Dowels are called special parts designed to connect the shaft and hub to transmit torque. Such elements can be wedge-shaped with a rectangular cross section, prismatic or segmented. The main advantages of keyed connections are their simplicity. Also, nodes of this type are not too expensive and quickly assemble / disassemble.
The strongest such compounds are obtained using segment keys, which, in fact, are a type of prismatic. Details of this type differ in that they have the deepest fit.
The main advantage of such dowels is, therefore, that they give the most stable connection. However, the use of parts of this type has one rather serious drawback. Due to the considerable depth of the groove in such nodes, the shaft may be somewhat weakened. Therefore, segment keys are used mainly for fixing only those parts that are not loaded too much during the operation of the mechanism.
What is a segment key
The details of this variety are made of a metal profile. Such dowels are low cylinders of a rather large diameter, divided into two equal parts. It is the segment key in the drawing below that is brought to the attention of our reader.
Key Dimensions
The dimensions of the part of this type may have different. Select this option when assembling the unit depending on the diameter of the shaft. The necessary values ββare then recognized from special tables. In technically justified cases, GOST allows the use of dowels with a diameter less than that provided. It is allowed to do so when used to assemble the shaft assembly:
Also, dowels with a diameter smaller than those indicated in the tables may be used in joints with a small torque. You can not use this method of assembly in all three cases only when the part is installed on the end of the shaft.
The dimensions of the segment keys, as well as any other similar connecting elements, of course, are standardized. What dependence the height, width and length of the part are in can be found in the table below.
Standard sizes of segment keys| Diameter (mm) | Width (mm) | Height (mm) |
| 4 | 1 | 1.4 |
| 7 | 1.5 ... 2.0 | 2.6 |
| ten | 2.0 ... 2.5 | 3.7 |
| 13 | 3 | 5 |
| 16 | 3 ... 4 | 6.5 |
| 19 | 4..5 | 7.5 |
| 22 | 5 ... 6 | nine |
| 25 | 6 | ten |
| 28 | eight | eleven |
| 32 | ten | 13 |
The symbol for the segment keys in the drawings includes:
- the word "key";
- dimensions - b * h (h1);
- designation of performance;
- designation of the standard.
According to GOST, among other things, there are permissible deviations in the dimensions of the keys and the grooves under them in the shafts. They are also determined according to special drawings and tables.
What material is made of
They make segment keys according to GOST 8786-68, from steel intended specifically for such parts. Also, sometimes other brands of similar material are used to produce these elements. Moreover, according to GOST, for the manufacture of keys of this kind it is allowed to use exclusively medium-carbon steels with a temporary gap of at least [Ο] β₯ 600 MPa MN / m 2 . This can be, for example, material grades St 6, 45 or 50.
Such standards are observed in the production of segment keys in a mandatory manner. In this case, the workpieces intended for the manufacture of such parts are usually also subjected to improved heat treatment. Performing this procedure, among other things, take into account the fact that the hardness of the surface of the keys should ultimately be lower than the same parameter of the mating elements of the assembly itself.
Under the usual dowels, the grooves in the shafts are made simple cutters. Under the segment elements, the grooves are cut using disk tools. Only milling cutters made in compliance with certain GOST standards are used for keyways.
Main varieties
To connect the shafts and hubs, two kinds of segment keys are used:
- normal, representing a whole segment;
- low, made in the form of a cut segment.
The keys of the latter type are rarely used when assembling parts. According to the regulations, they can be used only by mutual agreement of the producer and consumer. Cut keys are usually used simply to fix the parts of the assembly. That is, they are used in joints when the landing is stationary, when the torque is transmitted only due to friction.
Pairings
Assemblies are assembled using segment keys, taking into account the standards stipulated by GOST 24071. The main purpose of this type of element is to prevent the part from turning on the shaft. In this case, three surfaces participate in conjugation:
- grooves on the shaft;
- grooves in the sleeve;
- the surface of the key itself.
The joints of the dowels form loose. At the same time, they work, like ordinary prismatic, side faces. Segment elements are most often used in fixed joints. Moreover, the latter may be:
GOST also allows the free connection of a segment key with a sleeve and shaft. Such parts are usually used only in units with a small mating diameter of up to 38 mm.
Key calculation
Segmented key connections are, of course, carried out in compliance with GOST standards. When assembling nodes, such parts are first checked for crushing. In addition, calculations are made of the compound itself - for a cut. The formulas for checking the keys and pairing use the following:
- Q cm = 4T / d * h * l <[Q cv];
- r = 2T / d * l * b <= [r].
Here T is the torque on the shaft, measured in Nmm, h is the height of the key, d is the diameter of the shaft, l is the length of the key, b is the width of the key in millimeters, [r] is the allowable shear stress.
The values ββobtained as a result of applying the formulas are compared with the acceptable values. In the event that the connection strength is insufficient, two or even three dowels are installed along the length of the hub mounted on the shaft. Of course, this node allows you to create a fairly reliable and functional. However, with this method, for obvious reasons, the shaft is slightly weakened. Therefore, the keyed joints in such nodes are sometimes replaced with serrated ones. This reduces the risk of damage.
Installation of several parts
The dimensions of the cross-section of the keys are chosen, as already mentioned, in accordance with the diameter of the shaft, according to GOST. The length of the element depends on the length of the hub. Sometimes segment keys are mounted on shafts with a step diameter difference. In this case, dowels of the same dimensions are used for individual parts of the same part. At the same time, according to GOST, the step element of the smallest diameter is taken as the basis.
How to select the permissible voltage
This parameter depends primarily on the nature of the load and the operating conditions of the connection. When using steel shafts, the permissible voltage will be:
- for steel hubs in fixed joints - 130 ... 200 MPa;
- for hubs of cast iron or steel cast in fixed joints - 80 ... 100 MPa;
- in mobile unloading joints for steel hubs - 20 ... 40 MPa.
Large values ββwhen assembling nodes are taken into account at constant load. In shock or variable, smaller parameters are taken as the basis. With a reverse load, the permissible indicator is reduced by 1.5 times. On the cut of dowels, the ultimate stress indicators are taken as 70 ... 100 MPa. In this case, as in the first case, at constant load, a larger parameter is taken as the basis.
Milling cutters for segment keys
To hollow out grooves for parts of this type, tools are used that are made in compliance with the standards provided by GOST 66-4879. They make grooves for segment dowels with the help of mills with both straight and multidirectional teeth. In this case, tools with a cylindrical shank can be used:
The milling cutters themselves for the grooves of the segment keys are made of high-speed steel (GOST 19265). Tools whose diameter exceeds 13 mm are welded. Milling shanks are made according to GOST 4543 from steel grade 40X or according to GOST 1050 from material grade 50 or 40.
When assembling tools of this variety observe the following rules:
- on the working surface of the finished cutter there should not be places with reduced hardness or decarburized layer;
- surface roughness parameters of milling cutters must comply with GOST 2789;
- the tolerances of the radial runout of the edges of adjacent teeth relative to the shank should not exceed 0.03 mm, the opposite - 0.05 mm;
- the tolerance of the end runout of the edges should be no more than 0.02 mm.
When marking milling cutters for segment keys, GOST prescribes to indicate on the neck or shank the trademark of the manufacturer and the grade of steel used for the cutting part. The consumer should also be provided with such information as the width of the keyway with the designation of the tolerance field and its nominal diameter.