Building a drivetrain using crossed helical gears 1

[mccheynebeeman]

Hey all,

I'm trying to build a drive train for my 8 wheeled drive ATV. It uses the control arm and strut tower assemblies off of 89-93 gmc sprints as well as the differentials and CV axles that go with it. I plan on using one of the engines and automatic transmissions out of the same cars (993cc suzuki 55HP @ 5700 RPM). The problem I am having is how to get the drive to the 4 differentials. I was thinking of mounting the engine inline with axles to two 90 gear boxes, from there sprockets and chains to each differential gear set, but someone mentioned I could probably do it more cleanly with crossed helical gears.

This is the design I am thinking of now

http://199.126.216.230:81/pictures/random/helical-gear-design.JPG

I have been searching for a helical gear set, and it seems like most are custom manufactured. Since I don't know much about gears, I was wondering what I should be looking for as in dimensions, pitch, angle? so that it could work in my application. (the gear on the differentials is approx 6" in dia).

Thanks for your time,

Frazer RM Ross

 

[Philrock]

Crossed helical gears are not suitable for power transmission because they have only point contact, whereas parallel axis helicals have line contact. Crossed helical gears are only suitable for very light loads as in instrumentation gearing.

Hypoid gears, as in conventional rear axle differentials, have 90 degree non-intersecting shafts, and are quite compact in terms of power density, though less efficient than spiral bevel gears.

 

[mccheynebeeman]

Thanks Philrock, I'll have to investigate these hypoid gears a little futher.

On a side note though, when using a chain, can the tensioners be made out of smaller sprockets that are adjustable in the direction you would want the chain tensioned? Also, are there any concerns about angle or number of sprockets used in a chain system? (ie. If the chain was to connect all the sprockets on the diffs and the main drive sprocket with %50 of each sprocket in contact with the chain).

Frazer RM Ross

 

[Philrock]

Not sure what you mean by "...in the direction you would want the chain tensioned...." You can move the tensioning idler in any direction that tends to take up the slack. Best to have the tensioning idler in a slack span rather than in a taut span.

Serpentine chain drives are not the greatest. Chain works best when it's allowed to go slack right after a load cycle. This allows lubricant to re-enter the working surfaces where the load cycle has just squeezed it out. If you have it going taut over several sprockets in a row, the working surfaces will not be well lubricated after the 1st taut span.

 

[mccheynebeeman]

"...in the direction you would want the chain tensioned...." is what I meant by pulling up the slack. Sorry for the ambiguity.

If the diffs were labled 1-4, then I think I will build it connecting 1 to 2, 2 to sprocket 1 on the main drive axle (connected to the 90 connected to the automatic transmission), sprocket 2 on the main drive axle to 3 and 3 to 4 where diffs 2 and 3 have two sprockets. The tensioners will be placed on the slack span of each chain loop if the vehicle were to be moving foward.

As for calculations, the engine produces 58 lb/ft @ 3600 rpm. First gear reduction is 2.4:1 and the differential inside the automatic transmission is 3.16:1. That would give me a maximum torque of 58 lb/ft * (2.4 * 3.16) = 440 lb/ft at the output shaft. This shaft would be inline with the vehicle and change 90 degrees in direction via the bevel gear box, which needs to be rated at this maximum torque (440lb/ft or 5280 lb/in)..

I've found a gear box (

http://catalogs.hubcityinc.com/bevel/model800.html)

thats rated at 443 lb/ft @ 400 RPM.

Now given that I want a 1.45:1 reduction on the main drive axle, this would be putting 444 lb/ft * 1.45 = 643.8 lb/ft on some of the chains. The Main drive sprockets would be 22 teeth and the sprockets on the diffs would be 32. Using this ANSI Chain spec (

http://www.4wheeler.ca/index.php?act=Attach&type=post&id=24693)

I figured 80H Ansi size chain will work for the 22 tooth sprocket on the main shaft (can handle 446lb/ft torque) and 80H on the diffs with the 32 tooth sprockets (can handle 650 lb/ft).

Not being a mechanical engineer, does this appear to be the right way to do all the calculations?