Are these "shadow" lines on 5th gear bore?

[clarkd]

Please see this thread for reference:

915 transmission won't go into first (1st) gear



Is this a good candidate for honing if these lines would easily disappear with no more than a 1/4 mil ID increase?

[Geary]

IMO, this wear is indicative of an overheated top gear/transaxle. Extended driving in top gear (in which the idler gear, needle bearing, and mainshaft are stationary in relation to one another) can lead to gross heat build-up in the idler gear.

The primary path for heat transfer (from gear to shaft) is via the tiny needles which at some point begin to fuse .. first to the gear bore, and then to the shaft.

I would not reuse that particular gear, and I would also consider adding an oil cooler.

[Steve@Rennsport]

Originally Posted by Geary

IMO, this wear is indicative of an overheated top gear/transaxle. Extended driving in top gear (in which the idler gear, needle bearing, and mainshaft are stationary in relation to one another) can lead to gross heat build-up in the idler gear.

The primary path for heat transfer (from gear to shaft) is via the tiny needles which at some point begin to fuse .. first to the gear bore, and then to the shaft.

I would not reuse that particular gear, and I would also consider adding an oil cooler.

Spot on!!!!!!

Thanks Paul!

[Peter Zimmermann]

That gear looks pretty bad. My test is if I can drag a fingernail across the lines, and I can feel them, the gear is scrap metal. My repair estimate always includes the possibility of a bad 5th gear, on rare occasion I find one that's good enough to use again.

[Steve@Rennsport]

We see an increasing number of 915 5th gears looking just like this nowadays due to extended low RPM operation where there is high loading and insufficient splash lubrication.

[A horse with no name]

Originally Posted by Steve@Rennsport

We see an increasing number of 915 5th gears looking just like this nowadays due to extended low RPM operation where there is high loading and insufficient splash lubrication.

I was about to ask 'why' running in 5th would do this...Good to know the reason.
Would you please provide an example of what an 'extended low operation' would be?..Thanks!


PS - We have no issues with our 915...It continues to shift with ease.

[Matt Monson]

Originally Posted by Steve@Rennsport

We see an increasing number of 915 5th gears looking just like this nowadays due to extended low RPM operation where there is high loading and insufficient splash lubrication.

I think another additional reason for this is the increase of interstate cruising speeds over the last 10-15 years. With 75 now very common and some areas even 80mph, everyone is spinning them faster when they are actually in the gear and it is under load.

[Steve@Rennsport]

Originally Posted by matt monson

i think another additional reason for this is the increase of interstate cruising speeds over the last 10-15 years. With 75 now very common and some areas even 80mph, everyone is spinning them faster when they are actually in the gear and it is under load.

Amen!!!!

[A horse with no name]

Originally Posted by Matt Monson

I think another additional reason for this is the increase of interstate cruising speeds over the last 10-15 years. With 75 now very common and some areas even 80mph, everyone is spinning them faster when they are actually in the gear and it is under load.

Thanks Matt for the addition info - Appreciated!

[Geary]

I have to correct myself, and go with Steve's assertion that this is the result of continual low rpm operation, NOT severe overheating. Clark could provide some clues based on how he drives ..

[KTL]

Since this is the loose/free-spinning 5th gear, it's placed on the pinion shaft. Does the pinion shaft show these same witness marks on it?

Seems strange that 5th gear would suffer from this, considering it's placement down low in the gear housing where the oil is. So that begs the question from me, does 5th gear suffer from this due to the gear oil migrating to the back/bellhousing end of the trans resulting from the car traveling down the road?

[Geary]

I now don't believe this particular wear would be avoided, even with an oil stream directed onto 5th gear.

When an OP comes on for opinions & advice, I feel they owe the forum further information when it's asked. Merely sharing what driving style contributed to this wear (high speed highway vs low speed lugging) would have been huge for our collective knowledge. JMO

[KTL]

Here's another question (instead of answers, sorry) for consideration. I found in my racecar's transmission that the needle bearings had been "pre-greased" with actual grease.

915 gears


Would it be worthwhile to consider doing this on 5th's needle bearings since it's known to be hard to lubricate with the oil in the trans? Just curious since I didn't find any evidence in my racecar's trans that the pre-greasing did any harm and neither was free 5th gear showing any signs of needle shadows

[Geary]

In off-road racing with ultra-low gear ratios, we were forced to use two rows of tiny loose needles held in place with grease, for installation purposes. If this hadn't been necessary, I probably would have avoided doing so (just because it impedes oil flow until the grease washes out). However, I really appreciate the info you just provided (that it didn't seem to cause an issue).

Unless you lug your engine, I (now) wouldn't expect shadow lines to appear.

[oldie44]

Look in the internet under fretting and you will see what is happening. Many years ago in college, I wrote a paper on fretting and in researching it found a reference to automobile wheel bearing failures. Back in the 1930s, new cars were shipped by rail and wheel bearings would fail due to the rocking motion. Solution to the problem was better grease.
I have seen this sort of failure several times on 356 transmission 4th gears and think that part of the problem was lubrication. Oil technology was not the best back then.
I once repaired a couple of gear pairs by having the gears honed to remove the fretting and then making oversize inner rings to reestablish running clearance. Worked but really was not worth the effort.

[Geary]

This probably wouldn't happen with straight-cut gears, but the helical cut creates a slight cocking of the gear, as tooth pair contact moves from one side to the other. If bore clearance is excessive, the gear really cocks.

Better grease .. hmm. Perhaps a gun drilled shaft (oil fed from shaft through the needle bearing) really is the solution. I was initially thinking otherwise ..

[KTL]

Originally Posted by Geary

In off-road racing with ultra-low gear ratios, we were forced to use two rows of tiny loose needles held in place with grease, for installation purposes. If this hadn't been necessary, I probably would have avoided doing so (just because it impedes oil flow until the grease washes out). However, I really appreciate the info you just provided (that it didn't seem to cause an issue).

Thanks back at you for sharing your use of grease in the off-road needles. When I first saw the grease, I too was concerned that using it would have prevented the flow of gear oil into the needles. But you can see from one of the pics that the grease migrated out pretty well, as evidenced by the greenish-tan colored pasted on the left area of the gear



Unfortunately I don't know what kind of grease was chosen. So if I was going to do this? I would choose a wheel bearing EP grease and apply it as sparingly as possible to make sure oil can get in the needle bearings.

My data point should be qualified that it was in a racecar trans. So probably not much lugging going on in that situation. When 5th is used, it's usually used with a lot of throttle. Otherwise it's pretty much never used in a cruising style.