What kind of torture do your load cells have to withstand?
This article discusses how to select a load cell that can perform reliably under harsh operating circumstances and hostile situations.
In all weighing systems, load cells sense the weight of material in weigh hoppers, other vessels, or processing equipment. A load cell may be subjected to corrosive chemicals, heavy dust, high temperatures, or extreme moisture from washing down equipment with significant amounts of liquid in some applications. Alternatively, the load cell could be subjected to excessive vibration, uneven loads, or other difficult operating circumstances. Such circumstances can cause weighing errors or possibly harm the load cell if it is not properly selected. You’ll need a good understanding of your environment and operating conditions, as well as which load cell features are appropriate for handling them, to choose the right load cell for a challenging application.
What makes a tough application?
Examine the environment in which your weighing system operates, as well as the operational conditions in which the system must function.
- Will there be a lot of dust in the area?
- Is the weighing system going to be subjected to temperatures above 150°F?
- What are the qualities of the item being weighed chemically?
- Is the system going to be cleaned with water or anything else?
- What are the properties of cleaning chemicals that will be used to clean the equipment?
- Will the load cell be exposed to too much moisture as a result of your washdown method?
- Is the liquid going to be sprayed at high pressure?
- Is it possible that the load cell will be submerged in liquid during washdown?
- Will material accumulation or other factors cause the load cells to be loaded unevenly?
- Will the system be subjected to shock loading (huge loads applied suddenly)?
- Will the dead load (the vessel or equipment housing the substance) be disproportionately large in comparison to the live load (the material) in the weighing system?
- Will the system be subjected to high vibration from nearby processing or handling equipment or passing vehicles?
- Will the weighing system be subjected to high torque forces from the processing equipment’s motor if it’s for processing equipment?
You’re ready to choose a load cell with the proper features to not only withstand but also work reliably over time after you know the conditions your weighing system will endure. The following details the load cell features that are available to handle your demanding application.
Consult an experienced load cell provider or an independent bulk solid handling specialist for assistance in selecting a load cell that meets your stringent standards. Expect to provide extensive information about the material your weighing system will handle, your operating environment, and how the load cell’s operation will be affected by the conditions. A load cell is a metal element that bends as a result of the applied load. Tool steel, aluminium, or stainless steel can be used to make this element, which incorporates strain gauges in an electrical circuit.
Tool steel is the most commonly used material for load cells in dry applications because it offers outstanding performance at a reasonable cost and a wide capacity range. The tool steel load cell can be used for single and multiple load cell (also known as single- and multiple-point) applications. Because moisture can corrode tool steel, it’s best used in dry settings. Because it’s easy to machine and heat-treat, type 4340 is the most used tool steel alloy for these load cells. It also springs back to its exact starting position after the applied load is removed, preventing creep (a gradual increase in load cell weight readings over time when the same load is applied) and hysteresis (the difference between two weight readings for the same applied load, obtained by increasing the load from zero and decreasing the load from the load cell’s maximum rated capacity).
Aluminum is the cheapest load cell material and is commonly utilised in single-point, low-capacity applications. Wet or chemical situations are not suitable for this material. The most prevalent aluminium alloy is type 2023, which, like type 4340 tool steel, returns to its precise starting position after weighing, reducing creep and hysteresis.
Type 17-4 PH (prescription hardening) stainless steel – also known as grade 630 stainless steel – has the best overall performance of any stainless steel derivative for load cells due to its strength and corrosion resistance. This alloy is more expensive than tool steel or aluminium, but it provides the best performance of any material in wet (that is, chemically corrosive) and wet applications (that is, those needing a substantial washdown). Some substances, however, can harm the type 17-4 PH alloy. One alternative for these applications is to add a thin coat of epoxy paint to the stainless steel load cell (between 1.5 and 3 millimetres thick). Another option is to use a load cell composed of an alloy steel that is more corrosion resistant. Consult a chemical resistance chart (several are accessible on the Internet) and work closely with your load cell provider to choose the proper load cell material for a chemical application.
The cable that connects the load cell to the controller of the weighing system is also available in a variety of materials to withstand rigorous operating conditions. Most load cells employ a cable that is protected from dust and moisture by a polyurethane jacket. Some stainless steel load cells have a Teflon coated jacket for chemical resistance or will be used with high-temperature load cells (up to 400°F). If your load cell will be exposed to chemicals or extreme temperatures, you can request that it be made with a Tefl on cable, regardless of the load cell’s construction material.
From 0°F to 150°F, a load cell is temperature adjusted to provide accurate weighing data. Unless you choose a load cell that can endure temperatures up to 400°F, the load cell can give erratic readings or possibly malfunction when subjected to temperatures above 175°F. A high-temperature load cell is made up of high-temperature components such as strain gauges, resistors, wires, solder, cables, and adhesives and can be made out of tool steel, aluminium, or stainless steel.
To protect the interior components from the environment, a load cell can be sealed in a variety of methods. A rubber boot that fits over the load cell’s strain gauge cavity, a cover pasted onto this cavity, or a potted strain gauge cavity filled with material such as 3M RTV can all be used to create an environmentally sealed load cell. The interior components of the load cell will be protected from dust, debris, and moderate moisture, such as that created by splashing water during washdown, using any of these ways. A load cell that has been sealed against the elements will not be protected from high-pressure liquid cleaning or immersion during a heavy washdown. A hermetically sealed load cell provides additional safety for chemical applications or heavy wash down. Stainless steel is commonly used for this load cell because it is the ideal material for these demanding applications. The strain gauge cavity is encased by a welded cover or sleeve on the load cell. A welded barrier protects the cable-entry area of a hermetically sealed load cell from moisture wicking into and shorting out the load cell. The hermetically sealed load cell is more expensive than an environmentally sealed load cell, but it provides a long-term solution for this type of application.A welded seal load cell is suited for situations where the load cell may be exposed to water on a regular basis, but it is not suitable for heavy-wash down applications. The welded seal load cell is identical to the hermetically sealed load cell except for the cable-entry location, where it offers a welded seal for the load cell’s internal components. There is no welded barrier in this section of the welded seal load cell. The cable-entry location can be provided with a conduit adapter to assist protect the cable from moisture. The load cell cable can then be fed through the conduit to further protect it.
Overloads (caused by overfilling the vessel), minor shock load cells (caused by an outlet gate opening to discharge the entire load at once), excess weight on one side of the vessel (for example, from material buildup or a motor mounted on one side), or even live and dead load miscalculations can all affect the load cell in many tough applications. A weighing system with a high dead-load-to-live-load ratio (that is, where the dead load consumes a large amount of the system’s capacity) might also endanger the load cell because the high dead load affects the system’s weighing precision and accuracy. Any of these difficulties can cause weighing inaccuracies or damage to the load cell. To ensure that your load cell can provide accurate results under these conditions, it must be sized to handle the weighing system’s maximum live and dead loads, as well as an additional safety factor.
The simplest way to figure out the load cell size you need for your application is to add the live and dead loads (usually in pounds) and divide by the number of load cells in your weighing system. When the vessel is loaded to its maximum capacity, this is the weight that each load cell will be subjected to. To protect against overfilling, mild shock loads, unbalanced loads, or other severe loading conditions, add 25% to the amount you compute for each load cell. Also keep in mind that all load cells in a multiple-point weighing system must have the same capacity in order to deliver reliable data. To compensate for the excess weight, even if it is only applied at one load point, all load cells in the system must have a bigger capacity. Because this reduces weighing accuracy, avoiding off-balance loading is usually a better option.
Choosing the proper features and size for your load cell is only the beginning. Now you must carefully install the load cell so that it can withstand your harsh conditions.
Load Cell Installation
Installing the weighing system correctly will assist ensure that each load cell provides accurate, dependable weighing results in your demanding application. Make that the weighing system’s floor (or the ceiling from which it is suspended) is level and plumb, as well as robust and solid enough to support the system’s full load without flexing. Before you install the weighing system, you may need to tough up the floor or add bigger support beams to the ceiling. In some problematic applications, strong vibration is conveyed to the weighing vessel through the floor or ceiling from numerous sources, such as passing cars or motors on nearby processing or handling equipment. Large torque loads from a motor (such as on a mixer supported by load cells) are applied to the vessel in various applications. These vibrations and torque forces can cause the vessel to deflect non-uniformly if it isn’t properly mounted, or if the floor or ceiling isn’t sturdy enough to hold the vessel adequately. The deflection may cause erroneous load cell readings or overload and destroy the load cells. You can install isolation pads between each vessel leg and the top of the load cell’s mounting assembly to absorb some of the vibration and torque stresses on a vessel with compression-mounted load cells. Avoid hanging the weighing vessel from tension-mounted load cells at the ceiling in an application prone to severe vibration or torque forces because the forces can cause the vessel to sway, preventing accurate weighing and causing the suspension hardware to break over time. To prevent the vessel from deflecting significantly under load, support braces might be added between the vessel legs.