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Modern Weigh Modules: Never Compromise Safety for Correct Installation

June 28, 2013
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Weigh modules can be used in applications across powder and bulk solids industries when unique scales, such as tanks, hoppers and conveyors, are required. Weigh module users - like many businesses - are under increasing pressure to improve efficiency, quality, safety, and regulatory compliance. In addition, weigh modules often are installed by sub-contractors who may not know how to properly and safely install a weigh module for consistently accurate and reliable weighing. In response, a new generation of weigh modules has been designed with fool-proof installation in mind to greatly improve performance—and safety—during installation.

Design Integrity and Safety Requirements
Modern weigh modules are designed for easy installation the first time. When purchasing a weigh module, consider the following design integrity and product-safety requirements.

1) Whether your application requires a tank, hopper, or conveyor, the weigh modules you install are an integral part of its support structure and, in many cases, provide the only restraint available to prevent the equipment from tipping. Weigh modules should be designed and optimized using finite element analysis (FEA). In addition, prototypes should undergo a complete set of physical and metrology testing to verify the computer model and to ensure performance requirements are met. The manufacturer’s load ratings should be fully documented in datasheets; that along with drawings in various formats will ensure safe and easy integration into the overall scale design. [c1]

2) In the powder industry, it is very common to have applications that take place in a classified hazardous area. In those cases, it's extremely important to choose the right load cells. Load cells used in hazardous areas must have approvals from accrediting bodies such as FM and ATEX. In Europe, the weigh module hardware must comply with the requirements of EN 13463-1, “Non-Electrical Equipment for use in Potentially Explosive Atmospheres….” That standard considers the mechanical aspects of equipment, such as its ability to create mechanical sparks, heat or static discharges. Compliance should be demonstrated by availability of an attestation of conformity issued by a notified body.

Key Design Considerations
Not all weigh modules are created equal. There are several important considerations that impact the accuracy and safety of your weigh module.

1) The weigh module should use a load cell with a proven self-aligning rocker pin suspension or a load cell that is itself a rocker pin. The rocker pin has spherical surfaces at the top and bottom and rocks to allow free thermal expansion and contraction of the scale and to move to absorb side impacts. That minimizes the side forces applied to the load cells and the performance degradation that would result. In addition, the self-aligning feature always returns the scale to the ideal weighing position, ensuring best repeatability and accuracy. Furthermore, the load cells should be high-performance cells with NTEP and OIML approvals as standard, allowing them to be used in legal-for-trade applications, such as bulk bag weighing, if required.

2) To take full advantage of the rocker pin, a weigh module’s top plate should not be constrained. It should have 360° travel, meaning it should move freely in all horizontal directions to allow thermal expansion and contraction of the scale—up to a point. A weigh module also should offer 360° checking. That means it should stop the scale in all directions when travel is excessive. Those features combine to ensure high performance and safety, regardless of how the weigh modules are oriented during installation.

3) Weigh modules should have anti-uplift protection as a standard feature, not a field-installed option that may get lost or forgotten about on site. That feature protects the scale in applications in which excessive side force is present, such as tanks that are installed outdoors in windy conditions. Anti-lift forces should not be transmitted through the load cell and should still function even if a load cell isn’t installed within the module.

4) A weigh module should have a vertical down-stop to prevent the scale from tipping in case of load-cell or suspension failure. It is unlikely to happen, but considering how important efficiency is in the powder industry, that feature can help provide peace of mind for plant managers. A down-stop is when the top plate assembly sits down firmly on the base plate after approximately 5mm of downward travel.

5) The load cell should be suitable for all environments due to its stainless-steel body and welded covers. They conform to the IP68/NEMA 6/6P as standard, and protect the electronics, ensuring long life and excellent performance.

Shipping and Installation Considerations
In recent years, modern weigh-module design has improved greatly in regard to ease and safety of installation. The last section described how 360° travel and 360° checking make installation simpler and safer. But that is only the beginning; there are several other enhancements designed to simplify installation.
    The illustration shows a modern weigh module in shipping/installation mode. Its top plate is supported directly from the base plate by the insertion of fitted spacer plates. The centering washers align the top and bottom base plates horizontally and then the weigh module is locked into a rigid block by tightening down on the anti-lift screws. That arrangement has many advantages, including:

1) All components are locked in their ideal initial positions, ensuring perfect load introduction and equal top-plate travel in all directions. Also, the top and base plates are locked parallel when viewed from the side; during installation, when you level one plate, the other is automatically level.

2) The spacer plate thickness is chosen to create a gap above the load cell, making it impossible to damage by shock or overload during shipping and installation.
3) The alignment and rigidity does not depend on the load cell so the installation can be done equally well without the load cell installed. Customers can take advantage of this by installing the weigh modules without load cells and by only at the last minute inserting the load cells in preparation for calibration. This eliminates any possibility of damage to the load cell and its cable, including damage from welding on the scale structure, an all too common occurrence. That is especially important on large construction sites where sometimes many sub-contractors are working on the scales over the course of months or years.

4) After installation is complete, it’s easy to convert the weigh modules from the shipping/installation mode to the weighing mode in preparation for calibration. Simply relieve the tank weight with a jack, loosen the anti-lift screw, and remove the fitted centering washers and spacer plate(s). Then remove the jack and retighten the anti-lift screw. Do this in turn for each weigh module and the scale is ready for calibration and weighing with the assurance that everything has remained perfectly aligned through the installation phase. [c2]

Summary
Modern weigh modules are versatile and packed with useful features for various applications, including those requiring higher accuracy. At the same time, the task of specifying, purchasing and installing the product safely has been greatly simplified. In other words, when installing these weigh modules, installation will be right the first time.
    Jeff Capella is a market manager for the OEM and System Integrator segment at Mettler Toledo LLC, a leading global supplier of precision instruments and the world’s largest manufacturer and marketer of weighing instruments for use in laboratory, industrial, and food retailing applications. Capella has three years of experience in the weighing industry having held roles in industrial inside sales and segment marketing. He holds a B.S. in Business Management from Wittenberg University. For more infromation, visit www.mt.com.

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