Shafts in Machinery – Definitions, Design, and Applications

Cardan shaft application.
Image Source: Wikimedia.

Shafts are very important elements of most machinery. They are responsible for the transmission of power and moments between the rotating elements. For example, the power from an electric motor to a turbine system. So, we need to spare important attention to the design of shafts.

Here, you will find these topics about shafts;

  • What is a shaft? What are the general applications?
  • How to design them?
  • Which machine elements are attached to shafts?
  • FAQs about them.

What is Shaft?

As we stated above, shafts are circular and cylindrical machine elements that transmit power by rotating. You can see them in lots of kinds of machinery. Because power transmission in machinery is very important. So, shafts are very critical machine elements.

There are different designs of these machine elements according to the design considerations. For example, they are generally produced in one diameter. But also, some shafts have stepped designs.

What are the General Applications of Shafts?

Wind turbine power generation unit(Image Source: Researchgate.com).

We use them in power transmission in general. In all machinery, there is a shaft that carries power.

Automotive

In automotive applications, there is an engine that power is generated from fossil fuels or electricity. This rotational energy is transmitted to the crankshafts which is an also type of them. The rotation crankshaft transmits this power to the gear system which we call them transmission also. They attach the gears of transmission on smaller shafts. So, they are also transmitting power. Additional axles and shafts are transmitting this power to wheels.

Machine Tools

In machining operations, machine tools are very important. There are different kinds of machine tools such as turning, milling, drilling, etc. There is a common application for all of them: shafts. Because, they have electric motors to generate power to turn cutting tools, and move worktables. So, power must be transmitted to these portions of the machine. There is a main shaft that comes from the electric engine.

Clockwork Mechanisms

Clockwork mechanism.

You know that the old clocks which have lots of small gears are very cool. You are setting the clock by rotating the pin. This rotation collects potential energy from the spring of the clockwork mechanism. This energy is released in small amounts to work the clock. The clockwork mechanism works with this small energy release. There are lots of gears that transmit power. And these gears are attached to small shafts.

Pump and Turbine Systems

Compressors, pumps, and turbines are generally the same group of machinery. Always, there is a fluid that flows through the vanes. In turbines, high-energy fluids rotate the turbines to generate power. And in compressors and pumps, the low-energy fluid is taken to the high-energy state with an electric motor or internal combustion engine. So, there is always a shaft between the power generation unit and the propellers or impellers of turbines, compressors, and pumps.

Marine Watercrafts

There are lots of kinds of marine ships, boats, and yachts of different sizes. But their working mechanism is generally the same. There is an engine that generates power. A shaft that transmits this power to a marine propeller. So, they are a very common application for marine watercraft.

Aircrafts

Also, there are different kinds of aircraft applications that shafts used. The working principle is generally the same with the pumps or compressors. there is a propeller that rotated with a very high density and temperature of burnt gases. With this rotation, thrust takes place. So, they are the main elements of jet engines.

So, as you see above, they are a very important machine element. There is a lot of use of shafts in general.

Which Materials Used in the Design of Shafts?

In the design of them, we need to use generally high-strength materials. Also, it must withstand dynamic loads. Because fatigue may occur on shafts. Fatigue characteristics must be good.

Notch sensitivity is also a very important factor in their design of them. Because there can be notches or defects because of the manufacturing of shafts. Because of the dynamical loadings, fracture of them can take place. So, the material must withstand these kinds of situations.

Wear resistance is very important. In dynamic parts such as shafts, wear properties are very important. The shaft materials must be resistant to wear.

If we consider these factors of shafts, materials can be;

Mild steel: For average loadings of them, mild steels are among the most common materials. For higher strengths, chrome-vanadium steels and other alloyed steels are generally used.

How the Shafts are Manufactured?

Defects are very important to them. Because especially surface defects have a tremendous effect on their strength of them. So, it is very important to know the general manufacturing techniques to produce shafts.

For bigger shafts, turning is the general production technique. In turn, the raw billets of steel are attached to the machine. And the required diameter with the required surface parameters is produced.

Hot rolling is another manufacturing technique to obtain shaft geometries. It is very simple to obtain round and cylindrical geometries with hot and cold working techniques.

How to Design Shafts?

Design is a very important thing. You need to know how to design a shaft for a mechanical system. From the engineering viewpoint, the design of these systems depends on the different kinds of machine elements that are attached to them.

Draw a Free Body Diagram

First of all, we need to start by drawing free-body diagrams of them. In free body diagrams, we can show, bending moments, torsional loads, and other types of loads. We need to extract other ömachine elements. But we need to add the effects from other machine elements to the free body diagram.

With the free body diagrams, we can see the exact situation of our shaft. And also, we can make our design assessments according to this.

In two sections, we need to also draw a bending moment diagram. With these two sections, we can find in which section of them, we experienced the biggest bending moment. So, we can calculate it by using this formula;

Resultant bending moment on shafts.

In this equation, Mx and My are the biggest bending moments in X and Y orientations.

Calculate the Deflections

Deflections on them are very important. Lateral deflections of these elements take place because of the bending moments. So, you need to calculate the total deflection on a shaft with the effect of moments. There are contacts with other machine elements. The deflection must be in the allowable tolerances.

So, if you want to reduce the deflection, you need to take care of the places of other machine elements on them. Also, you can change the place of supports of the shaft to minimize the bending moments.

Torque Diagrams

Torque is quite different from bending moments. This torque is about the power that is transmitted by the system. We need to draw a torque diagram to see the total system. There must be a balance between the torque of the input power and the torque of the output power.

Torsion Deflection of Shaft

Torsion is another important problem of these elements. We need to calculate all the torsional loads on shafts. So, there will be a torsional deflection on them. We need to calculate the total deflections because of these loads.

After that, we need to re-design the diameter of these elements, if the torsional deflections are higher than the allowable deflection.

Torsion is a very important aspect to calculate.

Stress Calculations on Shafts

This is also a very important point in the design of shafts. You need to calculate the critical loads and critical sections to prevent the fracture. You can use the Maximum Shear Stress theory and Distortion Energy Theories to calculate the stresses on them.

Firstly, you need to define the critical point where the torque and bending moments are greatest on these elements. For example, stepping sections are the main points.

You can use the maximum shear stress theory and distortion energy theory to define the minimum diameter of the shaft.

These theories are generally for ductile materials. And lots of metals and steel are ductile materials.

You can use these two theories in the calculators below to see the minimum diameters. And the bigger result will be your minimum diameter. Calculate them below.

Maximum Shear Stress Theory Calculator for Shafts















So, the use of this calculator is very simple. You just need to enter these values;

Safety Factor: This is the value you need to define for your system. Unitless.

Yield Strength: Yield strength of the material of the shaft. The unit is Pa or psi in English units.

Bending Moment: Maximum bending moment at the critical section. The unit is N.m or lbf.ft.

Torque: This is the maximum torque that is applied to them in the critical section. The unit is N.m or lbf.ft.

And then, click on the ‘Calculate!’ button to see the minimum diameter of these elements. If you want to make further calculations, you can click on the ‘Reset’ button and then reenter the values.

Maximum Shear Stress Theory of Shafts Formula

The formula is like this;

Minimum diameter formula for shafts.

So according to this formula, with the increased safety factor, the minimum diameter of the shaft increases. Because, the biggest the safety, the biggest the diameter.

Also with the increasing yield strength, the minimum diameter decreases. Because their strength of them increases.

And also, with the increasing bending moment and torque on them, the minimum diameter increases.

Distortion Energy Theory Calculator for Shafts















The use of this calculator is the same as the maximum shear stress theory. The formula is like this;

Minimum diameter calculation formula for shafts.

Also in this formula, the only difference is 3/4 at the torque. All the parameters are the same for this formula.

Use Additional Machine Elements

Also, you can use additional machine elements to protect the shaft such as keyways, set screws, pins, or slip clutches.

Standard Sizes of Shafts in General Use

Like other machine elements, there are some standard sizes of them.

Machine types can have diameters up to 25 mm with 0.5 steps.

Transmission types;

  • If we use 5mm steps, the diameter is between 25mm to 60mm.
  • If the steps are 10mm, the diameter is from 60mm to 110mm.
  • For the 15mm steps, this span is 110-140mm.
  • For 20mm steps, this span is 140-500mm.

Advantages and Disadvantages of Shafts

There are advantages and disadvantages of using them on machinery.

Advantages

  • The most important advantage of the shaft is the high-strength structure. Unlike other machine elements, they are not prone to failure.
  • Maintenance costs are very low if we compare them with other complex systems such as chain systems.
  • If we compare it with the complex systems for power transmission, they have very basic constructions.

Disadvantages

  • One of the most important problems with shafts is vibrations. They’re very likely to vibrations occur in these systems.
  • Noise can be a problematic issue for their use of them.
  • Manufacturing of these systems is hard if we compare it with other transmission systems.
  • Speed change is not easy with the shafts.

Conclusion

So, they are maybe the most important mechanical elements for power transmission. There are various kinds of applications for them.

Also, designing shafts has a very different approach. You need to follow the required steps to design these systems effectively. Yıu needs to consider all the steps above.

Like other machine elements, they have also alternatives and advantages and disadvantages.

Above all, Mechanicalland does not accept any responsibility for calculations made by users in calculators. A good engineer must check calculations again and again.

Also, You can find out much more calculators like this in Mechanicalland! Take a look at the other engineering calculators available in Mechanicalland!

Finally, do not forget to leave your comments and questions below about shafts in machinery.

Your precious feedbacks are very important to us.

FAQs About Shafts

Why shaft alignment is important?

A successful shaft alignment is about the bearings and other bedding elements of shafts. It is very important to prevent any failures or other kinds of possible problems arising from eccentricities. Also, vibration can be a very big problem with the misalignment of shafts.

Why shaft made circular?

It is because the attachments of other machine elements are much easier than circular elements. Also, the production and assembly circular members are always easier.

How does the shaft work?

Shafts are the machine elements for power transmission. It takes power from an element to transmit to other elements. The RPM and the torque are very important for shafts. They transmit power by rotating with a torque. We can calculate this power with torque and RPM value.

Why do shafts get broken? 

It is because of the fatigue of materials. Shafts are generally designed according to the engineering principles that we defined above. So, the only reason that shaft is broken is fatigue or unexpected dynamic loads.

What are the advantages of shafts?

The most important advantages are; that they have very easy construction. They can transmit very high powers. They do not have a complex system.

What are the disadvantages of shafts?

The most important advantages are; that they have very easy construction. They can transmit very high powers. They do not have a complex system.

What are the applications of shafts? 

There are different kinds of applications of shafts in engineering. Machine tools, automotive, clockwork mechanisms, power transmission of turbines, pumps, compressors, and marine applications such as ships, etc. So in power transmission, the use of shafts is very common.

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