SHAFTS, General information, Shaft and axle materials, Shaft...

SHAFT

General

The shafts are designed to support the rotating parts of the machine and to transfer the torque from one rotating machine to another. The shafts bear the details of the mechanism, and therefore, depending on the design, they work either with the joint action of bending and torsion, or only with torsion.

The axis is a piece designed only to support the rotating parts, and does not directly participate in the transmission of energy. The axles, in contrast to the shafts, do not transmit the torque, but also are the elements carrying on themselves those or other parts. The axes work only on bending.

The shafts of the reducers are divided:

• on the input (high-speed);

• Output (slow-moving);

• Intermediate.

The shafts interact with the parts and bearings attached to them. Fixation of the fastened parts from the rotation is effected by keyed, notched (splined) joints and connections with guaranteed interference.

According to the conditions of assembly on the same shaft of parts with different plantings and types of joints, as well as the requirements for axial fixation of parts, overwhelming majority of cases take a stepped shaft design (Figure 5.1).

Most of the input shafts of the reducers are made in one piece with toothed crowns (shaft-gears, worms).

Straight shafts and axles in most cases have a circular solid section. In a number of designs, hollow shafts and axes are used (Figure 5.1) to facilitate the design, when other parts pass through the axle through them along the axis, for supplying oil and positioning control components in the cavity of the shaft.

Fig. 5.1

Materials of shafts and axes

Shafts and axles are mainly made of steels (Table A.1, A.2):

• Carbon 20, 30,40,45, 50;

• doped with 20Х, 40ХН, 30ХГСА, 40ХН2МА, 18Х2Н4МА, etc.

The choice of material is determined by the design of the shaft, the requirements for it, the conditions of operation, the necessary period of guarantee of failure-free operation.

Various types of heat treatment are used to improve the mechanical characteristics, including surface hardening to increase wear resistance.

The choice of the material of the pinion shaft (worms) is determined by the requirements for surface hardness and endurance when bending the teeth of the pinion shaft (worm turns).

Shaft Design

Approximate calculation of shaft diameter

At the first stage of designing shafts and axes make up a design and calculation scheme, determine the effective loads.

Approximate calculation of shafts is performed with the purpose of preliminary determination of diameters of their output ends, seats iodine bearings and cog wheels. At the first stage of the design, as a rule, the geometry of the shaft is not sufficiently defined, and therefore the calculation is carried out only over the tangential stresses arising from torsion.

Since the approximate calculation does not take into account the influence of the bending moment, the presence of attenuating factors of keyways, rings, transitions, etc.), the value m kr is underestimated compared to the permissible stresses <

t cr , given in the directories, for example, in Table. A.2, P.Z. The values ​​of t cr for

The rough calculation for shafts from medium-carbon steels is between 25 and 35 N/mm depending on the type of load and intensity of bending moments. Permissible stress t cr with rough calculation

is assumed to be low because it is necessary to ensure sufficient rigidity of the shafts as a condition for the correct operation of the gearing and bearings.

Approximately the minimum permissible diameter of the round shaft, without taking into account the presence of keyways or slotted grooves, is determined by the formula:

where T is the maximum torque on the shaft, H m; t to p - allowable tangent

voltage, N/mm; d t] n - the minimum permissible value of the shaft diameter, mm.

The resulting value d t [n for the cantilever input or output sections (Figure 5.2) is rounded to the nearest larger standard one (Table A.178, II.179), for intermediate shafts (Fig. 5.3) - to the nearest larger standard value of the inner diameter of the bearing (Table A.155-P.160).

For gearboxes connected by a coupling with an electric motor, the diameter of the cantilever section of the input shaft is coordinated with the seating diameter of the hub of standard couplings.

Fig. 5.2

Fig. 5.3

thematic pictures

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