Segmented key: definition, features, GOST and interaction

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Keys are special parts designed to connect the shaft and hub in order to transmit torque. Such elements can be wedge-shaped with a rectangular section, prismatic or segmented. The main advantage of keyed connections is the simplicity of the design. Also, nodes of this type are not too expensive and are quickly assembled / disassembled.

The strongest such connections are obtained using segment keys, which, in fact, are a kind of prismatic. Parts of this type are distinguished by the fact that they have the deepest fit.

The main advantage of such dowels is thus that they give the most stable connection. However, the use of parts of this type has one rather serious drawback. Due to the significant depth of the groove in such nodes, the shaft may be somewhat weakened. Therefore, segment keys are used mainly to secure only those parts that are not loaded too much during the operation of the mechanism.

What is a keyway

Parts of this variety are made from a metal profile. These dowels are low cylinders of rather large diameter, divided into two equal parts. It is the segment key in the drawing below that is presented to the attention of our reader.

Key sizes

Dimensions of this type of part may be different. This parameter is selected when assembling the assembly, depending on the diameter of the shaft. The required values are learned from special tables. In technically justified cases, GOST allows the use of keys with a diameter smaller than that provided. It is allowed to do this when using a shaft assembly to assemble:

• hollow;
• graded.

Also, keys with a diameter smaller than those indicated in the tables can be used in connections with a small torque. You cannot use this assembly method in all three cases only when the part is installed at 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 are the height, width and length of the part can be seen in the table below.

Standard key sizes

Diameter (mm)	Width (mm)	Height (mm)
4	1	1.4
7	1.5…2.0	2.6
10	2.0…2.5	3.7
13	3	5
16	3…4	6.5
19	4..5	7.5
22	5…6	9
25	6	10
28	8	11
32	10	13

Segment key designation in drawings includes:

• word key;
• dimensions - bh (h1);
• performance designation;
• designation of the standard.

According to GOST, there are, among other things, permissible deviations in the dimensions of the keys and grooves for them in the shafts. They are also determined by special drawings and tables.

What material is it made of

Segment keys are made according to GOST 8786-68, from steel designed specifically for such parts. Also, sometimes other brands of similar material are used for the production of these elements. At the same time, according to GOST, for the manufacture of keys of this variety, it is allowed to use only medium carbon steels with a temporary gap of at least [σ]≧ 600 MPa MN/m2. It can be, for example, material grades St 6, 45 or 50.

Such standards are observed in the production of segment keys without fail. At the same time, blanks intended for the manufacture of such parts are usually also subjected to an improving heat treatment. Performing this procedure, among other things, take into account the fact that the hardness of the surface of the keys in the end should be allsame below the same parameter of the mating elements of the node themselves.

Under ordinary keys, the grooves in the shafts are made with simple milling cutters. Grooves are cut out under the segment elements using disk tools. Only cutters made in compliance with certain GOST standards are used for keyway grooves.

Main varieties

To connect shafts and hubs, two types of segment keys are used:

• normal, representing a whole segment;
• low cut segment.

Keys of the last type are rarely used in the assembly of parts. According to the regulations, they can only be used by mutual agreement of the manufacturer and consumer. Sheared keys are usually used simply to fix the details of the assembly. That is, they are used in connections with a fixed landing when the torque is transmitted only due to friction.

Pairings

Knots are assembled using segment keys, taking into account the standards stipulated by GOST 24071. The main purpose of elements of this type is to prevent the part from turning on the shaft. In this case, three surfaces participate in conjugation:

• grooves on the shaft;
• slots in the sleeve;
• the surface of the key itself.

Connection segment keys form loose. At the same time, they work, like ordinary prismatic ones, with side faces. Segment elements are used most often in fixed joints. Whereinthe latter can be:

• normal;
• dense.

GOST also allows free connection of the segment key with the sleeve and shaft. Such parts are usually used only in nodes with a small mating diameter - up to 38 mm.

Key calculation

Connections with a segment key are, of course, made in compliance with GOST standards. When assembling assemblies, such parts are first checked for collapse. In addition, calculations are made for the connection itself - for a cut. The formulas for checking the key and mating use the following:

• Q cm=4T/dhl < [Q cv];
• r=2T/dlb <=[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 obtained as a result of applying the formulas are compared with the allowable ones. In the event that the strength of the connection is insufficient, two or even three keys 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 somewhat weakened. Therefore, key connections in such nodes are sometimes replaced by gear ones. This reduces the risk of breakdowns.

Installing multiple parts

The dimensions of the key section are chosen, as already mentioned, in accordance with the shaft diameter, according to GOST. The length of the element depends on the lengthhubs. Sometimes segment keys are installed on shafts with a difference in step diameters. In this case, keys of the same size are used for separate 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 choose allowable voltages

This parameter depends primarily on the nature of the load and the operating conditions of the connection. When using steel shafts, the allowable stress will be:

• for steel hubs in fixed joints - 130…200 MPa;
• for hubs made of cast iron or cast steel in fixed joints - 80…100 MPa;
• in movable unloaded joints for steel hubs - 20…40 MPa.

Large values when assembling nodes are taken into account at a constant load. With a shock or variable, smaller parameters are taken as the basis. With a reverse load, the allowable indicator is reduced by 1.5 times. For the cut of the keys, the limiting stress indicators are taken as 70 … 100 MPa. In this case, as in the first case, with a constant load, a larger parameter is taken as the basis.

Cutters for segment keys

To gouge grooves for parts of this type, tools are used that are manufactured in compliance with the standards stipulated by GOST 66-4879. Grooves are made for segmented keys using cutters with both straight and multidirectional teeth. In this case, tools with a cylindrical shank can be used:

• flat;
• smooth.

The cutters themselves for slotting segmented keys are made of high speed steel (GOST 19265). Tools with a diameter exceeding 13 mm are made welded. Mill 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, the following rules are observed:

• on the working surface of the finished cutter there should not be places with reduced hardness or a decarburized layer;
• surface roughness parameters of cutters must comply with GOST 2789;
• tolerances of radial runout of the edges of adjacent teeth relative to the shank should not exceed 0.03 mm, opposite - 0.05 mm;
• the end runout tolerance of the edges should be no more than 0.02 mm.

When marking cutters for segmented 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. Also, the manufacturer should provide the consumer with such information as the width of the keyway with the designation of the tolerance field and its nominal diameter.