A keyway is a kind of joining of two collapsible parts. It is important. In this case, the parts are connected using an auxiliary element - keys.
Application area
In this regard, everything is pretty simple. A keyway is used to dock the shaft and hub. Namely, to prevent their joint rotation during transmission of torque. These compounds are also used, although not so widely, to prevent shear of flat parts. In what follows, we will keep in mind only the first case.
Varieties
The keyway is divided into several groups:
1. Depending on the degree of mobility. This is an important feature. In this regard, distinguish:
- Mobile connection. In this case, with a guide or a sliding key.
- Fixed connection.
2. In the second group, the force that acts in the joint is considered. Two types are distinguished here:
- Tense. In such a connection, force is created during assembly. It exists separately from the workload itself.
- Loose. In such a connection, the force is created only in the presence of a workload.
3. The joints are separated by the type of keys used. Namely:
- Prismatic.
- Segmented.
- Cylindrical.
- Wedge.
- Tangential.
Next, we characterize each type of dowel separately. However, it must first 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 dowels, blanks for their manufacture are usually subjected to heat treatment. She improves their quality.
Prismatic key
This part is of three types. That is, it is divided into guides, mortgages and sliding. Due to the fact that the prismatic keys are difficult to replace with each other due to individual adjustment, they are not used in large-scale production. This is their main drawback. Another drawback in this case is the ability to tip over during wear.
Segment keys
This species does not have such disadvantages as the first. Therefore, they are easy to use in production. But they have a drawback that does not allow them to be widely used - this is a large section of the shaft. This does not allow them to be installed on bent sections of the shaft.
Cylinder keys
This is another important view. 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 makes it difficult to widely use this type of dowels in large-scale production.
Tangent key
This part consists of two parts. This is a kind of prismatic wedge that has a rectangular cross section. Set tangential keys in pairs with an angle of 120-180 degrees. The advantage of these parts is that their material works in compression, and there is also a better shape of the corresponding groove in relation to the concentration of the corresponding stresses. The minus of such a key can be considered its complex device. Apply these parts in heavy engineering.
Wedge keys
They transmit the moment using friction.
Pros of the specified part:
- The keyway in this case can withstand a small axial load.
- Good performance under variable loads was noted.
- No need to use additional parts that will keep the hub from axial movement.
The disadvantages of this key are:
- Difficulty in disassembling 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 small length of the hub, its essential skew is possible, and axial runout of the part that is fixed (pulley, gear wheel) is not excluded .
Key Tolerances
This definition is not insignificant. To ensure the quality of work, tolerances of keyed joints are prescribed. This is important to know. Defines key connections GOST 2.308–79 “Unified system for design documentation. An indication on the drawings of tolerances of the shape and location of the surfaces. " This is an appropriate 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 these designations include:
- T (w) - tolerance of the width of the keyway groove b.
- T (steam) - the specified parallelism parameter.
- T (sim) - the value of the tolerance of symmetry in the diametrical expression.
The obtained calculated parameters of these determinations are brought closer to the standard ones. We are guided for this by GOST 24643.
Keyway fit
The accuracy of centering parts is ensured by the choice of this parameter. This is important to consider. Tolerance fields define three types of joints or landings. It is regulated by standards. Next, we consider each view in more detail.
Loose connection
This fit is used under difficult assembly conditions with the same loads. It is used to obtain movable joints under the condition of easy operation.
Normal connection
It is characterized as a fixed landing that does not require frequent disassembly. It features good assembly conditions.
Tight joints
They are determined by the probability of obtaining approximately insignificant equal interference in the corresponding docking of the key with both parts (their grooves). Assembly is used with a small number of reverse loads.
The definition of these compounds
As mentioned above, the 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 subjected to heat treatment, which improves their quality.
When a key connection is developed, the drawing of which is preliminarily made, the height and width of this part are used according to GOST 2336–7, taking into account the diameter of the shaft used. In this case, there is nothing complicated. The length of the keys is taken depending on the hub used. Compare this with the norms of the relevant standard. The correctness of the selection of certain parameters of the keys is checked using an important calculation of the keyway strength. There is nothing complicated in this calculation. For example, the condition for strength in this case is the following formula:
σcm = F1 / Asm ≤ [σcm].
Here F1 is the circumferential force parameter on the pulley (N). 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 dowel with rounded ends. This parameter is measured in millimeters. l is the full keyway length.
Values in, h, t1 are standard dimensions according to GOST 23360 - 78.
[σcm] - parameter of permissible stress on collapse (N / mm²). When using a cast-iron hub [σcm], it is accepted in a certain limit: 55 ... 95 N / mm².
The key length is calculated as follows:
lp = 32 - 6 = 26 mm.
Determination of the area of crushing:
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 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).
Key Benefits
Here is the following:
- Designs are reliable and durable.
- They are easy to assemble and disassemble.
- Low cost.
- They are easy to make.
disadvantages
In this regard, they see the following:
- The keyway weakens the cross section of the shaft and the hub.
- In many compounds, decentration is observed. That is, the relative displacement of the axes of the hub and shaft by a half diametrical clearance.
- There is a high level of stress in the corners of the keyway.