Coupling two threaded shafts (engine and alternator)? 1

[robotbuilder]

I'm having some trouble finding an acceptable solution to direct coupling of an M8x1.25 Honda mini 4-stroke engine shaft to an M14x1.5 alternator shaft. I'd like to couple them end to end since the RPMs will be well-matched to obtain the output I'm expecting.

There are many spider couplings, but they all have a bore size for keyed shafts. Is there any such coupling for threaded shafts?

Ideally, there’d be a spider coupling which didn’t just have a bore, but a metric thread. And, I’d need two of them with different threads (M8x1.25 and M14x1.5) with the same OD so they can mate, using a spider between them to handle any offset. Threaded hubs simply are not available they’re all bored with/without an ANSI keyway.

One idea I have is to turn some material and make two sleeves with the same OD, but each would be tapped to the threads on the engine and alternator. I’d machine a flat spot for the set screw, too, and put a spider coupling over each machined shaft after I threaded them onto the alternator and engine. It's not a high-torque situation, but RPMs could reach 6000.

Does anybody have a better idea? What is the minimalist approach to this problem? I can turn, mill and tap without a problem.

Thanks,
robobuilder

 

[GregLocock]

Those thread sizes sound a bit marginal. Why not make an adapter to take the honda up to 14 mm, slide piece of fuel pipe on and use two hose clips?


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jet1749]

Look into a collet mounted hub system like the "Trantorque GT" marketed by Fenner Transmission in the UK. This is a three piece bushing system available in sizes down to 5mm dia. 8mm dia has a length of 22mm and 14mm dia has a length of 28.5mm. This should allow you to fit a hub of your choice

 

[DesignerMike]

You could buy an spider coupling with a smaller bore and have a machine shop tap whatever threads you need.

Personally, I avoid using threads to transfer power. But in this case is may be the most affordable method.

I have also seen some couplings that use "taperlock style" attachment methods like a ring-feder. The diameter will probably grow, but if you can stay in something standard you would be better off.

http://www.ringfeder.com/indexB.htm

 

[diamondjim]

Couldn't you simply bore and tap thru
M8 and then c'drill and tap to the M14
size in a one piece coupling or collar?

 

[MintJulep]

Both threads are right-handed? One of them will be turning in the come apart direction.

Arn't the shafts tapered, with the threads there only to draw-up a mating tapered hub or pulley?

 

[maytag]

We have several tach generators coupled to DC motors via rubber hose and hose clamps. With small assemblies like these it will be hard to get a proper alignment and 6000 RPM is going to be pretty unforgiving. Maytag

 

[JJPellin]

If both sets of threads are right hand, both will tighten with rotation or both will loosen depending on the rotatin of the engine. A ridgid coupling threaded on both ends for the two shafts would not be a good idea. You need some flexibility to accomodate misalignment or you will be breaking shafts. I would file or mill a flat spot on the threads lengthwise. I would take two coupling hubs bored too small, as suggested above and have them bored and tapped to thead on the shafts. Then I would thread them on and land a setscrew on the flat to lock them in place. Without the flat, the setscrew will damage the threads and make disassembly a problem. And the setscrew will hold better with a flat spot. I assume rotation will be tightening both sides. Otherwise, it will be very difficult to get this to hold up.

 

[MintJulep]

JJPellin,

You are correct regarding the rotation. I must remember "engage brain, then type."

 

[robotbuilder]

Excellent ideas and I thank you for the consideration.

Having considered everything, I'll buy two undersized couplings and tap them as appropriate. As JJPellin suggested, I'll grind a flat spot on each shaft for a set screw so the threads are not continually tightening since both are R-hand. This will let me take it apart.

Thanks much for the input.

- Ken