Losses due to bending a rotating shaft 2

[karting1]

I'm trying to find out how the bending of a rotating drive shaft effects the power output of a system in qualitative terms.

So, if we have a cantilever system with a rotating shaft and a load causing the shaft to bend - is there a formula for calculating the power required to rotate that shaft at a specific rate?

Many thanks

 

[drawoh]

karting1,

If your shaft is subject to elastic strain, all the energy you put into it should be recoverable. There should be no power losses, due to the elastic strain.

Your bearings and gears are another matter, and not as easily modeled with simple equations.

JHG

 

[TheTick]

drawoh:

Have you patented this apparently hysteresis-free and entropy-free material?

 

[rb1957]

bending of the shaft absorbs strain energy, which would appear as a loss (reducing the available power output)

 

[karting1]

ok guys thanks

 

[GregLocock]

The bending of the shaft is resisted somewhat by its internal damping. If there is no permanent deformation this is a tiny proportion of the energy flowing through the shaft, immeasurably small for any conventionally engineered system.

You might have more luck finding an approach if you look in the literature for torsional harmonic dampers, where the rubber insert does absorb a useful amount of energy.

Cheers

Greg Locock


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[karting1]

The issue I'm wrestling with is the mystery surrounding the effect of rear axles stiffness in racing karts.

Drivers report a significant difference in performance between axles with slightly different wall thickness. Generally we assume the stiffness of the axle has an effect on the overall flexibility of the kart chassis, but the deflection of these axles is very small under significant load so it doesn't seem plausible.

a typical axle would be in 4340 steel, 40mm x 3mm x 1040mm
Total weight of kart + driver 160kg.

So, I'm wondering if there might be significant efficiency losses due to complex harmonics, that can change with the stiffness of the axle.

 

[GregLocock]

Yeah I had a horrible feeling you were talking about karts. Basically karts are just voodoo science. FWIW I think that you are specifically on the wrong track here (hahaha) because the axle is in series with a much bigger source of compliance - the tire.

The axle's torsional stiffness probably has a significant effect on the instantaneous traction distribution left to right. You might try putting a second axle across the chassis and see if its stiffness helps.

Cheers

Greg Locock


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[karting1]

I think I might just stick with the voodoo approach then, you've confirmed to me that anyone who purports to know how these things work is delusional!!

thanks for your time

 

[potteryshard]

I have a gut feel that the deflection of the axle tube in the horizontal direction may be involved... As the axle defects under acceleration or braking loads, it is going to slightly allow the tires to lead or lag the chassis, in turn allowing instantaneous tire alignment to change, affecting tire scrub and total chassis road friction.

Vertical deflection would seem to be absorbed by the tire sidewalls for little overall effect. Torsional wind-up of the axle could affect instantaneous yaw, but because that effect is damped by the entire mass of the kart, it seems that this should be of little consequence.

If you arranged a test rig with a limber axle and then added supplemental outboard bearings near the rear wheels, and located those bearing via angle struts back to the chassis in a vertical and horzontal plane it might be possible to experimentally determine what factors are significant. With this rig, the compliance of the axle in the desired plane could be adjusted by substituting harder or softer steel, nylon, and various rubber washers to determine what is actually occurring.

 

[Tmoose]

"Power output" sure sounds like acceleration or maybe even top speed, but Is "performance" referring to acceleration, or handling as determined by butt dyno, lap times, or ??

 

[karting1]

Potteryshard: thanks for the alternative viewpoint, I would certainly like to experiment with how axles behave in terms of deflection in the horizontal plane.

Tmoose: Performance feedback from drivers and laptimes is what gives rise to the idea that different grade axles give different results.

I think my next step will be to do some track testing, and see if a data logger on the kart can quantify the reported differenes, and report back here.

Thanks again for your input everyone