PROVING RINGPROVING RING

TO MEASURE STRAIN DEVELOPED IN A PROVING RING AND TO PLOT THE GRAPH BETWEEN LOAD AND STRAIN

EXPERIMENTAL SETUP

Proving Ring

The Proving Ring is actually a mechanical type of force or load transducer which converts the applied force or load to corresponding Displacement and Strain. It uses a calibrated metal ring, the strain in which is measured with a precisiondisplacement transducer. Proving rings come in a variety of sizes. They are made of a steel alloy. Manufacturing consists of rough machining from annealed forgings, heat treatement, and precision grinding to final size and finish.

With the right design and over a limited range, a properly manufactured ring behaves as a nearly perfect elastic element with a highly linear relationship between applied force and resultant deflection. For a thin ring under tension load P, the increase in vertical diameter is proportional to PD3/EI, where D is the diameter of the ring, E is the modulus of elasticity of the ring material, and I is the moment of inertia of the ring in a cross-section. Thus determination of the applied force is possible if you can accurately measure the ring's deflection. The ring includes external bosses, internal bosses, or both; these bosses are threaded add-on sections, which connect and apply the external forces to the ring. Although it would be simpler to just weld these bosses onto the ring, such attachment would introduce uneven stresses and other mechanical problems. These problems would affect accuracy, so manufacturers machine the bosses as part of the ring itself. The strain in the ring is measured by mounting the strain gauges on the axis perpendicular to the loading axis and passing through the center.

Fig-2

Axial Loading of Proving Ring

Schematic diagram of the changes in the ring diameter as compression (push) and tension (pull) forces are applied

The strain distribution in the ring is a complex function of the geometry, and is significantly affected by the design details of the bosses, but the distribution illustrated in the figure is reasonably representative for constant-thickness rings. Because the bending moment does not vary significantly in the region of the horizontal diameter along which the strain gauges are mounted, the strain distribution is nearly uniform in this area.

The Position of Strain Gauges and Strain Distribution in various parts of the ring

The two gauges on the inner surface are in tension and the two outer gauges are in compression, all these experience identical axial strain, although the nature is different as inner gauges have compression and outer ones have tension. Using these, appropriately arranged in a full wheatstone bridge circuit yields four times the output of a single gauge

and also gives excellent thermal compensation and in cancellation of axial strain effects in the output of the bridge. This makes for a very stable force transducer that is easy to construct, is very robust and whose design is easily modified to handle a very wide range of applied loads. Proving rings are very commonly used for calibrating tensile/compression testing machines. Once the ring itself has been calibrated, its characteristics remain very stable.

The Wheatstone Bridge Circuit

E = Excitation Voltage(typically 10 Vdc.)O/P = Output Signal

Proving rings have been used as calibration devices for tension/compression testing machines for a long time – from long before strain gauges were available. Thedeflection measurement was made by putting a dial indicator across the inside of the ring at the locations of the gauges as shown above. It is easy to design the ring so that the change in diameter measured with the dial gauge is large enough to permit the necessary resolution.

E.g. one can readily use a dial gauge to measure to a resolution of 0.0001” over a range of 0.1 inches.

The ring design below offers several advantages over the classical form. To begin with, it is obviously easier, and much less costly, to fabricate. The squared ring also decreases the

compliance of the spring element, and correspondingly improves the linearity. At the same time, the flexural stiffness at the junctures of the bosses and the ring has been reduced to minimize the sensitivity of the element to off-axis load components

Improved ring design for low compliance, less sensitivity to off-axis loads, and reduced manufacturing cost.

PROCEDURE :-

1) Switch ON the strain indicator and let it stabilize.2) Place the hanger on the hook and note the reading of the strain gauge indicator.3) Further add the available calibrated weights very carefully and gradually without disturbing the system on the hanger and note the corresponding readings of the strain gauge indicator.4) Repeat the above steps a number of times and tabulate the readings.5) After loading all the weights start unloading and take readings for this also. Unloading should also be performed carefully and gradually.

PRECAUTIONS :-

1) Let the strain indicator stabilize before taking the readings.2) The loading and unloading should be performed very carefully.3) The wires should be tight and connected to the right arm of the strain indicator.

OBSERVATION TABLE :-

S.NoLoad Applied

(kgf)

Strain Indicator Reading On Display

Loading (x 10-6) Unloading (x 10-6)

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.