ON THE PERMISSIBLE WEIGHT OF HANDHELD MECHANIZED TOOLS

V. I. ELAZIN

(Presented by the scientific seminar of the Department of Labor Protection and Mining)

In various branches of industry, handheld mechanized tools have become widely used for the mechanization of assembly, locksmithing, installation, extraction, construction, finishing, and similar operations. This is explained by the fact that their use makes it possible to significantly increase the productivity of manual labor, reduce the cost of work, lessen worker fatigue, and raise the culture of production. At present our industry produces about 1 million units of handheld mechanized tools per year. However, the technical level of most of these products leaves much to be desired. Despite the rapid development of science and technology, the appearance of new advanced materials, and the development of progressive technology, some models of mechanized tools are produced for 10–15 years without any substantial change in design. Such a tool has impermissible weight characteristics, an unsatisfactory and inconvenient design in operation, and an unsuccessful form and external appearance.

A major shortcoming in the work of designing modern mechanized tools is the underestimation of the so-called “human factor.” Usually what is analyzed is not the “human being—mechanized tool” system, but the tool in isolation from the human being. Yet the functions of the elements of the system (human and machine) are so closely interrelated that the task of improvement cannot be solved solely through the technical factor. To improve a system means to improve all its links in the aggregate. For a design engineer engaged in designing mechanized tools, this task is beyond his powers. Being well acquainted with the technology, he does not know the characteristics of the human being and, naturally, cannot take them into account in his work.

Before designing a machine for any profession, it is necessary to study that profession from a psychophysiological point of view. This work should be carried out not by the designer, but by the appropriate specialists—physiologists and psychologists of labor, hygienists—though with the mandatory participation of the design engineer.

In the present article we shall dwell on only one question—the one that, in our opinion, is the most urgent and still unresolved: the question of the permissible weight of a handheld mechanized tool. Let us consider this question using the example of mining handheld electric drills, which have become very widely used in the mining industry.

The weight of domestically produced handheld electric rock drills has not changed over the last thirty-odd years. It is 15–19 kg. This is evident from Table 1. For drills with a feed mechanism it is 20–26 kg. If, moreover, the weight of the drill rod and of the cable supplying electrical energy to the drill is taken into account, then the value of the stated weight characteristics increases on average by 5–6 kg.

Table 1

During the same period the effective power of drills increased from 0.5 kW to 1.4 kW.

A new design range project provides for an even more powerful drill—1.6 kW (SET16-1), i.e., there is a persistent tendency toward increasing the power of handheld electric drills while keeping their weight unchanged, although a number of research studies have shown that drill powers of 0.9–1.0 kW are quite sufficient for work, and that in the further improvement of handheld electric drills it is necessary not to increase their power but to reduce their weight.

The motor power of foreign electric drills is lower than that of domestic drills, while their weight characteristics are approximately the same. This is clearly seen from Table 2.

Table 2

Manufacturers of handheld electric drills, when assessing the progressiveness of new designs, use indicators of specific weight. From Table 1 it is evident that this indicator is indeed continuously improving. This is explained mainly by the fact that the specific-weight indicator of an electric motor is always lower than the same indicator of the working machine. However, with improvement in the specific-weight indicator, the degree of fatigue of the driller does not decrease. Nor do his working conditions improve. The universal indicator of the progressiveness of a drill should be the labor productivity of the driller working with it and the degree of his fatig—

...fatigue. Thus, when evaluating the design of electric drills, a study of the degree of fatigue should be carried out.

Taking into account that the driller works under exceptionally difficult and unfavorable labor conditions, the weight of a “handheld” tool equal to 20–30 kg (together with the drill rod and cable) is impermissible. This is indicated by drillers’ comments, and also by the results of our research work carried out at the request of the manufacturers of handheld electric drills (the Tomsk V. V. Vakhrushev Electromechanical Plant and the Konotop “Krasnyi Metallist” Plant).

The question of diagnosing fatigue remains very difficult to this day. The signs of reduced working capacity, depending on the nature of the work, may be quite varied. In one case this is a decrease in the quantity of work; in another, a deterioration in quality; in a third, a change in the character of the work being performed. However, external signs alone are insufficient for diagnosing the degree of fatigue. Taking into account the leading role of central-nervous mechanisms in fatigue, in diagnosing it one must use tests characterizing the process of inhibition, which constitutes one of the “organic” components of fatigue. Such methods include determination of sensory-motor reaction time, the use of electroencephalogram and muscle biocurrent data, methods evaluating the state of various analyzers, etc. In assessing the severity of work, registration of pulse rate, blood pressure, the magnitude of pulmonary ventilation, and energy expenditure is of great assistance. Depending on the nature of the work, one or another diagnostic method is decisive.

Experimental work to determine the degree of fatigue of drillers as a function of working conditions and the weight of the drilling machine was carried out by us under mine and laboratory conditions. It was determined by means of time-and-motion study of production activity, determination of the “static endurance index,” and recording of pulse rate and reaction speed.

The “static endurance index” is a good determinant of the degree of fatigue. It represents the time during which the person being studied can perform static work to maintain a force equal to a certain fraction of the measured maximum force. It is determined using special dynamometers.

We used this index to determine the degree of fatigue of drillers as a function of their working posture. During 1.5 hours of continuous work (the average time for drilling a face), this index, when drilling upper blast holes (the drill is located overhead), decreases on average by 72.6% from its initial value. Such a value of the index characterizes the work as very severe. Less severe is the work of drilling horizontal blast holes while lying down (a decrease of the index by 62%). Work in drilling blast holes while sitting or kneeling, as well as at chest level, is approximately the same in terms of energy expenditure (a decrease of the index by 54–58%). And, finally, the easiest is work while standing, with the drilling machine located at a level from the knees to the abdomen (a decrease of the index by 35%). However, this ease is relative. For comparison, let us give one example: when working under conveyor-production conditions at a leather-goods factory, the average decrease of the “static endurance index” over a shift is only 19.4%.

To determine the degree of fatigue as a function of the weight of the drilling machine (different weight characteristics of the machines were achieved

with the use of a counterweight) time-and-motion measurement of labor activity and recording of the heart-rate frequency were used.

These data are presented in the form of graphs in Figs. 1 and 2. They refer to the most favorable case, when the driller works standing and the drill is located at abdominal level.

Fig. 1

Figure 1 shows the dependences of the main and auxiliary time spent on one hole on the number of holes drilled. With the help of these dependences one can trace the first signs of fatigue and determine the onset of its progression. From the graphs it is evident that when electric drills weighing 10 kg are used, the main drilling time changes hardly at all. These are normal working conditions. When drills weighing 15 kg are used, the picture changes. Starting from 40—

50 holes, the expenditure of basic time for each hole increases sharply, which characterizes the onset of considerable fatigue.

In determining the permissible weight of hand-held electric drills, data characterizing the functioning of the cardiovascular system (pulse rate) are of great help. In the given character of the labor process, this system is one of the most heavily loaded. Data (pulse rate) characterizing the functioning of this system as a function of various weight characteristics of a hand-held electric drill are given in Fig. 2.

Fig. 2

Vertical axis: Pulse rate, % of initial value
Horizontal axis: Time, min
Curves: 5 kg; 10 kg; 15 kg; 20 kg; 25 kg; 30 kg

The degree of severity of the work was determined according to the method of M. V. Leinik. The essence of this method is as follows. As the estimate of the degree of severity of work, one takes the degree of strain of the most heavily loaded system of the human body (in the present case, the cardiovascular system), and then, from Table 3, determines the degree of severity of the work.

Table 3

To express the strain as a percentage of the upper limit of strain of the most overloaded system, the percentage increase in pulse rate is determined relative to the maximum possible increase. The magnitude of the maximum possible increase is 140 beats per minute.

If we use the graph shown in Fig. 2, then when working with a drill weighing 10 kg, the increase in pulse rate from the initial value is 46 beats per minute. Relating this figure to the maximum possible increase, we find that it is 32.8%. Thus, work with a drill having such weight characteristics, in accordance with Table 3, is characterized as light. When working with a drill weighing 20 kg, the increase in pulse rate is 50.7%, and the work is characterized

as work of moderate difficulty. And, finally, when using drills weighing 30 kg (an increase of 70%), the work is characterized as heavy.

The graphs presented, as indicated above, correspond to the most favorable case of using electric drills. When working in other postures, the difficulty of the work increases. This is evidenced by data on changes in the “static endurance index.”

Using the data of the graph shown in Fig. 1, one can obtain an interesting regularity showing how the labor productivity of drillers changes as a function of the weight of the drilling machine. This dependence is shown in Fig. 3.

Fig. 3

As a result of the full scope of studies (taking into account various postures, additional fatigue during the remaining hours of the work shift, working conditions, etc.), we obtained the following data on the permissible weight of handheld electric drills: when working under the most favorable conditions, the permissible weight is 10–12 kg, and when working in difficult mining conditions in awkward working postures, it is 6–8 kg.

At present, the stated weight values for electric drills, at the required power, are entirely attainable. This is possible primarily by switching the motors of electric drills to a supply with increased-frequency currents on the order of 150–200 Hz, and by the broad use of plastics as structural material in the design of electric drills.