How does a ST4 Variator work?........ pics

Two drive train questions...
I am puzzled about the operation of a variator and how they function.
Can somebody give me a lesson on the physics behind it's function?
Seems to be reliable and simple. but looks like alot of stress would be put on those cast upright thingamajigs.

I know how the hydrostaics operate but why not use a system like this in the second pic? Seems like it would be the most simple to operate, no belts or fluid etc. What am I missing?.
I saw one cat with 2 dana 60's for each track as well. Why don't more units use an axle . Is there too much power lost to a pinion geared setup?
Just curious to know why things have evolved the way they have in the drive train dept...

I don't own a cat yet.. still looking. Thought I had a ST4 Located and went to the fellow's home only to find it was a Kristi. I did
not have a camera but will post a pic soon. The owner was not home.

The Variator uses the differential to do most of the work. The two chives atop the variator, are variable width, operated thru a convential steering box. It opens one pulley while closing the other. This speeds up one side and reduces the speed on the opposite, BUT it continues to provide power to both tracks at all times. With differential braking, the most common 2 track steering method( prior to hydrolic), only one track is powered during a turn and the other is dragged. This novel marriage between German and Swedish technology is what makes an ST4 one of the most aggressive machines of it's time. For more on the Variator there is an entire "Thread" on them eslewhere in this forum call VARIATOR.

great info... I just read it ...a very good FYI

http://www.forumsforums.com/3_9/showthread.php?t=4113

what units ( example a few) used differential braking and should one stay away from that tech?

Differential Braking, (Green unit, 2 Pic), really works the differential. Army Tanks had this and they tended to fail. Interestingly one of the larger Nodwell family of track machines used this same differential that was in american military tanks and had similar problems with failure. To combat this, both Bombardier, Thiokol, and Nodwell/Foremost later equipped their machines with a pump, filter, small radiator, and even temperature guages. They also substituted Automatic Transmission fluid for the original 90 weight Hypoid Gear oil. For no apparent reason the Snow Cat Manufacturers never did resort to the dual brake and clutch arraingement adopted by CAT, CASE, and John Deer. Where differential braking tends to preform the worst is going up a winding turn. Since you are dragging one track, more HP is required, and the other track can easily dig in and bury itself or slide off into the ditch. Bombardiere alone produced a special differential, that is Planitary, and has 6 sets of 6 each, (total 36 gears!) gears in a huge housing. This was standard equipment on J5's and their larger cousin the Dual Drive Muskeg Tractor. These hold up outstandingly well, even without the pump/cooler/filter arraingement. They are BIG, and look as if they would comfortably power an 18 Wheeler with a 525 HP Cat! Needless to say, differential braking faded out and was superceeded by several hydrolic schemes.

All controlled Differential whether Bombardiers T-16 or the OC4 OC12 OC15
OC21 or even The Military M-24 they are all the same design sure some of the sizing of the gears change but the principal is the same. when the steering
band is applied one side will go to 1/3 speed and through the planetary action the other will go to 2/3 speed radius of steering is effected by length to width aspect ratios and also terrain and bearing surface of snow or soil. Etc. The Bombardier Differential that Lydon has referred to is the same differential with exception of the Bombi and the big rigs like the B-8, B-15, B-20 of which run the M-24 tank differential with external planetaries on the differential and the sprockets . the B-20 is capable of hauling 20 tons and does so with ease.

The rest of the Bombardier machines all have the same Differential most with a 6/35 pinion and 35 tooth ring. of which it is a right hand ring and a left hand pinon tooth pattern. these units were first used in the T-16
Bren gun carrier so hence the Name T-16 has carried with it since the 30's
with little change except the cases they where put in.

The picture of the Green differential is from a Otter (also called Ferret and ranger)manufactured in Minnesota they worked very well for a light machine they went to a bull gear final drive at the wheel.

They are also used on some very big units in the Florida swamps
but they are using the 5ton Rockwell differentials

So to sum it up controlled differentials have proven themselves for over 75
years most of the manufactures went from 90 weight to ATF so that the steering bands would not go (soft) or turn mushy after a while since the newer steering materials have evolved. now some are using steering bands with some Kevlar in them to last longer up to (5X) but should be run with
a trans Hydraulic fluid not ATF atf does not provide enough protection for the harsh pressure on the Ring and pinion.

Preference of the controlled differential is still used today if you need a machine with greater speed. is needed that you can not get from a hydrostatic system.




Having consulted Bombardier for years these are some proven results

Thanks for the story on the T-16. I was beginning to feel somewhat concerned with all the talk on the forum about the OC-12 being such a strong differential. Apparently the T-16 in my Bombardier has a pretty good track (pardon the pun) record, too.