Wizard
SupporterI've opened up 4 centre diffs. The centre diff has two weaknesses.
First. The 4 brass thrust washers. One for each of the 4 gears inside the diff. In perfect condition they keep slack out of the mechanism. The washer sits between the casing and the rotating gear, stopping steel to steel contact. Washers are sacrificial. Washers wear over time. It is easier and much cheaper to replace a washer that is out of specification than worn gears and diff casing. Add to this some reports of poor case machining from factory.
Washers wear more quickly when lubrication is restricted through build up of sediment around them, or when oil levels are low. Each washer may wear at different rates. One may be missing altogether, another with sections missing, one cigarette paper thin and one only half worn - in the one diff.
When they wear away completely or fall apart in the final stages (bits of brass in your oil change) you then have a situation of steel gear running on steel case. If you never stress your drive train ie don't go off road, then you might run almost 'indefinitely' with no problems.
Take a vehicle with some hundreds of thousands of kms, with less than routine servicing of the transfer case. The centre diff is now 'loose' or out of specification because of worn thrust washers and slightly worn pins and gear centre holes. The gears not longer hold at an unflinching 90 degrees to each other. There may be varying degrees of friction on each gear, depending on if you are now missing any thrust washers.
Add to this the second problem. The two separate pins that hold the gears in place. The pins have to cross each other but remain on the same plane. To do this the pins are machined in the middle by half their shaft diameter ie only half their strength in the centre. The pins also have to hold the gears ridgedly at 90 degrees to each other, but they don't. There is a little bit of give. That is why Ashcroft sell a one piece replacement for the two separate LRover pins.
My feeling is that under stress the brass washers do not fuse under heat and jam the diff. They simply fall apart under pressure and rapid rotation, even when new, but particularly if already very worn. All this play then adds to the movement of the cross pins which then break in their centres where they have been machined to half their diameter. Locking the diff gets around the slackness or play.
On tarmac, I think you could drive all day with the diff locked. But you would have tyre wear and lots of vehicle vibration as the axles relieved them selves by forcing the tyre to lose traction by skidding. I drove an old Toyota like that after my friend took it for a test drive but did not disengage 4x4. Yes, you might break some other part of the drive train, but it would be along time coming under these conditions. So brief moments on hard pack between lots of soft stuff won't hurt.
On good gravel roads. I think not going to hurt if locked or unlocked. Differentiation unlocked will generally be within the units design range. Depends on whether you like the understeer.
On high speed corrugations, same as above. If locked, understeer probably not so obvious. On corrugations all wheels are momentarily losing traction to relieve any wind up, and wind up is unlikely as the time interval between crests is not sufficient to allow appreciable increase in rotation of one wheel against all others.
Off road, or generally good gravel but with say lots sand drifts or patches of mud, I would drive locked.
Again, Ashcroft make the solid pin centre, but you'll still have to maintain your thrust washers. They also make the auto torque biasing diff that can take some power away from the wheel losing traction - is good in changing road conditions without the need to be constantly locking and unlocking - but that can be locked in the really difficult stuff. Locking distributes power 50:50, but is also there to protect the Rover diff itself.
Wizard
SupporterIf my first reply was not long enough already...
On a slight tangent. Most of our vehicles rack up their mileage on good roads, high range, unlocked. The gears/dog clutch arrangements in these circumstances cop all the wear.
So your vehicle may well be able to climb that steep hill in high range. But if you could see the difference in wear between high and low, locked and unlocked components you would be quickly switching to the basically unworn configeration. Hope that makes sense.
Moderator
Also removes the thrust washers IIRCOriginally Posted by workingonit
The Phantom - Oslo Blue 2001 Td5 SE.
Half dead but will live again!
Nina - Chawton White 2003 Td5 S
Slowly being improved
Even driving around locked has reasons why it's hard on those little bronze thrust washers on the planetary gears. When the locking dog engages it locks the front output shaft to the diff hemisphere and for any drive to go to the rear, when the front doesn't have grip the transfer gears turn the diff hemisphere until the locking dog takes up the small amount of slack it needs to operate, then the hemisphere keeps turning until the side gears push the planetary gears out into the extremity of the carrier hard into those little bronze washers. This has the tendancy to hammer those poor little guys flat and split them around the edges. As said though they are sacrificial and cheap.
OldBushie
They are cheap to buy but not cheap to replace. Pat
Hate to be paying for someone professional to do it. Took me a couple days at home, by the time the floor and tunnell come out. Though it was out for a rear main seal and clutch I took the opportunity while it was out. As well as tidying up some surface rust where the floor panels overlap.
Wizard
Supporter2stroke, by the time you do it four times (four strokes!?) it becomes a breeze...well, actually, not really. Still a pain.
Was thinking on your description of the locked diff and washer stress. Never considered that effect. Would guess the compression of the washer would follow on from the wear of the dog clutch. Washer takes on stress and compresses until dog clutch faces engage - compression on washer stabilises with some thinning of the washer - washer thickness and load bearing remains the same as it now matches play in dog - then the dog clutch wears a bit - so the washer comes under renewed excess pressure again until it thins enough to match play in the dogs - an so on. But must say the dogs are very tough. My previous comment ties in here now I'm thinking about this? As I indicated above, most wear on the dogs is in the high range unlocked configeration - cause that's how most of us travel. Therefore hard wheeling in high and unlocked, with worn dogs, may put the most pressure on the washers. Changing driving habits so you tackle more things in low range and locked shifts you to virtually unworn dogs and less pressure on washers. Just postulating. Are you going to throw up some other interesting things to think about?You can tell my wife's watching 'the block' and I've nothing better to do.
Normally I drop the whole gearbox and t/c, to do all seals, and service the centre diff - although lately I just replaced the diff with an atb.
However, I once just pulled the diff with casing off the t/c by freeing the wires and levers, leaving the rest of the t/c and gearbox in situ. I was having trouble with a detent bearing. I'd do it this way if just wanting to open the centre diff for work.
Yes, expensive to pay someone else to do it, whether replacing the washers or the whole diff. Doing it yourself is cost effective.
Bye...'survivor' has just started - means I make coffee.
Must admit I was bored... I've only done 2 on an old 2 door Rangie and one on the Deefer. As for a point to all my posting, well I just think common sense is better than blanket statements and they are plenty strong enough if used in combination with that common sense.
I guess you'd be happy with the ATB then? The early LT95s had a regular clutch type LSD in them (was shown one once) but I'm told they all ended up wearing out when people didn't lock them when required.
Wizard
SupporterI'll be pulling my 5th centre in the next few weeks and replacing it with another atb. Basically you don't know its there. Seen this?
Review: Ashcroft LT230 ATB centre diff
Wizard
Obviously everybody's welcome to his or her own opinion on when to lock or not lock the CDL...
Land Rover's official line at the LR Experience training centers (unfortunately not available in Australia) is that the CDL on a Defender is locked the moment there is any chance of loosing traction - that effectively includes just about all surfaces other than black top...
There is a reason the vehicle has high range diff lock and I've done more than 300,000 km in a 300Tdi 90 following that principle, so its safe to say that it certainly doesn't do the car any harm to lock up the moment you leave the black top.
60,000 on my Puma and not a trace of abnormal metal content in the transfer box or gearbox oil at the last service (both original and 48 month health check)...
Cheers,
Lou
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