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Loose Shaft?

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Old 11-07-2009, 09:14 PM
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cheese_puddin
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Default Loose Shaft?

Hello,

I am part of the Milwaukee School of Engineering SAE Aero competition team. Our school has an old Jett 90 that we are using for propeller testing. The engine was running fine but noticed that the spinner is stuck to the shaft. The shaft had to be screwed directly into the engine. Is this normal? Most R/C engines we have worked with the spinner comes off not the shaft. We were testing it tonight and for some reason when we applied the electric starter the shaft spun right off. No matter how tight we try to screw the shaft on it continues to fly off. We checked for hydro-lock and it is not the case. After looking at the exploded view of the engine it looks like there is a locking cone, is this the issue? Any help would be greatly appreciated on keeping the shaft in place.

Thanks,

Brad
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Old 11-07-2009, 10:13 PM
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Default RE: Loose Shaft?

Thanks for writing.....

First ..... yes, the prop stud is removable. This is a design feature of Jett engines. In the event of a crash, the $2 stud might be bend or damaged, but the $70 crankshaft is not damaged, as would be the case with most other engines. Also, it serves for a bit of adjustment to accomodate different size prop hubs and spinners. Can be longer/shorter by turning it in or out a few turns, and can be replaced with a longer/shorter stud if greater adjustment is required.

The spinner nut should not be "part" of the prop stud. It is over-tightend and bottomed out on the prop stud, if I understand your description. The prop nut must tighten the assembly.

Try installing two 5/15-24 nuts on the prop stud, tighten them together in a locking fashion, hold with a wrench and then remove the spinner nut.

If you can not get it loose, your best bet is to replace the prop stud and spinner nut. Dub can send you a replacement. Or, if it is easier, you can purchase a 5/16-24 "set screw" from somewhere like McMaster Carr or a good hardware supply store - yes, that is all it is. A variety of lengths available.

An FAQ reference: http://www.jettengineering.com/faq.html#faq12

More important..

The prop stud should not installed tight into the crankshaft. Should not be bottomed out. Thread it in all the way (fingers) until it stops, then back it out 2-3 turns. You want it to float slightly in there. From there, mount the prop, and install and tighten the prop nut. When you install the prop, tighten the prop nut, wait about 20 seconds (Allow the prop to seat properly and conform to the drive washer), then tighten the prop nut slightly once again.

There is not a locking cone to secure the stud, there is a drive cone to mount the drive washer. The cone goes over the actual crankshaft, and the prop drive (where the back of the prop mounts) installs over this. When the prop is snugged down, this cone solidly grabs the drive washer. Always on center, and does not slip.

Let me know how you make out...

Bob
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Old 11-09-2009, 02:20 PM
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Default RE: Loose Shaft?

Thank you for your speedy reply. This clears things up quite a bit. Thank you again and we will let you know how it goes.

Brad
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Old 11-17-2009, 09:08 AM
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Default RE: Loose Shaft?

Hey...

Just wanted to check in and see how yall made out with this.

Bob
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