DEVICE FOR MONITORING THE HARDNESS OF SPRING BLANKS MADE OF 50KhFA AND 30KhGSA STEEL

A. M. GRUZNOV, V. D. SARTAKOV, I. G. LESHCHENKO, O. N. OLSHEVSKAYA

(Presented by the scientific seminar of the Department of Information and Measurement Technology)

The method for measuring the hardness of blanks for pressure-gauge springs made of 50KhFA and 30KhGSA steel is based on the experimentally established existence of a quite definite dependence between magnetic characteristics and hardness. In contrast to the known existing direct-current coercimeters, we have developed and built an instrument that measures hardness by the dynamic coercive force at industrial frequency. This method was developed and has already been used in the creation of a number of instruments at the Department of Information and Measurement Technology of Tomsk Polytechnic Institute [1, 2].

Fig. 1.

The method of measurement by dynamic coercive force used in the instrument that was created has sufficient sensitivity, a relatively small dependence of the instrument readings on changes in the geometrical dimensions of the items being inspected, on changes in the line voltage without the use of special stabilization measures, and the possibility of developing a measurement circuit having one sensor.

A differential circuit is used in the instrument, which makes it possible to measure not the entire value of the dynamic coercive force of the item being inspected, but only its increment relative to the coercive force of a reference item, for which the softest item is selected. Determination of the difference between the dynamic coercive forces is reduced to measuring the time interval between the instants at which the inductions in the reference and tested items pass through zero.

The block diagram of the measuring unit of the developed instrument is shown in Fig. 1. The element carrying information about the measured—

of my hardness, is a pass-through two-winding sensor with the following parameters: number of turns of the magnetizing winding, 1900; wire diameter, 0.2 mm; magnetizing current, 0.1 A; number of turns of the measuring winding, 6000. Structurally, the sensor is made as a remote unit and is connected by cable to a connector on the rear wall of the instrument chassis.

The emf from the measuring winding of the sensor is converted, by means of the integrating circuit IC, the low-frequency amplifier LFA-1, the Schmitt trigger ST1, and the pulse amplifier PA1, into a pulse signal that is fed to the input of the device for converting the duration of the time interval between pulses into direct current, measured by the M24 microammeter. The second channel includes the low-frequency amplifier LFA2, the Schmitt trigger ST2, and the phase inverter PI, at the output of which a voltage is formed proportional to the average induction in the reference article; a differentiating circuit DC2; and the pulse amplifier PA2. The presence in the circuit of the phase-inverting device PI makes it possible to make the instrument comparatively convenient for checking several types of articles.

Fig. 2. Functional diagram of the signaling unit.

The device for converting the duration of a time interval into direct current is built on the basis of the monostable multivibrator MM1, to one input of which pulses from the channel of the article being checked are applied, and to the other input, pulses from the channel of the reference article.

The amplified pulses from DC1 are also fed to the inputs of the static triggers T1 and T2 and of the monostable multivibrator MM2, which perform the function of the instrument signaling circuit.

The signaling unit (Fig. 2) serves to issue light signals if the article being checked has a hardness below 150 Vickers units and if the hardness of the article is greater than the maximum permissible hardness under the inspection conditions.

The signaling circuit (Fig. 3) makes it possible to regulate the upper limit of hardness control within a specified range. The lower limit remains constant, equal to 150 units HV.

On the front panel of the instrument there are two neon lamps (L9, L10), respectively engraved “Soft reject” and “Hard reject,” which signal that the article being checked has fallen outside the permissible hardness value.

Pulses from the reference and checked channels are fed to the inputs of trigger T1. If the article being checked has a hardness greater than 150 HV units, the pulses from the channel of the article being checked arrive at input T1 later than the pulses from the channel of the reference article.

Between the arrival of pulses from both channels at input T1 there is a time interval, the maximum duration of which is 400 μsec (provided that the hardness of the reference article is 150 HV units and the hardness of the checked article is 250 HV units).

When a pulse arrives from the reference channel, trigger T1 goes into the excited state and remains in this state until the arrival of a pulse from the channel of the article being checked, which returns trigger T1 to its initial state.

During the energized state of trigger \(T_1\), anode current flows through the right half of tube \(L_7\). All elements of the circuit are selected in such a way that, in this case, the voltage across capacitor \(C_7\), in parallel with which neon lamp \(L_9\) is connected, is insufficient for its ignition.

Fig. 3. Schematic diagram of the signaling unit.

If the hardness of the product being inspected proves to be less than 150 \(HV\) units, then the pulses from the channel of the inspected product arrive earlier than the pulses from the channel of the reference product (let us allow by 50 \(\mu\)sec) and return trigger \(T_1\) to the non-energized state, when the right half of tube \(L_7\) is cut off. After 50 \(\mu\)sec, the pulse from the channel of the reference product switches the trigger into the energized state, in which it remains for \((T - 50)\) \(\mu\)sec, where \(T = \dfrac{1}{f} = \dfrac{1}{50} 20000\) \(\mu\)sec.

At the same time, capacitor \(C_7\) has time to charge to the ignition voltage of neon lamp \(L_9\), which, lighting up, indicates that the hardness of the product has exceeded the minimum tolerance.

Pulses from the channels of the inspected and reference products, delayed by time \(\Delta T\) by the monostable multivibrator \(ZhM_2\), are applied to the inputs of trigger \(T_2\), assembled on tube \(L_8\). The time \(\Delta T\) is equal to the duration of the time interval between the pulse of the reference channel and the pulse from the inspected channel for a product with the maximum permissible hardness. Since the limit of the maximum permissible hardness when inspecting different types of products may be different, the delay time of the monostable multivibrator \(ZhM_2\) must be adjustable within the required limits.

Adjustment of the pulse duration of the multivibrator is carried out by means of resistor \(R_7\), the knob of which, bearing the inscription “Signaling adjustment,” is brought out to the front panel.

The circuit for starting trigger \(T_2\) is arranged so that pulses from the monitored channel bring trigger \(T_2\) into the excited state, when current flows through the right half of tube \(L_8\), while the delayed pulses from the reference-piece channel return trigger \(T_2\) to its initial state. If the item has a hardness greater than the maximum permissible hardness,

Fig. 4.

then the pulses from the monitored channel arrive at the input of trigger \(T_2\) earlier than the pulses from the reference channel by \(\sim 20000\ \mu\text{sec}\), and during this time capacitor \(C_8\) has time to charge to the firing voltage of lamp \(L_{10}\), which signals that the hardness of the item exceeds the permissible value.

The instrument underwent laboratory tests, the results of which for rods made of 50KhFA steel are presented in Fig. 4. The error in measuring hardness does not exceed \(\pm 15\) Vickers units.

REFERENCES

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

2. I. G. Leshchenko, B. B. Vinokurov, V. P. Dolgopolov. Alternating-current coercimeters for quality control of products made of ferromagnetic materials. Abstracts of papers of the IV All-Union Conference on methods for nondestructive quality control of materials and products. Kishinev, 1965.