Bolting a Load Cell Thread Class and Torque Applied
Let's start with the basic assumption that any cell that needs to be calibrated should be bolted down by the end-user of the equipment prior
to being sent anywhere for calibration. When you send a load cell that requires bolting, you should not expect the calibration results to be valid for your testing needs. If you are not testing the load cell in the machine it is being used in,
you will need to account for additional errors due to the following;
mounting considerations, variation between different bolts, material in the base, surface finish on the base, hardness, stiffness, alignment,
flatness, bending, and variations from using different bolting sequences may contribute to the uncertainty. Lets us not forget the torque wrench that may be accurate to 4 % of applied torque that was used to torque the bolts. This assumes a torque wrench was even used.
I hope by now most of you are starting to understand that this type of calibration has several major possible uncertainty contributors that all need to be addressed. Even if all of these potential errors are quantified, my bet is an R & R study has not been performed between the technicians installing the load cell in the machine, and the technicians in the laboratory performing the calibration. Is there a significant difference between these technicians? I'm going to go on record and say " You Betcha" unless a very rigid testing procedure was documented and you have SPC charts to prove it. My assumptions are that any lab going to these great lengths to quantify all of these errors has already realized that there is probably a better way to ensure more repeatable results.
Morehouse is always going to suggest purchasing the proper equipment that will allow for calibration of the load cells in the machines they are being used. There are two very good standards that will give you detailed instructions on how to use a load cell system to calibrate these load cells in place. These standards are ASTM E4 and ISO 7500. Morehouse can provide a complete load cell system with ISO/IEC 17025 accredited calibration in accordance with ASTM E74 or ISO 376 to perform in place calibrations of load cells in the machines, they are being used. If you are working to ASTM standards that reference ASTM E4 as a calibration requirement, it is important to note that the entire system needs to be calibrated if removed. When equipment is not available to allow for in-place calibrations, please remember to account for the various uncertainty contributors listed above in addition to environmental conditions, the uncertainty of the reference standards used to perform the calibration, the resolution of the device, the stability of the instrumentation and the reproducibility and repeatability of the measurement process.
Also, please remember that most load cell manufacturers, the removal of the load cell from the base voids the warranty
Unbolting these force-measuring devices is generally a bad idea as accounting for all of the additional error sources and measurement traceability can be problematic. If you are in a situation where unbolting is required, please feel free to contact the Morehouse support team to ensure you are following the best-recommended calibration guidelines in hopes to limit the overall measurement error and risk.