Load Cell Loading: Thread Engagement Errors
Morehouse has covered this topic in previous videos and blogs. However, it is important enough to write another blog about thread engagement. Why is it important?
In a typical design using female threads, the modeling is done with “assumptions” on the thread engagement. A standard rule is to assume the engagement will be at least equal to the thread sizes.
In general, the first handful of threads provides maximum strength. Engaging beyond this first handful often yields diminishing returns.
Therefore, most people live by the rule of thumb of at least engaging the thread by the diameter of the thread. In many load cells, engaging less or more can produce large changes in output. The loading conditions for almost any load cell are critical when trying to minimize the error.
Below are several tests conducted by Morehouse on various load cells showing the differences in loading conditions.
S-Beam example where loading through the threads versus against the base produces up to a 0.369 % difference in output.
10,000 lbf shear-web example showing a maximum difference of 0.044 % with an integral adapter installed versus engaging about 0.625” of thread.
This error can become larger, depending on the actual thread engagement. A standard rule of thumb is to engage at least the thread diameter. At 0.625” of engagement, we have observed errors as high as 0.2 % comparing three different setups of calibration with an integral adapter and a non-shoulder loaded full engagement of over 1”.
10,000 lbf load cell tested with two different adapters against the shoulder. The difference in output was 0.59 % using two adapters. One with 1.5” (10,001.5 lbf) of engagement and another with 0.5”(9,942.3 lbf) of engagement.
The error on this measurement was over 0.5 % on a device expected to be better than 0.025 % (20 times expected).
The best recommendation is to purchase and lock in an integral adapter as many Morehouse shear web load cells come standard with an integral adapter. Other manufacturers sell similar load cells, though the adapter is an add-on and is often not purchased with the load cell. This often creates a significant disconnect between what the customer will use with the load cell and how it was calibrated.
If an adapter is not purchased, or one is not installed into the load cell, Morehouse recommends having discussions with your calibration laboratory about how they calibrate such devices. The examples above show the drastic difference between different adapters and different loading conditions. The errors can be 20 times what might be expected. When this happens, there could be significant global risk passed via the next measurement which is eventually passed to the consumer.
Back to the general rule of thumb: if we do not know, we engage the thread by matching the diameter of the load cell. There are two very easy ways to do this.
- Measure the depth of the thread and put a line where the thread should stop. Electrical tape and thread tape can also work.
- Calculate the number of turns it would take to match the thread size.
The basic calculation is to multiply the number of threads per inch by the engagement required. Example 1: I have a ½”-20 thread and want 0.5” of engagement. Since I have 20 threads per inch, I would divide 1/20. This gives me 0.05” of engagement per full turn. Then take 0.5” / 0.05, which equals 10. Thus, for 0.5” of engagement on a ½”-20 thread, I would need to count 10 full turns.
Example 2: I have a 1 ½”-12 thread and want 1.5” of engagement. Since I have 12 threads per inch, I would divide 1/12. This gives me 0.083” of engagement per full turn. Then take 1.5” / 0.083, which equals 18. Thus, for 1.5” of engagement on a 1.5-12 thread, I would need to count 18 full turns.
Again, the best practice is to have an integral adapter locked into place. The next best solution is to purchase adapters such as a load button from Morehouse (shown below) or supply adapters for use with the calibration. Be sure to explain thread engagement and how the adapters are used.
If no information is obtainable, we recommend reverting to this general rule of thumb: match the thread engagement with the diameter of the thread. We caution everyone to make sure their equipment is safe and designed in such a way that this rule does not pose any safety hazards and that this amount of engagement does not damage the load cell.
Morehouse wants to help educate our customer base on what we do and how we do it. We are always looking to improve our capabilities and yours. Knowledge is power. We hope this article has been useful in understanding the differences pertaining to measurement errors from engaging threads at different depths that do not align with how the item was calibrated. We urge everyone to consider installing integral adapters or purchasing adapters from Morehouse or wherever to pair with your equipment for calibration. Please continue to check out our blogs, and the Documents and Tools section of our website for information that is guaranteed to help you start to make better measurements.
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