Key Connections - Applied Mechanics

Key connections

Keyway connections are designed to transfer torque from the shaft to the hub of the component mounted on it and vice versa. The joints are divided into non-stressed ones with prismatic (Figure 4.85, a), segmental (Figure 4.85, b) and cylindrical (Figure 4.85, o) dowels and strained wedge-shaped (Figure 4.85, d) with dowels. In mechanical engineering, the main application is non-stressed keyed connections for the transmission of small and medium torque moments.

Advantages: simplicity of design, low manufacturing cost, ease of assembly and disassembly.

Disadvantages: reduction of the fatigue strength of the shaft due to the presence of a keyway and the lack of complete interchangeability. The need for selection or manual fit limits their use in large-scale and mass production. The sizes of all types of keys are set by the corresponding standards.

Prismatic keys (see figure 4.85, a) have a rectangular section with the ratio of height to width h/b = 1 for shafts with a diameter of up to 22 mm and h/b = 0.5 for shafts of larger diameter. Dimensions of the key h and b are selected in accordance with GOST depending on the diameter of the shaft. The length of the key/is assigned from the standard length row and is taken 5-10 mm less than the length of the hub of the part. Keys are made of pure steel with a tensile strength of 600 MPa. The torque is transmitted by the lateral faces of the keys. In the groove on the shaft, the key is set to fit with interference, into the hub of the part - with a clearance. The depth of insertion of the key on the shaft is about 0.5 h.

The height of the groove in the hub is slightly larger than the protruding part of the key, i. e. A clearance is provided in the radial direction. The grooves on the shaft are milled by a finger

Fig. 4.85

or a disk mill. When milling with a disk milling cutter, the keyway has a more smooth output, which reduces the stress concentration, but the axial dimensions of the connection increase and the keying in the axial direction is required. Grooves in the hub are performed by chiselling or pulling. The width of the key is carried out with the tolerance field h9. Planting it on the shaft and into the hub is carried out in the shaft system. For prismatic keys, three types of joints are used: with clearance - the groove on the shaft is performed by H9, and in the hub - by D9; transient - respectively N9 and Js9; with interference - the same tolerances of P9 on the dimensions of the grooves on the shaft and in the hub. The shaft and the hub of the part are matched to fit with interference H7/p6 or transitional landings II7/k6, 117/js6. To facilitate the assembly of the connection in the radial direction, a gap is established between the key and the hub of the workpiece, which is ensured by the dimensions of the height of the key h and the depth of the grooves in the hub t 2 and on the shaft t 1.

Segment keys (see figure 4.85, b) are more technologically advanced, interchangeable, do not warp under load. However, a deeper groove on the shaft significantly reduces its strength. Therefore, these keys are used in shafts of low-loaded mechanisms. Landings on the lateral faces of segmented keys are similar to prismatic.

Produce dowels from clean-rolled steel bars. In lightly loaded mechanisms, dowels of carbon steels of 45, 50, 60 are used, and in loaded ones - of alloyed steel 40X with heat treatment up to HRC 35-45.

Keys work on the cut and crumple. Dimensions of the keys are set in such a way that the determining stresses are the stresses of crushing. The calculation is carried out assuming a uniform distribution of pressure along the side surface of the key contact with the shaft and the hub. For the assigned sizes, the calculation is carried out as a check:

where T - is the torque, N ∙ mm; с/- shaft diameter, mm; - working length of the key, mm; t2 - depth of insertion of the key in the hub, mm; - allowable stresses of crushing, MPa. Permissible stresses at constant load ; with the alternating sign -

If the strength condition is not met, then you can select a key of a larger standard section or increase its length.

If the hub of the part has a common end face with the shaft, it is possible to form the cylindrical key combination (see Figure 4.85, c). The holes in the ends of the joined parts are drilled in the assembled condition. The keys are arranged symmetrically along the circumference, the number of keys can be 1-4. The diameter of the key is , the length of the key is . The dowels are inserted into the hole in a fit with a tightness of 117/r6, and the joint is obtained by one piece. In order to avoid drifting the drill when forming a hole, the hardness of the shaft and the hub of the part should be approximately the same. To ensure axial fixation of the workpiece on the shaft between the cylindrical keys, set the setscrews. The load capacity of the connection can be increased by setting two or three keys at an angle of 120 or 180 °. Calculation of the connection is carried out on the stresses of crushing under the assumption of a uniform distribution of forces between the keys

where ζ - number of keys.

Strained keyed connections are formed by wedge keys, which usually have a slope of 1: 100 (see Figure 4.85, d). The working faces of these keys are not the side, but the upper and lower faces. There is a gap along the lateral faces. Wedge keys can transmit torque and axial force. When the part is mounted on a wedge, the workpiece axis moves radially relative to the shaft axis by the amount of radial clearance and contact deformation. Correspondingly, the radial runout of the part increases. Therefore, the keyed connection with wedge keys is of limited use. In responsible and high-precision mechanisms, these connections are not used.

thematic pictures

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