Designing Double Reduction Gear Drive: Input RPM 730, Output RPM 85.7

[fahadiliciouz]

i have to design a double reduction gear drive using a spur gear pair.Input RPM is 730 and output RPM is 85.7. s The pinion teeth are 18 and the module is 3. The problem is that the input torque is 144Nm due to which i am getting a very high tangential velocity (Ft) which results in a higher values of bending stress both for pinion and gear and the values for my bending strength are comparatively lower to the bending stress so i am getting a factor of safety that is below 0. The steel i am using to calculate bending strength is AISI 1050 tensile strength 725 MPa.
Can anyone advise me what changes should i make to get a higher value of bending strength.

 

[minger]

Are you using some sort of program or AGMA standards to calculate this, or is it basic Machine Design type stuff?

If the former, you can try increasing the addendum on the gear and decreasing it on the pinion.

What limitations do you have? Is material set? Pitch diameter? Face width?

 

[fahadiliciouz]

yes it is based on AGMA standards i used AISI 1050 steel which is hot rolled..
pitch diameter according to the values i gave you is 54 mm for the pinion and 159mm for the gear and the face width is 60mm.
what i was thinking was to use a steel which has a higher tensile strength compared to what i stated above so that i can get a higher value of bending strength..
what do u think ?

 

[minger]

Changing materials really is the easiest way to get more bending strength. How does the pitting resistance look?

 

[fahadiliciouz]

well my design is theoretical so pitting resistance is not included. u just have to find factor of safety using bending strength and bending stress...
according to what u said that changing materials would resolve the problem so can u suggest a material which can b used for gears and which has a very high tensile strength...

 

[minger]

the AGMA standards include two "strength" factors; one is bending, the other is pitting. They coincide and are theoretically based upon geometry, operation and material.

Anyways, in gear design, they really base all of their measurements on hardness rather than strength. Simply find a material with a higher hardness value. We recently analyzed a part that was made out of E9310 Ultra-High Strength Steel. It has a hardness value of HRC 58.

 

[fahadiliciouz]

the steel you mentioned that is 29310 ultra-high strength steel is double quenched...while finding the bending strength we require the surface factor which is based on a graph where we measure the surface factor using harness number and whether the steel is forged, hot rolled or machined.
So i wanted to know where can you find the surface factor for this material

 

[minger]

In practice, the surface factor, $$C_f$$ is typically just 1. In fact, looking through my literature, the AGMA standards do not address the surface factor (also called $$Z_r$$), so IMHO, just leave it 1.