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A Brief History of the Almen Stripby Jack Champaigne Since the invention of the Almen strip and Almen gage by J. O. Almen in 1942, the shot peening and blast cleaning industries have had a simple, inexpensive and reliable tool to gage shot blast stream intensity. Basically, the Almen strip is a small (3 inch by 3/4 inch) piece of spring tool steel that is blasted on one side and made to curve in response to the intensity of the impinging blast stream. The original strip thickness was 0.050 inch and had no letter designation until the "B" strip was developed. Although there doesn't seem to be any remaining records of the "B" strip, it can be surmised that it was quickly replaced by the "C" strip with thickness of 0.094 inch for higher intensity applications of the automotive sector. (It's not clear when the "A" strip thickness changed from Almen's 0.050 inch patent to the present value of 0.051 inch.) Thus the "A" and the "C" strip were in common usage, primarily in the automotive industry. Eventually a thinner strip was needed for lower intensity peening of aircraft structural parts and accordingly Nobel from Pratt & Whitney developed a strip of 0.031 inch thickness and the "N" strip was introduced. The most popular documents for controlling the characteristics of the Almen strip are the U. S. Government specification Mil-S-13165 and the Society of Automotive Engineers (SAE) J-442. The government specification dates back to August 31, 1944 when a document number AXS-1272 was published to describe a new technology for use by the U. S. Army. This document then evolved into Mil-S-13165 which was first issued on December 11, 1953. The Almen strip attributes in Mil-S-13165 Revision C are unchanged from AXS-1272. The Society of Automotive Engineers, SAE, was also active in preparing Almen strip specifications. Their primary control document is SAE J-442 with supplemental specifications for aerospace applications coming from SAE AMS-2430 and SAE AMS 2432. Unfortunately, the evolution of J-442 allowed the strip attributes to change relative to Mil-S-13165. See Table 1 below.
Since shot peening often makes a dramatic improvement in a metallic
part's performance, much of the early peening was done secretly. Eventually
proprietary peening specifications evolved to "enhance" the
peening process beyond Table 2 shows the Almen strip characteristics of hardness, length, width, thickness and flatness requirements. Only the "A" thickness strips are included in the table. It can be surmised that hardness and flatness variations reflect a desire for more consistent and accurate intensity measurements. SAE aerospace specification AMS-2432 was based upon SAE J-442 with enhanced hardness and flatness tolerance for the more critical peening applications using computer monitored equipment. Electronics Inc. Almen Strip Grading and Inspection The following narrative will describe how Electronics Incorporated became involved in supplying Almen strips and how they developed multiple grades intended to meet the various specifications. (See Table 3, below) Electronics Inc. began selling Almen strips in 1989 to help underwrite the cost of The Shot Peener Newsletter. As a result of Almen strip specifications' diversity, Electronics Incorporated developed three grades of strips that allowed users to choose the price/performance level best suited to them. Grade 3 strips were guaranteed to meet Mil-S-13165C specifications and were bulk packaged in bags of 1000 pieces without special inspection or measurement. Grade 2 Almen strips were packaged in specially designed smaller protective boxes of 50 strips, had excess oil removed and were inspected for nicks, scratches, burrs, length and width. It was found to be very important not to remove the oil, especially by vapor degreasing or similar technique since the strips are prone to rusting, even in a sealed zip-lock type bag. Grade 1 strips were 100% inspected for flatness to the more stringent SAE J-442 requirement (±.001 inch) and each lot of 1000 strips were sample inspected for hardness. When SAE aerospace created AMS-2432, a special grade of strip was required and grade 1S was developed. Again, the strips were 100% inspected for flatness (±.0005) and each lot of 1000 was sample inspected for hardness (45-48 Hrc). (Table 3. Electronics Inc. Grade Table)
It wasn't long after these four grades were established that special requests for other variations arose. Boeing requested a variation that didn't fall into the standard categories. General Electric asked for another variation, and differences in the Pratt and Whitney specifications were noticed. Someone noticed that SAE J-442 length, width and thickness didn't match the Mil-S-13165 dimensions when the metric measurements were converted to inch measures. The multitude of variations prompted the development of Table 2 which shows 12 distinct Almen strip requirements. The master flow chart shown in Figure 1 helps to visualize the differences required by the various specifications. At least 12 distinct specifications were identified for this article. To further illustrate the differences in individual attributes Figures 2 through 6 help to quantify the individual attributes of hardness, thickness, width, length and flatness. To help satisfy the various (and occasionally conflicting) requirements,
Electronics Incorporated began charting histograms of the length, width,
thickness, hardness and flatness attributes (30 piece sample per lot
of 1000 strips based upon MIL STD 105 inspection requirements). The
histograms, shown in Figures 7 through 11, provide the information needed
to certify that strips will meet any of the specifications that might
be required. Tables and Figures
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EI HOME | ABOUT | CONTACT US | DISTRIBUTORS SHOT PEENING & BLAST CLEANING UNIVERSE ©2004 Electronics, Inc. All rights reserved |
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