Force Proving Ring:
Proving Ring is a device that is used to measure force. It basically consists of an elastic ring of known diameter along with a measuring device present in the center of the ring. The entire assembly is usually made of steel alloy. This (electronic force sensor) can be used to measure both Compression & Tensile forces.
Proving ring is a force measuring instrument wherein the shape deflects elastically along the diameter when load is applied. This instrument also finds application for calibration/verification of Uni-Axial Testing Machines. It consists of a steel ring with a Dial gauge mounted in the center so as to measure the vertical deflection when the load is applied through the threaded blocks on the top and bottom. Applied load is known from its load and deflection characteristics.
Even though for very many years proving ring force measurements were considered as the standard of excellence for force calibration, the fact of the matter is it suffers from the following deficient attributes:
- Creep – If the force is maintained and we continue to measure the reading in the dial gauge that has a high degree of resolution, we would notice that the reading continues to change over a period of time.
- Deflection measurement – when we apply force on the proving ring, it deforms from its original circular shape and takes the shape of an ellipsoid. The determination of the deflection of the proving ring depends primarily on the subtraction of two numbers, namely, the inside diameter of the proving ring and length of the Dial gauge measurement assembly. But we have to keep in mind that the difference will be a very small figure. Hence any error whatsoever that creeps in while measuring either dimension would lead to a large percentage of error in the deflection measurement.
- Resolution - Most mechanical measurement systems would un-expectantly introduce certain errors which are in most probability difficult to control or overcome. One of the obvious problems would be resolution. And we know that resolution is usually limited by the fitness of the Dial gauge threads and the spacing of the indicator graduations marking. Non-repeatability of measurements taken multiple times in the same direction depends mainly how much force is applied to the threads of the Dial gauge screw threads. While we should be aware of hysteresis in the measurements taken at the same points from opposite directions which would be dependent on the preload, friction, and looseness in the plates.
- Temperature effect - Variation in the temperature of either the steel ring or the Dial gauge assembly can cause expansion or contraction. This could further result in a change in the deflection reading. Hence, we can see that proving rings are susceptible to temperature after-effects while taking reading. Also, there would be a change in the spring constant with temperature, which would thus bring about a change in the calibration.
- Response to extraneous forces – The very construction of a proving ring does not ensure the cancellation of extraneous forces such as side loads and moment loads. Any load other than purely a force through the sensitive axis of the ring, would result in a peripheral output.
- Specially trained personnel – the proving ring needs to be handled by specially trained personnel for proper operation of the equipment. This is basically because of the possibility of errors that maybe introduced by creep, and it also subject to errors due to temperature and extraneous loads. Normally proving rings calibration is as per the divisions on the gauge for specific forces and hence the service personnel should be aware of conversion of divisions to force or vice versa.
Electronic force proving Instruments:
Electronic force proving instrument system consists of a Force transducer and a high-resolution touch screen smart control display unit. These products have become quite popular in the market due to numerous advantages it offers.
These have become the most widespread of all force measuring systems and can also be used as force transferred standard also.
Unlike proving ring devices, these force transducers are less sensitive to temperature, creep, extraneous loads, side loads and moments. Also, the main limitation of proving rings is the resolution is limited to 2000 or 5000 counts or divisions. However, with a Force Transducer and Display Unit Combination you can have resolution up to 1,00,000 counts or divisions. This would ensure the working of the force transducers in the range of UTM Machine Classes i.e. from Class 0.5 to Class 3, with no loss of accuracy. Therefore now lesser number of force sensors (force transducers) can be used to cover a wide range of capacities and class of machines. And this would make the entire setup economical in terms of maintenance, service, running and calibration costs.
Also, with the advantage to interpolate necessary values in the equation, force can be calculated at any given step. Proving ring applications are limited to measuring specific forces for which they are calibrated. Unless otherwise the dial gauge used in the force proving ring is also calibrated every time.
SushmaTM electronic force proving instrument or the system usually includes high precision loadcells & its class of accuracy is defined as per ISO 376:2011 or IS: 4169:2014 & 2 lakh count Smart touch screen display.
The inimitable web design of the small profile sensor yields high performance and offers high resistance to extraneous forces such as bending, side / torsion loads, low deflection, high accuracy, and lowest creep. Compact low-profile design frees up valuable vertical testing space.
These loadcells are calibrated directly with Dead Weight Force Calibration machine utilizing our in-house test facility – Sushma Calibration & Test Labs Private Limited, (accredited as per ISO 376 in an accredited ISO 17025 (NABL Accredited Laboratory) traceable to National and International Standards.
This system is also suitable for metrology use as a reference standard or a transfer standard. And it can be used for calibrating or for verification of Uni-axial Testing Machines, Universal Testing Machines, Compression and Tensile Testing machine as per IS:1878-1:2005 / ISO 7500-1:2015 or as per any other international standards. It is also suitable for verifying test benches and fatigue testing systems, structural and material testing machines, Hydraulic/Pneumatic Press Force monitoring.