Force-Measurement Devices: Types, Devices, Characteristics, and Purpose
Force-measurement devices act to measure the torques, applied compression or tension loads, or forces due to gravity (“weight forces”) acting between two separate bodies. Weight forces can be measured as either tension or compression forces, depending on whether the measuring device is designed to be pulled on or compressed by the object’s mass. A variety of measuring instruments can be used to measure forces, as discussed in the article below.
This article explores the types, characteristics, and purposes of force-measurement devices.
A force-measurement device is a piece of equipment that is used to determine a numerical value for an applied load. The device will consist of a sensor unit that will detect the applied load using a force transducer and a display that can show the reading from the sensor. For analog force-measurement devices sometimes a spring system is used rather than a force transducer. Several types of force-measurement devices can measure a range of loads for different applications.
Force-measurement devices are used for a variety of purposes: research, testing, quality assurance, and production applications. Force-measurement devices can be used to determine whether a fastener is sufficiently tight, the tensile or compressive strength of a material, the tension in a cable, the power output of a motor, or the weight of an object.
The purpose of a force-measurement device is to accurately determine the forces acting on an object. This could include: the force required to break a material, the weight of an object, the load carried by a cable, the surface tension of a liquid, or the force applied through a fastener. Each of these purposes requires a different type of force-measurement device. Typical applications of force-measurement devices include: safety testing, quality assurance, in-service testing, research, and process control purposes.
Force testing is used by: production, research, and quality assurance personnel, as well as testing laboratories and organizations that set material and design standards. Each of these organizations uses force testing in every industry, including the: automotive, aerospace, oil & gas, medical, electrical, and construction industries. Each application may require a different force-measuring device but all force-measurement devices work by measuring a displacement to determine an applied load.
There is a wide range of instruments that are used to measure force across different applications and products. Some of these are discussed in the sections below.
A load cell uses a transducer to measure the tension, compression, pressure, or torque and uses either an electrical, pneumatic, or hydraulic signal to display the magnitude of the force in a way that can be read and recorded. Load cells are the most popular form of force-measurement devices by a long way. Load cells are used in most test equipment that measure either force or weight, including tensile test machines and scales.
Load cells can use an electrical transducer, or pneumatic or hydraulic pressure to determine the applied load. The main advantages of using load cells are that they are very precise, require little maintenance, have a long life span, and can be used for many different applications. The disadvantages of load cells are that they are bulky and that they require expensive signal-transfer equipment for electrical, hydraulic, and pneumatic signals. Load cells can be used to measure the forces on or the weight of any material. Most industries will use a load cell for one application or another, including: testing, quality assurance, and process control.
Spring scales, also known as spring balances, are analog force-measurement devices that determine the force applied by measuring the extension of the scale’s spring while under load. Spring scales are used to measure the force applied by gravity (weight), by connecting the attachment point of the device to a stable fixture and then suspending the material to be measured from the other end of the device. The device works on the principle of Hooke's Law, which describes the relationship between the extension of a spring and the force applied to the spring.
The main advantages of spring scales are that they are simple in design, cheap, and portable. The disadvantages are that they have low accuracy, they can not measure high loads, and they require frequent recalibration. Spring gauges that are exposed to large temperature changes are less accurate as the springs’ elastic modulus changes which changes the relationship between applied force and spring extension. Spring scales can be used in all industries to measure light loads and liquids, however, when measuring the weight of liquids a container of a known weight will be required.
To learn more, see our full guide on Spring Testing.
Force gauges, also known as push-pull force gauges or tension and compression force gauges, are used to measure an applied load in either tension or compression for testing, process control, quality assurance, or research. Force gauges come in two forms: analog and digital.
Analog force gauges use a spring or a load cell to detect the applied force and then mechanically display the magnitude of the force by moving a needle gauge. Analog force gauges are easy to use, portable, and low maintenance.
Digital force gauges work in the same way as analog gauges, however, a digital gauge uses a digital display to present the magnitude of force, whereas an analog gauge uses a mechanical indicator such as a needle and dial to display the magnitude of force. The movement detected by the load cell or strain gauge is converted into an electrical signal, which can be shown on a digital display. While digital force gauges are more expensive than analog gauges, they can record and store the measurements which allows the operator to see a change in force more easily. Force gauges can be used in the sports industry to test grip strength, in the manufacturing industry to conduct compression or tensile tests, and in research to test the strength of adhesives through bond tests.
A torque wrench is different from most other force-measurement devices as it not only measures an applied force, it is also used to apply the force. A torque force is an applied load that causes a body to rotate, the most common situation where torque is used is to tighten a fastener. A torque wrench, as the name suggests, measures the applied torque, specifically to fasteners such as bolts. There are several types of torque wrenches, including: click, electronic, and hydraulic. A click torque wrench works by setting the desired torque for a fastener and then tightening the fastener till you hear a click. This click confirms to the user that the fastener has experienced the right torque. Electronic torque wrenches work similarly but they can also record the applied load, which can be downloaded and stored for future reference. A hydraulic torque wrench works by placing the wrench drive socket on the fastener to be tightened and then a hydraulic pump pumps hydraulic fluid into a piston which extends and rotates the driving socket. Once the piston has fully extended it is retracted and the internal ratchet system prevents the driving socket from turning with the piston. The piston can then continue to be extended repeatedly until the desired torque force is achieved.
The advantages of using a torque wrench are that the operator can tighten the fastener and check the resulting value of torque at the same time; the tool is portable; and the torque wrench can also be very accurate (depending on the type of torque wrench used). Torque wrenches are used exclusively to tighten fasteners. They are used in many manufacturing and service industries that use fasteners including: automotive, aerospace, marine, and other heavy industries.
To learn more, see our full guide on Torque Testing.
Pressure sensors are very similar to load cells, however, the difference is that load cells directly measure the applied force, whereas pressure sensors indirectly measure the applied force. Pressure sensors measure the force indirectly by measuring the pressure of hydraulic fluid. This means that pressure sensors are less accurate as the pressure of hydraulic fluid is not exactly proportional to the applied load. This is because hydraulic fluid has weight which means it can exert pressure on itself. Additionally, there is friction to overcome when the cylinder is to be moved which means some of the force is used up overcoming the friction of the cylinder.
Piezoelectric force sensors work by using a quartz crystal to detect a change in applied loading. The pressure applied to the quartz crystal generates an electrical signal that can be digitized and stored or read from a digital display. Piezoelectric force sensors are used to detect rapidly applied loads in impact testing or for oscillating loads. This is because piezoelectric force sensors can rapidly detect the changes in applied loads.
The advantages of piezoelectric force sensors are that they are fast at detecting changes in force, they can measure a very large range of loads, and they are compact. The disadvantage of a piezoelectric force sensor is that it is prone to drift, which means when the voltage is removed from the quartz the dipoles of the quartz reorient. While the quartz re-orients itself the crystal will expand or contract. This slightly changes the relationship between the applied force and the electrical charge generated. So over time the amount of electricity generated for the same force is different, and this produces inaccuracies. This disadvantage is less of a problem when measuring large loads (kilonewtons and above) because the changes caused by drift do not make a substantial difference to the value of the applied load. Piezoelectric force sensors are used in: spot welding, impact resistance testing, ultrasonic transducers, diesel fuel injectors, grill igniters, microwaves, electric toothbrushes, and automotive crash testing.
Dynamometers are force-measuring devices that measure tension, weight, torque, speed, or power. They are used mainly in the automotive and construction industries. There are two main types of dynamometers: the first measures the power output of motors and the second one measures tension.
The motor-testing dynamometer is essentially an electrical generator that applies a load to the motor being tested and then reads the electrical power produced as the motor drives the dynamometer. Dynamometers are well known for their use in the automotive industry in measuring the speed and torque of vehicle engines to determine the power of an engine.
A tension dynamometer has a cable that runs through three pulleys. The two outer pulleys are stationary, while the middle one is spring-loaded. The more tension the cable is under, the more the middle pulley moves. The displacement of the middle pulley is measured and displayed as a tension force. This type of dynamometer is used to measure the tension in telephone lines, and cables in suspension bridges, cranes, ski lifts, and elevators.
Tensiometers are used to measure surface tension, dynamic contact angle, surface free energy, powder wettability, and density. Tensiometer test equipment details may differ, but they all work on the same general principle, which is to use a transducer to measure the force required to lift a test probe out of a liquid or to measure the weight gain of a submerged specimen.
For example, the wettability of fibers is tested by lowering the fibers into the test liquid and then measuring the increase in weight of the fibers. This determines how well the liquid wets (adheres to) the fibers.
To measure surface tension, a probe is lowered into the liquid to measure surface tension and then removed. The force required to remove the probe is measured, and the surface tension can be calculated using the force measurement. To achieve these measurements the tensiometer must be very sensitive.
The advantages of a tensiometer are that it is highly accurate and it is the only force-measurement device that can measure surface tension, wettability, and free surface energy. The disadvantages are that it is very expensive and it requires skilled technicians to operate it.
There are a few characteristics that all force-measurement devices must have, and that all devices have in common. These common features are listed below:
- Point for force application
- Force display
- A stiff case or frame to reduce inaccuracies
- Spring, transducer, or quartz crystal
The two main types of force gauges are listed and discussed below:
Spring-meter gauges are force-measurement devices that use the extension of a spring within the device to determine the applied force or weight of the measured material. Spring gauges are often analog and work using the principle of Hooke's Law, which describes the relationship between the extension of a spring and the force applied to it. A mechanism in the spring gauge, which is in parallel with and lies between the spring and the load, moves a needle within the device according to the extension of the spring. The more load is applied, the more the spring extends, resulting in a higher needle reading.
The advantages of spring gauges are that they are usually cheaper than digital gauges, they are portable and they are simple to use. Their disadvantages are that they may be less accurate and require more frequent calibration than digital gauges. Furthermore, they are usually only analog devices, and can not record data. Spring gauges are often limited in the force range they can measure. Additionally, spring gauges that are subject to large temperature changes become less accurate as the relationship between applied force and material extension changes due to the material’s change in elastic modulus. Also, metal fatigue can lead to the failure of the spring.
Digital meter gauges use transducers — which may be pneumatic, hydraulic, strain gauge-based, or piezoelectric to measure an applied load. The signal from the transducer is converted into an electronic signal that can be stored or displayed digitally. The advantages of a digital meter gauge are that it is usually more accurate, easier to calibrate, can measure a wider range of loads, and can record the forces measured for later review. The disadvantages include being more expensive and requiring a power source.
Force is calculated using Hooke's Law for spring gauge force-measurement devices. Hooke's Law describes the relationship between the extension of a spring and the applied load. Hooke's Law is displayed below:
Force = Spring constant * length of extension
The equation shows that if the spring constant is known, then the force can be determined by measuring the spring’s extension. This is how spring force-measurement devices work in practice, as they use a needle to measure the extension of the spring.
Yes, manufacturers conduct force testing on products to measure the difficulty or ease with which something can be pulled, pushed, compressed, bent, peeled, cut, etc. Measuring the force required to perform these actions on products can determine whether they meet design requirements and whether they are “fit for purpose.”
An example of manufacturer testing is tensile testing, which can be performed on a wide range of materials, such as plastics, composites, and metals. In this testing, the manufacturer applies force to the product until it breaks, providing them with valuable information regarding the material’s properties, including its yield strength, ultimate tensile strength, and ductility. This allows companies to determine if a material will have sufficient durability in an application.
This article presented force measurement devices, explained them, and discussed their various types and characteristics. To learn more about force measurement devices, contact a Xometry representative.
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