Calculation of keyed connections
Standardized dowels are made from medium-carbon pure-steel steels of special grade in accordance with GOST 8787-68 and GOST 8786-68. These steels are grades of steel: St5, Stb, 45, 50, 55 and 60. Other steels with a tensile strength a of = 590 MPa are acceptable.
In doweled joints, dowels made of alloy steels are used (for example, steel 40X with heat treatment up to 37-47 // LSE). Thermally processed keys are grinded along the working faces.
Allowable stresses of buckling in fixed keyed connections are found by the formula:
where ст т is the yield strength of the weakest material of the details - shaft, key or hub; 5 - safety factor; with exact account of the loads S = 1.25; in the remaining cases 5 = 1.5-2.
The allowable shear stress is determined from the relationship:
Due to a more accurate design, the permissible stresses for round and conical keys can be increased by (25-30%) compared to the permissible stresses for the prismatic keys.
The calculation of prismatic keys is performed as follows.
Improved crushing strength
Shear strength condition:
The calculation of the segmented keys is performed as follows. Crush strength condition
Shear strength condition:
where Г max - the maximum torque on the shaft, Н mm; (I - shaft diameter, mm;
/ p - working length of the key, mm; L - height of the key, mm; B - width of the key, mm; O - diameter of the key, mm; K - protrusion of the key from the keyway, mm;/- the depth of the keyway on the shaft, mm; сг см р - allowable stresses on the crushing, N/mm;
cpp - allowable shear stresses, N/mm.
3. Tolerances and planting of keyed connections
Application for keyed joints to fit the wheel to the shaft:
• with clearance - not allowed;
• transitional - highly undesirable.
It is recommended to take the following wheel alignment on the shaft:
Landings with high interference (shown in parentheses) are used for the wheels of reverse gears.
Selection of plantings & quot; dowel - shaft groove & quot; and & quot; dowel - sleeve slot & quot; are made depending on the desired type of connection, which in turn is selected depending on the purpose of the landing (for sliding keys, for fixed connection, for guide keys, etc.) and serial production.
In most cases, the connection of the key with the shaft is more dense than with the sleeves. This prevents the key from falling out of the shaft slot during installation and excludes its movement during operation. The clearance in the joint is "dowel - sleeve slot". It is necessary to compensate for inaccuracies in the dimensions, shape and relative position of the grooves.
In Table. A.145, A.146 shows the planting of the keys, the tolerance fields and the limiting deviations in the dimensions of the keys and keyways in accordance with GOST 23360-78.
The deviations of the depth of the groove on the shaft and in the bushing are given in Table. A.147.
The tolerance of the groove in the bushing or on the shaft is given in Table. A.148.
Key features of keyed connections
When landing with interference during the installation of the gears on the shafts, it is very difficult to combine the keyway groove of the wheel with the shaft key. To facilitate assembly, it is recommended to provide a guiding cylindrical shaft section with a tolerance of (1 1 (figure 4.6).
For the same purpose, the guide portion of the shaft is made conical.
At the beginning of the assembly, by free turning the wheel relative to the shaft, align the keyway of the wheel with the key, and then press the wheel onto the shaft.
The landing surfaces for gears and worm wheels are often polished. Therefore, before the thrust collar it is desirable to make a groove for the exit of the grinding wheel on the shaft.
The shape and dimensions of the grooves are given in A.172, A.173.
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