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If it should be thought that the item of depreciation ought to have been made larger for the high-speed spindles than for the slow ones, there is ample margin for this item to be modified to suit any reasonable opinions in this regard, without materially affecting the point of comparison which is the object of the table. The item of profits of business, if taken into the account, would offset any possible modifications of this sort several times over in favor of the high speed. Besides, the item of depreciation is intended to cover the cost of the continual changes and improvements being made in machinery, involving sometimes the discarding of the machines entire, and this item is more than half of the whole amount I have allowed ; so that the percentage of increase on account of wear and tear simply would be so small that I did not consider it worth while to calculate it for the purposes of this comparison.
A word in regard to the variation of 5 cents per spindle in the table, in the cost of power for modern mules. The existence of this variation is well known, and of course this difference in cost of power cannot be ignored, providing other things are equal, — quality of work, repairs, waste, wear and tear, etc., as it then represents a positive and considerable loss, without any compensating advantages. But, if it were a question of cost of power only, as between a slow and a high speed mule, other things being equal, the results of a comparison would be the same as with frames, only to less marked extent. The high speed would leave the slow ones “out of sight” as to economy of manufacturing
Taking now a general view of the entire mill, a comparison between the old and modern mills is no less striking. Going back to the time when both warp and filling were spun on mules (1869), my data show that what was then a good average mill of 44,000 spindles, all mules, was producing 37,700 pounds per week, and required 677 horse-power to drive it, - a product of .85 pounds per spindle per week and of 55 pounds per horse-power per week.
Another, with 28,000 spindles, all mules, produced 24,300 pounds per week, with 430 horse-power; or .87 pounds per spindle per week and 56 pounds per horse-power.
Another mill (1874), with old ring warp and mule filling, with 90,000 spindles, produced 78,000 pounds per week, and required 1,583 horse-power to drive it, – a product of .87 pounds per spindle per week and of 49 pounds per horse-power per week. These were all on print cloth numbers.
A mill with Sawyer warp spinning and fairly high-speed mules produced .93 pounds of cloth per spindle per week, and 1 horse-power produced 46.5 pounds of cloth.
And to-day a modern mill with all frames, at high speed, produces 1.17 pounds per spindle per week, and 1 horse-power produces 46.75 pounds. In round numbers, the product of a 30,000-spindle modern mill is equal to that of a 40,000-spindle mill of twenty years ago.
From these figures we find that the Sawyer warp mill required 19 per cent. more power to turn off a pound of cloth than the old all-mule mills. But this fact did not weigh a feather against the adoption of Sawyer spindles.
Since that period the product per horse-power has not changed materially. But the improvements made in steamplants meantime have reduced the actual cost of the power per pound of cloth, so that it is less than it was in the old slowspeed “all-mule” mill, the total cost of fuel for power alone being now about .41 cents per pound of cloth, while then it was .66 cents (taking the same price of coal in both).
The deduction to be made from the above table is, that, in regard to cost of production alone, any increase of speed and product will be in the line of economy, so far as cost of power is concerned, even if the latter should increase four times as fast as the production. And this is so unlikely a supposition that practically the question of power is not to be considered for a moment as against speed.
One of the items of power in a cotton mill, and not a small one, is the friction load of the shafting. Power expended for this is in a sense wasted. It produces nothing and costs a great deal. In the best mills it will be not less than 22 per cent. and often 25 per cent. of the total power. (This includes the friction due to the belts on the loose pulleys of all the machines, as this is the usual method of weighing this load, so that it does not of course represent the mere friction due to weight of the shafting.) In a mill requiring 1,000 horse-power, therefore, 220 to 250 horse-power will be expended in this manner, costing, at $19 per horse-power per year, about $4,200. Various methods have been tried from time to time to reduce this loss. One way has been by reducing the diameter of shafting, sometimes to extremes, and increasing the speed; but not much has been accomplished in results. The percentage remains about the same.
In the course of my work I have had occasion to test the power of a large number of mills of all descriptions, old and new, large and small, with excessively heavy and excessively light shafting, at extreme slow and high speeds, and medium heavy at medium speeds, and with all sorts of bearings, and in all sorts of conditions. I have found the friction load to run from 22 up to 39 per cent. The lowest I have ever found was 21.25 per cent., and this was in a very old mill, requiring 1,055 horse-power, with rather heavy shafting, but all at slow speeds, from 210 to 250 revolutions. The friction load was 224 horse-power. I have never found this result equalled in a modern mill. Several years ago I tested two mills in the same yard : one a very old one, with extremely, even ridiculously, heavy shafting, but at very slow speeds; and the other a new mill just completed at that time, with very light shafting at high speeds, and with bearings about 5 feet apart. I remember that I expected to find the friction of the older mill so much more than the new one that it would pay to change the shafting. The test showed so little difference between the two that it was not worth considering. This result was a surprise to me at that time, but would not be so now.
So far as the theory of friction is concerned, the laws that govern the driving capacity of a shaft and its friction are so related to each other that it makes no difference in theory whether a large shaft at a slow speed or a small one at a higher speed is used to convey a given amount of power, if the speed in both cases is in inverse ratio to the cubes of their diameters. But in a cotton mill this theory will not hold in practice. Increasing speed of the shafting is liable to increase the friction, and for obvious reasons. The friction of shafting in cotton mills is not due entirely to its dead weight, but more to the lateral stress of the multitude of belts on machines and counters. This stress is independent of the weight of the shaft and has no relation to it, and reducing its diameter will not affect it. On the other hand, when we reduce the diameter and increase the speed to give the same power, we decrease its circumference or rubbing surface only as the diameter, while we increase its speed inversely as its cube. Therefore we have increased the surface velocity in the bearings inversely as the square of the diameter. And, although the weight of the shaft is also reduced in the same ratio, yet the lateral stress of the belts is put upon the rubbing surfaces at the above greatly increased velocity. The friction therefore ought to be more, and I am satisfied that it is, other things being equal. Then, when in addition to this the number of bearings is increased, in order to properly support the reduced shaft, we magnify this evil; for in a cotton mill, with bearings suspended from wooden beams and floors, the conditions are far from that perfection which admits of the strict application of any mechanical formulas. Every unnecessary bearing increases the chances for greater friction. In lines which are merely carriers of power, with no pulleys or belts upon them, the above objections to high speed do not apply so forcibly up to a certain point. The fact is, in this as in all other mechanical constructions, it is impossible to apply any strict formulas. They must be materially modified by the carefully noted results of experience.
In the mill where I found the friction 39 per cent. the shafting was rather heavy, but not extremely so. I attributed the excessive friction here to the multiplicity of bearings, and all of a very bad construction, the boxes being absolutely rigid. Of course the difference between 39 per cent. and 22 per cent. is worth saving. It would represent 170 horse-power on a mill requiring 1,000 horse-power, and this would represent a needless loss of $1,700 a year for fuel alone, in the best steam mill. And, more than this, in some cases of partly water and partly steam power mills it may mean the still larger losses from stoppage of machinery or loss of speed, which might be overcome merely by reducing the friction down to a reasonable point. This was, in fact, exactly the case in the mill in question, which I tested recently on account of this condition of things.
High speed of shafting in carding and weaving rooms is especially to be avoided. I have in mind a large mill in which the shafting in these rooms runs over 320 revolutions. The loom pulleys are 5 to 6 inches diameter, the belts very tight; of course the shafting is very small, with bearings every 5 feet, and in one room with 384 looms there are over 500 bearings, whereas half that number with a proper arrangement would have sufficed. Every one of these 250 unnecessary bearings means more or less unnecessary friction.
In yarn mills the friction is of course less than in weaving mills. I have found it about 18 to 19 per cent.
The suggestion that has been made by electricians to overcome this waste of power in friction by attaching motors to each line will evidently not accomplish the purpose. The shafting and machine belts will still remain, besides having the loss from the conversion of power, - about 15 per cent. in the present state of the art.
Carrying steam to a number of small engines distributed about the mill was once suggested, I believe, in a certain mill, but of course is not to be thought of. At present the only thing to do is to avoid extreme speed, use bearings of good