Posted by Max on Monday, February 14, 2005 at 6:56AM :
Hi Folks,
re torque loading on the Dennan overdrive
I read the Dennan overdrive posts with interest.
I am in the jaguar restoration business, since the 60’s, and then got into the older chrysler products (and now went power wagon mad) After dealing with Lucas electrics and other brit car issues, the PW is refreshingly basic to work on. (Not even fuses to worry about). And real fun to bump and bounce round in. Great for seeing way over traffic. And as the seller said to me: ‘They get the respect they deserve."
Anyway this overdrive does look like the old Laycock de Normanville unit that’s been around for decades. If it is in fact, a Laycock unit, how these function very broadly is like this: The gear reduction is done by sun and planet gear. There is an oil pump driven by a cam, and operation is by a solenoid operated valve actuating pistons that engage a conical clutch. As mentioned in the thread, drive is also though a one directional sprag clutch.
This is how the o/d was setup in a production car: Electric operation by switch.
The reason for the electrical operation is if you forget to disengage, the override switch does it. If you had a hand lever you may forget. (although some were setup that way).
It needs to be disengaged when going in reverse because of the sprag clutch; as mentioned, if its engaged while reversing it will destroy itself as the shaft will be locked with full power and vehicle weight against it, something akin to pushing a vehicle under heavy load while in "park.." The circuit is automatically overridden in reverse by a cut-out switch the same way the reverse light is actuated at the gearbox. So don’t wire it directly to the o/d without the cut out switch or you’ll forget to switch it out when you back up and the next noise you’ll hear will be the sound of the bank account being sucked down.
There’s another cutout switch in the system also:- on top gear. Ie, it is wired so that it can only be engaged in top gear. All other gear positions cut it out of operation. The reason for this is that the box is NOT designed to accept the increased load in the lower gears, especially 1st and 2nd . I’ve had quite a few o/d’s come in over the years where the switch had failed in the "on" position and overdrive was in operation over all gears, with the result that the cone clutch burned out. This is why they won’t endorse using it in the low range and under heavy load. In my opinion it should be used ONLY in top gear in HIGH RANGE. Once your truck is lugging in top gear, that’s the signal that the o/d needs to disengage as you get into a lower gear.
Torque loading is dramatically increased under low-low gear surges. If you could eyeball the sun/planet gears you’d see that they’re just not of the size that would take heavy impacting. Nothing like the big clunking straight cut gears in the main box.
Please note, I’m not badmouthing the od, it just needs to used within specs. Like you wouldn’t try to pull a tree stump out with your winch while driving backwards. (I’ve seen a g/box after that was tried, not a pretty sight).
As evidence of this load issue, Laycock listed different units depending even on engine size in the same models. Eg Jaguar MK2 3.4 liter was different to MK2 3.8 liter even tho the torque and hp weren’t that different.
(All this assumes, of course, that these boxes are basically the same as the old ones. For all I know these are massively stronger- maybe)
But for freeway use, these overdrives are a great upgrade, save on gas and engine wear, and are very well made and reliable, in the Jaguar they would way outlast the cars, you can get where you’re going in reasonable time. (I’m having to restructure my eta’s with my PW at 45mph)
Anyway sorry for the length of this, can’t seem to say this stuff in one para.
Cheers,
Max
53 B4PW period custom extended cab.
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