How to Calibrate S-Beam or S-Type Load Cells and Achieve Good Results
Figure 1 General S-Beam Load Cell
General Overview of S-beam or S-Type load cells
S-beam or S-Type load cells were designed for several weighing applications. Most were designed to be mounted in a scale, used as a lifting system, or process weighing for hopper and suspended tank weight measurement. They are typically not the right choice for several force applications requiring calibration to the following standards such as ASTM E74, ASTM E4, ISO 376, and ISO 7500. These load cells work by placing a weight or generating a force on the load cell's metal spring element. The weight on the load cell causes elastic deformation. The strain gauges in the load cell measure the fractional change in length of the deformation. There are generally four strain gauges mounted in the load cell, and an indicator is used to measure the change in resistance of the strain gauges.
Figure 2 S-Beam Load Cell Loaded Flat on Bottom and with Morehouse Threaded Ball Adapter on Top
Why S-Type Load Cells are Not a Good Choice for Force Application
This article is going to show examples of challenges anyone using S-Beam load cells will face when trying to use this type of cell for force calibration. When the S-beam cells are mounted in place, they can be calibrated by weights, and the elastic deformation of the load cell will most likely allow the measurement to be repeated with acceptable results. However, when these load cells are used as transfer standards or people try to use them to measure force, they are typically not mounted in place and present several challenges. The challenges range from alignment as the load cells are susceptible to off-axis loading from various types of fixturing and adapters.
S-Type Load Cells Calibration Challenges
Figure 3 Slight Misalignment of S-Beam Load Cell Showing Large Errors (Slide Taken From Morehouse Training Class)
The S-Beam type load cells are susceptible to off-axis loading conditions, as shown in Figure 3. The S-beam was placed in our Morehouse deadweight machine accurate to better than 0.002 % of applied force, and readings were taken with the load cell aligned. Then we just slightly, probably around 3 mm and took another set of readings. The difference between the two measurements was approximately 0.752 %. This sensitivity to the slightest misalignment makes these cells very difficult to calibrate without using the proper adapters. The main issue is the calibration laboratory may have the appropriate alignment fixtures, but if this type of load cell was used in the field for ASTM E4 or ISO 7500, the technician would most likely struggle to achieve alignment. If the load cell was used to verify a force in a press or another force application, could proper alignment be achieved? We did the same test with a Morehouse shear web load cell and demonstrated an error of less than 0.0025 % in Figure 4, which is negligible. Both of these tests are shown via video and can be seen by following the links below.
Figure 4 Morehouse Shear Web Load Cell
Because the S-beam is so susceptible to off-axis loading, they are a better choice for applications where they can be permanently fixed into place. The best example would be mounted in a crane scale or other types of scales.
S-beam misaligned video link https://www.youtube.com/watch?v=15630Ab8YkU
Morehouse Shear Web misaligned video link https://www.youtube.com/watch?v=MgTWK2hRHLs&t=2s
Different Loading Conditions
Not only are S-beam load cells sensitive the slightest bit of side load; they have relatively large errors if loaded differently from how they were calibrated.