Epicyclic Underdrive, for LT76

[tony109]

There were a couple of Epicyclic Underdrive Gearbox options available or the LT230 transfer box. It fitted between the main gearbox output and the intermediate gear.

In trying to get a proper crawl Ratio I have the 2.888:1 Low with the lower ratio 3.6:1 1st, 3.01 :1 Rev suffix C main Box.... While The series 3 box does gives a lower 4:1 Rev ratio I didn't want to go syncro in the series 2.

In the UK, Ian Ashcroft supplied an underdrive for fitment to land rover Defenders with the LT230 Transfer box, as did East Coast Rover in the USA..... But they apparently only sold 2... and no longer sell.

From what I've seen, the underdrive centre shaft connects to the main gearbox Output shaft, then through epicyclic gears, connects externally with the transfer box Intermediate gear. It gave just a 2.69:1 reduction but this would be enough to turn 48.88:1 into 131.48:1..

For the LT76 the Underdrive input would need to connect internally onto the 1.15" diameter, 10 spline Gearbox main shaft. With the external output gear, meshing with the Intermediate gear cluster, matching the original 'Output gear' from the main gearbox. It's diameter being 3.4" with 27 Helically cut teeth. 

The Fairey overdrive output gear RTC7176 is already made for the job as is the overdrive main shaft RTC7168 to couple direct to the gearbox Main shaft. 

Is there a compact Epicyclic box strong enough to build such an underdrive for the Series LT76? 

[elbekko]

Might be worth looking into the "doubler" boxes the Americans like using?

[Daan]

Can't you re-arrange an overdrive so it works as an underdrive?

[tony109]

1 hour ago, Daan said:

Can't you re-arrange an overdrive so it works as an underdrive?

Effectively yes. The OD main shaft fits to the back of the main gearbox so already made, and the 27 helical tooth output shaft is perfect.. It's joining and finding a suitable epicyclic box..

[Snagger]

I think you’ll find it very difficult to source a suitable epicyclic gear system.  The problem is that to work as an under drive, the input shaft needs to be concentrically outside the output shaft, the opposite of the epicyclic overdrives and how the LR transmissions are built.  It maybe a lot easier tho have custom gears made for a Fairey or Toro overdrive, changing the rearmost independent gear for one with a smaller diameter and the lay shaft to have a rear gear of larger diameter.  The problems with that are closing ratios and tooth counts to retain the same main shaft to lay shaft spacing and fitting the larger lay shaft gear inside the casing.

[Daan]

On 4/27/2024 at 12:08 AM, tony109 said:

Effectively yes. The OD main shaft fits to the back of the main gearbox so already made, and the 27 helical tooth output shaft is perfect.. It's joining and finding a suitable epicyclic box..

What I meant was swapping the gears over so you reduce the speed instead of increase:

So basically, flip the bottom shaft, so the LH gear set sits on the right, and the RH gear set sits on the left, therefore reducing the speed.

[Snagger]

5 hours ago, Daan said:

What I meant was swapping the gears over so you reduce the speed instead of increase:

So basically, flip the bottom shaft, so the LH gear set sits on the right, and the RH gear set sits on the left, therefore reducing the speed.

You’d have to get a custom input gear (upper right) and output shaft (upper left) made with the swapped gears, but the lay shaft will fit the casing reversed.  You might need to have the inboard ends of the smaller gear trimmed on the lay shaft to ensure they don’t contact the synchro baulk rings - they do look longer on the right hand end than on the left of the lay shaft in that photo.

[ashtrans]

On 4/26/2024 at 7:18 PM, tony109 said:

 

In the UK, Ian Ashcroft supplied an underdrive for fitment to land rover Defenders with the LT230 Transfer box, as did East Coast Rover in the USA..... But they apparently only sold 2... and no longer sell.

 

Hi, we made the ECR ones, they sold a few to some rock crawlers

[FridgeFreezer]

Someone in our club had one but it didn't seem to get much use, I think it's great for dry rocks where you have lots of grip and want to crawl very slowly and carefully up something, but not super useful the rest of the time.

[landroversforever]

14 hours ago, ashtrans said:

 

Hi, we made the ECR ones, they sold a few to some rock crawlers

Ian, do you remember what sort of max torque input the under drives can take? 

[tony109]

On the Fairey Overdrive, Revolving the position of the layshaft and main shaft gears looks a possible way to gain a 21.7 % Underdrive, instead of an overdrive. This would give around a 1.25:1 step down.

Sadly as the output shaft and gear are in one piece, even if you could revolve the layshaft, the output gear would need a different solution. The reduction still falls a long way short of it's 'crawl/to Hassle ratio and bearing size I suspect would be a weakness. Although I see an underdrive could reduce the stress on the driveline in most cases.

@ashtrans very, very interesting to hear you made the ECR Underdrives..  Can I ask, did your 'Under-drives' use an epicyclic system.  And the box fitted between the Output and

intermediate gears of the LT230, so did that require you making gears or were you able to use stock parts? 

And how did it work and do you have any pics or information of it in operation..??

Would love to know more.

 

 

 

 

Edited Friday at 11:20 AM by tony109

[FridgeFreezer]

Also worth suggesting you look at @bill van snorkle's post history, he isn't active here any more but I am fairly sure he made his own Series underdrive as well as his own portal boxes and a ton of other amazing parts for his "Wild Fing" Series.

 

 

 

[tony109]

So it must have been @bill van snorkleheavily modified series 2 6x6 Land Rover, in Australia.  I read about it many years ago. Able to climb over meter wide tree trunks with PTO powered

Guide wheels and air bags to keep things level. The rear axels mounted to a fully compensating Rocking/Walking Beam rear axels, give amazing levels of articulation. And somehow used two Transfer boxes, but tech details were rather vague. 

As shown in the other video clip, the flexible series 2 with portal axel boxes, add clearance and no doubt reduce gearing further.. The ease his series 2 has Crawling through the course compared to the following, Clutch riding 90 shows the benefit of a decent Crawl ratio. Clearly he has some serious gear reduction.