Table of Contents
Electronic Weighing Systems Using Load Cells
Overload Protection in Weighing Systems
Spring Pre-Load Overload Systems
All weighing systems are developed in such a way that they have maximum accuracies or resolutions 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 example, if a high accuracy system can measure 1 ounce, then the full scale capacity is likely to be 10,000 oz. (or 625 lbs.). However, for a low resolution system (i.e. 1%), the full-scale capacity would only be 100 oz. (or 6.25 lbs.).
It is interesting to note that both 0.01% and 1% systems can measure a given weight to the same degree of accuracy and resolution. The major benefit of the higher accuracy (and much more costly) system 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 common things: they feature a structural element that acts like a spring which deforms when a force (or weight) is applied, and some type of sensing device that can measure this deformation. The resolution of the systems is controlled by the amount of deformation that the structural element can tolerate, and the sensitivity of the sensing device used.
In all these systems, the structural element is typically designed to have the maximum possible deformation when the expected full-scale load is applied and is also designed to be compatible with the sensing element used. The sensing element is the vital part of the system and determines the obtainable resolution or accuracy of the system. The problems related to overload are always associated with the system’s structural element.
Taking into account the fact that the structural element of the load cell deforms upon application of weight, four distinct regions of operation can be defined for the typical weighing system: The Normal Operating Range – The structural element deforms repeatedly and proportionally to the applied weight and can be considered 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. Operating occasionally in this region will not cause any detectable damage to the system. This region can be further divided into two sub-regions: the “Stated” and the “Actual” allowable overload range. The main reason for this is that it is not easy to accurately predict the upper border of the allowable overload region and therefore manufacturers apply a “factor of safety” and derate (hopefully) their stated range. The amount that different manufacturers “derate” varies considerably even among similar products. The factors considered in derating are those of unknown loading conditions, material property variations, potential risk costs, and unfortunately, sales appeal.
The Severe Overload Range – Here, the structure begins to show signs of permanent damage. After removing the overload, the structure may stop responding repeatedly to any applied load. While it is hard to detect minor excursions into this region, these are often associated with unexplained changes in “zero shifts” or calibration. Loads applied in the upper part of this region usually show signs of physical damage. Hence, it is sometimes helpful to get the manufacturer’s estimate of the upper part of this region, mainly if structural failure cannot be tolerated.
The Destructive Overload Range – Here, the structure fails but the main consideration is how it fails. Compression failure is often considered to be “fail-safe” as the load is being transferred automatically to the support structure of the weighing system. However, precautions should be taken to protect against outwardly thrown shrapnel. Tensile failure allows any suspended weight to fail, potentially with damaging effects. Clearly, this is a region that none wants to be in.
Among these four regions, it is obvious that none would wish to exceed the allowable overload range. The best way to protect the structural element in an electronic weighing system is to ensure that the element is never loaded beyond its allowable overload range. If the expected maximum overload falls within this region, there won’t be any problem 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, the weighting system will require some additional protection.
The easiest method of overload protection is simple derating of the system’s “Normal Operating Range”. For example: consider that a 100 lb scale on a production line is used 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 ensure that a 200 pound operator will utilize that scale as a seat for every lunch hour. If the manufacturer has applied an adequate safety factor to their stated overload range, the scale may survive in addition that the operator sat down gently.
However, if a 200 pound scale with the same allowable overload factor was installed, the operator could eat their lunch along with another 100 pound person without causing any damage to the scale. On the other hand, if the scale still had to weigh components with an accuracy of 1/10 of a pound, the 200 pound scale would require an accuracy which is double the original (i.e. the new scale would need an accuracy of 0.05%).
Overload Protection in Weighing Systems
The next method of overload protection is “Mechanical Stop” system. As weight is applied, the structure of a weighing system deforms. Hence, a mechanical stop could be installed that would be contacted at a specified force, which will prevent the structural element of the scale from carrying the additional force. A simple spring scale with this type of protection is depicted in Figure 1.
Although a spring scale was illustrated, this type of overload protection is often applied to “stiff” structural elements as 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” before being damaged. Therefore, if a mechanical stop is installed that would engage when the applied load causes the load cell to deflect between 0.005” and 0.0075”, the load cell could be protected from damaging the overloads.
Figure 2 shows this type of overload system, 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 often very difficult to execute in weighing systems that have very stiff structural elements. In the above case, the required gap must be set (or machined) with excellent accuracy. If the gap was less than 0.005”, the mechanical stop would be engaged prior to obtaining the full scale capacity of the scale, thus leading to inaccurate readings at high loads.
Conversely, 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. However, even if the mechanical stop system was set properly, a dirt particle with just the right size will still manage to lodge itself in the meticulously designed gap and result in premature gap engagement (Murphy’s Law).
The common 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 technique 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 illustrated in Figure 3.
In this type of system, the other design criterion is to protect the system from dirt particles either by using protective covers or by arranging for physical inspection and periodic cleaning of the gaps.
If the weighing system is designed for high speed operation, the method of softening the structure to make the mechanical stop system effective cannot be employed. High speed systems only dictate greater stiffness and not lower ones, making it difficult to effectively apply the mechanical stop system. In order to effectively solve this issue, “Preloaded spring” overload systems will be examined as a modified mechanical stop system.
Earlier, the possible use of a “soft” spring in series with a “stiff” load cell was explored to achieve adequate deflection enabling a mechanical overload stop to function efficiently. It was also determined that the addition of the spring decreased the natural frequency of the system. As a result, the speed of the weighing system was also reduced. For systems requiring high response rates, the series spring method for overload protection is practical only if there is a way to make the spring “stiff”, until an overload occurred. The “preloaded” spring system exactly does that.
The simplest form of a preloaded spring is a simple tension spring which has been wound with a controlled pretension. Figure 4 (above) shows a plot of the deflection versus the applied force for this type of spring. 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 provides tensile and side load protection for the load cell. A sketch of this system is shown in Figure 5 (below).
Moreover, a compression spring can be preloaded, but it needs some extra hardware to accomplish the task unlike the tension spring. Figure 6 shows a simple compression spring overload system, where an internal nut sets the preload.
It is quite easy to visualize and implement one directional overload system using preloaded springs. Although two directional systems are more complicated, they are still quite practical. Despite all the advantages of the preloaded spring overload systems, the reason for not using more of them is unknown.
This could be due to the fact that load cell manufacturers do not have to replace an apparently overloaded load cell under warranty. On a serious note, the preloaded spring systems have some disadvantages. For example, they tend to be restricted to the smaller capacity systems (under 10,000 lbs.), they add some mechanical complexity to the weighing system, 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, it would “reset” itself to a potentially different overall assembly length every time the overload system is activated.
This would cause an apparent “zero shift” in the load cell. The important point is that load cell applications such as the steelyard conversion also needs the installed length of preloaded spring system to be constant, or else varying tare weights will be suspended on the load cell. In the preloaded spring system, reset tolerances can be on the order of plus/minus 0.020 due to hysteresis losses between the spring coils or seating issues of various components. The relative suitability test of a preloaded spring system would be to establish the effect of adding a variable and potentially non-repeatable length load cell to the system.
For weighing systems that can endure physical separation of the load cell link, a simple shear pin system might offer a low-cost method of protecting 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 slowly applied.
The control or miss-control of shock loading can be witnessed or observed first hand by these two instances – when a man places a huge stone block on his stomach and then has an assistant break it in half with a sledge hammer or when a nail is driven through a thin plywood sheet that lacked any support behind it.
Shock loading plays a big role in determining whether or not a weighing system will bear the environment in which it is kept. The very characteristic that makes electronic weighing systems so fascinating (their high operating speed) makes them highly prone to damage from shock loading. To better understand the effects of shock loading, a fictional case of shock load damage on a 50 pound scale is studied.
First, a ten pound box of nails is unintentionally dropped from a height of 10 inches on a 50 pound (full scale) counting scale. After that accident, the scale has experienced a shift in zero reading that cannot be nulled out. To find out what precisely took place, the action was slowed down and the accident was repeated. As the box of nails is dropped, it starts to increase speed and gathers momentum. About one quarter of a second later, it travels at 5 miles per hour and contacts the top surface of the scale. As the scale is very stiff, the box of nails should at present come to zero velocity in a very short distance (typically 0.005”), meaning that the mass should experience remarkable deceleration, which is on the order of 160 times the acceleration because of gravity.
If nothing happened to release the force, the forces created by this acceleration (or deceleration) could build to 1600 pounds. Yet, the cardboard container starts to crush and many of the nails within the container begin to move, which takes in some of the energy of the fall. As the box starts to crush, the effective distance over which the weight is accelerated also increases, which further minimizes the acceleration forces. During all these occurrences, the upper surface of the scale, which possesses some amount of mass, develops 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 reduced so as to decrease the tare load on the load cell and better its high-speed performance.
This is a main reason as to why massive truck scales with their large platform masses are not very prone to shock loads; the mass of the platform is more probable to absorb them. The shock load force is at present 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 because of 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 then preserved by the frictional forces. The shock load force that enters the bolted connection can currently serve to tune the original quantity of energy stored in the connection.
If the load cell is properly designed, the bolting conditions should have just small effects on the operating zero point of the load cell. Conversely, because of space constraints, the load cell was designed in such a manner that the structural stiffness of the attached members can be used, which is all right as long as the attached members remain attached in the same way at all times. Although joint shifting is hard to visualize, it is a key cause of unexplainable zero shifting in a scale system that is exposed to shock loading conditions.
After the force passes the bolted connections, it presently enters the much stressed member of the load cell which is applied for measuring the load. If the force has been inadequately attenuated by this time, the stresses may be quite high to yield or break the load cell. In case the load cell does yield, more deflections are integrated to the system, which in turn helps to decrease the acceleration forces.
After passing the load cell, the force again moves through another bolted connection, causing the same problems as stated before, and reaches the base structure of the scale. If the base is big, it resists acceleration to react to the applied force, or if light weight 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 additional 10 lb load that would exist if the nails had been applied customarily 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 clear from this imaginary story of a shock loading incident. If the distances are somehow increased over which the unexpectedly applied load is ceased, the forces generated by this deceleration are significantly reduced. Key examples of this design concept are the bumpers on the newer model cars. Another possibility for decreasing the effects of shock loading is in the watchful management of the internal masses of the scale itself. For example, a preloaded spring overload system can be effective for shock loads, provided the load cell does not have to deal with large inertia loads.
Further, the suitable use of elastomer mounts can be an effective way of regulating shock loads by again adding to the effective distance over which the load is decelerated and also by altering some of the energy to heat in the elastomer itself.
Previously, things were dropped on scales and the action was slowed down so as to visualize how the force was channeled through the structure, with the impulse or shock being changed or absorbed through either the relative masses of the components of the scale or created as heat in elastomer (rubber like) elements. It was observed that to realize effective control over shock loads, the scale designer should pay attention to the masses of the scale’s many components and their relative positions in the scale’s force path. Figure 7 illustrates two types of spring preload overload protection systems.
Both of the designs illustrated will protect against generally applied overloads, but the system shown in Figure 7 (B) will also provide comparatively 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 created 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 created 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 main thing that the overload protector was designed to protect in the first place.
Conversely, the design illustrated in Figure 7 (B) was fundamentally 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 illustrated in Figure 8. Again, Figure 8 (B) is all right for shock loads and Figure 8 (A) is poor.
Spring Pre-Load Overload Systems
There are almost unlimited ways to design efficient spring pre-load overload systems, working in either tension, compression, or both. In all the designs, the one intrinsic feature in relation 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 offered by those designs where the base of the load cell is directly connected to the machine frame, or in some way is prevented from accelerating and producing damaging reaction forces.
After just being channeled through overload stops, shock loads typically continue to have damaging effects. The shock created should also be considered as a load is abruptly taken off from a scale. In such cases, the mass of the upper platform which helped in the normal overload situation, suddenly becomes a moving mass which, if not considered¸ can exert damaging forces to a load cell.
In order to guard against this condition, the best way would be to keep the upper platform as light as possible in order to reduce the return force (after removing an overload). Attenuating the shock load as it by-passes the load cell, or inhibiting the shock load from impacting 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.