Extension shaft - Help! 7

[CC9]

I need to extend the shaft on a 25cc methanol engine a distance of 600 mm. The shaft has to be light and strong (chromolly tube??) about 20mm o.d. and joined at either end by some form of universal coupling capable of 8000 r.p.m (hellical joint??)and will be turning an 500 cm dia.propellor so the torque load will be fairly high. It is for use in the rear of a large model aircraft, so being light-weight is of great importance. Any suggestions on the best cost effective method of achieving this ?

 

[NickE]

For the lightest weight and highest strength I would use a carbon fibre tube. You should easily be able to purchase a straight length of tubing. I woudl use your max torque output from the motor to size the wall and for needed strength. Depending on how much weight you cna tolerate would help you pick your safty factor.

I cant help you purchase one though but they should be available.

Nick
I love materials science!

 

[gwolf]

Try a carbon/epoxy composite tube with steel end fittings glued in with epoxy. The tube must have 45 degree fibres as well as axial. This sort of tube is easy to buy from stock and has the optimum strength/stiffness and weight/stiffness ratio. This will be very important - see next

I strongly suggest that you test the setup before installing in the model because with 25cc, 8000 rpm and only 20mm O/D, 600mm length and pin-jointed ends - shaft whirl is a possible problem. Shaft whirl is where the middle of the shaft bows out radially at the shafts first natural bending frequency. If the shaft's first natural frequency is below 133 Hz (8000 rpm) then you will have a strong resonance problem which will tear the structure to pieces very quickly. If you get into trouble like this you can solve it by splitting the shaft in half and adding a support bearing. This will increase the resonant frequency of each shaft by a large factor (4x on frequency, can't remember).

In terms of torsional strength it might also be worth doing some torsional stress calculations to ensure that you have adequate wall thickness. My gut feeling is that a 1 inch long bond for the end fittings is about right, again you would have to calculate this. The shear strength of epoxy is about 20-30 MPa, I would design to 10MPa. You could also add chicken rivets.

All in all 20mm diameter feels a little bit on the low side if you want to use a lightweight tube structure.

 

[SomeYahoo]

Check out good ol' McMaster Carr... they have carbon tubing in various sizes and walls. May be a place to start.

 

[CC9]

NickE / Gwolf /GensetGuy - thanks for your replies, excellent suggestions. I had thought of carbon fibre, but didn't think it would be strong enough.... I now have some more questions !
1. If I go to a 25mm OD cardon fibre tube, will this be significantly stronger?
2. If I use a bearing mounted in a firewall half way along the tube will this solve the "bowing" tendencies?
3. I am not an engineer (as you may well have guessed) so is there someone there that could help me with the torsional strength calculations? I understand the torque rating of the motor to be 240 Kgf. mm at 7900 rpm....if this helps?
4. Is a carbon fibre tube as true as a cromolly/aluminium tube? If the tube is more than .0005" out, then I think the whole setup will shake the aircraft to pieces...?
5. What are my best options for joing the shaft to the engine end, and to the propellor end - I haven't been able to find any small u.v. joints that will take that rpm ? although I did find some joints called hellical springs?? ANy suggestions?
Thanks for all your help, this will save me hours of trial and error, and a fistfull of dollars! Cheers, Chris.

 

[sreid]

I calculate 1.75 lb-ft of torque and 2.6 HP at 8000 RPM (someone should check my math).

I like bellows couplings for high speed applications-light, stiff, and no lubrication. As with helical couplings a bearing is required between the shaft and the coupling.

http://www.servometer.com/SMC-Series.html#integral

 

[CC9]

sreid
Thanks for the info. I like the couplers you have suggested. When you say a bearing is required between the shaft and the coupling, I am interpretting this as a simple sealed pillow block bearing secured to a former/firewall which the stud from the carbon fibre rod will pass through then attaching to the coupler..same at each end?
the studs at either end of the carbon tube would have to be shoulder studs, ie approx 20mm down to 3/8" for the coupler...will this be a weak point in the shaft?
This is great being able to discuss my venture with the experts...keep it coming folks!! I am learning a lot here.

To summarise the project to date I have:

25 mm o.d.carbon/epoxy composite shaft with 45 degree and axial fibres - i.d yet to be determined - any offers??

Steel studs at either end epoxied at least 1" into the shaft (and chicken riveted - what are chicken rivets?

3 sealed bearings along the length of the shaft (middle and either end firewall mounted in pillow blocks.

Studs shoulder turned down to 3/8" to suit Bellows coupler SMC-62 rated at 3500 oz torque, spring rate 112.4 lb/in

Thats it so far...keep it coming all you experts out there, I'm loving this!!

 

[unclesyd]

What are the physical properties of a Carbon Fiber Arrow Shaft? This might be starting point as the few I've seen were hollow and quite ridged.

 

[MikeHalloran]

Heli-Cal couplings are tough and strong, but like any helix, they try to change length when you twist them. When attached to a source of varying torque, like a stepping motor or an engine, the torque pulsations will be converted to axial force pulsations. In this case, they could be tough on your bearings and airframe.

The turboshaft installations that I have seen, universally use a metal diaphragm coupling, that is radially rigid but axially and angularly flexible. Given reasonable bearings on the propeller and the engine, a shaft with two such couplings would require no bearings of its own. The lightest bearing is the one that isn't there.

The diaphragm often takes the form of a square, often multi- ply, with holes at the corners, each diagonal pair of holes secured to a yoke on the end of a shaft, grossly similar to a Cardan joint's yokes. The plies of the diaphragm, basically shim stock, are separated from each other by shim stock washers.

The most efficient way to attach a c/f tube to a unit like that is to bond the tube into an eared metal tube, where the ears form the yoke arms that carry the torque to the flexures. The tube would have no center.

The mating yoke might have a hollow extension extending into the carbon tube, very loosely, just to keep the tube from flying away if a flexure fails. This would require the flexures to have a central hole.

Bearings on the shaft would be doubly inefficient because they add weight, and because you need to have a small hub to transmit torque through the bearing. The secret to lightweight design is to not allow force or torques to become concentrated like that.

Bellows couplings are radially flexible, so a shaft with two of those would require a bearing of its own at each end adjacent the bellows.

Three bearings on a shaft is never a good idea; you can't keep them aligned. If the critical speed of the shaft is too low, use a bigger shaft.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[CC9]

Mike - thanks for the tip on the bellows/helical couplers....now I have to make a decision. Is there a site where I can contact you for a diagrammatic representation of such a device...could I make one myself?. I am very interested to find out exactly how this can be done. Not being from an engineering background, it takes me longer to analyse and interpret advice given to me - but I appreciate all of it. I agree, now that you mention it, 3 bearings is probably asking for trouble - this sounds like a unique solution.

This shaft will be the best example of model engineering yet!! Can't wait to get a final solution together and start building it!

 

[CC9]

sreid - in your experience with the bellows couplings have you been aware of any expansion issues, or is this rpm too low to induce this? your thoughts would be appreciated too...

 

[gwolf]

CC9, I have some stock carbon tube in my attic which is about 30mm diamater and 2m long. It is rigid, strong and light. I'll take a look at it w.r.t. your issues.

I agree with Mike Halloran on the bearings and couplings issues - use the bearings on the motor and prop as your end supports, use "universal" type couplings which will not impose any end thrust on your bearings, and try to avoid a third centre bearing.

Regardsing the carbon tube dimensions, I will try and do some calcs sometime over the weekend for you. In general you should use as large a diameter tube as possible and get the weight out by having a thin wall. As I said, I'll try some calcs for you w.r.t. natural frequencies and strengths.

 

[gwolf]

CC9, how many pistons on your engine, how many blades on your prop - I'm looking at frequency stuff for you.

Regarding torques I did some very simple calcs based on expected performance of a 25cc engine at 100HP/litre. On this basis I get 1.64 ft.lb Torque and 2.5 HP at 8000 rpm which is very close to sreid's values probably calculated using prop data.

On shaft bending frequencies -

Length 600mm, 20 diam, wall t=1.5 => first bending=153 Hz
Length 600mm, 20 diam, wall t=2.0 => first bending=149 Hz
Length 600mm, 25 diam, wall t=1.5 => first bending=194 Hz
Length 600mm, 30 diam, wall t=1.5 => first bending=234 Hz

8000 rpm is 133 Hz, recomend 20% margin on this so looks like you need 25mm diameter or greater.

Stresses look ver low.

 

[CC9]

gwolf - What can I say? other than brilliant! Thank you very much for your advices, I am really enthused about this.
The engine is a 2 stroke single cylinder 25 cc methanol engine. bore 32.5mm, stroke 30mm max torque 240 Kgf. mm at 7900 rpm turning an 18" propellor with 6" pitch. I hope this is enough info. re the motor.
Re the tube, I'll go to whatever diameter it takes to be operationally sound. Thanks again, look forward to more calculations and advice. Cheers Chris.
I will wait a bit longer for some more info. from Mike O before I decide on couplings.

 

[gwolf]

CC9 glad to be of help, I still need the number of blades on the prop from you but will calculate for 2, 3, and 4 bladed.

You also need to look up some stock carbon tube and decide which ones you are going for. Out of interest the whole tube when made of carbon weighs only 100g! I love carbon.

Chicken rivets are extra rivets that you put through glued joints because you are scared that the glue won't work even though the calcs say it will.

 

[sreid]

CC9,

Years ago I used a bellows coupling to connect a motor to a spindle a 50 KRPM and it did not fail up to the time I was no longer involved (2 years).

Being a flexure, you don't need to be concerned about wear/lubrication issues. Since all the mass is at the OD they are very efficient re: weight vs. stiffness. The same is true relative to centrifugal effects.

For severe misalignment one has to look at fatigue life.

 

[sreid]

I would think that 4130 tube would be more than adequate for the application. Cheap, light, available and heat treatable to high strength after fabrication (welding and machining the end shafting).

http://www.aircraftspruce.com/catalog/mepages/4130tubing_un1.php

Lots of other good stuff too.

 

[NickE]

sreid- The thing about 4130 tube is that while being very strong and reasonably stiff, for strength/stiffness to weight you really cant beat carbon fibre composites. Especially since the amount of torque transmitted is so low.

At this point we have a tubing (30mm - gets us a natural bending frequency above the operational, adn a wall of 1.5mm should have a strenght far above whats necessary.)

Even aluminium would be a better material choice than steel, at least in my opinion based on the description of the application.

The issue has now become how to attach the ends to the motor and propeller. The mechanicals here should be able to help with that one.

 

[gwolf]

CC9,

I just got home and took a look at that bit of carbon tube that I had - it is 25mm O/D and 0.7mm thick, 1.23m long and feels like it's got cross-plies in it. It weighs 126g making it about 1g/mm. Your drive shaft would be only 60g!!! I'm not sure of the provenance on it but it's yours for free if the postage isn't too much - which country are you in, I am in the UK? The first natural frequency would be about 200Hz and the second about 800Hz. I would continue to source one locally though.

If you can find a local supplier, find out what sizes and thicknesses are available and try to get the manufacturer name so that the real material properties can be determined off their web site. I can then do some frequency calcs for you on the real thing - it only takes me about 5-10 minutes with the FE software at work.

You need to watch out for 2x133 Hz as well because you have a two-bladed prop although this is a smaller effect. So don't drive on with a 30mm because 234 Hz is getting close to 266 Hz. You also need to consider partial power and overspeed conditions, overspeed is more important in this instance, for example if you put a fine pitch prop on it, drop the torque and up the rpm. There is a type of diagram called a campbell diagram that you use to assess this, I'll do one for you when you know what you are going to make it out of.

Regarding end fixtures, I would use cylindrical aluminium inserts epoxied in with a good 24 hour epoxy - might be worth doing another post about this - automotive boys and helicopter people will know a lot about how to do this nicely, they may also have heard of chicken rivets.

You have my permission to ask the site moderator for my e-mail address if you feel that this would be helpful.

Regards, gwolf

 

[sreid]

For other parts you might need (self aligning aircraft bearings?) and all sorts of other aircraft hardware,

http://www.gen-aircraft-hardware.com/