The overwhelming reality of shock loads

At Interface, we know how versatile load cells are in Industry from precision weighing to force measurement in demanding industrial environments.

Although our load cells are designed for durability and high accuracy, there is a silent threat that can affect their performance and even lead to catastrophic failure: shock loads. Our engineers shed light on this danger in our Load Cell Field Guide, their destructive potential, and the critical steps you can take to protect your investment in interface load cells.

Beyond statics: The dynamic challenge of impact

We often think of load cells in terms of static loads - a constant weight or force. But what happens when a force is applied suddenly, such as from a dropped object or a sudden impact? This is where the concept of shock loading comes into play, and this is why it is a key innovation.

Unlike static loads, shock loads introduce a dynamic element that greatly amplifies the force absorbed by the load cell. The main difference is the speed of force application. A static load increases gradually, giving the load cell time to react. With a shock load, the force is applied instantly, concentrating the energy and potentially exceeding the capacity of the load cell, even if the weight appears small.

Understanding the Impact

Let's illustrate this with a simple example. Consider a steel ball falling onto an interface load cell. As the ball falls, its potential energy is converted into kinetic energy. Upon impact, this kinetic energy must be dissipated. Ideally, the load cell measures the force of the impact. However, the rapid deceleration of the ball creates a much higher force than the weight of the ball alone would indicate.

Here is a closer look at the physical relationships:

• Impulse and Momentum: Impact is related to impulse, the change in momentum of an object (mass x velocity). The impulse-momentum theorem states that impulse is equal to the change in momentum. In our ball example, impulse changes very quickly from "mv" to zero, resulting in a large force.
• Force and Time: The average force during impact is related to momentum by the equation: momentum = force x time. Since the impact time is so short, the force becomes large to provide the same momentum.
• Energy dissipation: The kinetic energy of the impacting object has to go somewhere. Ideally, it is measured by the load cell. However, a large part can be transferred to the load cell structure in the form of stress waves, which can cause damage even far from the point of impact if it is overloaded beyond its capacity.
• Stiffness: A stiffer load cell will experience a higher peak force during an impact than a less stiff one, even for the same impact energy. The stiffer cell will resist deformation more, resulting in faster deceleration and higher force. READ: https://www.interfaceforce.com/load-cell-stiffness-101/

Six precautions and preventive measures

Protecting your interface load cells is of utmost importance given the destructive potential of shock loading. Here are some important precautions:

#1 - Analyze your application for potential impact scenarios, including dropped objects, sudden starts and stops, and vibrations. For example, when using load cells in conveyor systems for quality control and safety, abrupt changes in motion can occur that can generate significant shock forces. Even seemingly gentle operations, such as placing heavy objects on a platform scale, can cause shock loads if not performed carefully. External factors such as gusts of wind, seismic activity, or accidental collisions can also cause shock.

#No. 2 – Evaluate the sensor features that include overload protection. Interface offers various load cells and Torque transducer with Overload protection in different capacities and configurations. These have mechanisms built into the design of the sensor to absorb overloads that can occur due to impacts. Our overload-protected S-type load cell SMT For example, it has internal mechanical stops that prevent overloads of up to 10 times the rated capacity. Our overload-protected MRTP miniature reaction torque transducer in flange design offers 7 times safe overload with a capacity of 0,2 Nm (1,77 lbf-in). The flange design allows for easy mounting. This product has low deflection and high torsional stiffness, making it an excellent choice for reciprocating measurements. READ: How does load cell overload detection work?

#3 - Consider load cells with a significantly higher capacity than required for static loads to create a safety margin. If your testing requirement calls for a maximum capacity range, make sure the force gauge has a higher capacity to accommodate potential overload risks. Determine the margin and plan by reviewing the capacities for the model you select. Consider a load cell with a capacity higher than the expected maximum impact force. This is especially important if your testing projects vary.

TIP: Our application engineers can help you determine the appropriate capacity for your specific requirements, taking into account factors such as the mass and velocity of the impacting object, the stiffness of the system and the desired safety factor.

#4 - Mount your Interface load cell to a rigid base. A flexible mount can increase shock. Use heavy-duty fasteners and make sure they are properly tightened. For torque transducers, the couplings designed for the sensor are a valuable accessory. There are bracket and mounting plate options for our load pins. Learn more about this by checking out the Load Cell Mounting 101 section. Interface offers a number of videos that provide installation and mounting tips.