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Thursday, September 15, 2011

Importance of Accurate Alignment

Your testing system represents a major capital investment for your organization. You make sure it is regularly calibrated for load, strain, and displacement, and that it is regularly serviced. But when did you last make sure that the alignment was correct?

Misalignment takes two forms: concentricity misalignment, in which the centerline of the upper grip or fixture is offset from the centerline of the lower grip or fixture; and angularity misalignment, in which the two centerlines are at different angles to each other. Both impose unwanted bending stresses into a test piece under load and can therefore affect the behavior of the material.

Load frame alignment can change for a number of reasons, including:
- Changing grips
- Installing new or replacement load string components (load cells, adapters, and fixtures)
- Repositioning the fixed crosshead
- Wear or damage to load string or load frame components

The importance of accurate alignment is recognized more and more by accreditation bodies, aerospace corporations, and others. You must be able to demonstrate that your systems meet the alignment requirements specified in many ASTM standards that reference tolerances for either bending stresses or alignment.

ASTM has produced ASTM E1012, which outlines the requirements and calculations for assessing load frame alignment. This standard is frequently quoted as an acceptable method for checking and quantifying materials testing machine alignment.

So, consider requesting an alignment check during your next service visit. You never know when you may need to show that your system is ready for everything.

2 comments:

Anonymous said...

is there an alignment kit ?

Lorenzo M. said...

Hi - interesting question! There are several ways to interpret this question, but I will presume that you are asking about either the presence of an alignment kit on the machine (being checked) or whether such a product is available. For those less familiar with this topic, an alignment kit is a device that fits between the load cell and the crosshead of a typical testing machine. This device allows you to adjust the concentricity and angularity of the upper load string in order to minimize bending in the specimen. If you check the alignment of a machine that does not have an alignment kit installed, it can be cumbersome to loosen/shim/adjust/retighten/recheck the alignment and it is not always assured that the results will be acceptable. With an alignment kit installed, the user can make adjustments and - with AlignPro software - monitor the bending on a strain gaged cell in real time - making the alignment process much faster/easier. For information on the availability of Alignment Kits, compatibility with machines in the field, and how it works with AlignPro, please peruse the following links or contact your local Instron representative. I hope this answers the question...!

http://www.instron.us/wa/product/Composites-Alignment-.aspx

http://www.instron.us/wa/acc_catalog/prod_list.aspx?cid=423&cname=Alignment Fixtures

http://www.instron.us/wa/solutions/Tensile-Testing-To-NADCAP-Alignment-Stan.aspx