Flexure Configurations: Bending Beams
The field of force measurement has the same types of constraints as any other discipline: weight, size, cost, accuracy, useful life, rated capacity, extraneous forces, test profile, error specs, temperature, altitude, pressure, corrosive chemicals, etc. Flexures are configured in many shapes and sizes to match the diversity of applications out in the world.
Bending Beam Cell
The cell is bolted to a support through the two mounting holes. When we remove the covers, we can see the large hole bored through the beam. This forms thin sections at the top and bottom surface, which concentrate the forces into the area where the gages are mounted on the top and bottom faces of the beam. The gages may be mounted on the outside surface, as shown, or inside the large hole.
The compression load is applied at the end opposite from the two mounting holes, usually onto a load button which the user inserts in the loading hole. Interface ME series cells are available in capacities from 5 to 250 lbf. SSB series cells have a splash-proof sealing cover and come in sizes from 50 to 1000 lbf.
Double-Ended Bending Beam Cell
A very useful variation on the bending beam design is achieved by forming two bending beams into one cell. This allows the loading fixtures to be attached at the threaded holes on the center line, between the beams, which makes the sensitive axis pass through the cell on a single line of action. In general, this configuration is much more user friendly because of its short vertical dimension and compact design.
The Interface SML cell is available in capacities from 5 to 1000 lbf. The 5 and 10 lbf cells can also be ordered with tension/compression overload protection, which makes them very useful for applications where they could by damaged by an overload.
The Interface SM(Super-Mini) cell is a low-cost, yet accurate, cell with a straight-through loading design. (See Figure 10). At slightly higher cost, the SSM (Sealed Super-Mini) is a rugged S-cell with splash-proof covers. Either series gives exceptional results in applications which can be designed so as to operate the cells in tension.
Although the forces on the gaged area appear the same as in a bending beam cell, the theory of operation is slightly different because the two ends of the “S” bend back over center, and the forces are applied down through the center of the gaged area. However, considering it as a modified bending beam cell may assist the reader in visualizing how the cell works.
Some caution should be exercised when using these cells in compression, to ensure that the loading does not introduce side loads into the cell. As we shall see later, the Low Profile series is better suited to applications which may apply side loads or moment loads into the cell.
SMT Overload Protected S-Cell
The incorporation of overload protection is a major innovation in S-Cell design. By removing the large gaps at the top and bottom, and replacing them with small clearance gaps and locking fingers, the whole cell can be made to “go solid” in either mode (tension or compression), before the deflection of the gaged area exceeds the allowed overload specification. Those gaps and fingers can be seen in Figure 11, which shows the flexure with the covers removed. The double-stepped shape of the gaps is necessary to ensure that overload protection operates in both modes.
The SMT series is ideally suited for applications that may generate forces as high as eight times the rating of the load cell. The two loading holes are in line vertically, which makes the cell easy to design into machines which apply reciprocating or linear motion, either from a rotating crank or from a pneumatic or hydraulic cylinder.
The covers provide physical protection for the flexure, but the cell is not sealed. Users should therefore be cautioned not to use it in dusty applications which might build up collections of dust in the overload gaps. Should a buildup occur, the overload protection would come into effect before the load reaches the rated capacity, thus causing a non-linear output.
The SMT series is especially suited for use in laboratories or medical facilities where large loads could be applied accidentally by untrained or non-technical personnel.
LBM and LBT Load Button Cells
Many applications require the measurement of forces in a very confined space. Where high precision is required, the Interface Low Profile cell is the obvious choice. However, where space is at a premium, the smaller LBM or LBT can fulfill the need for force measurements at a very respectable precision, sufficient for most applications.
These miniature compression cells range in capacities from 10 lbf to 50,000 lbf. Diameters range from 1 inch to 3 inches, with heights from 0.39 inch to 1.5 inches. The shaped load button has a spherical radius to help confine misaligned loads to the primary axis of the cell.
SPI Single Point Impact Cell
Although the SPI resembles competitive weigh pan cells, it was specifically designed to have greater than normal deflection at full scale, to provide for the addition of stops to protect the cell against compression overloads. This was necessary because the usual deflection of 0.001 inch to 0.006 inch of most load cells is much too small to allow for the accurate adjustment of an external stop to protect the load cell.
SPI cells with capacities of 3 lbf, 7.5 lbf, and 15 lbf contain their own internal compression overload stop which is adjusted at the factory to protect the cell up to four times the rated capacity. These cells have an additional bar under the lower surface, to provide a mount for the internal compression stop screw. Capacities of 25 lbf, 50 lbf, 75 lbf, and 150 lbf can be protected by placing hard stops under the corners of a weigh pan to catch the pan before excessive deflection damages the SPI cell.
Figure 14 shows the internal layout typical of the larger capacities of the SPI. The cell mounts to the scale frame on the step at the lower left corner, while the scale pan is mounted on the upper right corner with its load centroid over the primary axis at the center of the cell.
The center bar, containing the gages, is a bending beam. It is supported by the outer frame containing four thin flexure points, two on the top and two on the bottom, to provide mechanical strength for side loads and moment loads. This construction provides the superior moment canceling capability of the SPI, which ensures a consistent weight indication anywhere within the weigh pan size limits.
The SPI is also very popular with universities and test labs, for its precision and ruggedness. It is also very convenient for lab use. Fixtures and load pans can be mounted easily on the two tapped holes on the top corner.
1500 Low Profile Rotated Bending Beam
The Interface Model 1500 combines the moment canceling advantages of the Low Profile design, with the lower capacity desired by many customers who have precision testing applications.
Figure 16. Model 1500 outline
Although the external appearance of the 1500 is quite similar to the 1000 Series cells, the internal construction is quite different. Figure 16 shows the cross section of one of the two crossed beams, and the similarity to the SML double-ended beam is obvious. Moreover, the additional crossed beam, at 90 degrees to the beam shown in section, ensures moment stability in all directions around the primary axis.
The Model 1500 is available in capacities from 25 to 300 lbf to complement the Model 1200, whose lowest capacity is 300 lbf. In addition, the diameter of the Model 1500 is only 2.75 inches, and the connector orientation allows better clearance for the mating connector to clear nearby objects.
Note that the base is integral with the cell, which aligns the whole cell for straight-through applications. The balanced design around the primary axis ensures maximum cancellation of moment forces. The cell is sealed to protect it from the environment in typical production situations.