Calculating Drive shafts

[Mi_Ko]

Hi!

I found 2 other threads concerning calculating a fsae car's halfshafts, but I didn't get an clear answer.

I think we have to calculate it on the engine tourqe*gear reduction*chain reduction/2
2 because the moment split on 2 wheels!
So i got arround 500Nm * safety

Or do you simply calculate on the maximum force, that can be transmitted to the ground? (What's happening if you press both pedals? How do you calculate peak torque, torque when accelerating, braking)

Or did someone measured halfshafts with strain gauges?

Why did pat's team broke their shafts designed for 650Nm torque and why don't someone else have problems with 450Nm strong shafts?

You see, there are many questions.

2002/03 University of MARIBOR - Team Member

[Mi_Ko]

Hi!

I found 2 other threads concerning calculating a fsae car's halfshafts, but I didn't get an clear answer.

I think we have to calculate it on the engine tourqe*gear reduction*chain reduction/2
2 because the moment split on 2 wheels!
So i got arround 500Nm * safety

Or do you simply calculate on the maximum force, that can be transmitted to the ground? (What's happening if you press both pedals? How do you calculate peak torque, torque when accelerating, braking)

Or did someone measured halfshafts with strain gauges?

Why did pat's team broke their shafts designed for 650Nm torque and why don't someone else have problems with 450Nm strong shafts?

You see, there are many questions.

2002/03 University of MARIBOR - Team Member

[MattG]

Good point about the two pedals at once. It probably isn't a situation you would expect to arise but what WOULD happen if at peak torque you left footed the brakes?

Also, if you are running a car with a diff, couldn't the torque to one wheel be more than half the total torque as the diff could put more torque to one side?

[Denny Trimble]

The load case we design for is 1.5g's fwd accel, with a dynamic impact factor of 2.0, which usually gives us between 0.9 and 1.1 factor of safety, depending on the heat treatment specs. This means the tires are capable of a thrust equal to 1.5 times the car's weight (for example, driving through a slight g-out or depression on the track), and since we upshift without the clutch, we need an impact factor there.

We've found the only way to keep light halfshafts together is to follow these rules:
1) Reduce shock loading
2) Put a very large (CNC lathe) radius from the minor diameter of the shaft up to the OD of the splines; shaft minor diameter must be smaller than spline root diameter
3) Polish the shaft and protect it
4) Heat Treat 4340 to RC42 or higher (trying a higher number this year).

Anyone else with experience they'd like to share?

University of Washington Formula SAE ('98, '99, '03, '04)

[All Wheeler]

We've never tested our shafts, so take what I have to say with caution!
Last year we designed our shafts to take maximum torque loading before wheel slip. We calculated from weight transfer and our centre of mass that a maximum of 42% of the vehicles mass could be on one of the rear wheels under maximum acceleration and cornering loads at any one time (M).
Torque = M x 1.7 x 9.81 x rolling radius x sf
Where 1.7 is claimed to be the maximum number of "g's" experienced by other teams and sf ofcourse is your safety factor. This doesn't take into account any impact loading, so in the absence of any info on the moment of inertia of the engine components, we simulated the kinetic energy of the wheel assembly and half shaft spinning at 100km/hr being instantly stopped inboard. this can be converted to an equivalent torque load from any decent machine component design text. From there we calculated the expected out of plane loads from our CV joints (quite important as there is a reasonable bending load on the shafts particularly if they are thin and solid). There's a big long not as nasty as it looks equation to convert all of the above into a torque equivalent load (once again should be in most machine component design texts). All in all, the impact and out of plane terms aren't huge, and we got our static torque equivalent load without safety factor at around 650Nm. I managed to waz all of that into a Fortran 90 program to give a data sheet of possible alternatives attempting to minimise kinetic energy storage and considering torsional fatigue. Important things I learnt from that are:
*Hollow shafts are better for kinetic energy minimisation.
* The thinner the wall, the less significant the difference between cheaper steels and the more expensive materials.
*Design for some reasonable kind of buckling resistance.
As for why teams with 650Nm rated shafts break them, I would be looking at the interface between the shaft and the joints. Welded ends on shafts particularly thin solid shafts can fatigue badly, and welds don't always result in a straight shaft. If you connect by this method, make sure you spigot the ends and machine them flush. In the end, we made our rear half shafts of 4130 in a 41.3mm OD 3mm wall thickness (we had interfacing problems before the event and never ended up using the shafts that were designed for it bcos they didn't fit). each half shaft (excluding joints) weighed only 700g, but remember that the half shafts are pretty critical and it's probably best to make sure that they last (or fit in the car http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif)rather than weigh next to nothing.
Appologies for the psycho babel.
Dou

[Jimboblofski]

I agree totally! last year we did the same thing with 4130 tube and welded splines. under a contantly increasing load these shafts would take over 450 nm befor any sign of failure occured. This was deemed an acceptable level comparing the results to our calculations, however, this calculation did not take into account for the inertia of the engine/gearbox. we broke 2 sets by just spinnig the wheels!!! welded joint are as far as we are concerned a big no no!!
This year we have got spline data and are having them machined dirctly on to our shaft material.