A driveline component subject to fatigue would never be designed to operate close to its elastic limit, let alone beyond its elastic limit.
For example, an axle made from 300M alloy steel with an Ftu/Fty of 280/238ksi, a stress ratio = -1.0, and Kt = 1.0, would have an allowable fatigue limit of 130ksi at 10^5 load cycles (about 1 GP race distance). 130ksi is only about 46% of the material's yield strength.
riff_raff
Most obviously so, however when posters are describing xperiences of shafts with permanent twists, which surprises me indeed, I just thought it to be wise to differentiate apples and pears, that's all.
"I spent most of my money on wine and women...I wasted the rest"
xpensive wrote:Perhaps it's a good idea to point out the difference between elastic twist (like a torsion-spring) and plastic twist (permanent deformation)?
Two outrageously simplified xamples;
- A shaft made of mild steel, low yield strength with high elongation, is likely to leave a lot of permanent twist if torque reach such levels, but very little in terms of elastic twist.
- A shaft made from Titanium or high-strength aluminium, low modulus with high yield strength, could probably twist quite a bit elastically, but will not leave much in terms of permanent twist.
. Xcellent Point
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I'm not an expert in steel, but have done quite some work in flow properties of materials and would just echo some of the posts concerning linear and non-linear phenomena.
The drive shaft will experience torsional stress as it transmits the power from the engine to the wheel (and under braking in the opposite direction), the magnitude of which will depend upon the torque transmitted from the gearbox and the limit of adhesion of the tyre.
The degree of twist in the shaft will be a function of the lengtrh of the shaft and the Youngs modulus (limiting stiffness) of the shaft. Assuming the torque applied is below the Youngs modulus any deformation should be elastic, and therefore recoverable. It is evident from the pictures earlierr in this thread, that there is also some permanent deformation occuring in those shafts. This implies non-linear behaviour (i.e. that the stress/strain ratio is not constant at large stresses).
I think the mild steel and titanium, comparison was a good one to illiustrate the difference between modulus and strain. Given that the strain in the driveshaft, provided it is in the linear range, will be proportional to shaft length it follows that a longer shaft will twist more than a shorter on for a given percentage.
Non-linear phenomena, such as fatigue, are very tricky to model and introduce the problem of trying to predict when a component might fail. As PhilipM also mentioned, there is the complicating factor of non-torsional stresses (bending, as opposed to twisting) as the car goes over kerbs.
I quite like the idea of using spring steel as a drive shaft, I have visions of elastic band racers! although my sense is that it would be better to have a very stiff shaft and keep the strains within the linear range. I suspect this is the reason teams tend towards a larger diameter, hollow shaft than a smaller diameter solid one, and make it from a very stiff (high modulus) metal. I still maintain that they btyre will be the lowest stiffness material in the drive train and should undergo the largest strains as the power is delivered.
The drive shaft will experience torsional stress as it transmits the power from the engine to the wheel (and under braking in the opposite direction),...
That would be the case for half shafts with inboard brakes only, something you won't find in modern cars, but otherwise torque would only go the other way during engine braking, which typically is a lot less than what you get from forward acceleration.
Shafts inside F1 gearboxes are designed to 'twist' to allow the
sledgehammer shifts given by electronic control to occur.
Lock up diffs are mainly used to retain traction balance as these crude devices
operate under braking.
Hardly rocket science.
This is perhaps not the proper thread, but I hate to open a new one. Anyway, I'm discussing a new assignment in Stavanger right now, where I would help with developing a new type of transmission for windmills, when we came to input data.
Nominal power 3 MW at a speed of 15 rpm.
Which actually represents a torque of some 2 000 000 Nm...christ, never thought about it that way!
"I spent most of my money on wine and women...I wasted the rest"
xpensive wrote:This is perhaps not the proper thread, but I hate to open a new one. Anyway, I'm discussing a new assignment in Stavanger right now, where I would help with developing a new type of transmission for windmills, when we came to input data.
Nominal power 3 MW at a speed of 15 rpm.
Which actually represents a torque of some 2 000 000 Nm...christ, never thought about it that way!
I did a project with an emergency brake system on one of 4Mw turbines and had the same thoughts.
Please correct me if I am wrong but do F1 cars even have driveshafts?
The V8 engine has a crankshaft which is connected to a pilot shaft or main shaft in the transaxle which dispurses power to two axle shafts on each side.
Twist three times around? Someone has an incredible vivid imagination.
autogyro wrote:Shafts inside F1 gearboxes are designed to 'twist' to allow the
sledgehammer shifts given by electronic control to occur.
Lock up diffs are mainly used to retain traction balance as these crude devices
operate under braking.
Last edited by Richard on 14 Jul 2011, 22:49, edited 2 times in total.
autogyro wrote:Shafts inside F1 gearboxes are designed to 'twist' to allow the
sledgehammer shifts given by electronic control to occur.
Lock up diffs are mainly used to retain traction balance as these crude devices
operate under braking.
Why not just put some "real" springs in the cluth like it is done it customer cars?
