FSAE differential

[nohenriquefx]

I am designing a differential for my university's FSAE vehicle and have some doubts. First thing is that I got the CAD modeling of the torsen model that is sold to FSAE teams and I have verified that the large gears (attached to the semi-axles) are helical gears about 48 mm diameter with 15 teeth. I'm using MITcalc for the calculations and I can´t get even close to those dimensions (it indicates pitching failure). I am using an input torque of 900 Nm, which means a torque of 450 Nm at those gears. I am not understanding how can it be. I was thinking about maybe using a design similar to the wavetrac (http://www.wavetrac.net/technical.htm), that I know that have a large load capacity because is used in trucks and stuff. In fact, I found this post in the ford raptor forum (http://www.fordraptorforum.com/f5/my-ta ... ial-12856/) and you can see a picture of a guy holding one of the gears and, man, they're small!
So I'd like to ask what do you think about that and if anyne has any advice. Sorry about grammar mistakes, but I'm brasilian.

[ChrisDanger]

Your English is good. Most English people struggle with the language.

Amnyway, 900 Nm seems high. It might be more in the 100 Nm range. Do you have a reliable source for your figures?

[Pierce89]

I am designing a differential for my university's FSAE vehicle and have some doubts. First thing is that I got the CAD modeling of the torsen model that is sold to FSAE teams and I have verified that the large gears (attached to the semi-axles) are helical gears about 48 mm diameter with 15 teeth. I'm using MITcalc for the calculations and I can´t get even close to those dimensions (it indicates pitching failure). I am using an input torque of 900 Nm, which means a torque of 450 Nm at those gears. I am not understanding how can it be. I was thinking about maybe using a design similar to the wavetrac (http://www.wavetrac.net/technical.htm), that I know that have a large load capacity because is used in trucks and stuff. In fact, I found this post in the ford raptor forum (http://www.fordraptorforum.com/f5/my-ta ... ial-12856/) and you can see a picture of a guy holding one of the gears and, man, they're small!
So I'd like to ask what do you think about that and if anyne has any advice. Sorry about grammar mistakes, but I'm brasilian.

900 Nm is WAY too high for any FSAE vehicle I've seen. That's like an LMP1 diesel.

[spacer]

Why would that be a massively abnormal torque figure? Diff comes after both gearset + final drive reduction, multiplying shaft torque. I'd say 900Nm feels about right

[Tim.Wright]

Agree, I remember doing some diff calcs in my FSAE days and in 1st gear you are seeing numbers near 1000Nm.

I don't really understand your problem though. Are you concerned about the gears themself failing? Are you designing your own gears? or are you using the Torsen internals?

[autogyro]

Remember that the 'actual torque' seen by the gears in a lightweight racing car is going to be much lower than in a massive truck with unlimited traction.
It is why F1 powertrains are 'weedy' from end to the other, produce no more torque than an M3 BMW, seldom break geartrain components and yet still go fast.

Even with the turbo era F1 cars we were given 'torque figures' to design to that were way lower than one would expect.

Dont forget when the tyres light up there aint no usable torque, so the tyres set the figures.

[Pierce89]

Why would that be a massively abnormal torque figure? Diff comes after both gearset + final drive reduction, multiplying shaft torque. I'd say 900Nm feels about right

He said INPUT TORQUE

[tok-tokkie]

My only comment is that the torque is limited by the traction between tire & surface. The diff can't transmit more than that as wheelspin then takes place. Is there sufficient traction? Does wheel spin happen?

[autogyro]

My only comment is that the torque is limited by the traction between tire & surface. The diff can't transmit more than that as wheelspin then takes place. Is there sufficient traction? Does wheel spin happen?

It isnt just whether wheelspin will occur it also depends on the mass of the vehicle.
Try balancing a large truck on an F1 diff planet wheel.

[Tim.Wright]

Thats not entirely true. When the tyres break traction they do not limit the torque seen by the driveline. There is still excess torque applied to the diff, it is just spinning up the final drive assembly instead of accelerating the car. To paste something I wrote on another form;

When the tyre reaches its traction limit, the complete driveline will start to accelerate. Some torque will go into reacting the contact patch force, the rest will go into accelerating the rest of the driveline. The residual torque used to accelerate the driveline (e.g. 20% in a low friction condition) is not applied all parts of the driveline equally. The first component in line (e.e. clutch) will see all of the residual torque. The second component (eg gearbox) will see the residual torque minus the torque used to accelerate the first component. The third will see the residual torque minus the acceleration torque of the first and the second components etc until you reach the wheels.

The split of the torque depends on the split of the inertias in the system and the gear ratios. In this dynamic condition the diff will see more torque than the tyres can produce, but less than the full 900Nm the engine can provide statically.

[autogyro]

Thanks Tim, it is far more complex than simply using input torque from the engine (and other inputs) and doing a calculation based on a stationary final output as we both know.

I know this is selling my system again so I hope the mod will let it in.
If you use electro magnetic torque transfer between drivetrain components for shifts and energy harvesting and application the efficiency can be improved by a great deal and the trade off in components size load requirement and weight becomes even more favourable. For instance, you can harvest energy as you reach the programmed design limit of a component instead of breaking it.
The design requirements for old fashioned layshaft and diff geatbox drive line components are dictated by the basic math it is true. Shame realy.

We used to say that the final decision of gear size and strength was more a matter of practical experience than calculation.
Most gears, even in F1 diffs are over engineered by some margin.
It has been a continual fight between the car designer trying to shave off weight and size and the gearbox manufacturer trying to keep a reputation for a reliable gearbox. Many top gearbox designers bowed out of F1 years ago because of this.

When the tyres spin, there is a sudden reduction in torque seen by the diff gears. This increases as traction and vehicle acceleration stabalises. It is the tyre grip qualities and the road surface that dictates how much torque is then seen by the diff gears. It is not possible to 'not slip' either the clutch or the tyres from stationary to a speed in first gear that gives a decent rpm for the engine. Otherwise the engine would stall.
With an electromagnetic drive train system, either full electric motor torque can be applied from stationary with no need for slip (it is hell on old fashioned layshaft geartrains however, suddenly they have to work for a living), or if an ic engine is still needed for initial launch with all its faults (why?) the slip will then be harvested as energy for future use.

[mecharon]

I am designing a differential for my university's FSAE vehicle and have some doubts. First thing is that I got the CAD modeling of the torsen model that is sold to FSAE teams and I have verified that the large gears (attached to the semi-axles) are helical gears about 48 mm diameter with 15 teeth. I'm using MITcalc for the calculations and I can´t get even close to those dimensions (it indicates pitching failure). I am using an input torque of 900 Nm, which means a torque of 450 Nm at those gears. I am not understanding how can it be. I was thinking about maybe using a design similar to the wavetrac (http://www.wavetrac.net/technical.htm), that I know that have a large load capacity because is used in trucks and stuff. In fact, I found this post in the ford raptor forum (http://www.fordraptorforum.com/f5/my-ta ... ial-12856/) and you can see a picture of a guy holding one of the gears and, man, they're small!
So I'd like to ask what do you think about that and if anyne has any advice. Sorry about grammar mistakes, but I'm brasilian.

Wavetrack's diff is different than Mustang's in terms of construction, and gear profiles aren't same. I'm not sure, but they might be used a gear with different module and pressure angle, or completely different gear profile (direct gears, for example). They may also take advantage of construction to make gears with smaller diameter. Biggest stress occurs on gears at near tooth region, so they might be dimensioned them as small as possible. Mustang's diff is larger; it takes much more material to build it. Wavetrack may take advantage of smaller system by using higher grade material, or get them coated, which yields to be able to use smaller dimensions. My grammar is also bad...