Tips from the Cal Lab: How to use an Alignment Plug for Compression Calibration Setups

January 12, 2021

Centering a load cell is critical to obtaining the correct line of force and reducing calibration errors. An alignment plug along with an adapter plate can be used to center a load cell, whether it is a reference standard in a calibrating machine or a unit under test.

Why use an Alignment Plug?

No matter how good an operator may be, they are no match for something machined with precision. When someone must center everything with tools, it takes a bit of extra time to measure, center, measure again and then start the calibration. Quite often, misalignment still happens, and the technician is forced to redo the calibration. However, by using bottom adapters with concentric rings for alignment or alignment plugs to correctly center the force-measuring instrument, the operator can reduce misalignment error and quite possibly save on costly rework due to misalignment.

Keeping the line of force pure (free from eccentric forces) is key to the calibration of load cells. ASTM E74-18 in note 5 states, "Force-measuring instruments have sensitivity in varying degrees depending on design to mounting conditions and parasitic forces and moments due to misalignment. A measure of this sensitivity may be made by imposing conditions to simulate these factors, such as using fixtures with contact surfaces that are slightly convex or concave, or of varying stiffness or hardness, or with angular or eccentric misalignment, and so forth. Such factors can sometimes be significant contributors to measurement uncertainty and should be rejected in comprehensive measurement uncertainty analyses." Morehouse compression adapters such as alignment plugs and adapter plates are manufactured to match the calibrating machine’s load line, thus keeping the line of force pure.

How to Properly use an Alignment Plug

At Morehouse, we have received many load cells with an alignment plug stuck in the load cell’s bottom. Pictured below is a load cell with a Morehouse 5/8" -18 alignment plug (part # CA-1-0.625-18-01), which is not correctly threaded into the load cell.

Improper Way to Thread an Alignment Plug

When this happens, the bottom threads of the load cell can be damaged. If it is not damaged, then the incorrect loading condition will produce additional calibration errors because the load cell will not be sitting flush on the base. Two very different errors can occur together when this happens.

  1. The line of force is impacted by a non-flat surface, producing additional side-loading.
  2. The force is transferred through the threads of the load cell rather than the base.

Side-loading can cause significant errors of about 0.01 %. Loading through the bottom threads in compression accounts for an error of roughly 0.012 % of output. Therefore, improper threading will likely produce calibration errors of more than 0.02 %.

Proper Way to Thread an Alignment Plug

The solution to avoid these errors is straightforward. When using an alignment plug, make sure to thread the plug into the load cell base so that only a small portion of the plug is sticking out. The technician should easily align the load cell by sliding the load cell on the adapter until it sits in the centering hole. This will also ensure that the load cell is loaded on the base, just as it was likely done at the time of calibration. When used correctly, alignment plugs can be very helpful to reduce errors and save a technician’s time.


I take great pride in our knowledgeable team at Morehouse, who will work with you to find the right solution. We have been in business for over a century and focus on being the most recognized name in the force business. That vision comes from educating our customers on what matters most, having the right discussions relating to how to use calibration adapters properly, and helping the technician save time and costly rework.

Everything we do, we believe in changing how people think about force and torque calibration. We challenge the "just calibrate it" mentality by educating our customers on what matters, what causes significant errors, and focus on reducing them. Morehouse makes simple to use calibration products. We build fantastic force equipment that is plumb, level, square, rigid and provide unparalleled calibration service with less than two-week lead times.

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