Looking after an R380

First time doesn't count as that failure was due to spline wear due to lack of lube between output shaft and input gear to TC. But the second time I had my R380 rebuilt 5th on layshaft was worn, very dull on tooth surface of pinion (hope I have the terminology correct). But 5th was not the component that gave way. The input shaft to gbox the bit that drives the laygear and the driven gear on the laygears was where it all came to a noisy and hot end. Nowhere near as bad as another R380 that was in the gbox repair place where the owner of that one had continued to drive until the teeth melted, just a blacked area where the teeth once were.

I now use synthetic oil and am keen on getting a gbox oil cooler either that or an auto.

I spent many years work at the manufacturer of the largest gears in Australia. I left a long time ago, but at that time the largest gear made weighed over 200 tonne. Many of the gears were made for large mining equipment - grinding mills, draglines, bucket excavators etc. Some of the grinding mills I have worked on were driven with 4.5 MW motors (there are larger mills of around 7MW, but they have wrap-around motors, not gear drives).

The gear rating standard has been superseded since I last designed any gears, but the previous standard includes factors for gear precision. The new standard will still have factors for precision, but I am not familiar with it.

The gears rating (capacity in torque or power) is increased as the precision is increased.

There is a wide range of gear precision numbers, and the tolerances for profile errors and tooth to tooth pitch errors are incredibly small for the higher numbers and only achievable by finishing the gear on a gear grinding machine.

The gear cutting machines used can cut the gear teeth with camber to compensate for deflection of the pinion shaft between its support bearings, and can also with compensation for torsional twist across the width of the gear.

In the gear design process it is important to check for tooth deflection (bending from the root to the tip) and in some cases the profile has to be modified at the tip to compensate.

Gear tooth strength can be increased by increasing the gear width, within limits.

When the bearings wear in a gearbox, precision goes out the window and gear capacity is dramatically reduced.

But probably the greatest reduction in strength resulting from worn bearings, is due to the change in distribution of load across the width of the gear tooth.

This results in the load being transmitted through one side of the gear (equivalent to reducing the gear width) and leads to teeth breaking on that side.

The grinding mills that I mentioned above, use an array of sensors that continually measure the temperature across the width of the gear, close to the the teeth in mesh. These sensors are part of the control system and are detecting changes in load distribution due to deflection before damage can result.

One of the main things that Land Rover did when they upgraded the LT77 to the R380 was to increase the width of the teeth (made possible by moving reverse gear into the extension housing). I understand they also increased the width of some gears from the first R380 to current version.

In an earlier post in this thread, I explained how tooth load of 5th gear will be higher (for same vehicle load). It should be pointed out that due to the pressure angle of the gear tooth profile, the tooth load is not tangential (it is always directed along the line of the pressure angle). So for a particular tangential load for the required torque, there is a proportional radial (gear separation) force component. And with helical gears, a proportional axial load component.

These tooth forces (tangential, radial and axial components) are transferred to the bearings.

So if you have an increased load on the vehicle, whether due to gear in the back, trailer or hill, if you use use a lower gear than 5th, the gear tooth loads and resulting bearing loads are reduced.

The damage done to gears and bearings are cumulative over their life. High load events may not result in failure at the particular time, but some damage is incurred, affecting the life.

Bearings and gears are designed for a particular finite life. The cumulative damage of both are reflected in Miner's Rule, which is used to determine an equivalent design load (as the load spectrum gets complicated so do the calculations using Miner's Rule). Miner's Rule can tell us (as you would expect) that a high load over a short time does equivalent damage to a lower load over a longer time.

I think a little of my original statement has been missunderstood, I agree with the science on the gears themselves. No arguments there. My question was, "why do people believe that shifting down from 5 to 4 'at highway speeds' will prolong the life of your box?"
At highway speeds the engines torque is fairly low....well, below 380Nm.

I'll just end with, I've rebuilt a few R380's and a common problem that has raised it's head is that main, & layshaft tapered bearings had way too much endfloat, the bearings were showing signs of hammering and hardfacing chipping. One of the boxs was just having it's reverse gear syncro replaced, the other 2nd gear syncro. These box's were not old, one just over 10k. These are factory built items.
I subsaquently had to rebuild them both.
I think by setting all the clearance tolerances to the very min. and using synthetic oils, you should not have a problem with the box behind the Isuzu, just follow the golden rule of not towing in 5th.

Sorry to those that I confused

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Originally Posted by Larns

I think a little of my original statement has been missunderstood, I agree with the science on the gears themselves. No arguments there. My question was, "why do people believe that shifting down from 5 to 4 'at highway speeds' will prolong the life of your box?"

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In answer to your question... I believe it prolongs the life of the box because it prolongs the life of the box...:D

Quote:

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Originally Posted by Larns

I think a little of my original statement has been missunderstood, I agree with the science on the gears themselves. No arguments there. My question was, "why do people believe that shifting down from 5 to 4 'at highway speeds' will prolong the life of your box?"
At highway speeds the engines torque is fairly low....well, below 380Nm.

I'll just end with, I've rebuilt a few R380's and a common problem that has raised it's head is that main, & layshaft tapered bearings had way too much endfloat, the bearings were showing signs of hammering and hardfacing chipping. One of the boxs was just having it's reverse gear syncro replaced, the other 2nd gear syncro. These box's were not old, one just over 10k. These are factory built items.
I subsaquently had to rebuild them both.
I think by setting all the clearance tolerances to the very min. and using synthetic oils, you should not have a problem with the box behind the Isuzu, just follow the golden rule of not towing in 5th.

Sorry to those that I confused

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Its not so much that towing at highway speeds in 4th gear is the correct answer its more towing at XNm of Torque requires a downshift to 4th.

the problem comes about because of the thrust coming from the helical cut of the gears, the logic of engine torque being low for a highway speed in 5th is slightly off because it takes more torque at a lower number of engine RPMS to achieve the same amount of work as a lower torque at higher engine RPMS. (digression)A simple way of testing this is with a turbo diesel, get it wound up to a nice easy pace and try to acceleate away in 5th gear while you keep an eye on the boost gauge then do it in 4th. For the same amount of boost 4th gear accelerates faster or for the same level of acceleration 4th will do it with less boost (but more airflow) A better and more reliable way is to do the same test with an EGT gauge fitted.(/digression)

Its the thrust off of the gears that this torque creates that causes a problem for the box bearings. This is made worse when you stick a load on the back end and give it a bootfull in top cog.


From the roughnready Math Ive done off of the one failed dyno attempt I Had with Big Red and some seat of the pants work I always tell the missus, If your towing and the boost gauge is over 12PSI change out of 5th.

A good dose of common-sense combined with acquired mechanical sympathy pays dividends.

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Originally Posted by Bush65

A good dose of common-sense combined with acquired mechanical sympathy pays dividends.

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Thats exactly how we treat ours:D. Also it was refurbished if you like before it went in.

Justin

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Originally Posted by flagg

Hi Guys,

Don't sit on 110 in 5th for hours at a time, change down to 4th every once in a while to keep things lubricated, cool and happy.


Flagg.

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I believe what Larns is saying is that it won't hurt the box if you are just driving in fifth for prolonged periods.

As this was the original statement (not towing which come into later) that this thread was started with.

I agree with Larns that it would be ok to drive in fifth, towing is another story.

Quote:

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Originally Posted by Larns

I think a little of my original statement has been missunderstood, I agree with the science on the gears themselves. No arguments there. My question was, "why do people believe that shifting down from 5 to 4 'at highway speeds' will prolong the life of your box?"

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This stuff about changing down to 4th every now and again comes from a post in an old thread by Isuzurover regarding the LT85 gearbox. Flagg mentioned it in post #1 of this thread, but to be honest I think he's mixing it up with the LT85 advice.

I haven't dug up Isuzurover's original post on this, but I'm sure he'll be along shortly, however the advice was given to him by Mal Storey of Maxi-Drive fame who also use to re-build gearboxes. Now to be honest I think Mal's advice only applies to the split case version of the LT85 which doesn't have an oil pump, the solid case version (the stronger one fitted to factory Isuzu 110's) has an integral oil pump so shouldn't suffer from lube related problems.

EDIT - BTW & just to be clear, Mal's advice that Isuzurover passed on has nothing to do with labouring the engine / torque / towing or anything like that it was all to do with lubrication of 5th gear on the LT85.

Jon

Thanks guys, this thread is utterly fantastic.

:TakeABow: