GB738606A - Improved hardness tester - Google Patents

Abstract

738,606. Hardness testing apparatus. WALLACE & CO., Ltd., H. W., and MORE, A. R. May 6, 1954 [Feb. 6, 1953], No. 3438/53. Class 106 (2). A hardness tester suitable for micro-hardness testing comprises means for indicating when an indentor is in a predetermined position in relation to a support which allows it to approach and indent material under test without friction between relatively movable surfaces, means to cause and means to measure relative movement between the support and the material in the line of movement of the indentor and means for applying a load to the indentor. In the embodiment of Fig. 1, an indentor 19 with weight pan 26 attached is supported in an adjustable head 4 by a helical spring 27 with leaf-springs 20, 21 allowing free vertical movement and its position with the leafsprings 20, 21 horizontal is indicated by a device 39 connected by a cable 38 to a variable capacitor with one plate 36 fixed in the head 4 and a movable plate 35 fixed to the indentor. The material to be tested is placed on a table 18 integral with a vertical plate 15 rounded at the bottom for line contact with a wedge 6 which can be horizontally adjusted by a screw 9, helical springs ensuring contact between co-acting parts and a dial gauge indicating the vertical displacement of the table allowing for the reduction factor due to the wedge. To accommodate samples of various sizes the head 4 is vertically adjustable through rack and pinion means controlled by knob 32. In carrying out a test the indentor 19 is adjusted by turning knob 32 almost to touch the sample on the table 18. The device 39 is then turned to zero position and the screw 9 turned to raise the sample until a deviation of the indicator signals that a slight retraction is necessary just to make contact. The dial gauge is then set to zero and the test weight placed on the pan 26. The indentor penetrates the specimen which is raised by the screw 9 until the device 39 indicates that the indicator is back to the level at which the leaf-springs 20, 21 are horizontal and unstressed and the dial gauge indicates the depth of penetration. The test weight is removed from the pan 26 and if the material recovers at all the indentor will be raised and after lowering the sample by screw 9 to restore the position of the indentor the dial gauge will indicate the degree of recovery. The embodiment of Fig. 3 is adapted for standing on the material under test the indentor passing freely through a hole in the base. The indentor is mounted on leaf-springs 50, 52 secured in an adjustable frame 57 urged upwards into contact with a wedge 47 itself urged upwards so that adjustment of the frame is effected by turning the screw 9 bearing on the wedge. Capacitor plates 35, 36 are attached to the indentor and frame respectively. The leaf-springs 50, 52 are maintained in tension by a compression rod 61 pointed at each end to engage shallow depressions in the indentor and in a spring 62, the rod 61 inclining slightly toward the indentor effectively resists the weight of the indentor and pan 18 over the very restricted range of movement required. If the pan and indentor point are interchanged the tester may be inverted. The movementsensitive device for indicating the position of the indentor may take many forms, e.g. one supporting spring may be in the form of a strain gauge of resistor type; the indentor may carry a core or a coil displaced in relation to a fixed coil to change the inductance or mutual inductance or a differential capacitor may be connected to two sides of a bridge circuit by the adjustment of which a nil reading is obtained. Preferably an aural indication is used in conjunction with a single variable capacitor in a circuit including a pair of double triode valves one part of each of which acts as a high-frequency oscillator while the other part of each acts as an amplifier. One triode is tuned by the capacitor in the head of the tester, plates 35, 36, and the other by a manuallycontrolled capacitor, the outputs of the oscillators being combined to produce audio beat frequency which is amplified and head phones or a loud-speaker are provided for obtaining the nul point aurally, leaving the operator free to observe the dial gauge.