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 TENSION LINKS - ARE YOUR MEASUREMENTS ACCURATE? 


You are out in the field with a tension link, load link, or some type of digital dynamometer to use for a weighing application.  You need pins to engage into the unit.  What can you use?  Slings? Maybe one size pin with one diameter on one end and another pin with a different diameter on the other end.  As long as the fixtures are safe to hold the load it shouldn’t matter, should it?  

Pin Size is Critical
Loaded Without The Proper Pin Size
Here at Morehouse, we have done testing this issue in mind.  Forged pins, for example, can differ by small amounts in diameter from pin to pin.  Depending on the Scale manufacturer, we have seen differences up to 1.7% between different sets of pins (Picture above and below demonstrate this).  If your instrument has an accuracy of 0.1%, your error could be 17 times greater by substituting pins.   


Tension Link Loaded with Proper Pin Size
Calibration should be performed with the same load pins the end user is using the device.   Some links exhibit very high errors when substituting different types of pins. 


Okay, I have my pins.  I will always use the same ones, so now I am set.  Is there anything else to worry about?  Maybe.  If you are using forged pins and they are not aligned in the same position they were calibrated in, there could be additional random measurement error.     In our Lab, we have notes on each instrument and the way the loading pins should be aligned so we can duplicate calibrations.  We reference the pin diameter used on all our certificates and will put notes in with the instrument explaining the loading procedure.


Attention:


It is important to position any forged loading pins through the shackles the same way that we had the pins positioned during our calibration.  There can be variation in the surface of the pins from one pin to the next.  This variation may affect your ability to reproduce the calibration results.   We have a standard practice to label the pins.  Standard practice is to label the TOP PIN, BOTTOM PIN and mark the direction of each pin with an up arrow at the 12:00 position.  This helps the end user eliminate as much variation as possible, allowing for reproducible measurements. 


Tension Links Good Measurement Practice:

•  Using correctly sized pins is critical.
•  Do not use pins that are worn or bent.
•  If links are damaged, highly used, or worn, decrease the time between calibrations.
•  The same size and style of shackle and pin used during operation should be used for calibration.
•  Maintaining pin orientation is best practice



When the end user does not send in an adapter the calibration laboratory is left with having to load the device with some pin.  Luckily Morehouse has developed Clevis Kits to solve the this problem. 


Figure 1 Manufacturer's specification sheet for tension links

Finding the right pin size can be tricky as shown in figure 1 as the manufacturer’s recommendations may be counter-intuitive. In the figure 1 example, a 20-ton tension link requires a 2.0-inch pin and a 25 ton requires a 1.97 (50mm) pin.  One may say 0.03 inches will not make a difference and we would say go ahead and test it.  On a device with an accuracy specification of 0.1 % of full scale, we have observed a change of 0.03 inches to use up 70 % of that specification. 


Figure 2 Morehouse Clevis Kits

Knowing these issues, Morehouse has designed clevis assemblies for use with our Quick-Change Tension Adapters.  These assemblies cross reference the manufacturer’s recommended pin size and allow the calibration laboratory to calibrate hundreds of tension links, crane scales, dynamometers, and rod-end load cells all with using the same clevis.  Not only does this simplify the logistics of having the proper adapter, but it improves cycle time and standardizes the calibration process.   Link to Morehouse Clevis kits