Selecting the right load cell is crucial to ensure accuracy. Before selecting, carefully consider the manner by which the load is introduced to the load cell structure and installation issues. Also, pay attention to such design considerations as hysteresis performance, creep, temperature compensation, and mV/V/ohm and mV/V calibration.
First some definitions
The two main types of load cells are rocker column and double end shear beam load (DESB). A rocker column load cell is defined as one that uses a single compression column design with self-restoring alignment. DESB load cells are either secured at both ends and loaded in the middle or secured in the middle and loaded on each end.
Introducing the load
Rocker column load cells provide convex spherical upper and lower loading surfaces for load introduction. The load cells use a dual radius loading surface, with a large radius in the center where the load measurement is made and a smaller radius at the outer extremities where the rocking occurs. When choosing this style load cell, make sure the design provides accurate load measurement when the load cell has rocked to a non-vertical position.
DESB beams use a variety of load introduction methods. Some are supported at the center with a convex pillar and the load is introduced by links at each end. Other methods include bolting the load cell at the ends, or introducing the load in the center with a link or mounting the load cell at the ends with horizontal rods or pins. When using these methods, take care to ensure that there are no hysteresis errors that could affect the scale’s accuracy and performance.
Hermetic sealing key to performance
Hermetic sealing is extremely important to prevent entry of moisture and other external contaminants into the body of the load cell. If selecting rocker column load cells, be sure they are hermetically sealed at both their main sealing element and at the cable entry.
Most DESB load cells are potted (also called encapsulated), which means they have been immersed in a liquid resin that is then cured. In some cases, the potted seal may not hold up as well as hermetical sealing in truck scale environments.
Design and installation factors can affect performance
Several design factors affect rocker column load cell performance. These include hysteresis performance, creep, temperature compensation, and mV/V/ohm and mV/V calibration. For example, Fairbanks load cells can be replaced without the need for readjusting the corners or sections, and secondly, load cells can be replaced without the need for system recalibration.
Regarding installation, rocker column load cells require only simple upper and lower loading cups in most applications. Simple alignment fixtures are used for installing the load cells in a vertical position. By contrast, large castings are usually required for DESBs.
Overall, experience shows that the totally hermetically sealed rocker column load cell offers major performance advances over double end shear beam load cells. In addition to being totally hermetically sealed, rocker column load cells have fewer load introduction problems, and are easier and less expensive to apply.