Almen Strips Call to order: 1-574-256-5001 or 1-800-832-5653

Q: How do I choose the grade of Almen strip I should use?

A: Most general purpose applications, namely automotive and gears, can rely upon the A2 Grade of Almen strips. For more critical applications, such as aerospace, a tighter tolerance may be specified for flatness (pre-bow) and for hardness.

Summary

Notes:

Grades 3,2,1, and 1-S are service marks of Electronics Inc.
The Almen strip type (N, A, or C) is determined by its thickness

Detailed Information

1. Length
2. Width
3. Thickness
4. Flatness
5. Hardness

Width: The standard width of the Almen strip is .75 inches with a common tolerance of +/- .005 inch. Some specification have different tolerances and not all specifications use the .75 inch as the nominal width. For all practical purposes you can use .75 inch as the "width" of the Almen strip. Although no experiments are known to have been conducted to determine the influence of strip width on performance it is assumed to be negligible.

Thickness: The thickness of the Almen strip determines its type, namely "N"( .031 inch), "A" (.051 inch) or "C" (.094 inch). It is because of the thickness differences that each type performs at the different intensity levels. The "A" strip is deemed to be the "standard" strip and is valid for use in the range of .004 inch and .024 inch. Below the .004 inch "A" intensity you should use the thinner "N" strip. Above the .024 inch "A" intensity you should use the "C" strip. The most common tolerance for thickness is +/- .001 inch but some specifications require +/- .0005 inch tolerance. The Almen strip is very sensitive to thickness variations, as can be seen by the relationship between the three strip types. An "A" strip deflection of .005 inch would result in approximately .015 inch deflection of an "N" strip. This indicates that the strip will increase its curvature by .010 inch per .020 inch change in thickness (from .051 inch thick to .031 inch thick). The standard tolerance for thickness, +/- .001 inch, would therefore allow approximately +/- .002 inch variation in expected curvature reading. For automotive applications this may be acceptable but for aerospace applications it would be deemed excessive. Therefore the .0005 inch thickness tolerance, which would translate into a +/- .001 inch curvature tolerance, is required for aerospace applications.

Flatness: The initial pre-bow, or flatness, of the Almen strip acts like a latent bias in the measuring system. A strip that has a pre-bow of .001 inch will read .001 inch higher curvature than a strip with zero pre-bow. If the initial pre-bow value is negative than the resulting curvature will be diminished by the same amount. These readings are an approximation depending upon many factors, such as whether or not the strip has been peened on the concave or convex side and the nature of the residual stresses in the strip prior to its use. A compensation scheme may be used to negate some of these effects by taking a net reading of the strip curvature. The compensation practice should be limited to strips that exhibit less than .0005 inch pre-bow. Trying to compensate for excessive pre-bow above .0005 inch has not proven to be reliable.

Hardness: The hardness value of the Almen strip will affect its performance. Almen strips were tested at the low end of hardness HRc of 44 and high end of hardness HRc 50 and the results indicate that the curvature arc height varied by .0006 inch (total range) on the "A" strip at an intensity of .005 inch "A" and also at .010 inch "A". The aerospace grade strips, with hardness range limited to 45-48 HRc, would therefore limit this effect to .0003 inch (total) variation.

Order Almen Strips by phone at 1-219-256-5001 or 1-800-832-5653