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  • 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

Introduction to design of spur gears

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