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Old Dec 15, 2012, 11:32 AM
Flying = Falling (Slowly)
dharban's Avatar
Tulsa, OK
Joined May 2004
2,692 Posts
Discussion
Motor Mod For ALES

One of the things that I have observed and commented on here is the importance of properly attaching our spinner hubs to our motor/gearbox shafts. It can be argued that safest and most secure hubs use a collet system.

For better or worse, the hubs that I prefer (for reasons other than safety) are fastened to the shaft with set screws. As I have mentioned, most of these devices have two set screws placed at 180 degrees on the shaft. This is not a good and safe configuration. IT IS WORSE THAN A SINGLE SETSCREW. Good engineering would dictate two set screws placed 0 degrees apart or, at worse, maybe 90 degrees apart.

If you are contemplating using a hub that uses setscrews, use only one setscrew and mill a flat on the shaft on which to seat the setscrew. I do this and as an extra precaution, I do not mill the flat all the way to the end of the shaft. This way, if the set screw does loosen, it will still be captured on the shaft.

I have fashioned fixtures for doing this on 5 mm and 6 mm shafts (shown below). When I mill the flat, I tape the openings in the motor to keep the cuttings out. The finished product is a flat which is true and is probably your best bet on ensuring that the prop will not fly astray during use.

While the process of getting a true flat is very simple when you can mill it, filing can be iffy and result in a surface that is less rather than more likely to secure the set screw. For interested parties I would be pleased to mill a flat on your motor/gearbox shaft for $5 plus postage. Just PM me.

Happy Landings,

Don
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Old Dec 15, 2012, 12:17 PM
WINS - Winch In Nose Sailplane
jaizon's Avatar
USA, NH
Joined Mar 2008
3,109 Posts
Excellent info, Don. Thanks.

I always loctite the set screws.

One question. Why are two set screws set at 180 worse than one?
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Old Dec 15, 2012, 02:56 PM
Flying = Falling (Slowly)
dharban's Avatar
Tulsa, OK
Joined May 2004
2,692 Posts
Quote:
Originally Posted by jaizon View Post
Excellent info, Don. Thanks.

I always loctite the set screws.

One question. Why are two set screws set at 180 worse than one?
Everything we use in our construction is elastic to one degree or another. When we tighten a single set screw it realizes some of its holding power presumably by deforming the point of the setscrew into the shaft (but not very much on our hard steel shafts). The rest is realized by the "spring" force that our deformed hub exerts on the set screw. The screw pushes the shaft against the opposite side of the hub. The force screw exerts is proportional to the torque applied to it. If you place a second screw opposing the first screw (i.e. at 180 degrees) and tighten it to the same torque it will exert the same force. The thing is, without that second screw, that same force is already being exerted by the part of the hub opposite the first screw. There may be gains, but they are small. The hooker comes if one or the other screw backs out a bit. In this case the secure screw becomes less effective somewhat more quickly. Sometimes this can actually be observed by tightening one screw and then the second screw and then loosening the first screw -- the whole thing comes apart. How do I know? I have watched propellers fastened with two screws spiral nicely away from the front of the plane.

At any rate, this is a somewhat roundabout explanation of why engineering handbooks tell you to not use set screws placed 180 degrees apart from each other. There is actually design information which tells you how much you gain from placing a second screw on a shaft. Zero degrees (side by side) effectively doubles the holding strength. 180 degrees does almost nothing -- and for setscrews set on very hard shafts may be less than useful.

Happy Landings,

Don

BTW: I have observed that using set screws in any configuration on a shaft that does not have a flat ground on it can be a real problem if the hub ever comes slightly loose and is allowed to turn on the shaft. In that case, the set screw(s) can score the shaft and make it impossible to remove the hub from the shaft. I once had to grind a RF hub to pieces to get it off the shaft because of this very thing. I use a single set screw now and a flat. I use a little loctite on the screw. AND I CHECK TIGHTNESS AT LEAST ONCE EVERY FLIGHT SESSION. And every few months I discard the screw so I will not strip the hex out and have to grind the hub off.
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Old Dec 15, 2012, 03:01 PM
WINS - Winch In Nose Sailplane
jaizon's Avatar
USA, NH
Joined Mar 2008
3,109 Posts
Learn something new every..... . Thanks.
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Old Dec 15, 2012, 06:35 PM
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Lenny970's Avatar
Greeley, Colorado, USA
Joined Feb 2000
2,913 Posts
Good information Don, that many people are unaware of.
Most of our competition F5B props have setscrew type spinners and they hold reliably at 7000 watt power levels with a single setscrew and a good flat cut into the shaft.

Another important point is that only a good quality hex driver should be used on the setscrew.
Don't try to use a ball-end driver or a cheap "L" wrench

Lenny
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Old Dec 16, 2012, 09:21 PM
turn, turn, turn.
Athol, Massachusetts
Joined Oct 2005
10,492 Posts
I also grind down all my shafts.

If you do decide to grind your own, start at the motor end, and work your way towards the front... as the grinding stone will try and wander if you do it by hand.

Make sure you have a positive stop at the front... where the set screw meets the shaft at the appropriate spot.
It doesn't matter if the back end tapers.
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Old Dec 17, 2012, 12:47 AM
Tragic case
davidleitch's Avatar
Sydney Australia
Joined Feb 2002
5,875 Posts
The point about taping the motor openings closed is good advice. I recently shorted an F5B motor and the likely cause was filings and or dust being sucked into the rear of the motor on power up.
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Old Dec 17, 2012, 12:50 AM
Tragic case
davidleitch's Avatar
Sydney Australia
Joined Feb 2002
5,875 Posts
If the setscrew does somehow get locked onto the motor heating the shaft can help, as can a pinon puller.
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Old Dec 17, 2012, 08:17 AM
Flying = Falling (Slowly)
dharban's Avatar
Tulsa, OK
Joined May 2004
2,692 Posts
Quote:
Originally Posted by davidleitch View Post
If the setscrew does somehow get locked onto the motor heating the shaft can help, as can a pinon puller.
The problem I have had twice now has not been removing the setscrew from a hub which has spun on the shaft -- I have always been able to remove the setscrew. It was the scoring on the shaft which made it impossible to pull the hub off the shaft. And with the particular hubs that I was using, in one case it was not possible to use a puller without destroying the integral spinner and in the other case, there was not room to employ a puller at all.

In any case -- grind or mill a flat on your shafts, use a bit of Loctite, and check for tightness frequently.

Happy Landings,

Don

BTW: There are many different Loctite products which vary widely in holding power. Some are strong enough to secure our motor pinions to their shafts -- DO NOT USE THIS UNLESS YOU ARE PREPARED TO HEAT THE HUB TO REMOVE THE SETSCREW. One good Loctite product for this is Loctite 222 (Purple). It has low holding power and makes it possible to remove a fastener without heating it. Many local hobby shops carry Loctite. I use Mcmaster-Carr as my source.
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Old Dec 18, 2012, 04:32 PM
Electric Airplane Junkie
bhchan's Avatar
United States, CA, San Mateo
Joined Jun 2000
5,597 Posts
I found most gearbox has hardened shaft and the normal end mill would not even scratch the shaft. I just use a grinding disc to cut a small flat are on the shift where the set screw will be.

Brian, an EAJ
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Old Dec 18, 2012, 07:20 PM
Flying = Falling (Slowly)
dharban's Avatar
Tulsa, OK
Joined May 2004
2,692 Posts
Quote:
Originally Posted by bhchan View Post
I found most gearbox has hardened shaft and the normal end mill would not even scratch the shaft. I just use a grinding disc to cut a small flat are on the shift where the set screw will be.

Brian, an EAJ
I tried to use a regular steel mill cutter the first time. Result = Fail

I went to a carbide cutter, fairly high speed on the bit, lowest possible feed rate. So,far, no problems at all.

A word of warning. My preference would probably be to grind the flat with a grinding machine. But using a hand held grinder, sort of free style, may result in a less safe solution than no flat at all. Because of the hardness of our motor/gearbox shafts, the set screws we use only get a tiny, tiny bite on their seating surface (the shaft). If a hand ground flat is untrue to a larger dimension than the depth of the " bite" (probably 2 or 3 thousandths) striking the hub (not an uncommon experience for some of us ) may unseat the setscrew's tenuous grip on the shaft. Not good. Not good at all

Happy Landings,

Don
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Old Dec 19, 2012, 12:02 AM
Electric Airplane Junkie
bhchan's Avatar
United States, CA, San Mateo
Joined Jun 2000
5,597 Posts
I usually only cut a flat just wide enough for the set screw, not a long flat area. Some time I use a collet type prop hub, so smaller cut is better.

Brian, an EAJ
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