Almen Strip Comparator
This test is widely used on the shop floor as they are comparator measurements, calibrated originally from the MPa measurement results of the x -ray diffractometer.
The requirements for these checks, are specified in standards.
The aerospace industry applies the most rigid of standards when using Almen strips and checking devices.
The SAE standard defines the requirements for equipment and supplies to be used in measuring the shot peening arc height and other surface enhancement processes. Guidelines for use of these items can be found in SAE J443 and SAE J2597.
An Almen strip is a thin strip of SAE 1070 steel used to quantify the intensity of the shot peening process.
Originally, the measurement system was developed and patented by John O. Almen in 1944. Modern shot peening evolved in the automotive industry most likely due to failures of engine valve springs which were prone to failure in service. General Motors superseded the design of the Almen gauge for use in the automotive industry, and has since been updated and modernised to ensure that the reading is taken from the central portion of the strip.
The strip is placed in the chamber on the item to be shot peened, usually near to an area of the item where the result is deemed critical and sometimes located by a special fixture.
Compressive stress introduced by the peening operation causes the strip to deform into an arch, which is measured using a gauge.
Almen strips are classified into 3 types:
‘A’, ‘N’, and ‘C’.
They differ in their thickness but they have the same width and length.
“A” type is predominantly used for shot peening with cast shot or fully conditioned cut wire around shot.
“N” type are used mostly for glass bead peen and ceramic bead peen.
“C” type are used more rarely and are thicker than the other types.
Although similar, the specification for Almen strip dimensions of the same type slightly vary from one company to another. The Almen strips are made from plain carbon steel SAE 1070 and have a hardness of about 45 HRc
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Post written by
Shot Blasting Technical Manager