Keyed connection. Keyed connections - GOST. Keyway Tolerances

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

Keyed connection is a kind of joining of two collapsible parts. It is important. In this case, the parts are connected using an auxiliary element - keys.

Scope of application

In this regard, everything is pretty simple. A keyed connection is used to carry out the mating of the shaft and the hub. Namely, to prevent their joint rotation during the transmission of torque. These connections are also used, although not as widely, to prevent shearing of flat parts. In what follows, we will keep in mind only the first case.

Varieties

Key connection is divided into several groups:

1. depending on the degree of mobility. This is an important characteristic. In this regard, they distinguish:

• Mobile connection. In this case, with a guide or sliding key.
• Fixed connection.

2. In the second group, the force that acts in the connection is considered. Herethere are two types:

• Tense. In such a connection, the force is created during assembly. It exists separately from the workload itself.
• Loose. In such a connection, force is generated only when there is a working load.

3. Connections are divided according to the type of keys used. Namely:

• Prismatic.
• Segmental.
• Cylindrical.
• Wedge.
• Tangential.

Next, we describe each type of key separately. However, first it must be said that these parts are mainly made of high-quality, durable medium-carbon steels: 45, 50, 55. This is a reliable fact. In order to increase the strength of the keys, blanks for their manufacture are usually subjected to heat treatment. She improves their quality.

Prismatic key

This detail is of three types. That is, it is divided into guides, mortgages and sliding ones. Due to the fact that feather keys are difficult to interchange with each other due to individual fit, they are not used in large-scale production. This is their main disadvantage. Another disadvantage in this case is the ability to tip over when worn.

Segmented keys

This species does not have the same disadvantages as the first ones. Therefore, they are easy to use in production. But they have a drawback that does not allow them to be widely used - this islarge shaft section. This does not allow them to be installed on the sections of the shaft loaded with bends.

Cylindrical keys

This is another important look. They are mainly used at the end sections of the shaft. When using them, it is necessary that the joined materials do not differ in density and hardness. This prevents the widespread use of this type of keys in large-scale production.

Tangential key

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 in compression, and also there is a better shape of the corresponding groove in relation to the concentration of the corresponding stresses. The disadvantage of such a key can be considered its complex device. These parts are used in heavy engineering.

V-keys

They transmit moment using friction forces.

Pros of the specified part:

- The keyed connection in this case can withstand a small axial load.

- Noted good performance under variable loads.

- No need to use additional parts that will keep the hub from axial movement.

The disadvantages of this key are:

- Difficulty in disassembly during repair.

- Strong offset from the center of the hub in relation to the axis of the shaft. This is a significant fact.

- Whenthe presence of a small length of the hub, its significant misalignment is possible, and axial runout of the part that is fixed (pulley, gear wheel) is not excluded.

Keyway tolerances

This definition is not insignificant. To ensure the quality of work, tolerances for keyed connections are assigned. This is important to know. Defines key connections GOST 2.308–79 “Unified system for design documentation. Indication on the drawings of the tolerances of the shape and location of the surfaces. This is the corresponding documentary base.

The numerical parameters of the location tolerances are set taking into account the following relationships: T (steam)=0.6 T (w); T (sim0)=4, 0 T (w).

Where the indicated designations include:

- T (w) - keyway width tolerance b.

- T (steam) – the specified parallelism parameter.

- Т (sim) – symmetry tolerance value in diametric expression.

The calculated parameters of these definitions are brought closer to the standard ones. For this, they are guided by GOST 24643.

Key Fittings

Accuracy of parts centering is ensured by selecting this option. This is important to consider. Tolerance fields define three types of connections or fits. This is regulated by the standards. Next, consider each type in more detail.

Free connection

This fit is used for complex assembly conditions with the same loads. Used to obtain mobile connections under light duty conditions.

Normal connection

It is characterized as a fixed landing that does not require frequent disassembly. Features good assembly conditions.

Tight connections

They are determined by the probability of obtaining approximately insignificant identical interferences in the corresponding joining of the key with both parts (their grooves). The assembly is used with a small number of reverse loads.

Definition of specified compounds

As mentioned above, keys are mainly made of high-quality and durable medium carbon steels (55, 50, 45). To increase the strength of this part, the workpieces for them are usually heat treated, which improves their quality.

When a keyed connection is developed, the drawing of which is preliminarily drawn up, the height and width of this part are used according to the GOST 2336–7 standard, taking into account the diameter of the shaft used. In this case, there is nothing complicated. The length of the key is taken depending on the hub used. Compare this with the norms of the corresponding standard. The correctness of the choice of certain parameters of the key is checked by applying an important calculation of the key connection for strength. There is nothing complicated in this calculation. For example, the strength condition in this case is the following formula:

σcm=F1/ Asm ≦ [σcm].

Here F1 is the circumferential force parameter on the pulley (N). Asmis the collapse area (mm²). This value is determined by the following formula: (0, 94h-t1)lp.

In this case, lp=l-v is the working length of the key with rounded ends. This parameter is measured in millimeters. l is full key length.

Values in, h, t1 are standard sizes according to GOST 23360 - 78.

[σcm] – parameter of allowable crushing stress (N/mm²). When using a cast-iron hub [σcm] is taken within a certain limit: 55…95 N/mm².

The working length of the key is calculated as follows:

lp=32 – 6=26 mm.

Determining the crush area:

Asm=(0.94 6 - 3.5) 26=55.64 mm².

The value of F1 is taken according to the obtained measurements. In this case, F1=1200 H.

As a result of this, the calculation of the design stress 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 keyed connection

The following is seen here:

• Designs are reliable and durable.
• They are easy to assemble and disassemble.
• Low cost.
• Easy to make.

Flaws

The following is seen in this plan:

• The keyway weakens the section of the shaft and hub.
• Decentering is observed in many joints. That is, the relative displacement of the axes of the hub and shafthalf diametral clearance.
• The corners of the keyway are stressed high.