I don't understand synchros/lube.

[scout]

When a heavier lube is used (within reason), and the shifting effort increases, what is it that is being felt?
Is it metal against metal?
Is it oil layers being squeezed or sheared? (That's what I guess.)

Suppose the oil film remains intact throughout. Does this extra effort imply more wear, or maybe *less*?
(My guess is that there is more wear on the shifting parts, and less on the synchros and gears, adding up to less total.)

Sometimes, after pushing through this resistance, a metallic tooth-on-tooth grind is felt just prior to gear engagement. Are those syncho parts, or the gears themselves grinding?
(I guess that its the gears themselves.)
I wonder if that shows that the synchronizer did not do its job, and whether or not a lube weight change could help.
(I guess that thicker lube would help.)

I guess that the force exerted by the synchronizer is via, not sliding metal, but shearing lube, and that more or thicker lube gives the sychro more time to do its job before the gears touch. Therefore, at a given temperature, the lube should be heavy enough to do that, at the possible cost of greater shifting effort or time.
An alternative would be a "friction modifier" that *increases* friction. Is that sometimes done? It doesn't sound optimum to me.

With regard to the lube formulation, it appears that someone interested in long-term wear, and uninterested in quick shifting, could take a different mix. This would be analogous to the wear vs. gas mileage tradeoff with engine oil.

[ January 17, 2005, 08:54 PM: Message edited by: scout ]

 

[Gary Allan]

The gears are in constant mesh. The synchros lock the freely spinning gears to the shafts via forks.

I can't quite integrate the rest of your post (someone else may be able to) ..but I'll say that too little friction will increase effort. Whether or not too little friction is caused by a heavier lube or not ...I dunno

I'm sure someone will say that friction is indifferent to viscosity (although it may tend to follow it).

 

[Kestas]

A manual transmission is designed such that when you shift, the next gear first engages the synchronizer to the new gear. The first contact is two sliding metal conical surfaces to bring the clutch rpm up to speed with the new gear. Once this speed is matched, only then can the shift continue with a positive mechanical engagement on through the second set of synchros that are machined into the gear body.

If the lube is "too good" as one would expect with heavier oils, the two conical surfaces will continue to keep slipping. You'll either have to dwell a bit longer before finishing the shift, or you'll get gear clash (grinding).

Lubing a manual transmission is as balance of maintaining good lubricating properties of the transmission components and allowing enough friction for the synchronizers to do their job. That's why you'll often see ATF specified for manual transmissions. ATF has the balance engineers are looking for in their manual transmissions. Don't ever try to reengineer the lube requirements of a manual transmission. That includes staying away from synthetic lubes unless they are specified.

On a side note, after studying manual transmissions (I was once a summer student engineer and Ford Manual Transmission), I learned how to drive a manual transmission so it would last a long time. This included applying a moderate force on the stick during shifts to allow the synchronizers to momentarily do their job before complete engagement. I also down shifted to first just before coming to a stop, not after. One car I owned still had the original clutch and the transmission worked fine after 165K. I'm sure it lasted for a good bit longer for the second owner. I cringe whenever I see people slamming through the gears. This is downright abuse. Maybe they get a good feeling from it, but they'll have to pay for that feeling with an eventual transmission rebuild.

 

[scout]

Reply to Gary Allan:
I've seen inside a manual transmission, and even reassembled a Super T-10 from a pile of parts, and it worked. But I don't know much else.

When shifting, the shifter moves out of one gear, through a long free space, then hits a soft resistance. Once this is pushed through, there is often a tiny free space, then metal. It either slips through the metal, into the gear, or it grinds on it a little before slipping through into gear.
That "soft resistance" appears to be the one that increases when thicker gear lube is used. I posted because I'm curious about what this increased resistance tells me, if anything, about how well the parts are working.
My question might be nonsense.
Thanks!

Here is the question, abbreviated: Suppose my transmission grinds a tiny bit. What change in the lube, if any, might help that?

 

[Kestas]

That "soft resistance" you mention is where you get metal-to-metal contact. It's two conical metal surfaces spinning at different speeds. The heavy lubricant is too slippery for the two surfaces to come to the same speed. Because of this, you're getting gear clash. Don't use thicker lube, use a lighter lube like ATF. The gear clash should go away.

 

[scout]

Reply to Kestas:
Your post answers my question. It is the squeezing of lube between the two sliding conical surfaces that I'm feeling. And the torque that changes the speed of the clutch is applied through this lube film (like a fluid drive). While in this "zone", the ideal is to wait until that speed matching is done. (Coordinating vehicle speed, engine speed, and next gear.)

This is what I learned: The lube film and conical surfaces form a (momentary) fluid drive that runs the speed of the input shaft up or down as needed. Right?

quote:

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Originally posted by Kestas:
On a side note, after studying manual transmissions (I was once a summer student engineer and Ford Manual Transmission), I learned how to drive a manual transmission so it would last a long time. This included applying a moderate force on the stick during shifts to allow the synchronizers to momentarily do their job before complete engagement. I also down shifted to first just before coming to a stop, not after. One car I owned still had the original clutch and the transmission worked fine after 165K. I'm sure it lasted for a good bit longer for the second owner.

---

I like understanding how I can operate things that way. Thanks.

 

[scout]

quote:

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Originally posted by Kestas:
That "soft resistance" you mention is where you get metal-to-metal contact. It's two conical metal surfaces spinning at different speeds. The heavy lubricant is too slippery for the two surfaces to come to the same speed. Because of this, you're getting gear clash. Don't use thicker lube, use a lighter lube like ATF. The gear clash should go away.

---

My gear clash was hypothetical.
I intend to contribute to the people running this forum because of the many threads like this one. Of course, they should give you a cut.

 

[MolaKule]

There are two major points to consider for manual transmission gear lubes:

1. The proper viscosity of the lubricant to protect bearings and gear teeth;

2. The proper viscosity modifers so synchronizers can "shear" the FM for proper engagement.

I don't recommend ATF's in MT's to cure shifting problems; ATF's have minimal AW protection and NO EP protection whatsoever.

 

[mechtech]

That's some of the reasons why I just ordered MTL P from specialtyformulations.com.

 

[scout]

quote:

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Originally posted by scout:
Reply to Kestas:
The lube film and conical surfaces momentarily form a fluid drive that runs the speed of the input shaft up or down as needed.

---

(continuing) But this fluid is not pressure fed, or contained, like that in a torque converter. Here the fluid quickly slips out. Right?
So a thicker lube probably prolongs this phase of operation. This is what people are describing as harder shifting.
Sometimes in an old transmission, the "soft resistance" feeling is missing, and some gear clash occurs. A more viscous lube is justified, because the clearances in this fluid drive have increased. Yes?

Alright, I'm homing in here.
Suppose the lube is appropriate to lubricate the transmission parts. Along with that, it should be viscous enough to stick around in the fluid drive long enough, and should have some "friction modifier" to help it do the work, if needed. Since viscosity can only go so high, any additional capability here must be provided by the friction modifier. So, all the properties of these oils and lubes being compromises, what is the downside to the friction modifier?

 

[kevm14]

quote:

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Originally posted by MolaKule:
I don't recommend ATF's in MT's to cure shifting problems; ATF's have minimal AW protection and NO EP protection whatsoever.

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The T56 in my Camaro calls for an ATF, specifically Dexron IIE I think. I put some Motorcraft Mercon V in there and it shifts decently.

 

[scout]

Mine calls for "GL-4 or GL-5". I'll be obeying that. It says 75W-90 "recommended", and I'm looking for a rationale to use something heavier.

Toyota recently increased their recommendation for my differential from -90 to -140. That is good enough evidence to me that someone who had already been using -140 made a sound, safe decision. A similar decision might be appropriate for the transmission. My priority: long term wear, towing in mountains.

 

[Kestas]

Except for limited slip designs, the differential doesn't have parts like synchronizers that need "grip" action from the lubes. You can go with heavier lubes in differentials.... the only drawback would be more drag in the lube resulting is lower fuel economy.

 

[MolaKule]

Scout, all great questions.

quote:

---

Alright, I'm homing in here.
Suppose the lube is appropriate to lubricate the transmission parts. Along with that, it should be viscous enough to stick around in the fluid drive long enough, and should have some "friction modifier" to help it do the work, if needed.

Since viscosity can only go so high, any additional capability here must be provided by the friction modifier. So, all the properties of these oils and lubes being compromises, what is the downside to the friction modifier?

---

I am assuming you are referring to Manual Transmissions since your title is synchros/lubes.

What you want is minimal viscosity to get the job done, but enough viscosiy to quiet down the gears and to provide a sufficient hydrodynamic film for the bearings. Shifter forks like the thinner viscosity while bearings prefer a slightly thicker viscosity.

The viscosity needs to be thick enough to resist thinning in hot weather but not be too thick in cold weather; it's a balancing act for sure and one that comes about by formulator experience and testing.

You can reduce the viscosity of the base fluid somewhat by gving the oil sufficient EP and AW additives to take care of cold starts and high pressure (contact) areas such as gear teeth or shafts.

I think you may be misunderstanding the role of the friction modifier in a manual transmission. The role of the FM is to assist synchro engagement/disengagement. You want a fluid to provide just the right amount of the Dynamic Coefficient of Friction so you have smooth lockup and unlock.

The only downside to the FM in a manual transmission is that it wears out just like the other additives, which is why I recommend changing it out every 30,000 miles.

[ January 18, 2005, 02:45 PM: Message edited by: MolaKule ]

 

[scout]

quote:

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Originally posted by MolaKule:
Scout, all great questions.

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Thanks.

(Here's a tip for you: I also studied and tested waxes, and Meguiar's NXT won.)
Gotta go now.

 

[Lugosi]

quote:

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Originally posted by Kestas:

On a side note, after studying manual transmissions (I was once a summer student engineer and Ford Manual Transmission), I learned how to drive a manual transmission so it would last a long time.

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Well this sounds like some great information. How's about a "Tips and Tricks" writeup in the Mech/maint. section?