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WHEELED OR TRACK LAYING TRACTORS?
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For Work Under Extremely. Difficult Conditions, Track Machines are Essential, but for Lighter Duty the Higher Initial Cost is Hardly Merited. The Advantages of Pneumatics, Differential Locks and Fourwheel Drive are Considered. By D. N. McHARDY, A.I.E.E.
WERE I not a user of both types ot machine, I would hesitate to enter into a discussion of one of the most hotly debated aspects of tractor practice. Tracks versus *heels is a matter in which an extraordinary amount of prejudice has been displayed in this country. I hope, however, to put the various arguments forward solely in the light of practical results, and would, further, qualify all my remarks by saying that they refer to• the machines as they are to-day. Machines may come forward to-Morrow, or next year, • which will cause. us to revise our present opinions. The question of first cost may well be tackled at the outset. Track-layers. cost mare than wheeled tractors • of comparable 'drawbar h.p.; because they.include many, ' more working parts and often are sold in smaller num-. • hers.. Therefore, it may be Conceded that there is justification for the advice to use wheels where possible and 'crawlers " where one niust.
Where the Track laying Outfit Scores. . This leads one a step further to the suggestion that . track-laying outfits can do much that wheeled machines ' cannot. In speaking of *heeled ,machines, for the moment, I am referring to the conventional type with four wheels, of which the two at the rear are driven
and the front wheels steer. •
• It may make the relative position of, the two classes of machine clearer if it be pointed out that if both be powered by exactlythe same engine, it is possible to obtain some 25 per cent, higher drawbar pull (maximum). from the "crawler," the difference rising, under some ground conditions, to 100 per cent. Indeed, the track machine might, in certain situations, give its normal pull where the wheeled machine's .output was zero.
The tractor equipped with tracks can unquestionably operate to capacity under a far wider range of soil conditions than can the wheeled machine. Whilst the optimum condition for wheels is moist but firm soil, the " crawler " will tackle loose, wet and slippery ground with few failures. The light track-layer may, however, meet its Waterloo on` ground which is hard below and has about an inch of soft surface.
c24 The wheeled tractor, too, may be equipped to meet difficult ground conditions by means of a variety of special lugs, extension rims and pneumatic tyres, and is by no means finished when its standard wheels are
spinning. I have found, for instance, that when a medium-sized wheeled tractor with spade lugs was slipping to the extent of 80 per cent., when hauling a cultivator on loose ploughed ground, the substitution of pneumatics enabled the job to be carried ox i without any noticeable slip.
. Until we have explored all aspects of wheeled-tractor design, therefore, it would be unwise to condemn this class out of hand, because of occasional wheel slip. We all know the immense influence of the differential oil this problem and the possibilities of four-wheel drive, yet for .various reasons--mainly that of cost= little has been done towards the production of tractors with differential locks, or (with one or two exceptions) machines with four-wheel, drive. There is one generalpurpose four-wheel-drive tractor, available at a farmer's price, 'which certainly merits the attention of those who
have sandy conditions with which to contend.— .•
Scientific Aspects of the Problem .
The more scientific aspects of the use of wheels as against tracks may not at first sight appear of much interest to the farther, but it must be pointed out that the mere driving of steel lugs into the ground and pulling them out absorbs an appreciable amount of power, causing wasted fuel consumption, whilst the rolling resistance, in the case of wheeled machines, accounts for a large proportion of the fuel consumed. These factors cart both be reduced where pneumatic tyres are in use, but, nevertheless, the " crawler " scores appreciably on these points.
It is probably at this juncture that we can best raise another question of material importance, namely, that of the drawbar h.p. involved. What is a preferable design for one size of machine is not necessarily so for another, and my view is that this is fundamentally the biggest
deciding factor. -•
A wheeled-tractor is, to-day, capable of transmitting a drawbar pull which, in lb„ represents quite a large proportion of its actual weight, under favourable conditions. The weight must, however, always continue in advance of drawbar pull and there comes a point at which the practical farmer sets his face against any increase in. the weight of a wheeled machine. In Great Britain there are few agriculturists • who care to have appliances weighing over 2 tons on their land. Further, as the weight increases, in any but the best of going, the problem of rolling resistance becomes more and more serious.
These considerations seem to set a limit for this class of machine at drawbar pulls of around 4,000 lb. maximum. Above this figure I think we must, in general, look to the track-layer. Below it, except for special par:. poses, there appears no special justification for the extra cost of a "crawler."
It is when we enter the sphere of tractors offering 4,000 lb. as their minimum drawbar. pull that we find the machines that are really interesting, and here we meet. a field that is definitely the preserve of the "crawler." Whilst the general-purpose wheeled tractors. of . under 2 tons a,re replacing slowly but surely the farm horse, it is the big track-layer that is literally turning much of the
country inside out and bringing derelict tracts back into cultivation.
For subsoiling, breaking up rough land, and deep work generally, no tractor giving off less than approximately 50 h.p. at the drawbar is of much practical value, and where pulls of 8,000-9,000 lb. are in question, wheels —even if all four be driven—hardly bear thinking about.
It is very noticeable that the bigger th machine the worse are the conditions with which it can deal, and as an example I my mention that I have operated an Allis-Chalmers K raodeL on gradients of up to 1 in 3± (measured with t e Tapley meter), hauling a threebottom subsoiling plough to a depth of 16 ins, against the grade, without he slightest sign of instability. Such hillside work wou d be unsafe with a small wheeled tractor.
In rough land, 400, where such obstructions as tree stumps, rock, sm4 pits and so on abound, the larger machines are morel reliable and " crawlers " are preferable to wheeled tratitors. There is the summer fallow on heavy clay, which if broken up in big lumps, as it should be, may he impassable for wheeled tractors, as well as horses. Here, a big track-layer, which can pass over the top of the rough lumps, is again the most satisfactory.
In swamps and moorland the track-layer is naturally at an advantage and one would, but for the danger of setting up undue track wear, like to use it extensively on the loose sandy soil, where wheel grip is hard to secure.
The track-layer is always criticized on the score of heavy upkeep; and a careful study of tracks and soil is certainly necessary to keep this item within bounds. First, it is of prime importance to obtain a genuine " crawler " built by one of the concerns with an indisputable reputation. Secondly, do not misuse it. Because it can be made to turn square corners and pirouette on one track, do not allow your driver continually to turn on his brakes. The steering clutches should, whenever possible, be utilized to bring the tractor round in a smooth turn. This will also obviate undue wear on ploughs and other implements.
Then, as regards soil, it is not difficult for the majority of tractor owners to avoid the use of tracks. under adverse soil conditions. Where numerous stones abound on the surface, the soil is normally very abrasive, and stones and grit together play havoc with tracks. Fortunately, such land can be worked with the heaviest wheeled tractors, if big drawbar pulls be needed, and
the " crawler " is best kept away from it. The call for tracks is greatest on the clays, and here wear is at its lowest.
I notice a general tendency on the part of newcomers to farming to rush to track-layers, without any read need. The 'majority of farms can be efficiently operated with medium-sized wheeled tractors, with an exception in the favour of "crawlers," small, or large, where circumstances, such as heavy soil, or special crops like fruit or hops, call for short turning. For the specialist jobs, the big track-layer and for 'sandy land the four-wheel-drive tractor; are suggested:
For normal farming operations effected by a really general-purpose tractor, I have always looked upon wheels as a safeguard against spoiling soil textures. When the ground is too wet for wheeled-tractor operation, it is too wet to be worked by anything, including horses. The farmer who has a " crawler" will often be tempted to carry on when the ground is unfit.
A wheeled tractor is generally more suitable for mowing and binding and other jobs needing to be carried out at fairly high speed. The " crawler " is best operated with a full load for the lower gears.
On one point I think it is safe to be dogmatic. Neither type will oust the other. Both are too useful.