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gauge a yarn pretty accurately by breaking the material in the hands, but this faculty is possessed by few, and can only be acquired by long experience.
Again, the behavior of the material during its passage through the process of winding and warping is often taken as a criterion of its quality, and it may be admitted that this is a satisfactory test, because if it winds and warps indifferently, it is not likely that its weaving qualities will be good. One drawback to this plan is that the observations cannot be made until the yarn is in work, and if anything is wrong, it is not easily dealt with at this stage. These tests may be classed as empirical, and and not in any sense scientific, nor is it possible to keep any records. The following give some of the various forms of testing yarns for strength.
Lea Testing.–By far the most common form of strength testing at present in use is lea testing, which consists of breaking one lea (120 yards) on a specially constructed machine which indicates the breaking load in pounds, and the stretch to breaking in inches. Most makers of testing machinery construct a lea tester, and they differ only in minor details. Some of the machines are worked manually, whilst others are driven mechanically; the latter plan is by far the best, as it is then possible to get more regular and reliable results. One form of a mechanically worked lea tester is shown at Figure 18.
The machine is very similar in its main features to those commonly used for lea testing, probably the only difference being in the reversing motion, which automatically brings the moving hook G back to the starting place after the completion of a test. Provision is also shown for observing the amount of elongation up to the point of breaking. In some machines the latter is obtained by causing a plate marked in divisions of one inch to travel downwards at the same rate as the hook G, whilst a pointer overhanging the plate is attached to the upper hook B, and the elongation can be readily observed at the moment of the fracture of the lea.
Figures 2 and 3 show a series of lea breaks taken from two cops of 46s mule twist, the count of the yarn for each lea is also given underneath, and it may be interesting to note that the diagrams as might be expected follow each other closely, that is, as the count goes down the break rises.
Now, it would appear that the only result recorded on a lea tester is the aggregate breaking load of 80 double threads or
something approaching that. This may be useful for some purposes, and indeed it is practically the only recognized strength test at present, but what particular value has it for the manufacturer, seeing that when the threads are put into the loom each one has to bear its own share of the tension necessary for the production of the fabric? Of what use is the knowledge that 80 threads of a certain yarn will bear a given weight? What is likely to be of use to the manufacturer is some detailed statement of the breaking strength of a number of single threads along with the average as well, but first and foremost a statement, either graphically or in figures, which will disclose how that average is made up. It is much better to have a yarn which is regular in strength even if the average is slightly lower than another yarn whose average has been raised by the presence of a few abnormally strong threads, which are of no particular value to the warp, and which will certainly not assist the weak ones when they are in the loom. Single and Double Thread Strength Testers. — A consideration
. of the foregoing view of the situation has led to the introduction of a number of machines for testing the strength and stretch to breaking of single and double threads.
Some of these machines are elaborate in construction and are capable of giving very accurate results, but they are costly, and consequently not readily available for ordinary warehouse use. Such a machine is one made by SCHOPPER. Other machines are clumsy, and not sufficiently accurate for the work to be done.
Others again are handy, and made to bear the ordinary wear and tear of warehouse work, besides having the merit of moderate cost. Such a machine is shown at Figure 19, which is a front view of a single or double thread testing machine made by Henry Baer & Co., of Zurich, Switzerland, with a detail showing how the speed of the plunger is regulated.
The action of the machine is somewhat as follows: A thread is looped over a small pulley at B, and the two ends are clipped by a screw clip at S. This is attached to a frame which, when the apparatus is at rest, is held up by a catch K. This frame