On a Pneumatic Two-Stage Gear Motor

S. I. Shubovich

(Presented by the scientific seminar of the Department of Mining Machines, Mine Transport, and Mining Electromechanics)

Gear-type pneumatic motors with spur-toothed and helical-toothed rotors, which have found wide application in the drives of machines operated under conditions hazardous with respect to gas and dust, are the simplest and most reliable explosion-proof motors, but their economic and traction characteristics are relatively low. These indicators can be improved if the gear motor is made according to a two-stage scheme. The operating features and the principal characteristics of two-stage motors, as far as the author knows, have not been presented in the literature. The purpose of this paper is to fill this gap in part.

The schematic diagram of a two-stage motor is shown in Fig. 1. In the housing of such a motor, divided by a vertical partition into two independent chambers, two pairs of toothed rotors with straight or helical teeth are arranged coaxially. Each chamber with one pair of rotors forms a separate stage. One of the rotors of the first stage has a shaft that is common to the rotor of the second stage; therefore the torques developed on all the rotors are summed. Compressed air from the air network enters the inlet cavity of the first stage and performs work in it at a constant pressure \(p_1\), as occurs in an ordinary gear motor with spur rotors or with helical rotors having a small tooth inclination. The air that has performed work is carried by the depressions (at pressure \(p_1\)) into the exhaust cavity. Along its path, when the depressions filled with compressed air reach the special openings \(1\) (Fig. 1) in the motor housing, part of the air flows into channel \(2\) and expands there. At the same time, as one of the depressions of the first-stage rotors communicates with channel \(2\) until the next successive depression is connected, the pressure in channel \(2\) and in the inlet cavity of the second stage will additionally fall to the value \(p_3\). This will occur because, during the indicated interval of time, the rotors of the second stage will turn through an angle corresponding to half the pitch of rotor engagement, and the volume of the space of the inlet cavity of this stage will increase by the amount \(V_h\). Thus, air that has worked in the first stage will continuously enter the inlet cavity of the second stage, but at a lower and variable pressure.

It should be noted here that taking air from the first stage to supply the second stage has no effect either on the magnitude of the theoretical air consumption or on the magnitude of the work of the first stage. This can be verified by considering the analytical dependences—

of useful power and air consumption, obtained from an analysis of the working process of a gear-type engine with straight-toothed or helical rotors [1—3].

The theoretical work performed in the second stage by the air exhausted in the first stage is written by the following expression:

\[ L_2=\int_{p_2}^{p_3} p\,dV-p_0V_h, \]

Fig. 1. Schematic diagram of a two-stage engine.

and the work performed in the first stage is

\[ L_1=(p_1-p_0)V, \]

and, consequently, the theoretical work of a volumetric unit of compressed air in a two-stage engine will be equal to

\[ L_T=(p_1-p_0)V+\int_{p_2}^{p_3} p\,dV-p_0V_h, \]

where \(p_0\) is the pressure in the exhaust cavities of the first and second stages. For clarity, the magnitudes of these works are represented by areas on the theoretical diagram of the working process of a two-stage engine (Fig. 2), where the area \(ABED\) expresses the magnitude of the theoretical useful work performed in the first stage, the area \(GHFE\) that in the second stage, the area \(ABGHFE\) that in the engine as a whole, and the area \(ABCD\) the available work of the compressed air. It is not difficult to see that the useful work of the air in a two-stage engine is greater than the work of the air in a single-stage engine by the amount expressed on the diagram by the area \(GHFE\). This additional work represents a part of the expansion work of the compressed air which, in ordinary straight-toothed and helical engines, is not used but is lost uselessly in the exhaust process. Partial utilization of the expansion work in a two-stage engine gives it a higher efficiency and, consequently, a lower specific air consumption.

The second stage can be used not only to increase the economy of the engine, but also to improve its tractive properties, in particular, to increase the torque at starting. Increase

the starting torque of the motor, as is known, is highly desirable, since in a number of cases the dimensions of the motor installed for driving a given machine are determined not by the power, but by the magnitude of the starting torque. To increase the torque of a two-stage motor, it is sufficient to supply compressed air from the mains into the inlet cavity of the second stage and to raise the pressure in this cavity. The degree of increase in torque will obviously depend on the degree of increase in pressure at the inlet of the second stage and on the ratio of the geometric parameters of the first and second stages. The supply of air from the mains to the second stage can be carried out automatically, depending on the change in the speed regime of the motor caused by an increase in load, for example by means of a device (Fig. 1) consisting of a centrifugal governor 3 and a spool valve 4 closing channel 5. When air is supplied from the mains to the second stage in such an amount that the pressure in the inlet cavity of this stage becomes equal to the mains pressure, the torque of the motor reaches a limiting value, while the economic indices drop to those of an ordinary single-stage gear motor.

Fig. 2. Diagram of the working process of a two-stage motor

Thus, the second stage can be used at small and medium loads to increase the economy of the motor, and at large loads to increase its overload capacity.

For an experimental verification of the operation of the two-stage gear motor and to determine how great the influence of the second stage is on its economic and tractive indices, the author constructed and tested a model of a two-stage motor, which consisted of two production PRSh-10 motors connected according to the scheme described above (Fig. 1). One of the motors was used as the first stage, and the other as the second stage. Both motors were connected by their drive shafts to a single hydraulic brake; therefore the rotors of the motors had one and the same speed, and the torques developed by them were balanced by the hydraulic brake. On such an installation it was possible to carry out experiments with both stages operating jointly and with only one first stage operating. In the latter case the PRSh-10 motor used as the second stage was disconnected from the brake and air lines.

To supply air from the mains directly to the second stage for the purpose of increasing the torque, a special automatic device was used, which opened access for air from the mains when the rotational speed of the motor shaft decreased, beginning at 1350–1400 rpm. On the pipeline connecting the air mains with the spool-valve device, a valve was installed, with which it was possible not only to shut off this spool-valve device, but also to prevent the penetration of compressed air from the mains into the second stage when conducting those experiments whose purpose was to determine the economic efficiency of the second stage.

The experimental model described, in our opinion, corresponded to a two-stage motor to a sufficient degree; however, it should be noted that the parameters of the second stage of this model were not optimal from the standpoint of the fullest use of the energy of the air exhausted in the first stage. For this reason, the economic and traction indices given below, obtained in testing the model, cannot be regarded as the best for a two-stage motor.

Tests of the experimental model were carried out at a constant compressed-air pressure in the mains equal to 3.5 atm, which, according to the passport data, is optimal for the PRSh-10 motor. During the tests the following were measured: the per-minute consumption of compressed air, the torque on the brake shaft, the temperatures and pressures of the air before the first and second stages, and also the noise level of the operating installation. The experiments were carried out with both stages operating jointly, with the supply of air from the mains to the second stage disconnected and connected, and with only one first stage operating, i.e., with one PRSh-10 motor operating as the first stage.

The test results are presented by the graphs in Fig. 3, from which it is evident that, when the two stages operate jointly, the per-minute air consumption is practically equal to the consumption of the single-stage PRSh-10 motor¹, while the torque and the corresponding power are greater over the entire range of variation of the motor-shaft speed. The increase in power causes a corresponding reduction in the specific air consumption and an increase in efficiency. Thus, for example, at the nominal speed condition \((n = 1450\ \text{rpm})\) the reduction in consumption was approximately 18%. The increase in economy apparently could have been somewhat greater if the parameters of the second stage had been optimal for this particular case.

The increase in the starting-torque ratio \(K = M_{\text{st}} : M_{\text{n}}\) of the two-stage motor in comparison with the single-stage PRSh-10 motor was 7% without supply of compressed air from the mains to the second stage and 42% when air was supplied to this stage. From the graph (Fig. 3) it can be seen that the pressure in the inlet cavity of the second stage at \(n = 0\) was less than the pressure in the air mains, and therefore in our experiments the starting torque had not yet reached its limiting value. The reason for this was the insufficient flow capacity of the spool device used to control the supply of air from the mains to the second stage. Elementary calculations show that, with sufficient air supply to the second stage, the starting-torque ratio of the two-stage motor would have been 2.1 instead of the ratio 1.21 possessed by the PRSh-10 motor.

Analysis of the curves of air temperatures in the mains and in the channel feeding the second stage (Fig. 3) shows that the air taken from the first stage expands as it flows out of the cavities with absorption of heat from the surroundings, the polytropic exponent in our experiments being 1.21–1.28.

The noise level during joint operation of the two stages at the nominal speed condition was 93–95 dB, while for the single-stage PRSh-10 motor with silencers removed it was 98–105 dB, and with silencers installed, 90–95 dB. These measurements show that the second

stage lowers the noise level to approximately the same degree as is achieved by the use of relatively bulky mufflers.

Summing up the comparative analysis of the operating indices of single-stage and two-stage motors, it may be stated that the two-stage motor is favorably distinguished by higher efficiency, better traction properties, and lower noise

Fig. 3. Characteristics of single-stage and two-stage motors

during operation. Along with these advantages, there are also disadvantages, the chief of which are relatively large overall dimensions and weight, as well as the need for a comparatively complex device for reversing it while running. For these reasons, in our opinion, two-stage motors should be used in the drives of such machines as are characterized by severe starting conditions, large short-term overloads during operation, and do not require reversing while running.

LITERATURE

1. K. S. Borisenko. Pneumatic Motors of Mining Machines. Ugletekhizdat, Moscow, 1958.

2. A. M. Misharin. Pneumatic Gear Motors. Ugletekhizdat, Moscow, 1948.

3. S. I. Shubovich. Investigation of the Working Process and Characteristics of Certain Pneumatic Motors. Dissertation for the degree of Candidate of Technical Sciences, Tomsk, 1956.

1. In actuality, the air consumption with the second stage connected was somewhat smaller (by 4–5%) than with the second stage disconnected. This is explained by the fact that the second stage reduces leakage through the clearances between the rotors and the walls of the first-stage casing. ↩