Choosing the Right Load Cell for Your Job
You would not buy a sports car if you planned on towing a trailer, the same way you wouldn’t rent an excavator to dig a fence post-hole. The key to starting any job is having the right tools and equipment. This is especially true for load and force-measurement applications.
Not all load cells are created equal. Industrial load cells are designed for different tasks, sizes, orientations, durability, environmental factors, and signal outputs. The data from a measurement application could be useless without the appropriate load cell. Choosing the right load cell is critical to obtaining high-quality, accurate readings.
A load cell is a sensor device that converts the energy of the applied force into a quantifiable electrical signal. The strength of the signal is proportional to that force — compression, tension, pressure, etc.
In a perfect world, under ideal conditions, measurement device operators would control the orientation of an applied load and the method for applying it. Load cell designs would be much less complex if this were easy. Because there is no perfect approach, different types of load cells exist to compensate for irregularities and the uncontrollable factors in load measuring.
This article describes the categories of strain gauge load cells that measure normal (both tension and compression) forces, shear forces, and torsion forces, while first briefly explaining each of these forces.
Load Input Direction
When designing a load cell, engineers consider the physical principle of materials called stress. Stress describes the intensity of the internal forces acting on a specific section of an object subjected to an applied load. If you bolt a steel I-beam to a wall and hang 500 lbs. from it’s free end, you could examine individual ‘slices’ of this beam and see that the loading affects each one differently.
Knowing the type of stress a physical system undergoes influences the design of the various load cells described in this article and also determines the right load cell for the job.
Normal Stress and Normal Loading
Normal stress is stress due to a force perpendicular to a defined plane or surface. This is the simplest type of stress to calculate since it is based only on the force applied () and the cross-sectional area () affected by the force. That is,
Normal stress can be either tensile or compressive. Tensile forces pull or stretch the body, while compressive forces squeeze the body.
An example of a measurement system for normal loads is a grocery or postage scale, where the object weighs directly down on the plate of the scale.
It is important to note that normal stress differs from normal loading. Load cells designed for normal loading do not always experience normal stresses. They can undergo shear stresses and bending.
Bending is when an external force or moment applied to a structural element causes it to bend. An object’s reaction to this external force differs from its reaction to a normal load. Half of the element sees tensile normal stress, while half of it sees compressive normal stresses. See Figure 3.
The difference between tensile and compressive stresses is important in selecting load cells because not every transducer is capable of reading both. For structural components, it is important because the failure modes of tension and compression are different.
Shear stress is a stress component that acts in the considered plane, much like a sheet of paper between two opposing blades of scissors. The point of maximum shear is where the opposing blades meet and cause the paper to shear into two parts. Figure 4 below labels the the shear force created by the load and models the deformation in the structure created by it.
Torsion occurs when an applied external force causes the structural body to twist. This external twisting force is called torque. Torsion is a very important stress component to consider because all rotating or spinning objects experience it. This includes wheel axles, driveshafts, gears, motors, propellers, etc. The figure below illustrates this force.
Load Cell Types and Applications
Now that we understand the main forces creating stress on an object (normal, bending and shear, and torque), we can look at the various load cells designed to measure these forces. Every load cell is designed for a different application. Below are a few types of load cell transducers and the applications where they best perform.
Load Cells for Normal Loading
Any scale where the object is placed on top of a platform will bear a normal load. The weight of the object is a force that acts vertically downward, causing the load cell to react. The load cell will respond with an output signal proportional to the weight of the object applied. This section describes a few in the Tacuna Systems offering.
Single Point or Platform
Single point load cells are most commonly used in low capacity, compact weighing, such as balances, food scales, pricing scales, packaging scales, medical and pharmaceutical scales, on-board weighing, and other retail applications.
These load cells accept loads that are not perfectly centered on a weighing platform. Each single point load cell has a designated loading area to apply the force. This prevents the need for load cell trimming, making these devices ideal for single load cell applications. Load cell trimming combines measurement signals from various load cell devices through a summing box when a measuring system consists of multiple load cells; this ensures that a measurement from a scale is the same regardless of the placement of an object on the loading platform.
The Tacuna Systems product line of single point load cells ranges in capacities from 300g to 2000kg.
Canister and Disk Load Cells
Canister, or disk load cells are a type of load cell used specifically for compression loads. The load passes through the top of the “button” of the device. Strain gauges at the internal core of the load cell measure compressive normal loads.
Canister load cells are designed to handle much heavier loads, but are much bulkier than other compression load cells. Tacuna Systems products’ capacity rating ranges from 25 to 500,000 kg. The sturdy design is resistant to bending loads and side loads, making these devices ideal for systems with several directional load components where only one must be quantified.
Miniature Load Buttons
Miniature load button load cells are a type of small canister load cell for lighter capacities, between 5 to 45 lbs.
They commonly appear in small retail and postal scales. Like other canister load cells, miniature load button load cells can be used in systems where forces exert from different directions, but measuring only one of these directional force components is necessary.
Planar beam load cells are chosen specifically for their low profile. They frequently appear at the four corners of a scale, trimmed together through a junction box that combines their measurement signals.
Planar beam load cells work well in applications where goods or products are sold commercially by weight. These commercial applications require scales that are “legal-for-trade” or “trade-approved”. Examples include airport luggage scales and grocery scales.
Planar beam load cells typically weigh loads in the 10-400lb range. The lowest capacity in the Tacuna Systems product line is 3.75Kg and the highest is a 375Kg load cell.
S-Beam Load Cells
S-beam load cells are powerful because they can be used in both tension and compression. They are durable with high endurance, making them ideal for light performance testing such as testing the resistance of doors, hinges, or springs. They are, however, susceptible to large bending moments due to their geometry. Rod-end bearings and clevises help orient the load path in the desired direction.
The rated capacity of this type of load cell in the Tacuna Systems offering ranges from as low as 2Kg to 40,000Kg.
Load pins are designed for suspended systems where a pin supports the structure. They can be used in shackles with clevis pins, sprockets, or pulleys. When in use, the load pin experiences shear forces. Strain gauges in a bore through the center of the pin measure these forces.
Load pins lend themselves to large applications, as they have a capacity of up to 100 tons and handle both tensile and compressive loads. Their design is specific to systems needing real-time measurements where a support pin is part of the structure; in these cases the load cell replaces the pin as a structural element. In compression applications, the pin replaces a bolt to support the load.
Tension link load cells are similar to canister load cells, but are for suspension applications. Like canister load cells they handle much heavier loads, but are much bulkier than other types. They are designed for crane scales or cable strength tests for loads of up to 100 tons.
Tension link load cells typically operate in areas with a wide variety of environmental exposure including construction sites and ports. For this reason they have environmental protections within their design that allow them to withstand heavy amounts of humidity, salt from seawater, and high coastal winds. Their design also makes them resistant to bending loads, side loads, and torsion.
Bending and Shear Force Load Cells
Shear beam load cells are designed to take loads that cause shear stresses or bending moments. The alignment of the load path through shear beam load cells should be vertical. These devices do not compensate well for torsion or other significant side loads out of plane with the loading path. Proper mounting can help mitigate misalignment.
Single-Ended Shear Beam
Single-ended shear beams are a simple and affordable load cell option for medium capacities ( 2-2650 lb ). One end of the beam is fixed while a load is applied to the free end, as seen in Figure 15 below.
The shear beam deforms when the force is applied, allowing the strain gauges to equate a load based on the amount of bending it experiences. These load cells are ideal when there is limited physical space between the load cell and it’s mount or fixture. Single ended shear beam load cells are also practical for large weighing applications where multiple load cells combine to form a single measurement system. Examples of these applications include tanks, hoppers, and other vessels.
Double-Ended Shear Beam
Double-ended shear beam load cells function similarly to single ended. However, double ended beams are fixed at both ends and the load is applied to the midway point.
Double-ended shear beams have higher load capacity ratings than single-end, so when supporting greater loads with size constraints, implement double-ended load cells. These devices often appear in industrial weighing applications such as floor-scales, weighing tanks, and other vessels. Tacuna Systems offers double-ended shear beam load cells rated from 1000-400,000 lbs.
These transducers, like other load cell transducers, convert mechanical inputs into a quantifiable electrical signal. Torque cells specifically detect a torsional input, or a force that causes the body to twist.
Most torque transducers use full or half Wheatstone bridge strain-gauge configurations. Typically the gauges measure the applied loads causing shear strain and bending strain and combine these outputs to quantify torsion or torque.
The two main types of torque transducers are static and rotating.
Static Torque Transducers
Static devices are used when one end of the shaft or measured body is fixed to a non-moving frame. Creating them is simple and easy to accomplish with conventional strain gauges and wiring.
Rotating Torque Transducers
Rotating parts, like driveshafts or propeller masts, are more difficult to measure due to their rotation. Connecting wires to a rotating object creates a need for engineering solutions like slip-rings or wireless transmission.
- Mechanics of Materials, 9th edition, Russell C. Hibbeler
- Measurement and Instrumentation in Engineering: Principles and Basic Laboratory Experiments, 1st Edition, Francis S. Tse, Ivan E. Morse