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HARTNESS SCREW THREAD COMPARATOR
NO MYSTERY
There is no mystery about the functioning of
elements of machinery, but there are and have been
stages in the development of mechanism when we
have been baffled by the variable action of elements
that have seemed identically the same.
The yielding of screw threads we have generally
accepted as the inevitable-that screws yield just as
a wedge works out under some kinds of stress-and
yet the largest percentage of yielding threads has been
due to misfits of a kind that have put the stress on a
few threads or on a small area of all the threads en-
gaged, and this has resulted in an overstressing of
the holding surfaces.
We know that this is the result when we have
an excessive difference between the two mating parts
in one or more of the fit-controlling elements, such
as diameter, lead, form of thread, parallelism, round-
ness of helix, smoothness of surface, etc.
It is obvious that the general form and lead of
mating parts should correspond, SO as to obtain a
uniform distribution of the stress over the full length
of engagement of the mating parts. Since threads
cannot be produced with any degree of precision that
is comparable to that obtained with flat, cylindrical
or spherical surfaces, we must give special attention
to most essential elements. This requires a gaging
system that shows clearly the real characteristics of
the thread.
Heretofore we have measured threads with vari-
ous types of gages, all of which have been used with a
gentle pressure. This gentle pressure is unlike the
stress of service. The gages measure only the crests
of the irregular surfaces of the thread. They may bear
on some non-essential part like the root of a screw
8
Document source description
This is a publication about a comparator machine for the inspection of screw threads, with illustrations by Russell W. Porter.
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"ocrText": "HARTNESS SCREW THREAD COMPARATOR\nNO MYSTERY\nThere is no mystery about the functioning of\nelements of machinery, but there are and have been\nstages in the development of mechanism when we\nhave been baffled by the variable action of elements\nthat have seemed identically the same.\nThe yielding of screw threads we have generally\naccepted as the inevitable-that screws yield just as\na wedge works out under some kinds of stress-and\nyet the largest percentage of yielding threads has been\ndue to misfits of a kind that have put the stress on a\nfew threads or on a small area of all the threads en-\ngaged, and this has resulted in an overstressing of\nthe holding surfaces.\nWe know that this is the result when we have\nan excessive difference between the two mating parts\nin one or more of the fit-controlling elements, such\nas diameter, lead, form of thread, parallelism, round-\nness of helix, smoothness of surface, etc.\nIt is obvious that the general form and lead of\nmating parts should correspond, SO as to obtain a\nuniform distribution of the stress over the full length\nof engagement of the mating parts. Since threads\ncannot be produced with any degree of precision that\nis comparable to that obtained with flat, cylindrical\nor spherical surfaces, we must give special attention\nto most essential elements. This requires a gaging\nsystem that shows clearly the real characteristics of\nthe thread.\nHeretofore we have measured threads with vari-\nous types of gages, all of which have been used with a\ngentle pressure. This gentle pressure is unlike the\nstress of service. The gages measure only the crests\nof the irregular surfaces of the thread. They may bear\non some non-essential part like the root of a screw\n8"
}