strength of gears.. 1

[Chevykid18]

hi, im designing a gearbox for a small off-road car, with just two shafts, input and output using spur gears. I pulled a pair of gears from an atv, which are perfect for me, light weight and the ratio I want, 3.33:1. My concern is I don't know if they will hold up, the input shaft will see a max torque of 60ft-lbs at around 650rpm. Is there a formula using rpm, diameter, pitch diamter, etc...
any help is appreciated
Peter Ruggiero

 

[NickE]

One way to figure this out is to look at the output torque and weight of the atv you pulled the gears from.

Another issue is case flex and shaft mis-alignment. Are these helical cut? If so the alignment has to be much tighter than if they are not.

 

[EngJW]

Yes there is a formula, and I don't mean to come across as blunt about this, but go to a library and check out any textbook on machine design. You already know the load, so you have at least half the information.

 

[Chevykid18]

thank you for the replies, The only problem with using a formula from a book is that I don't know the actual strength of the material of the gear. I know that it is Rockwell C 35. Anyone with automotive experience may know what they make their gears out of?
thanks
Peter Ruggiero

 

[EngJW]

Do you know if it is steel? If so, the hardness can be cross-referenced to tensile strength. Machinerys Handbook has the tables. If it is powdered metal I'm not real sure, but it might be comparable to steel of the same hardness.

 

[TVP]

35 HRC equates to an approximate tensile strength of 970 MPa. This is not very high for a gear surface, and will only allow contact stresses of ~127-140 ksi and bending stresses of ~ 36-48 ksi. Most high performance gears are carburized, with a case hardness of 58 HRC minimum (typically 61-64 HRC), with a case depth of ~ 0.6-0.9 mm (depth where the hardness is equal to 550 HV or ~ 52 HRC). This allow contact stresses ranging from 180-275 ksi, and bending stresses of 55-75 ksi.

 

[ivymike]

35HRC does seem low, doesn't it? Might as well have made the gears from butter... or "I can't believe it's not butter."