Keys and keyways are a very important part of shafts and power transmission. We use keyed shafts in the gear and shaft assemblies in general. But also, there are lots of kinds of other applications. Here, you can find detailed information about these machine elements. This information is;
- What are shaft keys and keyways?
- Which applications use shaft keys?
- Types of shaft keys.
- How do produce them?
- How do design them at the engineering level?
- Advantages and disadvantages of them.
What is Shaft Key?
Firstly, we need to state that, shafts are very important machine elements. They attach, gears, pulleys, and cams on shafts to obtain machinery. In general, we call the assembly sections of these parts shafts hubs. So, we use keys in the attachment of these elements to shafts.
There is a section on the shaft which is carved in a specific shape. We call this shape a keyway. There is the same clearance on the other machine elements. So, we attach keys inside these keyways and clearances between the shaft and other parts to obtain assemblies. As you understand, the working principle of shaft keys is like this.
What are the Types of Shaft Keys?
According to the changing application, there are different kinds of shaft key and keyway applications. The general types of them are as below.
Sunk Type Shaft Keys
In general, sunk-type shaft keys are the most common type. The general working principle of the sunk types is, that there are two portions of the keys that lies at the hub and the other potion lies at the shaft. So, they transmit power as an intermediate element.
There are different types of sunk-type keys.
Rectangular Shaft Keys
This is the most classic shape of the shaft keys. So, the geometrical shape of the rectangular key shapes includes heights and lengths. In general gear and shaft assemblies, we are using these types of keys. In general, the material of the rectangular keys is much more hard and strong than the shaft and gear materials.
We use the rectangular keys for shafts that have diameters between 25mm to 500mm or 1 inches to 20 inches in English units.
The power of the gear or shaft is transmitted through the rectangular key mechanisms. So, the material must withstand the stresses due to the loads because of the rectangular keys.
If we compare with the square keys, the depth of the key is lower for these types of them But the lengths are higher. And the stress speeds along the length of the shaft.
Parallel keys are the parallel type of key family that are in the parallel direction to the shafts. Also, we use the parallel keys in general applications of shafts. In general, we use these types of keys in the transmission of one-direction torques and powers.
They are very useful in automotive applications because of their high strength. Also in pulley mechanisms, the use of parallel keys is very common.
In general, parallel types of keys are the easiest to install. We can use the set screws to prevent siling during operation.
Gib Head Shaft Keys
If we compare them with the other types of keys, there is a head section on them. This head provides a better ability to assemble and disassemble these keys. Also, it provides better vibrational characteristics.
Gib head geometry also provides a better fit for the gears, pulleys, and shafts. So, for the higher torques and power, we can select this type.
Feather keys are also very common in general mechanical applications. There are different kinds of advantages of feather keys over other types of keys.
If we take a look at the geometry of the feather keys, the edges of the rectangular geometry are in a semi-circular shape. This oval shape has the advantage to hold the shaft and gear assembly tighter.
The oval shape of the shaft key ends provides better and easy assembly. Aligning the feather keys is much easier if we compare it with other ones.
These types of keys are better in terms of stability. It can not get loosened or get ripped from the shaft and the gear assembly easily.
Axial motion is not a common thing for feather keys. This provides better accuracy of the assembly.
Disassembly of the feather keys is much easier. Also, there are three types of feather keys; peg feather key, single-headed feather key, and double-headed feather key.
At the peg feather keys, there is a round protrusion at the center of the shaft key. This protrusion assembled inside the hub provides additional intersection.
Single-head and double-head are very straightforward. There are two heads at the start and end sections for the doubleheader.
Woodruff Shaft Keys
The shape of the woodruff keys is quite different. The general shape is like half of a disk. But the use of the woodruff keys is generally for shafts that have diameters smaller than 2.5 inches.
In general, the circular section of the woodruff key is inside the shaft. And the straight side is in the gear or pulley hub.
Also, the main advantage of the woodruff keys is that it improves the concentricity of the shaft in high-speed applications. And also, the total stresses beside the woodruff keys are minimum if we compare it with other types.
In general, square keys are much deeper in the shaft and the part hub. This deepness provides the ability to transmit higher powers and torques. But also, we need to consider the general problems that can arise because of the loss of strength of the shaft because of this deepness.
We use the square keys in shafts which has a diameter below 1 inch or 25 millimeters.
In the applications of large torque transmissions, we generally use these types of keys. Also, applications can be in two directions. In general, also, we use this type in low-speed applications.
Explanation of the constructions of tangent keys is not an easy thing. There are generally two keys we place between the shaft and hub as a tangency. The one edge of the shaft is completely inside the shaft side. And the other edge is inside the hub side. So, there is a tangent construction of keys.
We generally use saddle keys for lightweight designs. Because they only use the frictional effect between the hub and the shaft.
Constrıctions of saddle keys are different from the sunk key designs. There is no interference between the saddle keys and shafts. There is interference between the hub and saddle key. So, power transmission takes place between this type and the shaft take place thanks to friction between them.
In general, there are two types of constructions of saddle keys; flat and hollow.
The hollow saddle keys have a concave shape at the shaft side. And the concave side sits on the concave surface of the shaft. So, friction takes place like this.
Also, in the flat type, there is no concave side. The surface that interferes with the shaft is flat.
This is another geometry and structure of hubs. These keys are circular. We use the round keys only in very low torque applications.
The advantage of the round keys, we can assemble these round keys by drilling the shaft and hub together. Also, we do not recommend using round keys in high-speed and direction changes.
You can use them in very basic applications.
In general applications, the diameter of the round keys is 1/6 of the diameter of the shafts.
There is a very different structure of spline keys in general. This structure is, designed in a 6 or 10-point star design. They provide much more torque and power transmission between the shaft and the key. Also, the attachment and detachment of these keys are very simple.
Like tangent keys, Kennedy keys have also very different structures. In the Kennedy key assembly, there are two square keys are placed 90 or 120 degrees apart from each other. Also, we place these keys diagonally, unlike the other keys.
The most important advantage of Kennedy keys, we can use them in high-load applications. So, we use them in industry-level applications. This is a very different and important key design.
How to Produce Shaft Keys?
In general, we use machining operations in the production of shaft keys and keyed joints. As you see above, there are simple geometries and also there are very complex geometries of keys. So, we can also use very basic machining operations and also complex CNC operations.
The surface qualities of shaft keys are very important. They affect the strength of the key directly. We need to take special care about the surface characteristics of these machine elements.
In general, we use steel materials to produce these elements. And steel is a very good material that we can apply machining operations.
Shaft Key Design
In the shaft key design, we need to use a structural design approach. In general applications, we need to understand the general failure scenarios. The most important failure scenario is if the torque exceeds the limit value failure occurs.
Also in the key design, we can use a static approach. Because we are not encountering cyclic loads on shaft keys to apply dynamic analysis such as fatigue. The torque values are generally constant.
The most important stress types on the shaft keys are compressive stresses and torsional stresses. Failure can occur because of the combinations of these two stresses. So, we need to calculate these stresses on the keys.
Also, we do not want to see any plastic deformation on keys during operation. So, we need to look at the maximum yield strength of the material. And also, the maximum torque that a key can carry is very important for us. To find the maximum torque, we need to take a look at the ultimate failure strength of the material. This is a very important consideration for a safe design.
Calculation of Shear Stress and Torque Deformation
Firstly, we need to calculate the shear stress and torque deformation of the key. Torque generally applies shear stress on the key. And we can find the maximum torque value that creates plastic deformation of keys with this equation;
If you take a look at this formula here;
Ys is the yield strength value of the key material. A is the cross-sectional area where the sheer force takes place on the key. You can this value different for different types of key shapes.
d is the diameter of the shaft.
S is the safety factor that you defined.
Failure of Compressive/Bearing Stress
Also, you need to find the shaft key failure due to the compressive or bearing stress on the key. This is the second consideration that you need to make in the key design. You can calculate this bearing stress with this formula;
In here, ‘l’ is the key length and ‘h’ is the shaft key thickness.
So, you find the maximum torques that you can transmit with a specific shaft key.
General Applications of Shaft Keys
There are different kinds of applications of shaft keys. Keys are the general type of machine elements such as gears, that you can find in different machinery. Here, you can find the different applications that we use them.
In most, automotive products and automotive applications, the use of shaft keys are very common. Because power transmission is very important for automotive applications.
The power is generated by an internal combustion engine or an electric motor if you are using electric cars. There is a transmission system that includes lots of gears. You can change the gear thanks to this transmission system.
These gears in transmission systems are attached to shafts. Between these attachments of gears and shafts, there are keys. So, we can conclude that the use of the shaft keys is very common in transmission systems of trucks, cars, motorcycles, etc.
Gear systems are also very important in aerospace applications such as airplanes. If we take a look at the main engines of the airplanes, we can see that there is a turbofan engine. In turbıfan engines, there is the main shaft that carries all the rotating elements such as turbine wings, stators, and rotors.
So, there are attachments of these elements to the main shaft mechanism. Between these attachments, the use of the keys is apparent.
Also, there are lots of other kinds of key applications in different aerospace applications such as helicopters. In helicopters, there is a gear mechanism that transforms the axis of rotation of the shaft of the turbojet engine. So, between the hubs of these gears and shafts, we can see the applications of shaft keys.
In power plant applications, the use of turbine systems is very common. For example in hydroelectric power plants, there is a fluid flow that flows through turbines. These turbines rotate because of this fluid flow. And the shaft of the turbine is connected to a generator.
As you see above, there is a shaft application again for power plants. In the attachments of turbines and other elements, we use keys and keyed systems in shafts.
Reduction Gears and Elements
The use of gears is very common in reduction gear applications. We use reducers to reduce or increase the speeds and torques of the transmitted power of an engine. In general, reducers are attached to electrical motors to obtain a specific speed or power.
In these reducers, there are lots of shaft and gear assemblies. So, as you know that if there is a shaft and gear assembly, there is a key application.
Advantages and Disadvantages of Shaft Keys
It is very important to consider the advantages and disadvantages of machine elements before applications. But, there are no such alternatives for keyed designs and keys in general.
- The most important advantage of the shaft keys is the costs of the construction are low if we compare it with other systems.
- We can transmit different kinds of high and low torques without any problem.
- There are different varieties and constructions of keys that we can use in different kinds of applications.
- The constructions of keyed shafts are very basic. So, we can dismantle easy and assemble them easily.
- The strictness of calculations for keyed joints is somewhat high. So the safety factors are high if we compare them with other mechanical elements.
- The strength of the shaft reduces because of the carving of key attachment points. In this section, the key section is considered in the shaft design.
- Shaft imbalance can be a problematic issue for keyed joints.
- If there is no fixing equipment such as screws, axial displacement of keys can be a problem.
- Shaft keys are not for alternating loads in two directions.
As you see above, shaft keys are very important in the design of different kinds of machinery. We use these systems in the assembly of the shafts and hubs of other elements such as gears.
There are various kinds of applications that we use keys. Around these applications, automobile, aerospace, and power plants may be the most important ones. Because all of these systems are carrying power through shafts.
The design procedure of shaft keys is somewhat basic. We generally use a static design approach in the design of the keys. So, we can use the yield strength value of materials. And also, the safety margins and factors are very high.
We generally use the machining operations to produce different kinds of shapes of keys. Also for complex shapes, we can use CNC machining centers.
There are different kinds of shaft keys. According to the constructions we can use them in different designs.
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FAQs About Shaft Keys
Shaft keys are very important machine elements in the power transmission between gears, pulleys, and shafts. So, we use keys to transmit power between the shaft and gears. Assembly and dismantling of these systems are much easier.
We use generally steel and other types of metals to produce these items. But in general, stainless steels are the most common application in the production of keys. Because they have very good corrosion resistance and mechanical strength properties.
There are different types of keys. These types are; sunk type, saddle type, round type, tangent, and Kennedy types. Also, there are different types of sunk keys; rectangular, parallel, gib head, feather, woodruff, and square.
Their working principles of them are very simple. There are clearances on shafts and hubs. And keys are placed in these clearances. So, we attach the keys inside these common clearances. If there is torque on the hub or shaft, the key will transmit it through itself.