helical gears in reverse

[rick gray]

i have a need to know if helical cut gears can be used at full tilt in reverse? i have a dana18 and dana20 transfer case that i want to use in a amphibious vehicle project and the have to turn in constant reverse. will they stand up?

Moved from TFI to The Garage
-ViRGE

 

[phucheneh]

Helical gears are stronger than straight cut gears, assuming similar size and materials. More surface area to engage.

The helical gear weakness is axial loading, which I'm sure is going to be tolerated differently by different transmissions. I would look for input from dedicated off-roaders using your parts. Kind of a really specific question for a general car forum, IMO.

 

[tcG]

^ Truly stellar advice.

 

[iamwiz82]

I would be hesitant to use a helical gear constantly if it were not made for such usage because I would not be confident that the Dana 18 was made for that kind thing, but how often do you think you will be in reverse?

On second thought, You will not see the same amount of load as on the street because the prop will be able to spin freely in the water, whereas the tires have a much higher level of power to break free of the friction.

 

[SpatiallyAware]

Some helical gears are cut to where the 'pull together' under load.

I would imagine reversing a full torque load for these types would cause them try and separate.

 

[rick gray]

Thanks for replies. good info. there is three gears side by side in the transfercase so clockwise rotation turns the centre gear in counterclockwise,which turns output gear in clockwise. yes i can see they would have stress to separate.

 

[rick gray]

I looked at the dana 20 transfer case ( run in reverse)and if the case is shifted in rear drive high gear rearwheeldrive the drive is straight through to an auto differential ( a slide gear locks the input and output into one straight through shaft) the other gears have no load on , but if shifted into frontdrive only high gear, the gears are spinning in reverse . since this front drive output is driving a jet pump with no immediate torque needed it should not have to much strain on gears until high speed.gears may howl? the gears are three side by side helical cut to drive the front output.

 

[phucheneh]

...okay, I want pics of whatever contraption you're making. I doubt I can offer much insight, I just wanna see it.

Three side by side means ones an idler to reverse rotation, right? So the driving gear pushes the idler in one direction, and the idler pushes the gear it subsequently drives in the opposite direction, with the net effect basically being the forces of the input and output gears working together to murder the idler, I would think. Which would explain why reverse is typically a spur gear; makes the needed idler subject to ZERO axial thrust instead of what I would logically assume is twice the load your foward gears would see.

So you'd want some damned beefy bearings on the thrust side of the idler. Or a set of six gears, set up as opposing pairs.

Again, though, I can't quite grasp what exactly it is you're trying to accomplish, TBQH? If you're literally running the gears backwards...input and output swapping jobs...the thrust bearing for your idler would be on the wrong side.

 

[rick gray]

Phucheneh:sounds aboriginal? if i can get my tongue around it. if you want a description of the gears look up a dana20 exploded view on google,lots of data. the idea is to power an amphibious vehicle. the v8 with a trans that is connected to a divorced dana20 transfer case that has an output shaft that goes aft to drive the differential( rearwheel drive output) if you shift to front wheel drive only,this output has a driveshaft that goes aft and drives a dana 18 transfer case that drives the jet drive.so you have wheel drive or prop drive with shifter control.if you send email address by private message? i can send photos.the cases are rotated so they are actually turning in reverse to enable drive line configuration.the original input shaft is now on output and the rear drive output is now the input.

 

[phucheneh]

Fileden (fileden.com) is a really easy image host. Sign up, click upload, pick files, and it spits out a link with the proper tags.

I understand what you're saying now. I think there would be a few concerns. The first one that comes to mind: how is the transfer case geared? If it's intended to increase torque (and thus decrease output shaft speed), running it in reverse would create an overdrive unit.

But more importantly, it would appear that you would indeed be dealing with the reversed thrust load issue, independent of how many gears you had in whatever configuration. See one of my patented glorious visual pictograms of stupendousness:

I wouldn't even bother trying unless you can take it apart and turn the gears around. May or may not be possible; I just don't know how the stuff is put together.

 

[SpatiallyAware]

I agree with phuc, from what I can determine from OP's description is sounds like it would be a lot of pressure on that idler gear. Definitely too much for a road/street type application, but it may be ok when pushing against water.


I am almost positive that I've seen someone reverse those gears somehow... For a dune buggy, or VW conversion, or something like that.