All weighing systems are designed in such a way that they have maximum resolutions or accuracies over a defined loading range. These systems have a typical resolving power of 0.01%, 0.1% or 1% of full scale range, which means for a high accuracy system (i.e. 0.01%), the full-scale capacity of the system is 10,000 times the minimum resolvable weight that the system can measure. For instance, if a system can measure 1 ounce, and is a high accuracy system, then the full scale capacity is likely to be 10,000 oz. (or 625 lbs.). However, the full-scale capacity would only be 100 oz. (or 6.25 lbs.) for a low resolution system (i.e. 1%).
It is interesting to note that both 0.01% and 1% systems can measure a given weight to the same degree of resolution and accuracy. With regards to the higher accuracy (and much more costly) system, the main advantage is the range over which the system will make measurements to this accuracy.
Electronic Weighing Systems Using Load Cells
Most electronic weighing systems using load cells as the sensing element have two things in common: they feature a structural element that serves like a spring which deforms upon application of a force (or weight), and some type of sensing device that can measure this deformation. The systems’ resolution is controlled by the sensitivity of the sensing device used and the amount of deformation that can be tolerated by the structural element.
The structural element in all these systems is designed to be compatible with the sensing element used and is typically designed to have the maximum possible deformation when the expected full-scale load is applied. The key part of the system is the sensing element and this determines the obtainable accuracy or resolution of the system. The issues related to overload are always connected to the system’s structural element.
Considering the fact that the structural element of the load cell deforms when weight is applied, four distinct regions of operation can be defined for the typical weighing system: The Normal Operating Range – The structural element deforms proportionally and repeatedly to the applied weight and can be believed to be perfectly elastic.
The Allowable or Moderate Overload Range – While the structure can still be considered elastic, the deformation may not be actually proportional to the applied load. Occasional operation in this region will not cause any detectable damage to the system. This region can be additionally divided into two sub-regions: the “Stated” and the “Actual” allowable overload range. The main reason for this is that it is difficult to accurately predict the upper border of this region and hence manufacturers apply a “factor of safety” and derate (hopefully) their stated range. The amount that different manufacturers “derate” differs significantly even among similar products. The factors that enter into the derating are those of unknown loading conditions, potential risk costs, material property variations, and unfortunately….sales appeal.
The Severe Overload Range – Here the structure begins to show signs of permanent damage. After the overload is removed, the structure may not respond repeatedly to any applied load. It is difficult to detect minor excursions into this region, but these are often associated with unexplained changes in “zero shifts” or calibration. Loads applied in the upper part of this region often display signs of physical damage. Therefore, it is sometimes helpful to get the manufacturer’s estimate of the upper part of this region, particularly if structural failure cannot be tolerated.
The Destructive Overload Range – The structure fails. Here, the main consideration is how it fails. Compression failure is often believed to be “fail-safe” with the load being automatically transferred to the weighing system’s support structure. However, precautions should be taken to guard against outwardly thrown shrapnel. Tensile failure enables any suspended weight to fail, potentially with damaging effects. Obviously, this is a region that one never hopes to be in.
Among these four regions, it is apparent that one would never wish to surpass the allowable overload range. The best way to protect the structural element in an electronic weighing system is to make sure that the element is never loaded beyond its allowable overload range. There is no problem if the expected maximum overload falls within this region, but if there is any possibility that a damaging overload can be applied to a system, then Murphy’s Law will ensure that it will be. Hence, some extra protection for the weighting system will be needed.
Simple derating of the system’s “Normal Operating Range” is the easiest method of overload protection. For instance: assume that a 100 lb scale on a production line is employed to weigh components with an accuracy of 1/10 of a pound (i.e. 1.0% accuracy). If the stated allowable overload capacity of the system is 150% of full scale (i.e. 150 pounds), then one can be sure that a 200 pound operator will utilize that scale as a seat every lunch hour. In case the manufacturer has applied an adequate safety factor to his stated overload range, the scale may survive provided the operator sat down gently. However, if a 200 pound scale with the same allowable overload factor was installed, the operator could eat his lunch and also have a 100 pound person join him without causing any damage to the scale. Conversely, if the scale still had to weigh components with an accuracy of 1/10 of a pound, the 200 pound scale would need double the accuracy of the original (i.e. the new scale would need an accuracy of 0.05%).
Overload Protection in Weighing Systems
“Mechanical Stop” system is the next method of overload protection. As weight is applied, the structure of a weighing system deforms. Therefore, a mechanical stop could be installed that would be contacted at a specified force, thus preventing the additional force from being carried by the structural element of the scale. A simple spring scale with this type of protection is shown in Figure 1.
Even though a spring scale was illustrated, this type of overload protection is usually applied to “stiff” structural elements like those found in load cells. Assume that a load cell deflects 0.005” at its rated capacity and has an allowable overload factor of 150%. This would indicate that the load cell can possibly deflect 0.0075” prior to being damaged. Hence, if a mechanical stop is arranged that would engage when the applied load causes the load cell to deflect between 0.005” and 0.0075”, then the load cell could be protected from damaging the overloads.
This type of overload system is shown in Figure 2 which is designed around a common “proving ring” structure as is normally found in low capacity load cell designs. Although “mechanical stop” overload systems are simple in concept, they are usually very difficult to implement in weighing systems that have extremely stiff structural elements. In the above case, the required gap must be machined (or set) with excellent accuracy. If the gap was less than 0.005”, the mechanical stop would be engaged before the full-scale capacity of the scale was obtained, thus resulting in inaccurate readings at high loads.
However, if the gap was just slightly greater than 0.0075”, the structure would be loaded into a region potentially causing permanent damage before the mechanical stop was engaged. Nevertheless, even if the mechanical stop system was machined properly, a dirt particle with just the right size will still manage to lodge itself in the meticulously designed gap and lead to premature gap engagement (Murphy’s Law).
The usual method of designing an effective mechanical stop system involves designing the structural element in such a way that it has the largest possible deflection that is consistent with the sensing method used. This could also involve adding a high deflection spring in series with the load cell to simply give an extra deflection to the system. This type of system is shown in Figure 3
In this type of system, the other design criterion is to guard the system against dirt particles either by arranging for physical inspection and periodic cleaning of the gaps or by using protective covers
In case the weighing system is designed for high-speed operation, the method of softening the structure in order to make the mechanical stop system effective cannot be used. High-speed systems do not dictate lower stiffness, only greater ones, making it difficult to effectively apply the mechanical stop system. Here “Preloaded spring” overload systems will be examined as a modified mechanical stop system, which effectively helps to solve this problem.
Earlier, the possible use of a “soft” spring in series with a “stiff” load cell was explored to obtain adequate deflection and thus allow a mechanical overload stop to function efficiently. It was also determined that the addition of the spring lowered the system’s natural frequency and, as a result, also reduced the weighing system’s speed. For systems requiring high response rates, the series spring method for overload protection is not practical…unless there is a way to make the spring “stiff” as well until an overload occurred. The “preloaded” spring system does exactly that.
A simple tension spring is the simplest form of a preloaded spring which has been wound with a controlled pretension. A plot of the deflection vs. applied force for this type of spring is shown in Figure 4 (above). In the case of low capacity tension load cells (i.e. below about 100 lbs.), this type of spring can be easily added to the load cell and offers tensile and sideload protection for the load cell. Figure 5 (below) shows a sketch of this system.
In addition, a compression spring can be preloaded, but unlike the tension spring, it needs some extra hardware to accomplish the task. A simple compression spring overload system is shown in Figure 6, where an internal nut sets the preload.
It is quite easy to envision and implement a one-directional overload system using preloaded springs. Two directional systems are more complicated but are still quite practical. However, with all the benefits of the preloaded spring overload systems, how come more of them are not used?
The reason is unknown. This could be because load cell manufacturers do not have to replace an apparently overloaded load cell under warranty. On a serious note, there are some disadvantages to the preloaded spring systems. For instance, they add some mechanical complexity to the weighing system, they tend to be limited to the smaller capacity systems (under 10,000 lbs.), and also they can have unusual effects on systems that are sensitive to relative deflections of components.
An example of the last problem would be found in “steelyard” conversions using load cells. If a preloaded spring overload protection system was also incorporated in the conversion, whenever the overload system was activated, it would “reset” itself to a potentially different overall assembly length.
This would lead to an apparent “zero shift” in the load cell. The important point is that load cell applications such as the steelyard conversion also require the installed length of the system to be constant, or otherwise varying tare weights will be suspended on the load cell. In the preload spring system, reset tolerances can be on the order of plus/minus 0.020 owing to hysteresis losses between the spring coils or “seating problems of the numerous components. The test of the relative suitability of a preloaded spring system would be to establish the effect of adding a variable and a potentially non-repeatable length load cell to the system.
For weighing systems that can withstand physical separation of the load cell link, a simple shear pin system might provide a low-cost method of guarding a high price load cell. Thomas Register lists more than a dozen suppliers under the category of Shear, Pins. All the discussions of overload, up until now as dealt with those loads, have been gradually applied.
Have you ever seen the act where a man places a huge stone block on his stomach and then has an assistant break it in half with a sledgehammer? Or have you ever tried to drive a nail through a thin plywood sheet that lacked any support behind it? If so, you have either observed or witnessed at first hand, the control or miss-control of shock loading.
Shock loading happens to be a major factor in establishing whether or not a weighing system will tolerate the environment in which it is placed. The very characteristic that makes electronic weighing systems so interesting (their high operating speed) makes them particularly prone to damage from shock loading. To better interpret the effects of shock loading, a fictional case of shock load damage on a 50-pound scale is examined.
First, a ten-pound box of nails is inadvertently dropped from a height of 10 inches on a 50 pound (full scale) counting scale. Following the incident, the scale has experienced a shift in zero reading that cannot be nulled out. To find out what exactly happened, the action was slowed down and the accident was repeated. As the box of nails is dropped, it starts to accelerate and gathers momentum. One-quarter of a second later, it moves at 5 miles per hour and contacts the scale’s top surface. As the scale is extremely stiff, the box of nails should now come to zero velocity in a very short distance (usually 0.005”), meaning that the mass should experience tremendous deceleration, which is on the order of 160 times the acceleration due to gravity.
If nothing happened to relieve the force, the forces produced by this acceleration (or deceleration) could build to 1600 pounds. Yet, the cardboard container begins to crush and many of the nails within the container begin to shift, which takes in some of the energy of the fall. As the box begins to crush, the effective distance over which the weight is accelerated also increases, which additionally reduces the acceleration forces. During all these occurrences, the upper surface of the scale, which possesses some amount of mass, creates a resisting force. This force is directly proportional to the amount of mass it contains, thus tending to guard the load cell which is directly mounted below. Conversely, the mass of the platform has been intentionally minimized so as to reduce the tare load on the load cell and improve its high-speed performance.
This is a key reason as to why massive truck scales with their huge platform masses are not highly susceptible to shock loads; the mass of the platform is more likely to absorb them. The shock load force is now attenuated by the nails shifting, the container crushing, as well as the resisting force of the upper platform to the load cell. The two surfaces, which are held together by the bolts in this connection, are stable due to the friction present between them. In the original process of tightening, these surfaces were preloaded in some way and the surfaces were deformed, which was subsequently preserved by the frictional forces. The shock load force that enters the bolted connection can now serve to tune the original amount of energy stored in the connection.
If the load cell is correctly designed, the bolting conditions should have only small effects on the load cell’s operating zero point. Conversely, due to space constraints, the load cell was designed in such a way that the structural stiffness of the attached members can be used, which is all right as long as the attached members stay attached in the same way at all times. Although joint shifting is difficult to visualize, it is a major cause of unexplainable zero shiftings in a scale system that is exposed to shock loading conditions. After the force passes the bolted connections, it now enters the highly stressed member of the load cell which is used for measuring the load. If the force has been insufficiently attenuated by this time, the stresses may be quite high to break or yield the load cell. In case the load cell does yield, more deflections are added to the system, which in turn helps to reduce the acceleration forces.
After passing the load cell, the force again travels through another bolted connection, causing the same issues as mentioned before, and reaches the scale’s base structure. If the base is huge, it resists acceleration to react to the applied force, or if lightweight tends to pass the shock force on through the rubber mounting feet, this further tends to attenuate the shock wave to the point that the table upon which the scale is placed, carries little more than the extra 10 lb load that would exist if the nails had been applied normally to the scale. In fact, there are numerous times where the scale can be assumed to be the protection device for the table upon which it sets.
Some true insights should be obvious from this imaginary story of a shock loading incident. If the distances are somehow increased over which the suddenly applied load is ceased, the forces generated by this deceleration are considerably reduced. Prime examples of this design concept are the bumpers on the newer model cars. Another possibility for reducing the effects of shock loading is in the vigilant management of the internal masses of the scale itself. For instance, a preloaded spring overload system can be effective for shock loads, provided the load cell does not have to compete with large inertia loads.
Further, the appropriate use of elastomer mounts can be an effective way of controlling shock loads by again adding to the effective distance over which the load is decelerated and also by changing some of the energy to heat in the elastomer itself.
Previously, things were dropped on scales and the action was slowed down in order to visualize how the force was channeled through the structure, with the impulse or shock being modified or absorbed through either the relative masses of the components of the scale or produced as heat in elastomer (rubber-like) elements. It was observed that to achieve effective control over shock loads, the scale designer should pay attention to the masses of the scale’s various components and their relative positions in the scale’s force path. Figure 7 (below) shows two types of spring preload overload protection systems.
Both of the designs shown will protect against normally applied overloads, but the system depicted in Figure 7 (B) will also provide relatively good protection for shock loads, whereas 7 (A) will not. This is because a shock force exerted to the input platform is reacted by the acceleration forces generated by the load cell housing, the upper platform, and the lower platform which is spring-loaded against internal “stops” in the overload protector housing. In this system, some of the shock load is absorbed with the help of the inertial force of the upper platform, whereas the inertia forces produced by the lower platform and the load cell housing only serve to prevent the applied shock load from reaching the spring, which is meant to collapse and protect the load cell. As the inertia of the lower platform and the load cell housing react to the applied shock load, the force is the very thing that the overload protector was meant to protect in the first place.
On the other hand, the design is shown in Figure 7 (B) was essentially only a single moving part under shock loading conditions…the upper platform, whose inertia is inclined to react to the applied force in a helpful way, to begin with. Two similar overload protectors for tensile shock load protection are shown in Figure 8. Again, Figure 8 (B) is all right for shock loads and Figure 8 (A) is poor.
Spring Pre-Load Overload Systems
Almost unlimited ways are available to design efficient spring pre-load overload systems, operating in either compression, tension, or both. In all the designs, the one intrinsic feature with regards to their suitability to shock loading can be identified by checking the location of the overload system with respect to the load cell housing. Generally, shock protection is only provided by those designs where the base of the load cell is directly linked to the machine frame, or in some way is prevented from accelerating and creating damaging reaction forces. After simply being channeled through overload stops, shock loads usually continue to have damaging effects. The shock created should also be considered as a load is abruptly removed from a scale. In such situations, the mass of the upper platform which aided in the normal overload situation unexpectedly becomes a moving mass which, if not considered¸ can exert damaging forces to a load cell.
In order to protect against this condition, the best method would be to keep the upper platform as light as possible so as to reduce the return force (after an overload has been removed). Attenuating the shock load as it by-passes the load cell, or preventing the shock load from affecting the surfaces of the overload gaps themselves is perhaps best handled by the careful use of elastomer elements, either in the overload gaps themselves or somewhere else in series with the by-pass force path.
This information has been sourced, reviewed and adapted from materials provided by HITEC Sensor Developments, Inc.
For more information on this source, please visit HITEC Sensor Developments, Inc.