The Quest for Quality with Force Measurement
Be it the strength of a packaging box, the sharpness of a scalpel, the holding power of an adhesive or the extent to which a screw tightens, all products have to conform to a certain set of quality standards established by their respective industries. Today, more and more companies are adopting or have been required to comply with these standards. For those who need to establish a quality control system from the ground up or are just looking to improve an existing system, some critical issues must be considered.
Even though many standards specify universal testing machines, when it comes to physical and mechanical properties, a set of basic instruments such as force and torque gages in combination with test stands are more than adequate to accurately and objectively qualify most products.
Before a set of instruments can be chosen, however a test standard needs to be adopted. One of the best places to turn to is the American Society for Testing and Materials (ASTM). ASTM International is a not-for-profit organization that provides a global forum for the development and publication of voluntary consensus standards for materials, products, systems and services. In the unlikely event of not finding an existing standard, mutually satisfactory criteria can usually be reached.
In general, force-measuring instruments are used to determine static or dynamic loads that are either compression or tension. With some small exceptions, most instruments can measure both and some are even configurable with loading shaft up or down to further accommodate the user. Commonly used units of measurement are pounds (lbs), kilograms (kg), Newton (N), ounces (oz), and typically electronic instruments are configurable in either one of these on the fly.
Instruments can either be all in one, sensor and indicator in one enclosure, or indicator with remote sensor. The all-in-one unit has the advantage of a lower cost and an easy setup, while the remote sensor arrangement is more desirable for production lines or when multiple and interchangeable capacities are needed. In some cases the remote sensor arrangement will have larger capacities then the all-in-one unit. As a rule of thumb, force-measuring instruments are recommended to be oversized by 25%, which minimizes the chance of accidental overload, and ensures that the instrument is used in its normal operating range.
One of the key qualities of any force gage is accuracy and resolution with a significant impact on the overall precision of the testing procedure. While high accuracy and high resolution are desirable, pricing will most likely be a factor in the final decision to purchase an instrument.
In addition to real time and peak-load measuring capabilities, today’s electronic force measuring instruments come with a variety of options. Some have the capability to electrically synchronize the instrument to external events. This is helpful in creating a simultaneous snapshot view of the applied loads in a multi-instrument system. This feature is also used for determining electrical contact activation force for switches.
Serial communication is likely to be another available feature. Through this two-way interface the gage can be configured or the status and measurement information can be sent to the computer or PLC. Certain gages are configurable for RS-232 or other serial communications ports.
Another possible option is averaging. This mode is used for obtaining an average force reading over a period of time. Its applications include measurement of peel force, muscle strength and frictional force. Three user–programmable parameters are associated with this mode: trigger force, which is the threshold above which the measurement starts; initial delay, which is the time delay to ignore the initial shock; and, the average time that is the time for the actual measurement to be averaged.
Averaging is helpful in coefficient of friction (COF) measurements. COF between two materials is determined by dragging a block of a known mass over a surface. Initially the instrument is used in its normal mode to determine the weight of the block and then, with the average mode set, the block is dragged until the average force is displayed. COF is obtained by dividing the average force with the weight of the block.
Set points are another practical feature and are used for tolerance checking (GO/NO GO) or as an alarm indication in process-control applications. User configurable, these signals are available on several output pins and will produce an appropriate output when the actual reading is either above, between or below their respective settings.
It is also common to have an analog voltage output proportional to the load to allow the user to customize the measurement information for unconventional or legacy systems.
Test stands are a convenient option for sample holding and applying either destructive or nondestructive loads. Equipped with travel indicators, travel limiter blocks and limit switches, these universal fixtures cover a wide range of applications. Some stands are modular and easily integrate into larger systems.
Typically operation is by lever or by wheel driven screw. Lever driven stands are quick and straightforward to use but often lack fine control and tend to back drive under larger loads. Screw driven stands in general are slower but provide more control and will most likely maintain the load without the operator’s help.
Often the most difficult part in testing products is maintaining parameter consistency. When comparing springs for example, the force developed at a given height is used as the main indicator. Repeating this height from sample to sample is critical and requires tightly controlled test systems. Motorized test stands interconnected with electronic instruments and limit switches, accomplish this task. Adjustable limit switches and set-point outputs consistently control location and load boundaries while repeatable test speed is ensured by the electronically controlled motor. With its high complexity, motorized test systems accomplish a great deal more than their manual counterparts.
Grips and accessories come in a great variety. Some are available from the instrument manufacturer providing a quick solution. However, in many occasions a more complex or specialized version needs to be fabricated.
In any case, before a grip is purchased it is a good idea to supply the manufacturer with samples—be it a packaging box, scalpel, adhesive or screw—and have them suggest an off the shelf or a custom solution.
1. When it comes to physical and mechanical properties, a set of basic instruments such as force and torque gages in combination with test stands are more than adequate to qualify most products.
2. Before a set of instruments can be chosen a test standard needs to be adopted. One of the best places to turn to is the American Society for Testing and Materials.
3. In general, force-measuring instruments are used to determine static or dynamic loads that are either compression or tension.
4. Instruments can either be all in one, sensor and indicator in one enclosure, or indicator with remote sensor.