THE POWER OF A TRACTOR.
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A Few Complexities Concerning Drawbar Pull Explained. By Henry Sturmey.
AGREAT DEAL of nonsense is being disseminated conceining the'power of tractors, and many wild and exaggerated claims are -being made as to their capacity. These claims, it may be stated without hesitation, are doing an enormous amount of harm to the tractor movement, Farmers naturally accept broad statements, made without qualification, as applying, to all conditions. When their conditions of situation or soil are beyond the normal and tractors purchased on the strength of broad statements fail to perform the work which is demanded, naturallythey condemn all tractors, as such, as being unsuitable for farm work.
What is Ploughing Capacity ?
The ploughing capacity of any tractor depends pri • manly upon the width of cut and the numben of ploughs. Thus, it will be readily perceived that an American plough cutting a swath, as some of them . do, 16i ins, in width, will cut twice the acreage in any given time as an English plough cutting an 8-in., furrow. It goes without saying that, given the same furrow width, a tractor pulling a four-furrow plough will do twice the acreage in the same time as one pulling only a,two-furrow plough. Therefore the question for any intending purchaser of a tractor is to ascertain just how many ploughs a particular tractor will pull in the particular soil and under the particular conditions with whichihe,has-to deal. This introduces freah factors, because different soils have vastly different powers of resistance. The depth of ploughing also Jaas to be taken intb consideration. These factors must be weighed against the ascertained draw-bar pull of the tractor in ques tion. In testing the resistance of the soil against the plough, it is usual to reckon per square inch section of cross-furrow. The reason is easy to perceive, because, the greater the depth and the width, the greater will be the Cea of resistance. Thus, with a, 9-in, plough working 6 ins, deep, we get 'a-cross-section of 54 sq. ins. (6 ins. by 9 ins.). With a 14-in, plough working at the same depth, we, get 84 so. ins. The total resistance which the tractor will have to overcome is pounds per inchmultiplied by this area and by the number of ploughs which it is desired to pull. II If it is possible for the investigator to carry out some tests on the particular land in question with a dynamometer, the actual resia. tance of the soil can, be aseeis tained.; but to illustrate how it, works out, if we take a wheat stubble showing a resistance of say, 5 lb. per sq. inch and it is desired to pull a four furrow 9-in, plough, set13•ins. deep, through it, we get the following equation : — • Area of cross-section of furrow x draught x number of ploughs = power required in pounds. Thus, with a 9-in, plough cut ting 6 ins, deep, in 5 lb. resistance soil and working with four ploughs we,get : 64 X 4X 5 = 1080.
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On the other hand, if we are propesing to plough up virgin sod—in other words, permanent grass land— our 5 lb, resistance becomes 15 lb., and we shall require 3240 lb. drawbar pull to haul the ploughs through it, Therefore, • it is manifest that to make a broad statement to the effect that any particular tractor will pull any particular number of ploughs, or turn up any particular acreage per hour, or per day, is to make a statement which without very considerable qualification, as to width and .dapth of cut and nature of soil cannot be substantiated.
Without carrying :out dynamometer tests, we are accustomed to speak of land in units of horse capacity, working .on. a single plough. Thus, in ordinary light soils,4two horses are sufficient to pull a single plough, and such land is spoken of as "two-horse land," More horses are required according to the consistency of the soil to be worked, and we get "three" and "fourhorse land "; whilst, if we wish t go to a very much greater depth in ploughing—as is done, for example, in potato cultivation in the Channel Islands—we get up to " five-horse " and even " six-horse " land, cutting that particular depth, which is usually about 11 ins.
Draw-bar Pull Fluctuations.
If we can approximate the resistance of the land. and we know the drawbar pull of the tractor to be employed; we can,ascertain fairly correctly if it will, or will not, pull the numberof ploughs we have in eye ; or, alternatively, exactly boav many ploughs it will pull. But drawbar pull itself may vary under different conditions of work, and this is a point of considerable importance.
Makers' drawbar pull is usually ascertained when working, on firm ground. It will not, however, take much perception to see that the resistance of the soil to the progress of the tractor itself must vary with the nature of the soil on which it is working, just as the resistance does to the passage of the plough, and it will also be clear that, the more a tractor sinks into the: ground by reason of its weight, the greater the ,proportion of power absorbed in propelling itself.