Why is a driveshaft hollow instead of solid?


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95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

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Justin - One Happy FTE Member

 

Just guesses on my part (of course) but maybe easier to balance (?) - weight reduction (?) Good question though . . .

 

a steel hallow tube is stronger than a solid one when it comes to twisting it long ways

its also harder to bend the hallow shaft out of place than it is for a solid one

 

If it was solid, it would take two big guys to pick it up. If you dumped the clutch, all that mass would probably snap off the u-joints.

 

Strength in a round member is derived from the outer portion. Therefor if you make the driveshaft just a hollow tube you can have close to the same strength as a solid stock driveshaft. and if you've ever picked up a piece of solid 3 inch round stock of any length, you know how heavy it is. your driveshaft would probably weigh close to 200 lbs. any mass that is rotating takes power to turn it, therefore engineers try to make as little rotating mass as possible when it comes to things like driveshafts and axleshafts. Chevy for a while in the 90's made their driveshafts out of a carbon material for the ultimate lightness. you'll also see alot of aluminum driveshafts in mustangs and impalla ss's. In trucks we use hard we try to find a balance of strength and weight. Ford in my opinion was kind of dumb when they designed driveshafts for our trucks. If you've ever looked at one of them they use like a 3 inch OD tube and neck it down to 2.5 inch OD at the ends. They might as well make the whole shaft out of 2.5 inch tube cause its would not lose any strength since the weakest part will break first. When i had my custom driveshafts made i had it made out of 4 inch OD tube. But anyhow...this is kinda long so i'll shut up for now.

Ryan

 

So it's stronger, but then how come they don't make axle shafts hollow, because they can't make them large enough with the axle tubes? I do know that they want to keep driveshafts light, but seems to me that a solid piece of stock is stronger twist wise then a hollow tube, but i guess i'm wrong. I looked at it like, if it was stronger, than you wouldn't need a 3" piece of steel stock, you could use something MUCH smaller. But i see where you're going. Just a curious question.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Thats it for now, saving for a stang. Check out my Gallery for a look-see.


Justin - One Happy FTE Member

 

haha, know you get into my realm, the mechanical engineering stuff. I worked for American Axle for a couple years. Here's the skinny:

Axle shafts are solid for strength reasons. The shaft is extruded to the necessary shape and then heat treated. So you actually have a soft inner core of steel that allows the axle shaft to twist, while you have a hard heat treated ouside to provide stiffness and strength. If the axle were heat treated all the way through, it would shatter at the slightest dump of the clutch. No heat treat and it would twist and break like a tootsie roll. The actual depth of heat treat penetration is also critical. I watched these wicked sweet tests called ultimate torsion. Put the axle in a hydraulic tester, and turn the shaft till it explodes, the ultimate force (and degrees of rotation) indicate it's strength. The dual strength nature of shafts is why they are solid. Driveshafts however are hollow since they see 3-4 times less torque (3.55 or 4.10's, whatever your R&P ratio) and are a long rotating mass. Like someone else already mentioned. Now chrome alloy and other aftermarket shafts are much stronger for the above reasons, but typically not needed and are "expensive" to manufacture.

 

The polar moment of a shaft is the measure of it's strength in torsion. It is a function of it's diameter to the fourth power (J = .098 x D^4) for a hollow shaft it's given by(J = .098 x (OD^4 -ID^4))

A 3" solid shaft would have a polar moment of 7.938 and weigh 24# per foot. A 3" x 1/4 wall shaft has a polar moment of 4.110 and hvve a weight of 7.3# per foot. Better than half the strength for less than 1/3 of the weight. A solid shaft equivalent in strength to the 3 x 1/4 wall would be 2.54 diameter and weigh 17.3# per foot.

The heavier smaller diameter shaft would deflect significantly more under its own weight and would have a much shorter fatigue life as a result. This was the common failure mode on the early 60s GM compacts with the torque tube drive.

 

Very Interesting, thanks for your imput.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Thats it for now, saving for a stang. Check out my Gallery for a look-see.


Justin - One Happy FTE Member