EXPERIENCE IN DESIGNING MULTIPLIERS WITH HELICAL GEARS FOR POWER PLANT DRIVES

A. E. Belyaev

(Presented by the scientific seminar of the Department of Applied Mechanics)

According to [1], the greatest load-carrying capacity of gear wheels operating at high speeds \((20 \div 60\ \text{m/sec})\) occurs in the range \(18 \div 20^\circ\). This makes it possible to assume that the traditional scheme of multipliers used in power-machine building (drives of gas blowers, turbocompressors, etc.) can be changed: instead of double-helical gears, helical gears may be used. In this case, the axial dimensions of the drive are reduced and gear cutting is sharply simplified.

In connection with this, under the direction and with the participation of the author, a large number of multipliers for drives of stationary power plants were designed. The design of these multipliers with helical wheels and thrust collars, which absorb axial loads, is described below.

The multiplier housing is made cast, with a horizontal split, the plane of which passes through the axes of the shafts. Such a design is very convenient both in manufacture and during assembly, although it is somewhat overweight in comparison with other designs of single-stage reducers.

For centering, the housing cover is mounted on dowel pins and fastened to the base with bolts. To facilitate disassembly, jack bolts are provided in the cover. Both the cover and the housing are made sufficiently rigid to avoid possible deformations under the action of internal and external forces. The housing is mounted on a special frame, which will make it possible to avoid frequently encountered difficulties caused by deformation of the housing during running-in and installation.

On the side walls of the housing there are special bosses in which the bearing assemblies of the gears are located. The practice of plants (and first of all the V. I. Lenin Nevsky Machine-Building Plant—NZL), manufacturing turbine-type reducers, has shown that the most reliable design is one in which the bearings have covers not connected with the common cover of the multiplier. Such covers are fastened to the base on studs either for each bearing (when the distance between the housing bores is sufficiently large), or for two bearings.

The babbitt lining of the bearings (Fig. 1), 2, is made on the smooth surface of the shell with a diametral clearance along the shaft of \(0.3 \div 0.4\ \text{mm}\) (experience in operating similar reducers and multipliers has shown that such a lining is in no way inferior to the previously used lining on an expensive “dovetail”). One of the supports of the multiplier...

is made in the form of a bearing with thrust collars to eliminate the possibility of axial displacement of the shaft; the other is without thrust collars.

In the designs described, the specific pressures do not exceed 20–24 kg/cm²; the ratio of the length of the liner \((l)\) to the shaft diameter \((d)\) is 0.8–1. The journal speed does not exceed 50 m/s. For ease of operation, mercury thermometers or resistance thermometers (for remote measurements) are installed on each bearing; the corresponding drilled holes are provided for them.

The multiplier covers on the input- and output-shaft sides are made as embedded covers with shaft seals in the form of rings (radial clearance 0.15–0.2 mm); the remaining covers are of conventional design.

The gears are made with the standard pressure angle (the use of pressure angles greater than 20°, at which the load-carrying capacity of the mesh increases, requires a special tool and can be recommended only if production has the necessary general technological preparation; in this case no design changes need be introduced, and the use of a nonstandard profile will make it possible to increase the strength margin). The small gear is integral with the shaft (pinion shaft) and is made of steel 30Kh2N2VFMA with nitrided teeth (surface hardness \(HV = 750 \div 900\)). As the experience of the Kirov Plant shows, this steel is the most successful variant of all nitrided steels (in particular, at the levels of the load-carrying-capacity coefficient \(K_0 = 20 \div 22\) kg/cm² used in transmissions with a high hardness difference, no flaking of the hardened layer is observed). The rejection of steel 38KhMYuA is explained by its poorer hardenability in large cross sections of blanks. Nitriding is the last operation before inspection. The gear wheel is made of steel 34KhN3M (a steel grade used for high-speed transmissions). Heat treatment: quenching and tempering to a hardness of \(HB \approx 270 \div 300\). Assembly of the wheels with the shaft is carried out by heating the wheel in oil to a temperature of approximately 150°C, using fit A/Pr. The degree of accuracy (according to the standards for smoothness of operation) under GOST 1643-56 and GOST 1758—56 is the fourth or fifth. The wheel and pinion have special annular collars at the ends for checking the center distance with a special template.

The multiplier provides for forced lubrication from the central oil system of the entire installation. The pump (of centrifugal or gear type) is mounted on the gear shaft.

Because the proposed multipliers have smaller overall dimensions than existing ones, while the oil flow rate (and, consequently, the dimensions of the oil pump) is fairly large, the center distance (when the pump is mounted on the low-speed shaft) is determined only by its dimensions. As a result, the load-carrying capacity of the transmissions may prove to be extremely underestimated. Another possible arrangement of the pump is to take off power for the pump. But in this case it is necessary to design the multiplier with a parasitic wheel (since the pump speed must be the same as that of the low-speed shaft), which sharply increases the dimensions of the housing.

Both of the described variants, in our opinion, are unsuccessful. Probably the most expedient (from the standpoint of design, technology, and economic considerations) is the variant in which the pump (with its motor) is installed as a separate unit.

Lubrication of the bearings and gearing is carried out with the same oil (turbine oil L), pressure 2 atm, inlet temperature 25–30°C. Oil is supplied to the bearings through drilled passages in the multiplier housing, the cross section of the passages being selected on the basis of an oil velocity of 1–2 m/s. Lubrication of the mesh is carried out through a special

oil line and a spraying nozzle. Oil may be drained from either of the two end faces (for the convenience of operating personnel), and the oil-drain opening in the housing is designed so that the entire quantity of incoming oil exits at a velocity of about 5 m/s.

The output and input ends of the multiplier shafts are connected to the units of the installation by toothed couplings.

The device for absorbing axial forces (since the shafts are mounted on plain bearings) consists of a conical washer 4 (with cone angle \(\gamma = 40'\)) and a special conical groove (thrust cones) made on the end face of wheel 5 (Fig. 1). The conical washer is mounted on pinion shaft 1 with a press fit and is locked (depending on the magnitude of the axial force) by 4–6 pins 3. The cones are ground, and deviations of the angle (to ensure the specified contact line) must not exceed \(2 \div 5'\). The axial clearance between the cones is \(1 \div 1.5\) mm.

Fig. 1. Design of the thrust device (for absorbing axial forces) used in single-stage helical multipliers for power-plant installations

Operation of the multiplier does not require reversing. However, the design of the thrust device was selected taking into account a possible change in the direction of the axial load. This is due to the fact that, in the event of a possible loss of power to the electric motor (with which these drives sometimes operate), the compressor will begin to rotate in the reverse direction because of excess pressure; this, in turn, will entail rotation of the gear transmissions in the opposite direction.

REFERENCES

1. A. E. Belyaev. Influence of the tooth helix angle on the load-carrying capacity of gear transmissions. Power Engineering Machinery, No. 9, TsNIInformtyazhmash, Moscow, 1965.