INSTRUMENT FOR MEASURING THE HARDNESS OF STEEL PRESSURE-GAUGE SPRINGS

V. D. SARTAKOV, V. I. URUSOV, I. G. LESHCHENKO

(Presented by the scientific-technical seminar of the Faculty of Automation and Computer Engineering)

The hardness control of pressure-gauge springs made of steels 50KhFA and 30KhGSA at the Tomsk Pressure-Gauge Plant was carried out for a long time selectively on Rockwell and Vickers hardness testers. Such a method of measuring hardness is low-productivity and belongs to destructive methods.

At the Department of Information-Measuring Technology, studies were carried out and the dependence of various electromagnetic characteristics of pressure-gauge springs made of steels 50KhFA and 30KhGSA on hardness was analyzed. On the basis of the results obtained, mock-ups of instruments were developed for the pressure-gauge plant [1, 2]. Making use of the sufficiently satisfactory relationships between the hardness of pressure-gauge springs and the magnitude of the emf of the fundamental harmonic for a two-winding through-type transducer (Fig. 2), the authors developed an instrument for monitoring the hardness of pressure-gauge springs of several standard sizes made of the indicated grades of steel. The instrument underwent tests under factory conditions during control inspections of 16 standard sizes of springs: MGn 160×160, MGn 160×250, MSSh 100×100, MSSh 100×160, etc.

The schematic diagram of the instrument is given in Fig. 1, where \(D_0\) is the transducer into which the reference spring is placed, and \(D_k\) is the transducer with the spring being inspected. As the reference spring for steel 50KhFA, a spring with a hardness of 42 Rockwell units is used; for steel 30KhGSA, one with a hardness of 40 units.

The measuring windings of the transducers are connected in a differential circuit. The magnetizing windings are connected in series and are supplied with a voltage of 220 V, 50 Hz from a ferroresonant stabilizer. The magnetization regime of the pressure-gauge springs was selected as optimal \((I_d — 0.1\ \text{A},\ W = 1500)\) [2].

Operating Principle of the Circuit

Before measurements are made, a spring is placed in transducer \(D_d\) whose hardness is equal to the hardness of the reference spring of the given standard size. Because of the nonidentity of the transducers and the difference in the chemical composition of the steel of the given batch, a certain differential emf \(\Delta E_2\) will appear, which is compensated by a compensating circuit. The latter consists of a voltage divider \(R_1 \div R_{13}\) and a bridge phase shifter \(R_{14}, R_{15}, R_{16},\)

Fig. 1. Schematic circuit diagram of the instrument for measuring the hardness of pressure-gauge springs.

Visible inscriptions in the diagram:

• “Defective”
• “Hard”
• “Soft”
• “Adjustment”
• “Sensitivity”
• “Measurement”
• “Setup”
• “Phase, kN”
• “Mains”
• “~220 V”
• “50 µA”
• “−30 µA”
• “Modulation, 6.3 V”

$R_{17}, C_1$. With the aid of a voltage divider the amplitude is varied, and with the aid of a bridge phase shifter, the phase of the compensating voltage is varied. Structurally, the voltage divider is assembled on an 11-position switch $P$. The knobs of potentiometers $R_{16}, R_{17}, R_{13}$ and of the switch are brought out to the front panel of the measuring unit of the instrument, with the common engraving “Zero setting.”

After the zero has been set, the springs being checked are placed in turn in sensor $D_k$. The unbalance signal from the measuring windings of the sensor, proportional to the hardness of the spring being checked, is fed to the input of the selective amplifier ($L_1$). The gain is adjusted by potentiometer $R_{18}$ “Gain.” The amplifier is covered by negative feedback for all frequencies except 50 Hz. A double T-bridge ($R_{32}, R_{33}, R_{34}, C_7, C_8, C_{20}$) serves as a notch filter for the frequency 50 Hz. The second bridge ($R_{27}, R_{28}, R_{29}, C_5, C_6, C_{19}$) is a notch filter for the frequency 150 Hz and suppresses the third harmonic remaining in the unbalance signal after the selective amplifier.

The voltage from the output of the second bridge, through the divider $R_{30}, R_{31}$, is fed to the second amplification stage, built on tube $L_2$. The knob of potentiometer $R_{31}$ is brought out to the front panel of the instrument with the engraving “Sensitivity.”

From the output of $L_2$ the voltage is fed to the inputs of two phase-sensitive rectifiers built on semiconductor triodes PT1–PT8. Their purpose is to convert the amplified ac voltage signal into a direct current of the corresponding polarity. The load of the phase-sensitive rectifiers consists of the series-connected resistances of the microammeter $\mu A$, whose scale is graduated in units of hardness, and the windings of the polarized relay RP-5. To set the relay to a definite operating current, a potentiometer $R_{54}$ “Signal setting” is placed parallel to the relay winding. Toggle switch $T_3$ “Measurement—setting” connects the circuit of the microammeter and the relay to the output of one or the other phase-sensitive rectifier when performing the “Zero setting” operations.

The commutating voltages required for the operation of the phase-sensitive rectifiers are taken from the secondary windings of transformers Tr1 and Tr2 and are shifted by 90° relative to one another. The primary windings of the transformers are the cathode loads of tube $L_3$, to whose grids voltages shifted by 90° are applied from two series-connected phase-shifting circuits. The first circuit ($R_{47}, R_{48}, R_{49}, C_{10}$) is powered by a voltage of 26 V, 50 Hz from the power-supply unit. Potentiometer $R_{49}$ “Phase KN” makes it possible to smoothly adjust the phase of the commutating voltages in order to obtain the greatest sensitivity of the instrument to the hardness of the springs. The second circuit ($R_{50}, R_{51}, R_{52}, C_{11}$) shifts the voltage arriving from the first circuit by 90°.

The average value of the current flowing through the microammeter connected as the load of the phase-sensitive rectifier is

$$ I = \kappa \cdot U_c \cdot \sin \varphi, $$

where

$\varphi$ — the angle between $U_c$ and the commutating voltage,
$U_c$ — the signal voltage, proportional to the hardness of the spring,
$\kappa$ — the conversion coefficient of the measuring converters.

Thus, the current flowing through the indicating instrument is proportional to the hardness of the spring being checked.

When the current through the microammeter exceeds $\pm 100$ μA, relay RP-5 operates and, by its contacts, closes the power-supply circuit

Fig. 2. Dependencies between the instrument readings and the hardness of springs:

● — for springs of type MGn 160×160 made of steel 50KhFA;
+ — for springs of type AM 160×10 made of steel 30KhGSA.

signal lamps \(L_4\) or \(L_5\), depending on the polarity of the current. The lighting of the lamps indicates the presence of the defects “soft” or “hard.”

The power-supply unit includes a ferroresonant stabilizer \((C_{21}, C_{22}, Dp_1)\), power transformer \(Tr3\), and rectifier \(D_1 \div D_4\) with filter \(Dp_2, Dp_3, C_{23}, C_{24}, C_{25}\) for supplying the anode circuits of the circuit. Stabilization of the alternating voltage is provided in order to reduce the error of the instrument due to fluctuations in the line voltage. Deviation of the voltage-curve shape at the stabilizer output from a sinusoid does not affect the operation of the measuring unit because a selective amplifier tuned to a frequency of \(50\ \mathrm{Hz}\) is used.

To create the specified-current mode, a resistance \(R_{55} \gg Zw\) is connected in series with the magnetizing windings of the sensors.

Structurally, the instrument is made in the form of two units: a power-supply unit and a measuring unit with remote sensors.

It should be noted that the diameter of pressure-gauge springs changes significantly in passing from one standard size to another. In order to cover the entire range of diameter variation, the instrument provides for the use of 3 pairs of sensors.

As an example, the dependence of the instrument readings, in scale divisions, on Rockwell hardness for control springs of type MGN—\(160 \times 160\) made of steel 50KhFA and springs of type AM \(160 \times 10\) made of steel 30KhGSA is shown in Fig. 2. The hardness was determined as the arithmetic mean of 7–9 measurements along the generatrix of the cylindrical surface of the spring.

The error in measuring spring hardness with the developed instrument (on the basis of an investigation of different standard sizes of pressure-gauge springs) is \(\pm 1.2\) Rockwell units for steel 50KhFA and \(\pm 0.6\) Rockwell units for steel 30KhGSA.

A drawback of this instrument is that the hardness measurement of a pressure-gauge spring is carried out only in the middle part of the spring. In reality, the hardness of springs is nonuniform along their length and, thus, a defective spring of normal hardness may be encountered. Elimination of this drawback is possible either by using a new type of local-action sensor or by applying a new method of hardness control.

References

1. A. M. Gruznov, V. P. Dolgopolov, I. G. Leshchenko, V. D. Sartakov. Measurement of the hardness of steel products using an alternating-current coercimeter. Defektoskopiya, No. 4, 1967.

2. I. G. Leshchenko, V. D. Sartakov. Measurement of the hardness of pressure-gauge springs by electromagnetic methods (in this collection).