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Prop Thread Engagement on Motor Shaft
1133 7 2015-10-12
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liningiv
Second Officer
Flight distance : 329409 ft
United Kingdom
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There have been a few threads recently on the Forum concerned about when and if you should replace the Props on the P3 series.
After 125 flights I noticed that one of the props on my P3P, a black one felt a little more stiff than normal, so I decided after 125 incident free flights and an this stiff thread, I would change them all.
I cut through one of them to show just how much, or how little, of the thread is engaged when fully fitted to each motor.
I must say I was a little surprised.

First shot shows just the prop hub cut down it's axis, I have marked the start of the thread with a horizontal pencil line on each side, and also used the pencil to highlight the thread shape.

Second shot shows the Prop hub fitted to the motor.  About 3 full threads are engaged.

Third shot shows the assembly in an isometric view so you can better see what I am trying to illustrate.

Your comments would be interesting......?
Ivor Linington Phantom Prop Engagement 1.jpg
Ivor Linington Phantom Prop Engagement 2.jpg
Ivor Linington Phantom Prop Engagement 3.jpg
2015-10-12
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JtrJr-UAV Pilot
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Lining,

Interesting. Thoughtful of you to do that and post the pics. I'm a bit surprised as well. It seems to me (from a layman's perspective) that leaving such a large gap at the top of the hub could cause premature stress and wear on the threads since the props are self tightening. It would also seem that it may be cause for some people to put more stress on the threads by over-tightening the props. Interesting to me anyway. Thanks for sharing.

Jerry
2015-10-12
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Bittsandbobs
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Good work!  That really is interesting.   Looks more frail than expected.
I agree that three threads into a plastic hub is gonna fail sooner or later.
Too bad that the female threaded section does not extend further down so that the male threads could engage into more material.

Cheers,
B&B

2015-10-12
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acenothing
Second Officer
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Very interesting pictures.  Do you see stress/wear on the plastic threads?  jtrjr make a good point too about the gap.

Thanks to both.
2015-10-12
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liningiv
Second Officer
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acenothing Posted at 2015-10-12 18:28
Very interesting pictures.  Do you see stress/wear on the plastic threads?  jtrjr make a good point  ...

No obvious signs of wear, the irregularities that seem to indicate a damaged thread are the rough edges left by my saw cuts, and by using abrasive paper I was still unable to remove this debris any claener.  The material is very tough and difficult to cut so there is comfort to be had from that.

Surprise for me is the double contact areas, one at the base of the thread on the steel shaft, and another on the aluminium motor rotating body.
Normal thread engagement is equal to the diameter of the thread, in cases like this where the nut is softer than the bolt then more engagement in the nut is recommended.


2015-10-12
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gil
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liningiv Posted at 2015-10-12 14:14
No obvious signs of wear, the irregularities that seem to indicate a damaged thread are the rough e ...

As I seem have an abundance of time, power tools and broken props I was going to illustrate the 3 different kind of props and the double contact areas at the base of the thread and on the motor housing.  But while I was sawing through the props, taking the photos, editing, overlapping and labeling... aww dang, you already mentioned it!
PropCutaways.jpg
2015-10-12
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liningiv
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gil Posted at 2015-10-12 19:54
As I seem have an abundance of time, power tools and broken props I was going to illustrate the 3 d ...

Hi Gil
Good to see the evolution of the system like this.  Seems the double contact area is present in each design.  I don't think that there is anything "wrong" with this design, and by the forum comments not many people are suffering from props coming off or breaking, its just not conventional engineering practice.  Thanks for the views of the Mk1 & Mk2.
2015-10-12
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greenbean
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Just a guess for the possible engineering with this application. Assuming there are no deficiencies with the mold or threads of either, from the limited experience I've had in toolmaking, the thread depth need not be any deeper than the outside diameter…..further,   prop being plastic would also help in any equation regarding vibration.
2015-10-12
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