By David Seidel (6x6aatv) on Unrecorded Date: Edit

I would like more ideas and discussion of the dual differential drive system. This system really interests me. I know that tanks use this but what kind of drive setups do you use to steer the vehicle(to drive the 2nd differential).
Is there an easy way to incorporate two differential together or what vehicles used this setup that you could get a used one besides a tank?
Thanks, David

By Eugene Kochnieff on Unrecorded Date: Edit

Hello David,
You need a low speed high torque drive to the steering diff. Hydraulics or electric motor and wormdrive reduction are my favoured options.
As for how to do it i have been toying with the idea of joining two small car diffs, the sort that is used in and independant suspension car like the rear diff out of an old subaru 4X4. These diffs have flanges that will allow the use of sprockets and chain on one side and gears on the other. Open gears will wear badly though.
I would like to build something up into a single casing so as to keep everything clean and lubricated. Any ideas??
The only other vehicle that uses this system is the argo centaur as far as i know.

By Douglas MacCullagh (Dougmac) on Unrecorded Date: Edit

The tanks work differently. Tanks typically have 3 or more forward gears. The second differential is driven directly by the engine, and whether you turn left or right is controlled by steering brakes. You get a 0 radius turn in neutral, and progressively wider turns as the gears go up. However, you still have only one turning radius for each gear. The best diagrams & discussions I've seen have dealt with the Merritt-Brown gearbox used in WWII Churchill tanks. Continuously variable turning radii in all gears requires a triple differential gearbox, and I still do not have a real strong handle on how they work.

By Eugene Kochnieff on Unrecorded Date: Edit

Hi Doug,
I dont argue with your description of tank steering.
The system i describe was designed for tanks in the early eighties as far as i know and was designed with tanks in mind.
It can be considered as the final drive and can be driven by any form or variable or multi ratio transmission. as for the steering aspect this system relies on both diffs being coupled.
1:1 on one side and 1:-1 on the other.
One is driven the other sums the left and right axle speeds. The input to the second diff determines the steering action. locked it steers straight (full all wheel drive on an aatv) any input forces a difference in axle speeds and causes a turn the rate of turn is dependant on the speed of both the drive and the steering input.
This is why hydralic drive is a good choice as flow rate can be controlled and tends to track engine speed.Also this system eliminates entirely the need for steering brakes which turn valuable engine power to waste heat, a loss that a tank can not afford given the overheads it must cope with such as armour etc. I have written at length on this system in this forum on two other occasions. just search on my name to find the other posts.

By Douglas MacCullagh (Dougmac) on Unrecorded Date: Edit

Eugene -
I understand your idea completely, and I think it sounds great. However, all of the tank drivers I've talked with agree that they do steer using brakes and there is no separate hydraulic steering support. None of them are quite sure what exactly the brakes brake, and I haven't gotten to talk to a mechanic or engineer yet. Interesting fact - some tanks with steering wheels actually use steering brakes - the wheel controls two master cylinders! My points are that tank steering systems won't necessary do what we want in our AATVs, and that what you are describing is closer to what tank designers call Triple Differential Steering than it is to what they call Double Differential steering. Since our AATVs are light weight and don't see combat, we don't need to work under the same rules tanks do. I do wonder how to connect both the CVT assembly and the hydraluic pump onto the engine, given the length of the engine shaft and the space available in the engine compartment.

By Eugene Kochnieff on Unrecorded Date: Edit

Hi Doug,
Once again i am not questioning what you know as fact about actual tanks. I am unaware of the nomenclature used by tank designers, but the system i describe is corectly called a differential/integral drive and it only requires two diffs to fully implement.I can only postulate that a third diff could be used to split drive to the other two and brakes could be used to force the split in the desired way. however this would not provide for a reversible drive to the steering input and i wonder how it is done.
Do you have any links to any site with details of such a drive if i can work it out i would be happy to write a descriptive explanation for this forum.

By Eugene Kochnieff on Unrecorded Date: Edit

Hi All,
I found a link to a site for meccano modellers of all things. apparently they take their engineering very seriously.
www.erector.webnexus.com/diffs3.html

this page has a reprint of what is a basic run down of all skid steer systems as applied to tanks.
It is hard to read but if you right click and view image you will get a quite good full sized version of the gif image it has been saved as.

It's great reading!!!!

By Douglas MacCullagh (Dougmac) on Unrecorded Date: Edit

Eugene -

Thanks for the nomenclature. I'll start researching differential/integral drive. As for links, everything I have is in books or libraries. After fighting with computers all day at work, I usually don't use the home computer for much. As for triple differential drive, it uses a wheel for input, and is horribly complex. Also, I'll check out the Meccano site during lunch. Thanks!

By Douglas MacCullagh (Dougmac) on Unrecorded Date: Edit

Eugene -

I just read the Meccano article on tank steering. Nice article! Thank you for the link. I don't remember having all the necessary parts in my Erector set, though. I may need to take another look.....

By Kevin Chandler on Unrecorded Date: Edit

Just an update for the meccano site: www.scmec.us/diffs3.html

Also, a site based upon the info of the meccano site: www.gizmology.net/tracked.htm

By Jim Davis on Unrecorded Date: Edit

Could someone please tell me how the dual differential steering method works, and if it is able to do with commercially available automotive parts.Also is their a web site that could explain how the dual differential steering works. Thanks. Jim Davis

By Douglas MacCullagh (Dougmac) on Unrecorded Date: Edit

Jim,

There are two websites that include really good information on steering differentials, double differentials, etc. One is http://www.gizmology.net/index.htmand follow the link down the page. The other is a Mechano site that includes a nice report. I have the URL at home, not here. However,. the gizmology site included that report in their new discussion of track vehicle steering. Hope that helps.

By Jim Davis on Unrecorded Date: Edit

Dear Doug thanks for the information I will be connecting to the web sites as soon as I can. Thanks again Jim Davis.