Making a shaft that will run 20K rpm in one of my R/C boats, it'll b
1/4" diameter with a keyway and threads on one end to hold gear. I've had 2 break right at the end of some threads but wil eliminate them as a possible stress point.
My first was made of 416 stainless and didn't last long at all, th
next was 1144 "stress relieved" and did better bu eventually broke too. I need something that can be turned and no warp and that is tough. A toolmaker buddy suggested 414 "pre-heat treat", what's the thinking here
Watch your stress risers, and if the part is failing due to fatigue from vibration, balance the shaft. 6-4 AL-V titanium might work, though it would need to be a third larger for the same stiffness.
If the stressproof shaft (SAE 1144) broke, odds are that the pre heat-treated SAE 4140 will fail also. The physical properties of the two are quite similar except for their machinability.
If you can make the shaft larger in diameter. You can even use an aluminum tube of the right dimension!
If that is not possible you will need to remove any stress risers at the end which keeps breaking. Is a keyway really necessary? Would loctite do? How about a collet-type connection which would eliminate threads and keyways.
The gearbox photo doesn't come up -- just a picture of a (very nice) RC boat.
How long is the shaft that's breaking, and how long is the drive shaft? If they're long enough to be resonant at 333.3Hz or it's close multiples (667, 1000) then they'll really whip around; no amount of stress relief would help you then. For that matter the motor power pulses will still be coming at 400Hz, so you have to worry about those resonances, too. Solving that would involve making the shafts more torsionally stiff, to move the resonances up.
If that's not it then the suggestion to balance everything (your shaft _and_ the drive shaft) would be good.
Greetings Terry, You can also use 4340 in the heat treated condition. I think that's what your toolmaker buddy is talking about when he says pre-heat treat. I.E. the material is heat treated before machining. The stuff machines nicely, though it is tough because of the heat treated condition. I would rather machine it than 4340 in the annealed condition because of the much better finish attainable just turning with carbide. Since the shaft broke at the threads it looks like you have a stress riser there. What does the thread relief look like? And how about the threads themselves? Are they smooth? Ideally, your thread relief should be cut with a full radius tool. And the threads need to have as large as possible the radii in the roots and on the crests. A good finish is very important. Fine scratches can lead to cracking. So remove them with 600 or finer wet or dry sandpaper. The keyway could also be a problem. How is the finish in the keyway? Does the keyway start at the end of the threads? Have you looked with a magnifier to determine exactly where the failure started? Cheers, Eric
================ You may be encountering "shaft whirl" which causes the shaft to vibrate or flex like a plucked guitar string at certain rpms. All steels are about equally "stiff" or resistant to bending [although some are much more to take a permanent set or bend] so switching materials if this is the case won't help. How long is your shaft [l/d ratio is critical parameter here]. Can you install an intermediate bearing? It won't have to handle much load, just enough to keep the shaft from starting to bend/twang.
Unka George (George McDuffee)
...and at the end of the fight is a tombstone white with the name of the late deceased, and the epitaph drear: ?A Fool lies here, who tried to hustle the East.?
Rudyard Kipling The Naulahka, ch. 5, heading (1892).
Can the shaft be made a little bigger between the bearings? You don't want stress risers so if possible make a smooth transition to maybe 2 mm greater diameter.
That's why I was asking about the shaft that he's driving -- if _that's_ vibrating it could take out the gearbox shaft before it dies. And I would expect a 'driveshaft' to have a longer l/d ratio than a 'gearbox shaft'.
Thanks again guys, looks like I'll go with "heat treat-stres proof" 4140, or "normalized" as it's described in th Machinery's Handbook. This material has a tensile strength o
148,000 and yeild strength of 95,000 whereas the 1144 I used befor is rated at 96,750 & 58,000 respectively
The big help should be the elimination of the threads on the end tha
keeps breaking, I'm goin to fix the collet to the shaft wit set-screws instead. If this set-up breaks I'll have to go with bigger shaft..
With setscrews, you will have a bit more difficulty balancing the shaft -- but I'll bet that the keyway introduced enough of an imbalance at the speeds which you mentioned.
If you *must* have a keyway -- can you cut *two* of them at 180 degrees -- and use two keys as well -- to maintain the balance?
And another thing which *might* improve your reliability with the threaded end of the shaft. Look into having the threads rolled, instead of cut. This makes for a more rounded root of the thread, and the rolling process makes for greater strength beyond that of the stress riser problem.
If you want to gild the lily, make your new shaft from VascoMax C-300 or C-350 (Teledyne Vasco). These materials have high yield, tensile, toughness, ductility and impact strengths, as well as high fatigue and compressive strengths. Could be your breaking problems will come to an end.
It will require heat treatment, but VascoMax requires temperature no higher than about 925 degrees F for several hours to achieve maximum strength.. Distortion is limited or eliminated, and shrinkage is predictable.
On Fri, 28 Jul 2006 04:40:26 GMT, snipped-for-privacy@spam.invalid (Terry Keeley
You can also use 4340 in the heat treated condition. I think that' what your toolmaker buddy is talking about when he says pre-hea treat. I.E. the material is heat treated before machining. The stuf machines nicely, though it is tough because of the heat treate condition. I would rather machine it than 4340 in the anneale condition because of the much better finish attainable just turnin with carbide. Since the shaft broke at the threads it looks like yo have a stress riser there. What does the thread relief look like? An how about the threads themselves? Are they smooth? Ideally, you thread relief should be cut with a full radius tool. And the thread need to have as large as possible the radii in the roots and on th crests. A good finish is very important. Fine scratches can lead t cracking. So remove them with 600 or finer wet or dry sandpaper. Th keyway could also be a problem. How is the finish in the keyway? Doe the keyway start at the end of the threads? Have you looked with magnifier to determine exactly where the failure started Cheers Eric[/quote:a51be0eec1
Sorry, I wasn't seeing all the posts looking through the DIY forum.
Yes, i tryed to make the threads as smooth as possible and ground small radius in my threading tool so the bottoms weren't sharp.
The keyway end isn't a problem..
Think eliminating the threads completely will help a bunch
On Fri, 28 Jul 2006 04:40:26 GMT, snipped-for-privacy@spam.invalid (Terry Keeley
You can also use 4340 in the heat treated condition. I think that' what your toolmaker buddy is talking about when he says pre-hea treat. I.E. the material is heat treated before machining. The stuf machines nicely, though it is tough because of the heat treate condition. I would rather machine it than 4340 in the anneale condition because of the much better finish attainable just turnin with carbide. Since the shaft broke at the threads it looks like yo have a stress riser there. What does the thread relief look like? An how about the threads themselves? Are they smooth? Ideally, you thread relief should be cut with a full radius tool. And the thread need to have as large as possible the radii in the roots and on th crests. A good finish is very important. Fine scratches can lead t cracking. So remove them with 600 or finer wet or dry sandpaper. Th keyway could also be a problem. How is the finish in the keyway? Doe the keyway start at the end of the threads? Have you looked with magnifier to determine exactly where the failure started Cheers Eric[/quote:7aae61f5b4
Sorry, I wasn't seeing all the posts looking through the DIY forum.
Yes, i tryed to make the threads as smooth as possible and ground small radius in my threading tool so the bottoms weren't sharp.
The keyway end isn't a problem..
Think eliminating the threads completely will help a bunch
You can also use 4340 in the heat treated condition. I think that's what your toolmaker buddy is talking about when he says pre-heat treat. I.E. the material is heat treated before machining. The stuff machines nicely, though it is tough because of the heat treated condition. I would rather machine it than 4340 in the annealed condition because of the much better finish attainable just turning with carbide. Since the shaft broke at the threads it looks like you have a stress riser there. What does the thread relief look like? And how about the threads themselves? Are they smooth? Ideally, your thread relief should be cut with a full radius tool. And the threads need to have as large as possible the radii in the roots and on the crests. A good finish is very important. Fine scratches can lead to cracking. So remove them with 600 or finer wet or dry sandpaper. The keyway could also be a problem. How is the finish in the keyway? Does the keyway start at the end of the threads? Have you looked with a magnifier to determine exactly where the failure started? Cheers, Eric[/quote:bd1f56389a]
Sorry, I wasn't seeing all the posts looking through the DIY forum. Yes, i tryed to make the threads as smooth as possible and ground a small radius in my threading tool so the bottoms weren't sharp.
The keyway end isn't a problem...
Think eliminating the threads completely will help a bunch!
On Fri, 28 Jul 2006 04:40:26 GMT, snipped-for-privacy@spam.invalid (Terry Keeley
You can also use 4340 in the heat treated condition. I think that' what your toolmaker buddy is talking about when he says pre-hea treat. I.E. the material is heat treated before machining. The stuf machines nicely, though it is tough because of the heat treate condition. I would rather machine it than 4340 in the anneale condition because of the much better finish attainable just turnin with carbide. Since the shaft broke at the threads it looks like yo have a stress riser there. What does the thread relief look like? An how about the threads themselves? Are they smooth? Ideally, you thread relief should be cut with a full radius tool. And the thread need to have as large as possible the radii in the roots and on th crests. A good finish is very important. Fine scratches can lead t cracking. So remove them with 600 or finer wet or dry sandpaper. Th keyway could also be a problem. How is the finish in the keyway? Doe the keyway start at the end of the threads? Have you looked with magnifier to determine exactly where the failure started Cheers Eric[/quote:1750266bf0
Sorry, I wasn't seeing all the posts looking through the DIY forum.
Yes, i tryed to make the threads as smooth as possible and ground small radius in my threading tool so the bottoms weren't sharp.
The keyway end isn't a problem..
Think eliminating the threads completely will help a bunch
How long is the shaft that's breaking, and how long is the driv
shaft? If they're long enough to be resonant at 333.3Hz or it's close multiples (667, 1000) then they'll really whip around; no amount of stress relief would help you then. For that matter the motor power pulses will still be coming at 400Hz, so you have to worry about thos
resonances, too. Solving that would involve making the shafts more
torsionally stiff, to move the resonances up
If that's not it then the suggestion to balance everything (your shaf
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