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Old 03-17-2017, 02:00 PM   #1
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torque convertor gear ratios?

I was playing with some numbers, trying to see what it would take to satisfy the conditions someone reported that Allison placed on unlocking the 6th gear. (Looks doable).

But in searching out the ratios for gears 1-6 in a 3060 I came across a listing of the available torque convertors that would fit, and their ratios. Torque convertor gear ratios? We don' need no steenkin' torque convertor gear ratios!

Really, I thought that torque convertors were there just to provide "slippage" like teasing a clutch as you take off from a stop. And, I thought that "lockup convertors" were desirable because they had the ability to stop slipping and pass the rotation straight through. Like, 1 rotation in, 1 rotation out.

Do TC's actually step the shaft rpm up or down on the way through? If so I would think that we would be talking about swapping one for another to get that last few MPH. A TC is a heck of a lot easier to in&out than a diff. Also why don't the gear/speed calculators ever ask for that spec?

Teach me, O ye elders of the Net!

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Old 03-17-2017, 04:27 PM   #2
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I'd expect the "ratio" of a torque converter to refer to its maximum torque multiplication ratio, not to a gear-like ratio between input and output RPM.

Yes a TC does step the shaft rotation speed down. I suppose one could be engineered to step the shaft speed up, but I don't know whether there's a useful application for that.. Anyway the term "slip" refers to the difference in input and output RPMs.

Slippage would be greatest at a stop and diminishes as the vehicle gains speed. Normally slippage should get down to just 100-200 RPM when the vehicle is at speed. As you said, a lockup clutch serves to reduce slippage to zero.

It might make sense to factor in the TC slip percentage as "gear ratio" in a case where there's a non-lockup type transmission/TC mated with a low speed engine: if an engine were limited to 2000 RPM and the TC was projected to slip 5% (100 RPM) at cruising speed, the top cruising speed would be 5% lower than what's calculated using the gear ratios alone. If target cruise speed were 60 MPH, 5% down is 3 MPH. I'd ignore it, but it might be meaningful to some.

A likely reason for having a range of converters that fit the 3060 is that this transmission finds itself mated to a variety of engines and propelling a variety of trucks. Different torque converters would probably be selected for a garbage truck doing frequent starts in hilly country vs a straight truck doing regional delivery service in flat country.
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Old 03-17-2017, 05:35 PM   #3
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Quote:
Originally Posted by family wagon View Post
I'd expect the "ratio" of a torque converter to refer to its maximum torque multiplication ratio, not to a gear-like ratio between input and output RPM.
How can a passive device multiply torque while holding RPM constant? Where would that energy/work/power come from? I'm still perplexed.

Quote:
Originally Posted by family wagon View Post
Yes a TC does step the shaft rotation speed down. I suppose one could be engineered to step the shaft speed up, but I don't know whether there's a useful application for that.. Anyway the term "slip" refers to the difference in input and output RPMs.
"Step it down" would refer to a ratio IMHO. Slippage is slippage; losses are losses. Were you using "step down" and "slip" interchangeably here? Again, just trying for clarity.

Digging around the Web a little more, I copied this information right out of the Allison website: (http://www.allisontransmission.com/d...k.pdf?sfvrsn=2)

TORQUE CONVERTER
Type: One stage, three element, polyphase.
Includes standard integral damper which is operational in lockup.
MODEL STALL TORQUE RATIO
TC-411 2.71
TC-413 2.44
TC-415 2.35
TC-417 2.20
TC-418 1.98
TC-419 2.02
TC-421 1.77

MECHANICAL RATIOS
(Gear ratios do not include torque converter multiplication)
Range
First 3.49:1
Second 1.86:1
Third 1.41:1
Fourth 1.00:1
Fifth 0.75:1
Sixth 0.65:1
Reverse -5.03:1

(apologies for the tabular data looking like s**t)

Now "STALL TORQUE RATIO" doesn't necessarily mean "rpm change ratio". I find the phrase "Gear ratios do not include torque converter multiplication" intriguing, and no help in clarifying this. I agree that "lockup" must mean 1:1, because the speeds I got out of an online calculator with the above ratios and some educated guesses for other values made sense with no fudge factor for the TC dialed in.

I'll keep poking around, but I don't think the idea of a "slip ratio" is the right one.
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Old 03-17-2017, 06:57 PM   #4
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Quote:
Originally Posted by dan-fox View Post
How can a passive device multiply torque while holding RPM constant? Where would that energy/work/power come from? I'm still perplexed.
It doesn't hold the RPM constant: it multiplies input torque by some amount greater than 1.0 and at the same time multiplies the input speed by some amount less than 1.0. Torque increases and rotation speed decreases.

Quote:
Originally Posted by dan-fox View Post
"Step it down" would refer to a ratio IMHO. Slippage is slippage; losses are losses. Were you using "step down" and "slip" interchangeably here? Again, just trying for clarity.
Yeah, to me the two terms mean effectively the same thing: that when the converter is not locked, the output speed is less than the input speed.

Maybe "slip" is distracting because it carries negative connotations: if one thinks of a clutch operating with its output slower than its input, that condition is called "slip" and it's a bad thing. Here in the context of a torque converter we have the same condition input vs output speed but now it's normal and desirable. We could just as well call it a "step down" or "multiplication (by less than 1.0, which many people would strongly prefer to call division)" instead.

Quote:
Originally Posted by dan-fox View Post
Now "STALL TORQUE RATIO" doesn't necessarily mean "rpm change ratio". I find the phrase "Gear ratios do not include torque converter multiplication" intriguing, and no help in clarifying this. I agree that "lockup" must mean 1:1, because the speeds I got out of an online calculator with the above ratios and some educated guesses for other values made sense with no fudge factor for the TC dialed in.
That's a handy document from Allison. Stall torque ratio is definitely not the same as "RPM change ratio." Stall torque ratio happens when the converter is "stalled." That's the condition in which the drive wheels are stationary because you're standing on the brake, while also standing on the throttle with the other foot to rev the engine up for a launch when the traffic light goes green (for example). The converter's output RPM is zero and its input RPM is whatever its stall speed is. Probably 2000+ RPM. Remember that one can apply torque to a bolt.. or a driveshaft.. without actually turning it. In this stall condition the TC-411 torque converter will multiply the engine's torque by 2.71, and its RPM/speed by 0. As conditions move away from stall, ie applied torque from the engine goes down and road speed goes up, the amount of torque multiplication decreases from 2.71 toward 1.0 and the RPM multiplication rises from 0 toward something on the order of 0.92 to 0.98. Those are the acceleration and coupling operational phases mentioned in the wikipedia article.

The doc mentions three shift sequences. Ours all seem to use option 2: 1C–[1L]–2C–2L–3L–4L–5L where C denotes converter mode and L denotes lockup mode. When the transmission is in 3L, 4L, or 5L the converter is locked and both the RPM ratio and torque ratio across the converter are 1.0. In positions 1C and 2C the converter is unlocked and in that case the RPM ratio across the converter, what they ambiguously call "torque converter multiplication," is less than 1.0. The note means to clarify that for example when shifted to 1C, the transmission output speed will be less than 3.49 times the input speed. I say the note is ambiguous because when we read "torque converter multiplication" we mentally make the leap to "torque converter torque multiplication." It's my opinion that they mean "torque converter speed multiplication (or division)" there.
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Old 03-17-2017, 10:50 PM   #5
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Familywagon pretty much nailed it. A few points.

Torque converters aren't passive devices. They do more the just take the place of the clutch that would be found in a manual trans.

A torque converter will never spin the input shaft faster then engine rpm. It will either match engine rpm and transmit the engine torque unchanged(minus slip, its negligible here), or it will transmit a slower rpm and multiply the torque delivered. This multiplication creates alot of heat though so they're rarely doing it.

The torque converter has a pump, stator and rotor. It uses the same principles that a portapower uses in hydraulic multiplication. You input a small force in a large movement and the psi created is multiplied to a larger force at the expense of less movement.

The different ratios are essentially the different pump/rotor angles inside the converter. They'll choose the ratio based off of application, engine torque, and vehicle weight.
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Old 03-19-2017, 07:57 PM   #6
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Thank you thank you thank you!

I decided that it wasn't manly of me to just throw myself on your collective mercy (effective as that turned out to be) so I spent Saturday evening soaking up the Internet on this topic. Specifically, Youtube. Lots of good stuff there, just like on any topic.

It appears that the torque ratio is a torque multiplication ratio, and it's more of an empirically derived SWAG than something that you can come up with an exact number for, like the ratio between the number of teeth on two adjacent gears. It's done with something called a stator, which is a set of turbine-looking vanes with a sprag clutch on the input(?) shaft. That changes the angle that the pumped fluid impinges on the turbine. The lock-up is a separate mechanism around the outside of the mechanism that is driven by an external source of fluid pressure coming in through the hollow xmsn shaft. In other words, something somewhere else makes a decision as to when to lock up.

One remaining question: is the operating fluid in the convertor transmission fluid? Does it circulate between the transmission and the convertor, or is the convertor a sealed unit?

Thanks!
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Old 03-19-2017, 08:37 PM   #7
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It's not so much of a swag as you call it. The ratio is something that is calculated. That's the max multiplication ratio that can be achieved. In operation, the torque applied to the input shaft depends on the current input and output RPM and the torque and load being applied. So the amount that is applied part is what's the guess.

Fluid is ATF, the same ATF used in the transmission. It's circulated by the pump into the converter and dumps back through the snout and into the pan. The converter is the major source of heat in the transmission, especially if it is in torque multiplication mode(below stall speed)
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