Baby Safemount load cell

A load cell is the element that measures force or weight. Standard load cells are metallic bodies that are deflecetd in a defined and elastic way by the force applied to them. The deflection is usually measured by glued-on strain gauges and converted into a voltage signal. When the load cell is loaded with a defined force, it emits a standardized signal (mV/V) proportional to the load.

For small to medium forces, bending beam or shear beam load cells are used. For higher forces, cylindrical bodies or double shear beam load cells are usually used.

A weighing assembly consists of the load cell and a mounting assembly, also known as mounting kit. The mounting assembly has the function of directing the force in the desired direction and optimally into the load cell. For silos and other bins, the assembly has the function of fixing the silo on the load cell and preventing it from tipping or shifting. The accuracy of the load cell should not be affected, or only very slightly. Some weighing assemblies also have integrated transport, installation and overload protection.

Regardless of the accuracy of the load cell, the assembly is of fundamental importance for the system accuracy of the weighing process. In addition, there are special weighing assemblies with vibration-damping properties, for example.

To achieve best accuracy, all support points should be weighed. Apart from the cost, there is no real limit to the number of load cells. Since the load on each support should be the same, 3 or 4 supports are easier to level than 8 supports, for example. 

For indoor bins and vessels, 3 support points are ideal because, with a symmetrical load and arrangement of the load cells, each support point automatically is loaded with the same load. For large external vessels and silos, a 4-point support is often chosen, as this provides a more stable support, which is advantageous if additional loads due to wind and earthquake apply. When erecting the structure, care must be taken to ensure that the load cells are loaded uniformly by levelling the wiehging system. 

With analog load cells, due to the parallel circuit, care must be taken to ensure that the measuring amplifier can supply the required number of cells.

The Baby Safemount weighing assembly has a significantly lower overall height. Furthermore, installation is much easier to carry out, as there are generally no stay bars or end stops to be adjusted. This is where mistakes are often made in practice. Baby Safemount modules can often be installed by the silo manufacturer or the company erecting the silo without the need for special knowledge of weighing technology.

Cylindrical load cells with so-called "pendulum bearings" require stay bars or end stops to prevent the silo from rotating. The stay bars limit the horizontal movement of the silo and allow vertical movement with as little friction as possible.

In Safemount and Baby Safemount weighing assemblies, the top and bottom mounting plates of the assemblies are connected to the load cell via hardened bolts. This allows the assembly to compensate for a certain amount of expansion (e.g., due to different thermal expansion coefficients between steel and foundation material) of the overall structure, while at the same time limiting the movement of the silo. A load table lists the maximum horizontal loads that may be applied to the weighing assemblies. This is relevant for structural design and approval.

A lift-off protection device is used on silos and other constructions when, for example, wind loads or earthquake loads apply and there is a risk that the silo could tip over case of uplifting forces. In the case of wind, for example, the wind exposure area is large in relation to the total weight of the silo, especially when the fill level is low, which is why the tendency to tip over is at its highest in these cases. Weighing assemblies with and without integrated lift-off protection are available. Lift-off protection is often not necessary for indoor vessels and silos. 

Integrated lift-off protection:
Lift-off protection prevents the silo or vessel from tipping over. The "Safemount" and "Baby-Safemount" weighing assemblies used for silo weighing allow integral lift-off protection due to their design. The top and bottom mounting plates of the modules are connected to the load cell via hardened bolts. However, lift-off protection must be provided on the vessel or silo on site to protect the cells against wind forces and earthquake loads. Lift-off protection is easy to implement in steel structures. 

As an alternative mounting assemblies with lift-off protection are available for Safemount load cells with a nominal load of up to 40 t. Two vertical threaded rods are screwed into the bottom assembly plate. These protrude through a hole in the upper mounting plate without touching the top plate. When the silo is empty, the maximum lifting distance is limited to 1–3 mm by a nut above the top plate and additionally secured by a lock nut.

Lateral forces such as wind loads are mostly balanced out by the parallel curcuit of the load cells. When one side of the silo is lifted by the wind, the load on the opposite side increases. The sum of the forces therefore remains the same. Wind loads are only noticeable in strong gusts when the averaging (filtering) of the measuring amplifier is significantly reduced. 

All "modern" weighing amplifiers have "sense" input. Only four wires are required to measure the weight with standard load cells. Two wires are provided for the power supply (5–15 V) and two wires for the output signal. For distances higher than 10 m, we recommend 2 additional wires, the "sense" wire. The weighing amplifier "measures" how much voltage is lost along the wire via the sense wire and compensates for the voltage loss mathematically. With a sufficient cross-section of each wire and separate laying of a shielded connection cable, distances of up to 200 m are possible. For longer distances, we recommend using an amplifier on site at the silo or a bus connection via our Intelligent junction box. 

There are also other options for transmitting the weight signal over longer distances. These include Profinet and Ethernet interfaces, radio, etc.

No. Each load cell in a weighing system should have the same capacity. Using load cells of different capacities will result in varied outputs among the load cells, causing weighing errors.