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.Â
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.Â
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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