I believe that skin effect or something like it is involved, giving the shaft a much higher shear fail rate.
This where we need a college student to step in and tell all
Quick question that I need the engineering reason for!
Why do gundrilled driveshafts not shear off in conditions that cause solid shafts to shear?
I know all the theory behind tubular shafts being lighter and obviously that a thin walled shaft of the same cross sectional area as a solid one will
be much stronger in torsion BUT
experience (not just by me of course) shows that gundrilled solid shafts will withstand vastly higher loadings that solid shafts of the same diameter,
why is this?
It was explained to me ages ago and I have forgotten!
excuse my ignorance but by gundrilled do you mean drilled through the centre (like hollow kart axles)? if this is the case I believe its because there
is no material in the centre of the twisting force (moment) to sheer and the further from the centre the loads are applied the less sheer (is that
what i mean) force is applied.
or i may be talking bollox but i'm sure i rememebr an explanation some time ago along those lines..
Ned.
Gundrilled is the right term - I was given the choice of solid or gundrilled driveshafts. The gundrilled were nearly twice the cost but were ok for
about 500bhp IIRC.
I believe that skin effect or something like it is involved, giving the shaft a much higher shear fail rate.
This where we need a college student to step in and tell all
without going into anything too complex (i.e. sticking with what I can remember - its been more than 3 years since I did this stuff)
if the hollow shaft has the same x-sectional area then it has the same amount of material (i.e. is the same weight) but the material is better
distributed to where it can do more work
(i.e. the material in the centre of the solid shaft does next to nothing)
if the shafts were the same diamiter the solid one would be slightly stronger but much heavier
other than that I'd need to get my old books out 
and I havn't gone into what happens when you heat treat a thick bit of metal compaired to a thin one
it's one of those mysterious like how come the buttered side of a piece of toast always ends up on the floor when dropped.
Sounds like it shouldn't be true if the shafts are the same diameter - hmmmmmmm...
Do they just make gun drilled shafts out of better material cos you're gonna pay more anyway?
Does the drilling somehow heat treat the wall of the bore?
Do torsional shear failures generally propogate from the centre of the material, therefore with no material in the centre a shear failure will be less
likely to form?
Or something entirely different?...
Liam
complete guess..........
the drilled shafts have more flex in them so can withstand the torque being applied better - the solid shafts can't twist as much therefore they
start to shear and then break.
Failure is nearly always at a splined section where stress is highest. Gun drilling a shaft will somewhat reduce it's torsional stiffness compared to a solid shaft of the same diameter and material. My guess is that the "springier" drilled shaft results in lower peak stress under shock loading conditions.
I don't think it does. 
Do you have any data from controlled tests to support this theory other than "experience"?
This arguement that the tube is stronger than the rod is only true if the same or more material is used. If the same amount of material (cross section
area) is used then the material is further from the center(has to be a larger diameter), ie the polar moment of inertia is higher(.. if thats right...
1st year mechanics getting in here)
BUT and heres the thing.... if you have two shafts of same diameter a rod and a tube, the tube will transmit less torque, but only MARGINALLY. This
means that if you increase the diameter slightly while still keeping the weight down you dramatically improve the torque transmitting capabilities.
badly described but i hope you get the idea. any Q's
PS, if you want some maths i could dust off some books.
[Edited on 29/8/06 by Findlay234]
quote:
Originally posted by 02GF74
it's one of those mysterious like how come the buttered side of a piece of toast always ends up on the floor when dropped.
quote:
Originally posted by MikeRJ
Failure is nearly always at a splined section where stress is highest. Gun drilling a shaft will somewhat reduce it's torsional stiffness compared to a solid shaft of the same diameter and material. My guess is that the "springier" drilled shaft results in lower peak stress under shock loading conditions.
I've never heard of gundrilling an axle to make them stronger. Lighter, but not stronger. All the hollow axles Ive seen are made of better
alloy and usually have a larger diameter. When drag racing guys go to hollow axles they convert to 40 splines and better material which makes them
stronger than the 35 spline solid axles.
Out of Strange Engineerings catalog.
"A solid axle shaft is stronger than an equivalent diameter gun-drilled shaft; therefore, when weight is not an issue a gun-drilled axle should
not be considered."
Having said that, I remember the sprint car guys using hollow torsion bars and there being something to do with them springing back from deflection
faster.
Maybe in rallyX that plays a factor in axle life?
Don't know enough about springs to give any real numbers. Sorry.
For the same reason that toilet paper never tears where it is serrated.
And, I've decided...I'm either going to make all my taables twice as high, or not to put anything on toast. On the other hand, the dogs get
to anything dropped faser than I can. And they clean up whatever food mess happens on the floor...
Toast only ends up butter down if it topples off the edge. If it gets thrown it tends to land butter up.
quote:
Originally posted by Liam
Do they just make gun drilled shafts out of better material cos you're gonna pay more anyway?
Does the drilling somehow heat treat the wall of the bore?
Simple. Greater surface area...
quote:
Originally posted by C10CoryM
I've never heard of gundrilling an axle to make them stronger. Lighter, but not stronger.
"A solid axle shaft is stronger than an equivalent diameter gun-drilled shaft; therefore, when weight is not an issue a gun-drilled axle should not be considered."
Having said that, I remember the sprint car guys using hollow torsion bars and there being something to do with them springing back from deflection faster.
Maybe in rallyX that plays a factor in axle life?
The shaft itself can't be stronger if the O/D remains the same. What probably makes it last longer/fair better is the fact it's weaker but more capable of taking the shock loads that smash driveshafts. Ie either is capable of the power, but the gundrilled one can take the clutch being dumped because it flexes rather than breaking.
Heat treatments only affect the 'skin' of the metal don't they? So if the gundrilled shaft is heat treated it effectively has twice the
skin area and is therefore stronger than its solid counterpart.
That's what I was getting at in my first post 
Fair enough. But does the extra strength from the heat treatment counter the loss of strength from not having the centre? How small is the hole in a
typical gun-drilled shaft?
Oh, and the extra surface area in the middle will be relatively small, so proabably won't add much.
[Edited on 30/8/06 by DIY Si]
This where my info runs out I'm afraid I can vaguely remember a conversation with an engineer some years ago and he was trying to explain it
to me but my eyes must have glazed over or something
I was hoping an engineering guru would have stepped in by now with all the facts & figures. 
quote:
Originally posted by RazMan
Heat treatments only affect the 'skin' of the metal don't they?
I think there needs to be a clarification of strength VS life. For example, drag racers are very hard on axles. You generally have lines marked on
the axles so that when they twist to a certain point (and stay there) you replace them. Drag racing axles are designed to twist rather than snap.
This makes them weaker in that they can handle less load before deforming, but they won't snap off and send you into the wall. Same goes for
gears. The differential gears are very soft compared to a street car. This keeps them from snapping under the shock load, but makes them easier to
deform. Needless to say as far as wear goes, you have to replace them much more often than harder gears.
Drag racing axles are generally drilled .8" or so. Axle OD range from 1"-2".
I had an article somewhere explaining how torsion worked in shafts. The majority (I want to say 80%+) of the load is dealt with by the outer edge of
the shafts. This is why driveshafts are 3-5" and hollow rather than 1" solid. Also why shafts tend to fail at the splines or anywhere the
diameter is reduced. Leverage and all. If I remember where I saw the article Ill post it.
The sprint cars went to rules permitting hollow torsion bars. With hollow torsion bars they were getting faster acting springs and having to adjust
their dampers accordingly. The spring rate, and load rating were the same (they used larger OD hollow bars to get the same ratings as solid), but the
frequency wasn't. Again, sorry to not have numbers to back up. Oval track guys arent engineers... They just make it work (sometimes).
Cheers.
The drag racing comparison is very relevant as grasstrack cars have a suspension setup almost enbtirely devoted to getting to the first corner first,
and develop enormous traction even on very slippery surfaces.
Cars with lots of power and locked diffs (i.e. most top grassers) tend to snap shafts at the startline with some regularity, and gundrilling prevents
this, as does making the shafts as long as possible and of not too great a diameter.
Just chatting to a fellow engineer at work and coming to the conclusion that the benefit if gundrilling is probably a combination of REDUCING
torsional rigidity of the shaft so reducing shock loading by enabling increased "spring" of the shaft, coupled with removing any stress
raisers left in the centre of the bar during production (which is where nearly all bar impurities end up)
................however we are still guessing, and the next question is........why is an annulus a better torsional spring than a solid bar????
for anyone else like me who doesn't know what annulus means:
http://en.wikipedia.org/wiki/Annulus
quote:
Originally posted by NS Dev
................however we are still guessing, and the next question is........why is an annulus a better torsional spring than a solid bar????
quote:
Originally posted by 02GF74
I cannot see how removing material from an object can make it stronger than the original no matter where it is remove from.
Man, I searched through a pile of books and could not find the info I wanted.
I did find this which is similar enough to the article I remembered.
http://instruct1.cit.cornell.edu/Courses/virtual_lab/chalktalks/theory/1.shtml
Page 8 shows solid VS hollow.
Cheers.
At some conditions you can get a stress concentration at the center of a round bar. IIRC, typically if you press the bar between two flat surfaces. I don't know if/how this in anyway applies to a drive shaft.
quote:
Originally posted by MikeRJ
quote:
Originally posted by RazMan
Heat treatments only affect the 'skin' of the metal don't they?
Not always, you are possibly thinking of case hardening which does just affect the surface. Steel is a very complex metal, heating it to various temperatures and letting it cool at different rates can significantly change it's properties.