Thought I'd post up one of the more interesting pieces of drive-train equipment I've run across. I have had a number of people over the years ask about how the 'factory' PTO overdrive works on my old CJ. First and foremost - its not truly an overdrive, but rather a gear splitter or reducer (20%) that works on every gear in the transmission no matter whether in hi or low on the transfer case. So, normally a 3 speed transmission with 2 speed transfer case would have 6 forward gears - but with the OD unit, it has 12 forward gears (and 4 reverse of course). This is pretty useful on trail when trying to find just the right gearing and speed - especially for old school 3 speed transmissions that have some gap between the gears, and when used on the highway it reduces 3rd gear (which is 1:1 drive) to 0.8 which is pretty standard for an overdrive. Plus it gives me 3 shifters which is cool - but not as cool as 4 shifters which is normal with a twin stick transfer case - or even 5 shifters if a PTO unit also used on top of the OD! Additionally, CJs from the factory with the Warn OD also came with a lower diff gearing than without - so for my 1970 CJ5 with V6, I have stock 4.88 gears instead of 3.73. (Note that reducing 4.88 gears by 20% you get 3.9 gears which is pretty similar to the 3.73). Originally the unit was built by Warn, but the patent was eventually sold to Saturn and new ones are still being built today on limited production.
Anyway - It was designed for use with the fairly ubiquitous Dana/Spicer 18 transfer case. Although a bit antiquated by today's standards, it was simple and strong and worked as the standard 4x4 TC for over 30 years, and its subsequent redesigns as the Dana 20 and Dana 300 give it an almost 50 year run. But the defining characteristic of the D18 (but not the D20 or D300) is the offset rear output that is inline with the offset front output and sets both differentials off to the passenger side (a nice feature sometime when trying to finagle through large rocks). Of course, because the rear driveshaft comes from the TC, the rear TC gears must be constantly driven from the transmission output (even when in two wheel drive). Directly inline with the transmission output is the PTO port - many early 4x4s (and especially jeeps) were viewed with more agricultural application than 4wd recreation or use. Its off this PTO port that the OD connects.
Some pics: Here is the rear of the D18 that has the transmission connected on the front. You can see the output yoke and driveline mounted parking drum brake to the right and the hole on the left is the PTO port with the transmission output shaft sticking in there and the large input gear for the TC that must be driven (that is my new and larger Tera Low transfer case gear that I installed a few weeks ago).
Up close of the transmission output and PTO port:
Normally there would be a standard looking drive gear on that transmission output that drives the transfer case gears. But for the OD, that gear looks more like this:
The top of this 'barrel' gear has the main drive gear teeth that drive the TC gears. The transmission output shaft slides through the those drive gear teeth here:
Ok, so it can drive the TC from the transmission output - but how does it reduce the gearing? Well, look into the other end of the barrel and you see a 4 gear planetary assembly that goes between the transmission output shaft in the center, and the outer barrel with the large drive gear teeth for the TC.
When engaged, the rotation of these planetary gears allows for the transmission output shaft rotation to be different than the outside of the barrel gear which of course drives the TC gears and thus the rear driveshaft. This is why it can be used for any and all gears - it just acts in between the transmission output and the TC input (which is again always used on the through drive D18).
Once the barrel gear is slid on the output shaft and meshed with the TC gear, this rear cap is bolted on which houses the shifting mechanism and a small synhronizer ring.
The reason I thought I'd post now - I have been rebuilding my TC and transmission, and am at the point of putting my OD back in so I can put everything back under the jeep and DRIVE IT! But, I needed a new barrel gear for it to work with the new Tera Low gears, and the wrong one got sent to me. So to discharge my frustration at this never ending project I thought I'd post here. Herm Tilford (the overdrive guy in WA) is going to exchange again for the correct gear (hopefully) - and I am stuck in place for a week waiting.
now back to your regularly scheduled web surfing...