While selecting the appropriate load cell model for an application is important, if the load cell is not properly mounted and installed, its output measurements could be useless. Properly installing a load cell is critical to obtaining high-quality, accurate readings. It also guarantees ease of use and safe operations.
This article will give helpful load cell installation tips but is not intended to be an installation guide. For best results always follow the manufacturer’s installation instructions, ensure that installers are properly trained prior to installation, and if questions or concerns arise while installing our products, contact Tacuna Systems immediately.
The Right Frame and Fixture
Every load cell is designed for a different environment, set of conditions, and mechanical orientation. Setting up the proper fixture (or load cell mount) and frame is critical to maximize load cell performance and maintain safe operating conditions.
The fixture should be designed to minimize unintended inputs from the surrounding environment. Vibrations, thermal expansion, mechanical deformation, and stray electrical current can cause false readings and damage to or failure of the load cell.
An optimal frame design should maximize stiffness but minimize weight and costs. The frame is required to be sturdy enough to support the maximum weight capacity of the measuring device, and rigid enough to withstand deformation and flexing. The supporting features to the structure should be rigid, ultimately mitigating vibration issues.
A variety of sources can introduce vibrations that affect the fixture, including compressors, pumps, actuators, and engines. Any of these can lead to inaccuracies in the load cell measurements. Additionally, ground vibrations from seismic activity can skew measurement results.
Securing the frame to a hard flat surface will reduce vibrations from passing vehicles and local equipment. This will also prevent component and hardware failure that could cause personal injury or damage to surrounding equipment.
If possible, turn off vibrating or rotating equipment during measurement taking and calibration of load cells. If equipment cannot be turned off, select load cells that compensate for dynamic loading.
Aligning Loads in the Proper Direction
Ideally, in a measuring system all of the load or force will be transmitted through the load cell. Failure to set up the fixture in this way will cause inaccurate “force shunts” that interfere with measurements.
A force shunt is any device or path through which a portion of the force flow or load is diverted. While this term is typically used in electrical systems, the concept remains true for the “flow” of force. The force being considered is mechanical, and can be created by a physical load, weight, or pressure.
If the force to be measured is not fully applied through the load cell, the measurement system cannot capture the correct reading. Avoiding force shunts is critical for weighed objects such as tanks, platforms, or scales.
For a more in-depth explanation of force shunts, see Measuring Forces in the Force Shunt.
Place load cells at all corners of the supporting structure to maintain the full weight or load. While some manufacturers recommend placing “dummy” cells on supports and deriving the total load, dummy cells can prevent proper calibration. When using load cells under structural supports, always arrange them symmetrically around the plane perpendicular to the force flow, and level to this plane.
If there are attachments to the measured fixture due to process connections, (i.e., piping, hosing, ducts per Figure 1) they should remain flexible to reduce force shunting. Small diameter piping, or long segments without significant supporting structures, can also cause significant force shunting. If possible, use larger diameter connections at all times. Connect piping with hoses or accordion tubing for best results.
Ladders, pipes, rods, and catwalks can all improperly load, or shunt, a portion of the measured structure. Remove these when measuring or compensated for the errors they introduce to the output measurement.
Despite these advisories safety is a high priority; therefore use safety features even if they create force shunts. Choose flexible over rigid structures, such as cables or chains, to help reduce force shunts while assuring high levels of safety for operators.
Environmental Factors Affecting Load Cell Fixtures
Every load cell will perform differently depending on the environment. Before installing the measuring system, planners should do a thorough analysis of the environmental factors influencing the load cell and fixture. Keep the load cell application in a controlled environment and when possible use devices indoors to reduce environmental effects. When the load cell application must be outdoors, the operation of the measuring system (including calibration) will need to account for the effects of temperature, wind, precipitation, sunlight, humidity, electrical systems and ground conditions to derive accurate readings.
If ambient temperatures fluctuate where devices will operate, use load cells that are temperature compensated. This information is found on the load cell model datasheet. Uncompensated devices will behave poorly at extreme temperatures, so it is important to cover them from radiant heat and cooling systems. Installing a shield or insulation where the device is housed can provide protection from extreme temperatures.
Temperature extremes may affect not just the load cell itself but also the frame and fixture. When thermal expansion of the frame or joints occurs, ensure that the mount movement will not be hindered. If the load cell mount does not allow for mechanical expansion, permanent component damage could result from fixture movement. To maintain a safe operating environment, always design for the appropriate amount of expansion clearance.
If the operating environment will be exposed to moisture or precipitation, discuss with the manufacturer how well the load cell performs and compensates for moisture. Moisture can lead to shorting in the electrical current causing errors in measurements, and can often damage the load cell.
Take precautions to avoid corrosion of both the load cell device and the mount, and inspect them frequently. Some steps include using non-corrosive metals such as aluminum and stainless steel when the device will be exposed to moisture, and keeping the load cell and surrounding surfaces dry. Also check for cracks caused by corrosion pitting that could lead to damage of the load cell and equipment failure.
Stray electrical currents could interfere with load cell readings and are another possible source of damage to the device. Properly cover and secure all wiring and lighting, and ground the load cell fixture to the specifications outlined by the manufacturer. Avoid welding the frame, fixture, or mounts while the load cell device is connected. Stray currents from welding can significantly damage components.
Properly Preparing the Mounting Frame
The preparation of the surface and structure of the load cell mount is critical to delivering accurate results. When installing mounts, this surface should always be clean, even, and level. This will allow the total load to be transferred through the bottom mount of the load cell and to the supporting structure.
Mount the load cell so that it acts as the intermediate point between a fixed surface and the introduced load path. The load direction relative to the load cell body’s orientation should always comply with the installation manual provided by the manufacturer. These axes are often clearly marked or labeled on the device. The loading direction will ideally be perpendicular to the loading surface.
For load cells designed for axial loading (see definition in our Force Measurement Glossary), avoid significant side loads and bending moments. Use suitable mounting to mitigate misalignment. If side loading or torsion cannot be avoided, use the appropriate load cells to capture these inputs. Rod ends or clevises (Figure 2) can also help reduce the bending moments experienced by load cells.
The load cell should be mounted to heavy plates attached to the fixture. The surface should be level and machined. As mentioned before, these plates should be rigid and non-deformable. Tacuna Systems offers stock or custom-designed mounting kits as necessary for each specific application.
Drill and tap holes in the mounting plates based on the dimensions in the load cell’s installation drawing or data sheet. Maintain tight tolerances when drilling. Loose tolerances could prevent the proper installation of, or cause unnecessary residual stress in the device and hardware.
Select a size and capacity of the load cell appropriate for the application without significant oversizing. The dimensions of the load cell will affect clearances of the fixture when it is loaded to capacity.
Single point load cells are rated for a specific loading area; therefore keep the load or weight within these margins. Top plates and scales should be designed with these capacities and limitations in mind. While load cells are rated for some margin beyond their capacity, it is best practice to keep the max load and location of the load safely within margins.
Properly securing the load cell to the support plates will limit unintended movement in the device. This keeps the load cell aligned with the load path, maximizing accuracy and preventing damage to the component.
Always use the provided device hardware or other standard hardware approved by the manufacturer. Install all hardware to the specifications outlined in the installation manual. Engage the full threaded section of bolts connecting the support plates, and apply the proper preload before allowing the system to support the full weight of the measured entity.
If bolts are not preloaded or preload is not maintained, the hardware could experience joint-separation or a fatigue failure. It can also lead to the bolts self-loosening, ultimately causing a failure in the bolted joints. Also, torque the hardware to the proper specifications. Utilize jam nuts and other locking hardware to prevent connections from loosening or separating. Lastly, do not allow suspended systems to rotate, as this might also loosen hardware.
Standard hardware should not be the weak point in the load path. Yielding, shearing, or fatigue failures could cause damage to the load cell device. If standard hardware is potentially inadequate for the load cell application, upgrade hardware before beginning installation.
The attachment plate used to mount the load cell should be thick enough to have a significant thread engagement with attaching hardware. If the plate is thin it will prevent the component from being properly secured.
Best Practices for Installing Various Types of Load Cells
Load cells are designed for different applications and therefore, require different mounting and installation.
Axial Load Applications
When installing shear beams, single-point, platform, canister, and disk load cells, keep the lower mount plate level and flat. The top plate that will translate the load should always maintain a parallel position to the bottom plate, and the load path should be perpendicular to the plates. This will keep a predominately axial loading.
A failure to maintain axial loading could result in bending moments. As mentioned before, rod-end bearings and clevis mounts can be installed to prevent these bending moments. Misalignment of the mounting plates and the load path can also cause inaccuracies, especially in devices that are sensitive to non-axial loading.
When operating double-end shear beams, the load should be aligned vertically through the center, and should not cause twisting or torsion. The load should be kept fixed relative to the cell body. Double-ended shear beams are rated for higher load capacities than single-end, so when supporting greater loads with size constraints, implement double-end beams. Canister or disk load cells are typically rated for higher capacities, but are larger and bulkier.
Tension and Suspension Applications
Load cells that measure in tension are good for lifting, suspending, and hanging applications. For these applications the measuring system is usually permanently fixed to a frame above or is hoisted at the time of measurement. Examples of these applications are pulleys, hoists or cranes, and fork lifts.
S-beam load cells can be used in both tension and compression. They are susceptible to large bending moments; to prevent this, install with rod-end bearings. Usually a strain gauge is located on a specific end of S-beam load cells; always verify it is oriented correctly. If the strain gauge is oriented incorrectly the device cable can interfere with the accuracy of measurements.
Load pins can be used in suspended systems where a pin supports the structure. They can be used in shackles with clevis pins, sprockets, or pulleys. In these cases the support pin can be replaced directly by a load pin load cell to obtain real-time application measurements.
As with structures supported on top of load cells, fewer suspended supports will deliver more accurate results. Install suspension systems with adjustable linkages to maintain the ability to evenly distribute loads among load cells. Additionally, limit the amount of rotation in suspended systems to prevent hardware from loosening over time.
If load cell component failure would cause injury, have the proper structural backup in place. Secure the fixture with cables or stops to prevent harm to operators and equipment.
Lastly, always contact Tacuna Systems with installation or application questions regarding our load cell products.
- Measurement and Instrumentation Principles, 3rd Edition, Alan S. Morris
- Introduction to Instrumentation and Measurements, 3rd Edition, Robert B. Northrop
- Elements of Electronic Instrumentation and Measurements, Joseph J. Carr