PCE Instruments PCE-2500, Руководство пользователя

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Manual
9
6.4 Supporting the Samples during testing
When measuring hardness with this tester, the following has to be noticed: Despite the low mass of the
impact body and low impact energy, a relatively large impact force of short duration is generated when
the impact body hits the measuring surface. The max. impact force of impact device D/DL is 900N.
For heavy samples of compact shape, no particular precautions are necessary.
Smaller and lighter samples or work pieces yield or flex under this force, producing L-values which are
too small and of excessively large variation. Even with big or heavy work pieces it is possible for thin-wall
regions or thinner protruding parts to yield upon impact. Depending on the frequency of the resilient
yielding action, the measured L-value may be too small or too large. In many situations, potential
problems can be checked in the following manner:
a) Medium-weight samples and also heavier samples with protruding parts or thin walls should be placed
on a solid support in such a manner that they do not move or flex during the test impact.
b) Light-
weight samples should be rigidly “coupled” with a non-yielding support such as a heavy base
plate. Clamping in a vice is of no value, since the samples become exposed to stress and because
complete rigidity is never attained. As a rule, the measured L-values would be too small and show
excessive variations.
6.5 Samples with Curved Surfaces
Impact testers only work properly, if the impact body has a certain position in the guide tube at the
moment of impacting the test surface. In the normal position, automatically present when testing flat and
convex-cylindrical samples (such as round samples), the spherical test tip is located exactly at the end of
the guide tube.
However, when testing spherically or cylindrically shaped concave surfaces, the impact body remains
further within the guide tube or protrudes further therefore. Thus, with such types of curved surfaces, it is
to be observed that radii of curvature do not drop below the values indicated in the following Fig.
Curved surfaces should always be tested with the small support ring.
Impact device types D Rmin=30mm
For impact devices D, special support rings are available to accommodate smaller radii on convex or
concave surface.