Force gage may not be the most familiar term to engineers and quality control professionals. When posed with the question of how to measure force, a load cell may come to mind, or, on the other end of a spectrum, a tensile tester or materials tester. One could be forgiven for not being aware of force gages, as they were relatively basic until the digital revolution changed things dramatically in the 1980s and 1990s. And today, they still serve a unique and subtle purpose that can be difficult to neatly define.
How does a digital force gage work?
At the heart of a digital force gage is a strain gage-based load cell. Strain gages applied to the load cell stretch and retract as a variable force deforms its structure. Deformation of the strain gages changes the electrical resistance in the circuit, thereby correlating force to voltage. This signal is converted into a digital signal, further manipulated by a microprocessor, and converted into units of lbF, kgF, N and other measurement units displayed on an LCD. The load cell is mounted to a metal case sturdy enough to withstand the maximum force rating. Attached to the load cell is a shaft with a threaded end to accommodate attachments, such as hooks, clamps and specialized fixtures.
Selecting an appropriate capacity
Force gages are available in a range of capacities and resolutions to accommodate a wide variety of applications. For example, a suture manufacturer may need to measure 50 N of force, while a manufacturer of wire harnesses may need to measure 5,000 N.
With any application, a gage should be selected with a capacity that is as close as possible to the maximum expected force. Since accuracy is typically specified as a % of full range, lower force readings are subject to an increasingly large error as a percentage of reading. For example, if the choice of force gages includes 5, 10, 20 and 50 N capacity models, and the requirement is for 15 N of force, the 20 N model is most appropriate. In general, a force gage should be used between 20% and 80% of its capacity, for best accuracy and margin of safety.
In many applications, the peak force is of most interest, that is, the force at which a sample broke, engaged or loosened. This force is often how manufacturers gage the performance of their product. In other cases, force gages are used to apply a specified force to a sample to determine if the sample remains intact, such as in wire crimp pull testing.
Although force gages may be handheld for less demanding applications, it is considered best practice to mount the gage to a test stand, actuator, or other fixture. This type of mounting reduces the influence due to operator variability, which can lead to inconsistent readings. It also helps to ensure axial alignment with respect to the load cell shaft. Side loading, which can be difficult to avoid during hand testing, can skew the force reading and potentially damage the load cell.
Measuring force can quantify the behavior and performance of a product. A force gage is an invaluable tool for manufacturers of materials, assemblies, and finished products in virtually every industry.
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