The Lea Test
In the older woollen, cotton, and flax, the lea or
hank test on a pendulum lever tester is the standard method of measuring the
'strength' of the yarn.
In this test, a hank of yarn, with its starting and
finishing ends knotted, is placed over the hooks of a lea tester. As the lower
hook descends, a load is imposed on the loops of yarn constituting the hank. At
some point one of the threads breaks, yet the hank remains intact and capable
of sustaining further loading. The hank suffers a succession of thread breaks
and ultimately becomes unable to sustain any further increase in load. When
this happens the pendulum stops moving and the maximum load is indicated on the
dial, the 'lea strength' of that particular hank.
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| LEA STRENGTH TESTER |
The single-thread strength test
The disadvantages of the lea test-a
complex result, no measure of extension-are eliminated in the single-thread
test. For critical testing where the maximum amount of information is required,
the single-thread test should be used. It must be borne in mind that the level
of skill and care should be high, especially if the instrument is non-automatic,
because the manner of handling the threads can affect the results. The aim is
to give each individual thread identical treatment.The minimum number of 20 in. test
specimens for single spun yarns is 50. Thus, the actual length tested is relatively
short and must be regarded as a disadvantage of the test. The chances of
spotting a weak place, which may only appear once every 1,000 yd or so by
testing only 1,000 in. might seem small. Even so, by calculating the
coefficients of variation of the strength and the extension, a measure of the
regularity of the material is obtained.It has been noted that there is no
simple relation between the single-thread strength and the lea strength. The
strength per thread for the lea test may be calculated by dividing the: lea
strength by the number of threads between the hooks, e.g. by 160 if the hank
was a 120 yd hank wound in 80 revolutions of the wrap reel. This value of
strength per thread can be expressed as a percentage of the average
single-thread strength. Usually, about 70 per cent is obtained but it may vary
between 50 per cent and 90 per cent. The ratio of lea breaking length:
single-thread breaking length is quoted for a number of experimental yarns, the
ratio varying from 0.56 to 0.90.
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| SINGLE THREAD STRENGTH TESTER |