Weak manual transmission, how to strengthen?

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ptuomov
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Weak manual transmission, how to strengthen?

Post by ptuomov » Sat Aug 25, 2018 6:10 pm

I've got a weak manual transmission (more precisely, five-speed transaxle). It's old, about 30 years or so. Spare parts are insanely expensive, when available. What can be done to make it handle more torque? The center-to-center distance between the main and lay shafts should stay constant, and I want to keep it five speed. What to do?
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU

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Re: Weak manual transmission, how to strengthen?

Post by PackardV8 » Sat Aug 25, 2018 6:18 pm

Describe the failure modes and that will point toward what might be possible.
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Re: Weak manual transmission, how to strengthen?

Post by ptuomov » Sat Aug 25, 2018 6:23 pm

PackardV8 wrote:
Sat Aug 25, 2018 6:18 pm
Describe the failure modes and that will point toward what might be possible.
Gears come out the drain plug hole in small pieces.
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU

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Re: Weak manual transmission, how to strengthen?

Post by bigblockmopar » Sat Aug 25, 2018 6:33 pm

Can you weld shut the drain plug to prevent that? :wink:

Are you sure this isn't a "design-fault" that the trans can't handle more torque?
Can you beef-up the bearings saddles in someway?
Use custom gears (with maybe a changed pitch angle)?

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Re: Weak manual transmission, how to strengthen?

Post by ptuomov » Sat Aug 25, 2018 6:59 pm

bigblockmopar wrote:
Sat Aug 25, 2018 6:33 pm
Can you weld shut the drain plug to prevent that? :wink:

Are you sure this isn't a "design-fault" that the trans can't handle more torque?
Can you beef-up the bearings saddles in someway?
Use custom gears (with maybe a changed pitch angle)?
The transmission was poorly designed as the idiot designer didn’t anticipate the inevitable situation in which someone decides to put 2.5x the stock peak torque thru the transmission a quarter century later.

Bearing saddles as in the bores in the aluminum case that accept the shafts? One might be able to bore some of those larger and interference fit in steel sleeves/inserts and then bore those exactly at the right location and “square” such that the shafts are exactly parallel and the exactly right distance from each other.

Cooling the aluminum case would help against extreme thermal expansion and shafts pulling apart, but the latest failure was with the transmission at about ideal temperature.

Why would changing the pitch angle of the gears make a big difference? An honest question, I don’t know much about gears.
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU

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Re: Weak manual transmission, how to strengthen?

Post by gunt » Sat Aug 25, 2018 7:15 pm

polish and chryo everything

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Re: Weak manual transmission, how to strengthen?

Post by ptuomov » Sat Aug 25, 2018 7:22 pm

gunt wrote:
Sat Aug 25, 2018 7:15 pm
polish and chryo everything
I haven’t seen much credible evidence in favor of cryogenic treatments, but, hey, any port in a storm! ;-)

Micropolishing or superfinishing or whatnot the gears makes sense.

The next set of weak factory gears (probably about $4000 per set) will get systematically checked for cracks before anything else is done.

I think but don’t know that the problem is not so much shafts pushing towards the end bearings, it’s more the basic problem if torque and the gears pushing the shafts away from each other. So I don’t think reducing the gear helix angle will really help that much?
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU

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Re: Weak manual transmission, how to strengthen?

Post by ptuomov » Sat Aug 25, 2018 8:39 pm

Here are example failures from one transmission:
DSCF5088.JPG
DSCF5086.JPG
DSCF5085.JPG
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Re: Weak manual transmission, how to strengthen?

Post by ptuomov » Sat Aug 25, 2018 8:50 pm

Here's another one that hasn't been opened yet, only the drain plug is off:

http://speedtalk.com/forum/viewtopic.ph ... 45#p761410
Image
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU

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Re: Weak manual transmission, how to strengthen?

Post by user-23911 » Sun Aug 26, 2018 2:04 am

Does it use tapered roller bearings?
When they get worn, they need re shimming otherwise the gears don't mesh properly.
Then they chip bits off.

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Re: Weak manual transmission, how to strengthen?

Post by ptuomov » Sun Aug 26, 2018 7:06 am

joe 90 wrote:
Sun Aug 26, 2018 2:04 am
Does it use tapered roller bearings?
When they get worn, they need re shimming otherwise the gears don't mesh properly.
Then they chip bits off.
I know next to nothing about transmissions so my description below may have non-sensical parts.

The main shaft assembly is the input. That input has a large ball bearing relative to the case. The other end of the main shaft is the pinion assembly that gets bolted to the case. Inside the pinion assembly, there appears to be two tapered roller bearings that face each other. The lay/countershaft has needle bearings (or maybe they should be called non-tapered roller bearings, I don't know) and a thrust washer.
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU

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Re: Weak manual transmission, how to strengthen?

Post by bigblockmopar » Sun Aug 26, 2018 7:25 am

A different gear pitch could load the gears differently which may make them handle the load better, but I was thinking the transmission was something 'simple' so custom gears don't look too feasible.

Could the gears be heat treated too hard (for the current purpose)?
Would a softer heat-treatment make them hold up better and prevent chipping/breaking, eventhough they would wear quicker?

Just thinking along here. My metallurgy knowledge is limited to hobbying in my garage and some "Forged in Fire"...

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Re: Weak manual transmission, how to strengthen?

Post by ptuomov » Sun Aug 26, 2018 9:10 am

bigblockmopar wrote:
Sun Aug 26, 2018 7:25 am
A different gear pitch could load the gears differently which may make them handle the load better, but I was thinking the transmission was something 'simple' so custom gears don't look too feasible.

Could the gears be heat treated too hard (for the current purpose)?
Would a softer heat-treatment make them hold up better and prevent chipping/breaking, eventhough they would wear quicker?

Just thinking along here. My metallurgy knowledge is limited to hobbying in my garage and some "Forged in Fire"...
Sometimes it seems to take the teeth right out the root when you apply torque to it. I don’t know if that’s because the shafts pull apart or because the gears are just weak or if gears come out of the factory with cracks.
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU

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Re: Weak manual transmission, how to strengthen?

Post by ptuomov » Sun Aug 26, 2018 11:43 am

Leftcoaster wrote:
Sun Aug 26, 2018 10:47 am
ptuomov wrote:
Sun Aug 26, 2018 10:31 am

This one went out at only 500 lbf-ft at the rear axle and 5000 rpm with transmission near ideal operating condition. So it was likely cumulative stresses and damage.
Would this application benefit from cryogenic or meta-lax treatment?
In my opinion, there isn't much credible evidence proving that cryogenic treatments work. However, it seems unlikely that it'll hurt, either, as long as the subsequent heating isn't done at a too high or a too low of a temperature. Often, gears are "superfinished" with REM micropolishing treatment, and that's less controversial in terms of its benefits.

I don't like free hydrogen in the gears... Do you guys bake the gears to get hydrogen out? How long and at what temperature? 24 hours at 375F?

There's also another question which is the right temperature for the manual gearbox oil.

Water in the oil will cause hydrogen embrittlement or rather hydrogen assisted cracking in the case of steel. To get rid of any water in oil, the transmission oil needs to be above minimum temperature and the transmissions needs an effective breather. The temperature of the tranny oil increases by about +100F when it runs thru the gears, so the minimum safe temperature for transmission oil is somewhere between 110-160F in the sump. From those temperatures, the oil between gears heats enough to boil out any water. The water vapor then needs to be vented out thru a vent in the gearbox that is shielded from oil spray

Does anyone actually positively vent the gearbox by moving clean dry air into the gearbox and hot water vapor out?

On the other hand, various manufacturers seem to say that the gearbox oil in the sump should never go over 250F. If you say that the gears then heat that by about +100F, that would mean 350F for the oil. That would lead to breakdown of most oils, no? So I'd say that maximum allowed temp should be about 230F in the sump.

Based on this logic, one should look for 160-230F temperature for transmission oil in the sump. Do you agree with this? And if I am using an external oil cooler with a thermostat, the usual thermostats seem to turn the cooler on at 180F. That seems a little low if the heat exchanger has big capacity? Any thoughts on that?
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU

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Re: Weak manual transmission, how to strengthen?

Post by Kevin Johnson » Sun Aug 26, 2018 12:06 pm

https://www.researchgate.net/publicatio ... rMoV_Steel

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I have to laugh when software deletes abbreviations of analysis with [Blank Post]. I fear for the survival of our species.

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