NS Dev
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| posted on 29/8/06 at 03:01 PM |
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Why do we gundrill driveshafts to prevent torsion failure?
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!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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ned
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| posted on 29/8/06 at 03:10 PM |
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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.
beware, I've got yellow skin
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RazMan
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| posted on 29/8/06 at 03:19 PM |
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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 
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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mcerd1
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| posted on 29/8/06 at 03:35 PM |
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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
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02GF74
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| posted on 29/8/06 at 03:48 PM |
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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.
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Liam
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| posted on 29/8/06 at 04:24 PM |
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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
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MikeR
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| posted on 29/8/06 at 04:34 PM |
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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.
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MikeRJ
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| posted on 29/8/06 at 04:49 PM |
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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.
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02GF74
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| posted on 29/8/06 at 05:03 PM |
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I don't think it does. 
Do you have any data from controlled tests to support this theory other than "experience"?
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Findlay234
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| posted on 29/8/06 at 05:05 PM |
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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]
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Avoneer
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| posted on 29/8/06 at 05:15 PM |
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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.
This has been proven and due to the average height of a table, the toast only has time to do half a revolution.
If you drop if from a table twice as high, it will land the right way up again.
Pat...
No trees were killed in the sending of this message.
However a large number of electrons were terribly inconvenienced.
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iank
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| posted on 29/8/06 at 05:21 PM |
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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.
This sounds plausible, the quickest way to snap a shaft is to suddenly make a large changes to the forces applied to it - hitting the brakes on a
spinning wheel being favorite, dumping the clutch on a monster engine second. Anything that absorbs some of those forces has to help.
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C10CoryM
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| posted on 29/8/06 at 05:34 PM |
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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.
"Our watchword evermore shall be: The Maple Leaf Forever!"
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whitestu
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| posted on 29/8/06 at 07:26 PM |
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For the same reason that toilet paper never tears where it is serrated.
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zetec7
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| posted on 29/8/06 at 08:08 PM |
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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...
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DIY Si
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| posted on 29/8/06 at 08:13 PM |
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Toast only ends up butter down if it topples off the edge. If it gets thrown it tends to land butter up.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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mcerd1
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| posted on 30/8/06 at 07:59 AM |
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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?
You'll pay alot more because its expencive to drill a hole that small through a shaft that long
As for heat treating - I meant that if the shaft was properly re-heat treated after drilling it could improve it properties slightly
This happens because the centre of a bit of metal will take longer to cool, so the thiner the metal the more consistant and controled its properties
will/can be
(thin metal allows better temp. control, so better control of the structure of the metal)
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Stu16v
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| posted on 30/8/06 at 07:40 PM |
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Simple. Greater surface area...
Dont just build it.....make it!
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02GF74
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| posted on 30/8/06 at 08:34 PM |
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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?
I cannot see how removing material from an object can make it stronger than the original no matter where it is remove from.
did wonder about the springyness idea but surely the spring is stronger if there is m ore steel so it will a) deflect less and then b) spring back
quicker?
Again goes back to my prevous post - show me the data - any measurements/test on the origianl shaft and an identical one but drilled.
I am kinda siscounting any heat treatment caused by the drilling since this would surely be drilled with coolant?
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DIY Si
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| posted on 30/8/06 at 08:47 PM |
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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.
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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RazMan
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| posted on 30/8/06 at 09:17 PM |
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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 
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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DIY Si
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| posted on 30/8/06 at 09:23 PM |
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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]
“Let your plans be dark and as impenetratable as night, and when you move, fall like a thunderbolt.”
Sun Tzu, The Art of War
My new blog: http://spritecave.blogspot.co.uk/
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RazMan
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| posted on 30/8/06 at 09:30 PM |
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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. 
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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MikeRJ
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| posted on 30/8/06 at 10:49 PM |
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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 properites.
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C10CoryM
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| posted on 31/8/06 at 05:05 AM |
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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.
"Our watchword evermore shall be: The Maple Leaf Forever!"
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