 83 per cent.
Tyres for tractors operating over difficult country are produced by the Avon India Rubber Co., Ltd., of Melksham, Wilts. One is the Chevron-grooved 32-in. by 4i-in. high-pressure pneumatic, the other, intended for unmetalled roads and rough territory, is ft solid tyre with clean-cut chevron-shaped grooves.
Henley's Tyre and Rubber Co., Ltd., 20-22, Christopher Street, London, E.C.2, has found that a type of tyre which is particularly useful in this country when the roads are in bad condition or snow-covered, is the Air Cushion type, which cannot burst or be punctured, whilst it can be fitted to standard solid-tyred wheels without alteration. It is claimed to possess the good points of both the pneumatic and solid, viz., the capacity to absorb road shocks and the covering of great mileages without failures.
For some years past much experimental work has been carried out by the Goodyear Tyre and Rubber Co. (Gt. Britain) Ltd., Wolverhampton. The new crossbar-treaded tyre is designed to be, to a great extent, self-cleaning. It has been found that ordinary squaresection ribs expand at the point of contact, thus forming dovetail spaces between them, which collect mud and even tear lip grass, causing wheelspin and chatter. The Goodyear bar has a rounded "U" formation which narrows towards the walls. These crossbars close in under pressure and force the mud out at each side. Each crossbar is buttressed at one side, the supports being arranged alternately. The bars are also linked up alternately.
When a tyre sinks the tread is not much good for driving, but in the case of this Goodyear construction the drive is taken through the "U's" of the side walls. Such a tyre is especially suitable for surfaces over broken ground, mud and clay, whilst it permits the employment of bracelet or over-all chains. Tyres of this description are being used on Seammells for work in Iraq.
We illustrate the employment of a purely experimental and very advanced tyre equipment on a 12-15-ewt. van. Each rear tyre is 35 ins, in overall diameter. 15 ins, in cross-section, and has a hub diameter of 6 ins., runs at a pressure of 12} lb. per sq. in" and is held to the hub by serrations on the beads, which lock on similar serrations on the hub flange.
This Goodyear Airwheel is purely an experiment as applied to a transport vehicle, although already used with great success for aircraft, in which work it has to carry 3,000 lb. per tyre, whilst on the vehicle it is called upon to bear only 1,200 lb.
Whilst for ordinary transport it may not be necessary to go so far as to utilize tyres of this type, they seem ideal for difficult cross-country transport over soft sand and deserts, presenting the additional advantage that they can be used just as effectively on ordinary roads, having run at 45 m.p.h., practically the speed with ordinary tyres.
In the vehicle illustrated the old hubs were left intact and special aluminium hubs fitted over them, so that the original wheels could be replaced. The front tyres are 9-in, cross section and 13-in, diameter wheel fitting. These large Airwheels have run 1,500 miles and are still in good conditon.
The company is absolutely satisfied with its experiments to date, and is convinced that tyres must become larger and more flexible. When operating in soft sand, etc., a tyre with a less flexible cover scuffs the sand to the rear and merely digs itself in, whilst in running forward it builds a mound in front of itself.
The B.F. Goodrich Rubber Co., Akron, Ohio, recently designed a new cushion tyre, known as the ZeroPressure, for agricultural and industrial tractors. It has an arch formation, the pillars preventing excessive deflation. As there is no air pressure, under load the tyre can assume a concave tread, It is claimed to pack soft soil or sand under the centre, thus preventing sinking.
For some time the Firestone Tyre and Rubber Co., Ltd. Great West Road, Brentford, Middlesex, has been dealing with the question of special tyres for traversing difficult country. It has found that for a vehicle to traverse soft ground without bogging is entirely a question of the area of contact of the tyres, and the largest is obtained by
fitting large tyres run at abnormally low pressure. These have been tried out in connection with log pulling over virgin ground, mechanical shovels working in sand and ballast, running over clay, and carrying pipes over desert sands.
The tyres used are, of course, larger than those required to carry the actual load imposed, good traction is given by the special non-skid pattern, and the flexible walls give to any abnormal contour.
In the case of mechanical shovels, the difficulty has been increased by the fact that the machines pass backwards and forwards continually over the same soft ground with loads swung high at the ends of derricks.
A company .which has done most creditable work in connection with track-type vehicles is Roadless Traction, Ltd., Gunnersbury House, Hounslow, Middlesex. Perhaps the most striking of its developments in this direction lies in the invention of the rubberjointed track. Some years ago the company took the view that if the tracklaying principle of construction were not to remain of very limited utility, a radical departure would have to be made from the conventional construction, which consisted in linking together a series of metal track plates by metal pins.
After innumerable experiments the company produced a track consisting of metal plates connected together purely through the medium of rubber blocks, so that movement between the links is taken by distortion of the rubber, there being no metallic contact. The result is longer life, higher efficiency and greater adhesion ; wear on the rubber blocks is negligible because there is no skin friction and the material is shielded from light. Some tracks have been in use since 1928 without any wear affecting the working of the machines to which they are fitted.
The rolling path is separate from the ground plate, so that engine power is not wasted. The better adhesion is partly due to the track forming an elastic girder, and no " cackling " of the rolling path is caused by the weightcarrying rollers. Road plates or wooden swamp shoes can easily be fitted to the track.
In tests at the National Physical Laboratory with a track link submerged in sand and water and moved as if it were in service, under tension loads ranging from 4,600 lb. to 12,500 lb., and at rates of flexing equivalent to 10.65 m.p.h. and 32 m.p.h., it survived flexures equivalent to travelling a distance of 30,040 miles. Now we come to non-skid devices
which can be applied to ordinary tyres. Of these, one of the best known is the Sheppee, made by The Sheppee Motor and Engineering Co., Ltd., Thomas Street, York. Two main types are made, the crossbar for twin tyres and the " Safety First" chains for singles.
The spud for providing driving grip titt soft ground is particularly useful, although not intended for hard roads or surfaces on which chains alone are quite sufficient.
The spuds are made of malleable iron. They are held in place by chains which pass around the tyres, and are fixed by nuts and eyebolts under the rim, a leather protector with projections which fit into the links preventing the tyre walls and rim from being damaged. They are moderate in price, the spuds costing 1s. fid. each.
Amongst the manufactures of Kennedy and Kempe, Ltd., Harewood Forest Works, Longparish, Hants, are non-skid bands for single and twin tyres and for the bogie tyres of six-wheelers, either of the single or of the twin pattern.
Two of the patterns are known as the Packflat, one is for six-wheelers and the other for single tyres. Each consists of track plates carried on chains, one at each side of the tyre. The side flanges of the plates are ninged so that, when not in use, they can, as their name implies, be packed flat.
The K.M. pattern for six-wheelers resembles the Packflat except that the sides of the track plates are in one with the plates.
Finally, there is the Cupid's Bow pattern for six-wheelers having twintyres, the shape of the tread accounting -for its curious name. This also has hinged side pieces. Both this and Ole K.M. Pattern have been supplied to the Iraq Petroleum Co., Ltd., for Scammell and other vehicles.
Tracks and bracelets are made to fit single tyres up to 13.5 ins., and twin tyres up to 12.5 ins. Special coupling hooks and adjustments are provided for this attachment.
The Parsons Colonial-type non-skid chains, made by the Parsons Chain Co., Ltd., 25, Victoria Street, London, S.W.1, are specially designed to overcome transport difficulties under the worst possible conditions of snow, mud, ice, sand, wet grass, etc.
Each consists of two side chains of welded steel with a number of crosschains of high-quality steel welded and tempered. The couplings are non-rusting and easily operated by the thumb and finger.