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Helical Gears vs. Spur Gears


If you have observed a spur gear application, you may have noticed that the spur gear can be replaced by a helical gear. Where should a helical gear should be used instead of a spur gear? What are the benefits and disadvantages of doing so?

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    A gear is a positive power- transmitting member of mechanical systems. Unlike belt drive it does not have any slippage, and that’s why it is positive.

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    What is a spur gear?

    Most of the time when you see gears, they are spur gears. You can identify a spur gear from its teeth. Teeth are projected radially and are parallel to the axis of gear. The teeth of a spur gear are exactly perpendicular to its flat faces.

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    Advantages of the spur gear

    • Manufacturability: Spur gears are simplest, hence easiest to design and manufacture

    • Efficiency: A spur gear is more efficient if you compare it with helical gear of same size.

    Efficiency of gears: (Output shaft power / Input shaft power)* 100

    Efficiency of helical gear is less because helical gear trains have sliding contacts between the teeth which in turns produce axial thrust of gear shafts and generate more heat. So, more power loss and less efficiency.

    • Easy to assemble: Spur gear teeth are parallel to its axis. Hence, spur gear train does not produce axial thrust. So the gear shafts can be mounted easily using ball bearings. On the other hand, helical gears generate axial thrust so the bearing arrangement of the shafts in this case should handle the axial thrust along with radial thrust. You may have to go for a roller bearing or a thrust bearing in this case.
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    What is helical gear?

    The teeth of helical gears are not parallel to the gear axis; they are set at an angle. Since the surface over which the teeth are made is cylindrical, the teeth take the shape of a helix.

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    Advantages of the helical gear

    • Silent operation: In a helical gear train, the teeth engage a little at a time rather than the entire face at once. This causes less noisy power transfer in case of helical gears.
    • Non-parallel shaft: Helical gears can be used for transferring power between non parallel shafts. Though you have to compromise on efficiency in this case.
    • Strength: For same tooth size (module) and equivalent width, helical gears can handle more load than spur gears because the helical gear tooth is effectively larger since it is diagonally positioned.

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    Spur gear application

    In mechanical systems where you will require simple design, more efficient power transfer, and low speed applications where noise does not matter, you should go for spur gears.

    Example: robotics applications, machine tools etc.

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    Helical gear application

    If noise is the main design criteria and you can compromise on power, you should go for helical gear. Helical gears normally used for high speed, high power mechanical systems.

    Example: Car gear box, machine tools etc.

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    Spur and helical gears are mostly used all around. You should go for spur gear for simplicity and efficiency. On the other hand helical gear should be considered for smooth high power applications.