Key connection. Keyway connections - GOST. Shank key tolerances
The keyed connection is a kind of joining of two demountable parts. It is important. In this case, the parts are connected with the use of an auxiliary element - dowels.
In this respect, everything is quite simple. A keyed connection is used for jointing the shaft and hub. Namely, to prevent their joint rotation during the transfer of torque. Also these compounds are used, although not so widely, to prevent the displacement of flat parts. In what follows we shall keep in mind only the first case.
The key connection is divided into several groups:
1. Depending on the degree of mobility. This is an important characteristic. In this regard, distinguish:
- Mobile connection. In this case, with a guide or a sliding key.
- Fixed connection.
2. In the second group, consider the force that acts in the joint. There are two types:
- Stressed. In this connection, the force is created during assembly. It exists separately from the workload itself.
- Straightened. In this connection, the force is created only when there is a workload.
3. Connections are divided by the type of keys used. Namely:
Next, we will characterize each kind of key separately. However, it must first be said that these parts are mainly made of high-quality, durable middle carbon steels: 45, 50, 55. This is a reliable fact. In order to increase the strength of the keys, the preforms for their manufacture are usually subjected to heat treatment. It improves their quality.
This part can be of three types. That is, it is subdivided into guides, mortgages and sliding ones. Due to the fact that prismatic keys are difficult to replace with each other due to individual fit, they are not used in large-scale production. This is their main drawback. Another disadvantage in this case is the ability to overturn when worn.
This species does not have such drawbacks as the first ones. Therefore, they are easy to apply in production. But they have a drawback that does not allow them to be widely used - this is a large cross-section of the shaft. This prevents them from being installed on bent-loaded shaft sections.
This is another important species. They are used mainly on the end sections of the shaft. When using them, it is necessary that the combined materials do not differ in density and hardness. This prevents widespread use of this kind of key in large-scale production.
This part consists of two parts. This is a kind of prismatic wedge, which has a rectangular cross-section. Install tangential keys in pairs with an angle of 120-180 degrees. The advantage of these parts is that their material works on compression, and also there is a better shape of the corresponding groove with respect to the concentration of the corresponding stresses. The disadvantage of such a key is its complicated device. Apply these details in heavy engineering.
They transmit the moment using frictional forces.
Pros of the specified detail:
- The keyed connection in this case can withstand a small axial load.
- Good work under the influence of variable loads.
- Do not use additional parts that will keep the hub from axial movement.
The minuses of this key are:
- Difficulty in dismantling during repair.
- Strong displacement from the center of the hub in relation to the axis of the shaft. This is an essential fact.
- If there is a short length of the hub, it may be significantly skewed, and the axial runout of the part, which is fixed (a pulley, cog wheel), is not excluded.
Shank key tolerances
This definition is not significant. To ensure the quality of work assign the permissions of keyed connections. This is important to know. Identifies keyed connections GOST 2.308-79 "Unified system of design documentation. Indication on the drawings of the shape and surface tolerances ". This is the corresponding documentary basis.
The numerical parameters of the location tolerances are set taking into account the following relationships: T (pairs) = 0.6 T (w); T (sym0) = 4.0 T (w).
Where the indicated designations provide:
- T (w) - tolerance of the slot width of the keyway b.
- T (pairs) - specified parallel parameter.
- T (sim) - the value of the symmetry tolerance in the diametrical expression.
The calculated parameters of these determinations approximate the standard ones. Oriented for this at GOST 24643.
Fitting of keyed connections
The accuracy of the centering of the parts is ensuredby selecting this option. This is important to consider. The tolerance fields define three types of connections or landings. It is regulated by standards. Next, consider each species in more detail.
This landing is used in difficult assembly conditions with the same load. Used to obtain mobile connections under the condition of easy operation.
It is characterized as a fixed landing, which does not require frequent disassembly. It has good assembly conditions.
They are determined by the probability of obtainingapproximately insignificant identical strains in the appropriate joining of the key with both parts (their slots). The assembly is used with a small number of reverse loads.
Determination of these compounds
As already mentioned above, the dowels are mainlyare made of high-quality and durable steels of medium-carbon (55, 50, 45). To increase the strength of this part, the workpiece for them is usually subjected to heat treatment, which improves their quality.
When you develop a keyed connection, a drawingwhich is previously made, the height and width of this part is used according to GOST 2336-7, taking into account the diameter of the shaft used. In this case, nothing is complicated. The length of the key is taken depending on the hub used. Compare this with the standards of the corresponding standard. The correctness of the selection of certain parameters of the key is checked by applying an important calculation of the keyed connection for strength. In this calculation, there is nothing complicated. For example, the strength condition in this case is the following formula:
σcm = F1 / Асм ≤ [σsm].
Here F1 is the parameter of the circumferential force on the pulley (H). Asm is the area of crushing (mm²). This value is determined by the following formula: (0.94h-t1) lp.
In this case lp = l-in is the working length of the key with rounded ends. This parameter is measured in millimeters. l is the total key length.
Values in, h, t1 are standard sizes in accordance with GOST 23360 - 78.
[σsm] - parameter of allowable stress on the crushing (N / mm²). When using a cast-iron hub [σsm] is taken in a certain limit: 55 ... 95 N / mm².
The working length of the key is calculated as follows:
lp = 32-6 = 26 mm.
Determination of the area of crushing:
Асм = (0,94 · 6 - 3,5) · 26 = 55,64 мм².
The value of F1 is taken from the measurements obtained. In this case, F1 = 1200 H.
As a result, the calculation of the calculated voltage will look like this:
σcm = 1200 / 55.64 = 21.56 N / mm².
This indicates the fulfillment of the strength condition:
σcm = 21.56 <[σcm] (55 ... 95 N / mm²).
Advantages of the keyed connection
Here we see the following:
- The structures are reliable and durable.
- They are easy to assemble and disassemble.
- Low cost.
- They are easy to manufacture.
In this regard, they see the following:
- The keyway section weakens the shaft and hub cross-section.
- In many compounds, there is a decentralization. That is, the relative displacement of the hub axes and the shaft is half the diametric gap.
- In the corners of the keyway, a high level of stress.