How should I build the driveline?

[mccheynebeeman]

Hello all,

I have a project I am building that has 8 wheels & 8 wheel steering. I have used 8 sprint control arm assemblies (strut towers and brakes and rotors.. etc) along with their CV axles to join up to four seperate differentials. I was origionally thinking that I would have an engine & transmission inline with the vehicle, droping the drive using CV axles to two 90 gear boxes, which will drive two sprockets with 80 size chain to each corresponding diff. The problem I don't like about this idea is that if I need to tension the chain (which I would) I'd need to either move both diffs side to side for each chain, or move the 90 gear box up and down. Since I wanted everything sealed in a custom box, this would make it really hard to seal.

So, I was thinking about using 3 carrier diff gears (same ones as on the current diffs) inline instead of chain to connect everything. The middle gear would connect to the 90. This way, everything can be secured down without needing future adjustments.

Anyway thoughts?

(to see some pictures of this machine for a reference of what I'm talking about got to

http://www.4wheeler.ca/index.php?showtopic=20755)

Thanks in advance,

Frazer RM Ross

 

[Warpspeed]

Could you drive each diff centre from a mutistart worm reduction of suitable ratio ?

The drive to each worm would be above each crown wheel, all four worms could then be coupled together in line down the length of the vehicle.

 

[mccheynebeeman]

Warpspeed,

Interesting.. I like that idea. That would be great if I could do it that way! But I wanted to get at most a 2:1 reduction in speed from the output of the automatic transmission to the wheels. Is this possible with a worm gear assembly? I could of course, play with the gears on the output of the worm drive and on the diff, but my size on the diff is limited to what is on there right now (approx 6") and I not sure how much it would cost to get a bigger gear than that on the output of the worm drive.

Thanks for your response, this is definitly something I hadn't thought about yet.

Frazer RM Ross

 

[Warpspeed]

Yes it is possible. A worm gear can be a simple single start, or multiple start worm. When the number of "starts" becomes quite large, the drive worm begins to look more like a simple helical cut gear than a worm gear.

Two crossed helical cut gears can be meshed to have the shafts at ninety degrees (or any other desired angle), and the ratio can be anything you want to make it, even 2:1

Here is a picture of some crossed helical gears:

http://auto.howstuffworks.com/gear3.htm

 

[mccheynebeeman]

Ahh I get the picture now. Never thought of a worm gear drive in that way. So, just to get a picture now, how about this:

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

Is this roughly the same thing you were thinking?

 

[Warpspeed]

That is how I believe I would do it. The crossed helical gears could be cut to any ratio that you decided to use.

Helical gears generate considerable end thrust, so at least some of your bearings may need to be tapered roller bearings.

The whole drive system might be enclosed in a sturdy gear case that could form a box shaped backbone chassis. All the major suspension loads could be fed into it as well.

There is also the possibility of extending that long driveshaft through CV joints to power some additional wheels in a powered trailer.

 

[mccheynebeeman]

Then I guess the final question would be, where would I be able to get gears like these?

Thanks for your help!

Frazer RM Ross

 

[aviat]

There is a vehicle called an Argo which is similar to your idea. It uses a skid steer and chain drive. You may be able to get some ideas from it.

http://www.4x4mag.co.uk/marfeatures/magargo.html

 

[Warpspeed]

Rather a lot depends on where you are in the world. I am in Melbourne.

The internet or telephone book may lead you to businesses that cut custom gearsets. If you are patient, it may be possible to track down some existing gears, or something that could be readily adapted.

There is a "gear and pulley" Forum here on eng-tips, someone there near your location may be able to help better with this.

 

[mccheynebeeman]

Aviat,

I currently own an old praire bobcat (basically an argo), which is exactly where I got the idea from. I wanted to incorporate steering so I could safely get higher speeds, and will use two brake systems (left and right) to be able to skid steering if I want. Thanks for the suggestion.

I'll be looking around for gears in the next little while.. thanks for all the help!! you guys have been great!

Frazer RM Ross

 

[Warpspeed]

I have never seen an argo, but have driven a thing called a wildcat. This had four driven wheels, and chain drive with hydraulic transmission and skid steering. At speed, the skid steering was pretty frightening. Bumps would also throw it off line in a rather alarming way, as it had no suspension.

Eight wheels, properly ackerman steered, with independent suspension on each wheel sounds like a vast improvement on something crude like an argo.

 

[mccheynebeeman]

Warpspeed,

You might have some appreciation then of why I crashed my prarie bobcat into a telephone pole at around 50kms. A little younger and wilder, I decided ditch the current rotax 503 and throw in a 993 4 stroke suzuki engine and the autotranny that went with it. After many many trail and error tests with short little drives, I finally got it working quite well. This was her madian voyage:

http://www.4wheeler.ca/index.php?act=Attach&type=post&id=20497

Hense the reason for a whole new design in the first place (glad you agree with the idea!!)

 

[Warpspeed]

Oh golly !!! And that thing has skid steering ?

You most certainly deserve the Eng-Tips Evil Kinevil award for that.

I often "located" a stump or a boulder in the wildcat, and it would stop from around 20Kmh to zero in about an inch, whereas I would continue on going at undiminished velocity over the dashboard.

All jolly good fun when you are young.

 

[mccheynebeeman]

"whereas I would continue on going at undiminished velocity over the dashboard"

You should write a book. Hehe. Yeah, that was skid steering and yes, quite crazy to think back on it. Needless to say she has a hard time floating now. This new one should alleviate the steering and rough riding issues.

 

[Warpspeed]

It certainly should.

Eight independently and very softly sprung wheels should give amazing traction. Eight rigidly mounted wheels are likely to see most of them up in the air on any very uneven surface. Fully independent suspension should give a nice smooth comfortable ride too.

If you plan to drive it fairly fast, some appropriate spring damping may be well worth the trouble, or it might go into eight wheeled pogo stick mode, and jump like a spider.

I suppose only six wheels really need to be steered, and with a bit of careful design, all the correct akerman steering angles could be incorporated. That would create an amazingly agile and stable vehicle on all sorts of surfaces.

 

[ca2sbl]

You could look into using differentials from the M151 military quarter ton truck.

The M151 had full independent suspension and, since the differentials were interchangeable from front to rear of the vehicle, they have power input flanges on the front and rear.

You would not need any fancy chain or gear box for each pair of diff's. You could drive one differential directly from the other with your transmission or transfer and all 4 differentials in line

The M151 had only a 70 hp motor so the differentials may not be able to handle a lot of torque. Also, I do not know what the gear ratio was.

Have fun,
Russ

 

[amorrison]

ca2sbl got me Googling m151 differential
where this was found
- might be of interest

8 wheels driven
8 wheel steering.

http://www.mil-plus.com/DesktopDefa...em WHERE ItemId=?&ConnString=ShopDbConnString

 

[amorrison]

See also

http://www.army-technology.com/projects/mrav/mrav3.html

 

[Warpspeed]

Wonderful amorrison4 ! I have been searching the internet for a picture exactly like that.

"Normal" direction rear wheel steering is unstable at speed because it promotes over-steer. The rearmost pair of wheels should definitely be fixed. For eight wheels, the front four or front six should ideally be steered. For very low speed vehicles, it probably does not matter.

The other point, is that for normal heavy duty road transport trucks with Bogey drive, the front diff is a "through" design with front and rear facing drive flanges. But the way they do it is with a normal spiral bevel gear plus a drop gear from the input/output shaft. That may end up being too heavy for your application.

I still feel that crossed helical gears may be a suitable solution for a very light weight low powered vehicle.

 

[amorrison]

There are problems in accessing the pictures of the 8x8 on my first post.

Solution:

1. Copy all of this test (not just the underlined part)

http://www.mil-plus.com/DesktopDefa...em WHERE ItemId=?&ConnString=ShopDbConnString

2. Paste in as an address
3. Select M151 Sales on the LHS
4. Select the 8X8