Complete Guide To Design Spur Gear Systems

Spur gear systems are one of the most used engineering systems in the machine elements world. To design successful spur gear systems, an engineering approach must be adopted. By following the instructions that are given in this article, you can design spur gear systems according to your system’s parameters. 

Important Points To Consider Before The Design Of Spur Gear Systems

To start to design a spur gear system for a specific system, you need to know about the fundamentals of spur gear systems. Here you can find very useful and detailed information about spur gear systems. 

Torque And Speed Calculations

First of all, you need to know the torque and speed requirements of your system. This is about the input speed and input torque and output speed and output torque. Calculations can be made with the calculator that is provided by Mechanicalland.

Spur Gear Types And Tooth Form

There are various types of spur gears available in engineering and design. Selection of the proper spur gear type is a very important step of the design process. 

Also, you need to be aware of the special tooth form of spur gear systems which provides specific features to these systems. These are also called involute-shaped teeth.

Pitch And Metric Module Concept

Pitch is the specific identifier of spur gear systems that can be selected from the manufacturer’s catalog according to the pitch of spur gear systems. Pitch is for the US Customary system. The corresponding notion of pitch is the metric module in SI Units.

You will deal with these values of spur gear systems in various geometric and strength calculations. 

Teeth Geometry Calculations

Geometric calculations are very important to start the design of spur gear systems. Geometric calculations are made according to the required geometric requirements of the constructional systems. Teeth geometry calculations are a very important part of geometric calculations of spur gear systems. 

Contact Ratio Specification

Contact ratio is the very important numerical value of the spur gear design to identify the boundary geometrical limitations of gear selections. The contact ratio must be around 1.2-1.5 for the selected spur gear pairs. 

Avoiding Interference Between Spur Gear Pairs

Actually, you do not need to consider this step in gear design. Because it is about the design of the tooth shape of spur gear systems. Special shape characteristics are given to spur gear tooths to prevent interference problems.

Also if your current system has an interference problem, you can apply the recommendations to alleviate interference of your spur gear system. 

Velocity Ratio Calculation

You can use our calculator to see the velocity ratio of your spur gear system. 

Train Value Calculations In Spur Gear Trains

If you will use gear trains in your design which will provide transmission of power between shafts that far away from each other. So, you need to be aware of the train value for this system. 

Idler Gear Requirement Check

Idler gears are generally used for changing the rotation direction of the output gear. You need to decide to use whether your system needs idler gears inside it. 

Stress Calculations 

The strength of designed spur gear systems is a very important aspect. You need to consider lots of kinds of parameters in the stress calculations and safety calculations. 

All the calculations must be done below to make sure about the strength and safety of spur gear systems. 

Bending Stress Calculation Of Spur Gears

Gear tooth root bending FEA analysis(Image

First of all, you need to define the occurrence of forces on spur gear teeth during operation. There are three types of forces that appear on spur gear teeth. To calculate the bending stresses acting on specific spur gear, you need to know the tangential force acting on that tooth. 

Also, you need to know the parameters below to calculate bending stress with its calculator that is provided by Mechanicalland; 

  • Diametral Pitch: Which we defined above. 
  • Face Width Of Tooth: The total width of teeth in specific spur gear. 
  • Geometric And Safety Factor: All the required factor must be calculated or specified to obtain accurate bending stress calculation. Take a look at the calculator article. 

The calculated bending stress must be smaller than the gear material’s safe bending stress value which is dependent on the material properties and safety factor. This is also called maximum allowable bending stress which requires another calculation. Mechanicalland provided another calculator to calculate this maximum allowable bending stress value. Compare the calculated values. 

Contact Stress Calculation For Spur Gears

Contacting between mating spur gears(Image

Contact stress is another type of stress that occurs between spur gear mates. Like bending stress, this contact stress must be also calculated accurately. Calculation of contact stress required more parameters; 

  • Velocities of pinion and gear: Circumferential velocities of pinion and gear must be calculated on the pitch diameter.
  • Elasticity Modulus Of Gear And Pinion Materials: The elasticity modulus value of gear materials is specific for gear materials must be known.
  • Pinion And Gear Diameters: These diameter values must be measured.
  • Pressure Angle: Pressure angles are specific for selected gear sets. Generally, pressure angle values are given in the gear manufacturer’s data. 
  • Factors: The same factors with bending stress have the same effects on contact stress calculations. 
  • Total Load: Resultant of three loads acting on spur gear teeth in working conditions. 
  • Face Width: The same as the one in bending stress. 

Just like bending stress one, this contact stress must be smaller than the allowable stress value of the gear material. 


Right after the geometrical calculations and definitions of the required spur gear system, stress calculations for both bending stress and contact stress must be established to show the safety of the designed spur gear system. 

In the design process, you must apply reasoning to obtain a spur gear system from scratch. 

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

You can find out many more calculators in Mechanicalland! Take a look at the other engineering calculators available in Mechanicalland!

Do not forget to leave your comments and questions below about the spur gear design guide and share your problems. 

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