Gear load: Paper limit and breaking load?

Hello,

When it comes to TDI tuning, there is often a focus on the durability of the transmissions, for example, a torque limit of 350 Nm for 6-speed transmissions.

What do these 350 Nm mean?

I can imagine that even with a standard standing start, even with a 75 hp 3-cylinder engine, significantly higher values would be achieved if the clutch is released "quickly" – and this is due to the stored kinetic energy resulting from pressing the accelerator pedal while the car is still stationary.

If you try to induce a burnout (by flooring the accelerator and letting the clutch slip around ~3000 rpm), the input torque to the transmission will likely be limited only by the clutch. As long as the clutch is overloaded, it will slip momentarily instead of following the pedal and fully disengaging.
(Based on my gut feeling, I would estimate the peak torque to be around 500 Nm or even higher.)

These are, for me, the maximum loads that a transmission must be able to withstand – it would be embarrassing if every new car scattered its transmission onto the road after just ~3 such aggressive or panic starts...

That the 350 Nm at 6th gear only represents the long-term sustainable value does not, in my opinion, contradict anything. In this case, the material will wear out faster (initially) compared to when the 350 Nm limit is observed.

Question: By how much higher than such paper limits is the torque at which spontaneous material failures must be expected?

Hi Ulf,

Think about the Gremlins scene with the broken engine part... so, shattering is possible.

I would rather think that it's about stresses that *can* leave damage, and a "caviar start" might certainly be one of those things if it's done frequently.

"Nothing is going to break immediately, because if it did, the Ibiza-ASZ faction from the Ibi-Forum would definitely have already destroyed some gearboxes, because I wouldn't put it past them to drive like that."

My last time doing something like that was with the AEX (60 horsepower) in a Polo... I wouldn't even consider doing it with the Ibiza, just to protect the tires and the drive shafts. He usually gets going properly on his own, so you don't need to resort to childish tactics.

Best regards,

Jan.

My thought is as follows:

I can subject the transmission to the maximum load test by using the most powerful engine possible (350 Nm), which, according to your description, means suddenly putting the drive onto the transmission at maximum torque. And doing this continuously! Then I can assume a manufacturer's tolerance of 20-30%, which means it should still handle 420 Nm without issues.
But?
Can I cause a gearbox failure with a 500Nm engine using a 'caviar start'? I highly doubt it. I believe that either the clutch slips beforehand, or the insufficient friction of the tires prevents the force necessary to cause a breakage from reaching the gears.
What kind of stress or strain would be placed on the system if, while driving downhill at 120 km/h in third gear with the clutch pedal depressed and a vehicle weight of approximately 2 tons, I suddenly release the clutch? That would have nothing to do with the power of the machine, but would still represent an abnormal load until the motor reaches its operating speed.

To do that, you would need to determine the moments of inertia, and then you could calculate the loads.
'Anyone with the corresponding input is hereby invited...'

CU Gremlin.

According to your description, at the maximum torque, the drive

ObenbeiMutti wrote:

suddenly engages with the transmission. And that's permanent!

In my opinion, that's not possible.

Applying sudden force to the transmission means that the clutch was disengaged beforehand.
Then the engine is running in a loaded idle state (similar to the AU test), but in that state, the engine's RPM increases faster than the turbocharger can keep up.

If at all, the boost pressure required to achieve the Nm maximum is only reached well above 1900 rpm.

I think that the total rotating mass on the engine side of the clutch can release such high forces during a sudden engagement (braking) at approximately 3000 rpm (depending on the clutch's grip) that the proportion of the "active" engine torque is almost irrelevant.
But this power surge can, in principle, not be generated continuously.

Approximate analogy:
Let's see a 150 horsepower TDI engine, with a measly 20 km/h (approximately 12 mph) speed, disengage from its transmission and roll into a wall without brakes.
Then, you place a similar, undamaged car against the wall and try to push the front of the car without any sudden movement in first gear, pressing it in as far as possible (at full throttle @ 1900 rpm in first gear). If you let the clutch slip or the wheels spin, you have no chance, even though the engine can accelerate the car to over 200 km/h.

Quote:

What kind of stress is actually put on the clutch if I, with the clutch depressed and a vehicle weight of approximately 2 tons, am driving downhill at 120 km/h in third gear and suddenly release the clutch? That would have nothing to do with the power of the machine, and would still represent an abnormal load, until the motor reaches its operating speed.

This is probably essentially the same thing as releasing the clutch quickly when starting, but "in reverse" .
"The 'natural Nm limit,' in my opinion, is likely also the maximum grip of the clutch in this case."

Hello Ulf!
Torque specifications for gearboxes in the automotive industry do not indicate either a breaking load or the maximum continuous torque.
Automotive components are typically designed for fatigue strength, meaning that... They must withstand a specific load collective for a certain period without component failure.
Regarding automotive transmissions, I imagine that each automotive and transmission manufacturer has its own specific approach when it comes to defining the load spectrum.
However, it is certain that a vehicle transmission will never reach its design lifespan (e.g., 2000 hours) when subjected to continuous stress from the specified "rated torque."
Furthermore, there are likely statistical data available on which gear ratios are used with what frequency, which means that differences between gear pairs in the design could also be considered based on these statistics. Simple example: Try to achieve the specified rated torque continuously in reverse.

Regards,
Alex.

donalexo wrote:

With a vehicle transmission, I can imagine that each vehicle/transmission manufacturer has its own specific approach regarding the definition of the load spectrum. It is certain, however, that a vehicle transmission will never reach its design lifespan (e.g., 2000 hours) under continuous stress from the specified "rated torque."

Oh my goodness - that means the specified input torques can be interpreted almost arbitrarily by outsiders...
As a "constant" load, it can handle even less, but surprisingly, nothing breaks immediately above that point.

So, the best thing to do is not to worry about it at all, but rather tune your TDI as much as you like (of course, everything within the legal limits ), and if the transmission fails...
. . . the tuning data of the engine, its driving profile, and the lifespan of the transmission under the tuning load are entered into a yet-to-exist experience database, so that future tuning customers can roughly estimate what they can expect when they add a certain amount of Nm and horsepower.

But what do you think about the Nm safety valve called "Kupplung"?

Some 74 kW PD models, even without any tuning, already put a strain on their clutch.
"It is reportedly possible to tune engines to produce well over 300 Nm of torque without the stock clutch slipping."
How can this be explained? Are sprag clutches with their "holding torque" (what is the technical term for this?) designed with such a wide range of holding torque values?

Are there any practical guidelines to determine how much Nm (or rather, what percentage) the holding torque of a clutch should exceed the torque of the engine?

In particular, some 74 kW PD models already put a strain on their clutch, even without any tuning.
'It is reportedly possible to tune engines to produce well over 300 Nm of torque without the stock clutch slipping.'
How can this be explained? Are sprag clutches with their 'holding torque' (what is the technical term for this?) designed with such a wide range of holding torque values?


It's enough if the installer has just left their greasy fingerprints on it...
or the simmering pot is sweating...

CU Gremlin.

Gremlin wrote:

ulf wrote: In particular, some 74 kW PD models already put too much strain on their clutch, even without any tuning. "It is reportedly possible to tune engines to produce well over 300 Nm of torque without the stock clutch slipping." How can this be explained? Are sprag clutches with their "holding torque" (what is the technical term for this?) designed with such a wide range of holding torque values? It's enough if the installer has just left their greasy fingerprints on it... or the simmering pot is sweating...

Follow-up question:

A leaking oil seal provides a constant supply of lubricant, which means the clutch should theoretically be prone to slipping.

With Glibberpratzen ( ), on the other hand, no additional lubricant is added, so the lubricant, in my opinion, should gradually evaporate along with the wear, and the clutch should eventually "grip" fully again.

Does this roughly correspond to actual practice?

Hi,
"Under full load, the first one would..." "They told us in college that it only takes about 10 hours to walk the entire thing..."

'By the way, my mother wrote the following:'
According to your description, suddenly engaging the drive at maximum torque and sending it to the transmission. And permanently!

In my opinion, that's not possible.


I'm sorry, I misphrased it.

My statement should be more in the direction of DonAlexos' 'Lastkollektiv.'
This means that the worst-case scenario is occurring. For example... B. Mirtl, a 21-year-old from Hinterdupfing, demands the maximum possible load from the vehicle at every conceivable traffic light.
In the long run, this will likely cause the bearings to wear out to the point where damage becomes unavoidable, but I think a sudden failure only occurs with severe material defects and errors. And since there's no specific manufacturer involved, the maximum load specifications are likely conservative and cautious.

In Hintedupfing, there's probably only one pedestrian traffic light, and recently, they added a roundabout.

So...
Since I once had to do a rather silly gearbox design (though it wasn't a multi-speed one), I'll chime in with my opinion now.
When designing a new gearbox, engineers typically start by making rough estimations based on experience. A predetermined load is specified, often statistically determined, that the gearbox must withstand. During the design process, various shafts and notches inevitably arise, which represent potential weak points and are checked based on a material fatigue life curve. Of course, this is initially done purely through calculations. Ultimately, the resulting gearbox, with the safety margin applied, is likely to survive the intended lifespan with a reasonably high probability when subjected to the specified load. In a standard, two-stage gearbox, there are so many assumptions and estimations involved that even the designer wouldn't be able to accurately predict how long it will last. This is especially true when considering different loads. Typically, the safety margin against breakage is higher than the safety margin against deformation (flow). In normal operating conditions, there is usually no deformation at all. When the material reaches its flow limit, it will yield slightly (very slightly), but then it will solidify, preventing any immediate breakage.
Of course, new transmissions are only truly tested in practice, and one also has a certain amount of experience that a student might lack. This makes the whole process more predictable, and allows one to better manage the weaknesses. But there are also material and manufacturing tolerances to consider.
I once read that VW stated a lifespan of 500,000 km for the Golf 4. They probably based that on a transmission survival rate of somewhere between 90% and 100%. However, increasing the load definitely reduces the probability of survival. In this case, the higher peak loads are much more damaging than an increased average load.

Conclusion:
If you treat your transmission properly, it should last a good while even with normal tuning.

Sure, here's the translation:

"PS:"
I'm currently studying for my materials science and component strength exam.

@ulf

You should try driving our Passat. When you shift gears quickly, nothing slips. It directly transfers the power to the transmission. It's a very aggressive clutch, possibly one that comes from racing. It's quite difficult to pedal and gets very aggressive in the last quarter of its range.

Best regards, WarLord.