In common terminology, a load cell is a device which converts a weight into an electrical signal. A more technical term would be a type of force transducer. In the broadest sense, a transducer converts energy from one format into another. Ideally, the transfer happens through the base of the load cell as force is applied. These load cell mounting, installation tips and best practices will help you with load cells in compressive, torque and tension-based weighing systems.
The most standard type of load cell acts as a transducer in that it uses strain gauges to convert the force of gravity acting on an item on a scale platform or other weighing system into a small differential voltage. Typically, this output signal will be amplified by a precision amplifier and then sent to an analog to digital converter in a weighing indicator. This results in accurate measurements of the weight.
There are technologies other than strain gauge that are used for load cell designs. The other type of load cells includes pneumatic load cells, hydraulic load cells, and Surface Acoustic Wave (SAW) load cells, which is one of the newest and provides the best measurement results. All of these technologies may also be used in other types of force transducers, including pressure gauges.
Load Cell Mounting Considerations
There are a variety of common load cell configurations. Shear beams are often used in platform scales and floor scales. A beam load cell is generally not suitable for a bench scale or cylinder scale. High quality units used in harsh industrial environments are more likely to use stainless steel single point load cells.
Selecting the Right Frame for Your Load Cell Systems
A load cell is really a spring. When a load is applied, it bends. When the load is doubled, the amount of bending doubles. When the load is removed, the load cell returns to its original position.
This is the case, at least in theory. It is dependent on many other factors, such as the material used, the modulus elasticity of that material, and what percentage of yield strength is reached by the load, amongst others. A very important consideration is mounting the load cell.
Since the load cell is the spring element, the support structure should have virtually no characteristics of a spring. It should be built in a manner that is very rugged and stiff. When a load is applied, there should be little or no deflection. If the load remains on the scale for long periods of time, there must be no measurable relaxation of the frame.
Clearly, a frame that is designed and built for a 10 lb scale should meet these expectations. Just as clearly, a frame built for a 1000 lb scale has the same expectations. And equally clearly, a frame designed for a 10 lb scale will not be suitable for a 1000 lb scale.
In general, the heavier the capacity of the scale, the heavier the frame must be to properly support it. Also, different materials can be used. An aluminum frame will be lighter in weight than a steel frame. Stainless steel frames can be used in environments that are subject to moisture or corrosive conditions.
The nature of the design of the frame will also be influenced by the size of the scale platform. Most bench scales include just a single load cell mounted in the center of the platform. The frame does not have to be very robust if the scale platform is small, because there will not be a large moment arm to the edge of the platform.
On the other hand, for bench scales with large platforms, the demand on the frame increases significantly. Arlyn’s large Ultra Precision SAW scale boasts a 31” x 31” stainless steel platform. With a capacity of 1000 lb, it is evident that the frame must be extremely strong. It is constructed from welded stainless steel tubing.
This 1000 lb scale just about reaches the limits of a reasonable frame for a single load cell. For larger capacities or larger platform sizes, the design switches to one with four load cells. In this case, there is one load cell in each corner of the scale, as with platform scales, floor scales and drum scales.
Scales with four load cells can achieve virtually any weight capacity. Standard units range from 1000 lb up to 20,000 lb, with platform sizes ranging from 20” x 27” all the way to 7’ x 9’. These are most often used for weighing pallet loads of materials. The frames may be welded aluminum, welded steel, or welded stainless steel.
Preparing Your Load Cell Mounting Frame Before Installation
As was noted above, only the load cell can be a spring element. The frame should be very rigid, and should not deflect under load. But even if the frame itself is extremely strong, unwanted side effects will appear if the frame is not properly prepared for the load cell installation, and if there is improper mounting of the cells to the support
The mounting area of a load cell should be very flat. Generally, load cell manufacturers will machine these areas to a very tight tolerance. There should be no visible ridges, bumps, or other markings from the machining process.
Similarly, the areas on the frame which accept the load cells should also be flat. The attachment screws should be tightened to a specific tolerance of torque. The electrical cable from the load cell must be attached to the frame.
Creating the Best Environment For Accurate Load Cell Readings
Strain gauges are special types of resistors. As with all resistors, the value of resistance changes with temperature. Therefore, strain gauge load cells are compensated to minimize the effect of changes in ambient temperature. Nonetheless, best results will be achieved when the temperature of the environment is as stable as possible.
Ultra Precision SAW load cells are much less impacted from environmental issues. They are much more suitable whenever superior performance is required amid thermal expansion or excessive humidity.
Common Mistakes When Installing Load Cell Systems
The most common error made when installing a load cell is choosing one with the wrong capacity. If the load cell has a lower capacity than the range of weight expected on the scale, it will likely be damaged easily. If the capacity is too large, the output under full load will be too low, and the resolution of the scale will suffer.
Another error is neglecting to take safety issues into account. If the environment includes explosives or is considered to be hazardous due to exposure to flammable or explosive materials, then an Intrinsically Safe scale and load cells must be chosen.
Contact Arlyn Scales for Your Load Cell Product Needs
At Arlyn Scales, highly experienced sales technicians are available to help you choose your best solution. Give us a call or fill out our online form to talk to our representatives. Learn about the compressive loading applications preferred for business operations.