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Friday, 1 January 2016

DIFFERENCE BETWEEN HELICAL GEARS AND SPUR GEARS

We were discussing in our previous post about the basic of helical gears and also basics of spur gears and we have also seen the various advantages, disadvantages and applications of spur gears and helical gears.

Today we are interested to make a comparison between spur gears and helical gears on the basis of their various features. We will discuss the various features of spur gears and helical gears respectively to secure the better understanding of various differences between spur gears and helical gears.

So let us start our discussion with the help of following mentioned points

Gear teeth

Spur gear will have a cylindrical shape, as shown in following figure, with straight teeth made parallel to the axis of gear; spur gears are always used to connect two parallel shafts. While if we consider helical gears, teeth will be cut over the surface of helical gear at an angle with the axis of gear as shown in following figure.


Spur gear teeth design and manufacturing is quite easy as compared to designing and manufacturing of helical gear teeth.

Gear teeth engagement

Spur gear teeth engagement will not be gradual but also one tooth of driving gear will mesh with tooth of driven gear each time and will produce noise and also deliver the impact load over the meshing teeth. 

Therefore spur gears are not preferred in driving mechanism of vehicle such as car.
While in case of helical gears, there will be gradual contact between helical gear teeth i.e. engagement will start at one end of a gear tooth with other mating gear tooth and will spread gradually and continuously as both mating gears rotate and hence helical gears are used in driving mechanism of vehicle in order to reduce the impact loading, noise and stress on gear teeth.

Silent Operation

Spur gears will not be preferred to use at high rpm as it will create more abnormal sound and hence if noise restriction is the main design criteria then we will not prefer to use spur gear. Helical gearbox operation will be quite silent and smooth as compared with spur gearbox operation.

Axial thrust during operation

As we have discussed above, spur gear teeth made straight and parallel to the axis of the gear and therefore there will not be any issue of axial thrust and we can use ball bearings to install the spur gears in our drive mechanism.

But in case of helical gears, there will be creation of axial thrust load on gear due to helix angle of gear teeth and therefore gearbox designer has to select such bearings those are able to absorb and support this axial thrust load.

Gear fixing between shafts

Spur gears are only work between two parallel shafts but helical gears may work between parallel shafts and non parallel shafts also. Helical gears those are connecting two non-parallel shafts will be termed as crossed helical gears.

Strength and load carrying capacity

If we compare the spur gear with similar size of helical gear, we will observe that spur gears are quite more efficient as compared to helical gear

Power loss and efficiency

As we have discussed in our previous post that there will be sliding movement between mating gear teeth in case of helical gear and hence heat generation will be more as compared to spur gear application. Therefore in case of helical gears operation, Power loss will be more as compared to power loss in spur gears operation. 

In order to reduce the heat generation we need to provide good quality lubrication between helical gear teeth during operation.

One pair of mating helical gears will have less efficiency as compared to efficiency of mating spur gears of similar size.

Cost of designing and manufacturing

Manufacturing and designing cost of helical gears will be more as compared to cost of designing and manufacturing spur gears.

Do you have suggestions about differences between spur gears and helical gears? Please provide your suggestions and feedback in comment box which is provided at the end of this article.

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