Introduction to design of spur gears
In this video we will learn
how to design spur gears.
Here we will learn the design of spur gears for its
mechanical strength, for dynamic loading conditions, and finally
for surface durability.
We will meet all the design requirements conforming to
the AGMA standard.
In a really simplified manner.
So, let's get into it.
Before starting the design will have the following design requirements.
or design inputs.
Here the gear should be able to transform an angular velocity to another angular velocity
RPM1 to RPM2
and it should transmit a specified amount of power
also there will be some space constraints
so this design should be able to meet that also
And most of the time
material of design is an input to the designer
So using this design data, the designer has to find out
The following gear parameters such as
Number of teeth on both the gears
Pitch circle diameter
addendum and dedundum
face width
and tooth thickness
so these are the outputs we are expecting from a gear design
so lets see how we can deduce values from this input data
We will start the design by considering the space limitation constraint
the designer cant design a gear of any size
if he does so there can be two problems.
First, the gear may not be economical at all
Second, the gear may not get fit into the space which is supposed to get fit
so there is a space constraint when designing the gears
Assume the desired gear should go within the desired area. Something like this.
So the designer cannot xxxx say
if some this diameters of these two gears
they should less than the allowed width,
say eigthy percent of the allowed witdth
so this is one equation
And we also know the speed ratio of the gear
so we can write another equation like this
Next step, determination
of number of teeth on each gear
first step, you have to assume
a number of teeth t one for small gear
here, I should say something to you: desing
is an open ended problem
there can be many solutions for a single
problem and all of them can be right
according to your first assumption
the design may change but
you have to star like this
if you know t one you can find out
number of teeth on second gear using this relation
and this ..
diametal pitch
Now we got the number of teeth on both of gears
but still there is a small issue
it has to be checked for
The issue of interference
Interference means if
your gear has got profile below base circle
that won't be involute shape
so if this portion of the gear comes in contact
with the meshing gear it can
cause material erosion and huge problems of noise
but you could avoid this interference
if following conditions are satisfied
if number of teeth are more ....greater than
this value then
there won't be any interference
So for check for this
if this conditions is true evertything
is perfect you've got number of teeth in both gears
but if this condition is wrong
then you have increase value of t1
and have to redo this calculation again
and here aw means addendum value
and xxxx per xxx of gear
and usually sin of phi takes 20 degres
in that case you can take addedum of one module
and deddendum one point two module
and module is defined like this
Next, design for strength of gear
or your gear should able to withstand
the force acting on the gear
here, if there is a force acting on the gear
the gears has two components one: tangential
and one radial
the tangential ....induce a bending stress
over the xxx of the gear
and if the maximum value is bending stress induced over here
is more than the allowed stress limit
the gear will fail
you can avoid this can of failures
by designing with help of
an equation call Lewis bending equation
before preceding to the Lewis bending equation we
should determine tangential force first
you know tangential force at gear multiplied by xx velocity
And you know the power transmitted
so you can solve for Ft
Now, according to Lewis bending equation
maximum bending stress induced over here
will be this