Featherweight Screw Switch - Safety FYI/Alert

The Rocketry Forum

Help Support The Rocketry Forum:

mrwalsh85

Well-Known Member
Joined
Apr 30, 2012
Messages
1,252
Reaction score
279
All,

I wanted to share a safety concern that I have encountered.

I have a rocket that utilizes these PCB mount screw switches. Twice now, I have had the rocket arm itself during transit. I keep my Av Bays prepared and in the house until it's time to transport as I don't want to expose my batteries to colder temperatures if I can avoid it.

During transit, it would appear that these switches have vibrated enough to allow themselves to close. I have heard of rocketeers replacing the switches with longer screws and deforming the threads on the backside of the switch so they can back out fully. I haven't done this and am concerned because I do not want to introduce undue force to the PCB mounted nut - I've had this nut break loose in the past as the screw somehow would not release.

In the first instance, I immediately pulled over and opened all the windows to help equalize air pressure, then carefully opened/closed the doors that I utilized to remove the av bay from the cabin. The second instance, it was in my front seat. I kept a screwdriver with me in my cupholder in the event that it armed itself, and it did. Thankfully I caught it during the startup sequence so I was able to disarm it.

I'm not sure what I'll do in the future to eliminate this safety risk. This av bay is due for a rebuild anyway so I have opportunities for kaizen. One thought I have is to introduce a shunt for all of my charges so that if the altimeter does arm itself and fire itself due to doors opening/closing, it won't fire the charges. Other options are to explore other manufacturers.

I am not throwing Featherweight out the window as an option, but also don't want anyone else to have a nasty surprise, either.

YMMV. Feedback and/or constructive criticism is appreciated.
 

Titan II

Well-Known Member
Joined
Aug 24, 2013
Messages
1,220
Reaction score
356
The following refers to the ones sold by MissileWorks.

I have used dozens of these switches since they were introduced. I "slightly" deform the screw threads for resistance.

I typically, but not always, use the switch cover-guide. If you snug the screw against it, it also helps the screw from vibrating in (which it will not do anyway with the "slightly" deformed threads).

I have never had any problems.

Here is a previous thread that discusses this issue:

 
Last edited:

rharshberger

Well-Known Member
Joined
Oct 13, 2014
Messages
10,963
Reaction score
2,794
Location
Pasco, WA
Staking the threads aka deforming the end of the threads just makes it very difficult/impossible to back out the screw, however it also has the effect of helping to keep the switch in the disarmed position. These type switches are very reliable but be aware of their limitations.
 

timbucktoo

Well-Known Member
Staff member
Global Mod
Joined
Jun 13, 2014
Messages
9,380
Reaction score
1,848
Location
Cocoa Beach
I forget who makes them but there is a plastic insert you can add to these screw switches that allows the screw to snug up against a backing in the off position.
 

Voyager1

Well-Known Member
Joined
Oct 23, 2015
Messages
816
Reaction score
297
Location
ACT, Australia
I have had the same issue with the screw switches. There are a few 'fixes' that can reduce or eliminate the problem. Some of those have been mentioned above. Personally, I've found the best solution is to use the plastic cover guide. However, there are instances where the cover will not fit between the switch and the body tube.

Another method is to use a drop of Loctite to prevent the screw from freely rotating while in its OFF position. This might also increase the friction between screw and switch body and prevent the screw from freely rotating even after several activation cycles. Additionally, a drop of Silicon sealant in the nut thread might be useful, too - just don't get it on the underside of the screw head and the PCB contact.

I have since changed all my avbay switches to either WiFi (Eggtimer) or microswitch pull pins (Lab Rat).
 

manixFan

Not a rocket scientist
Joined
Feb 15, 2009
Messages
2,519
Reaction score
1,690
Location
TX
This is a well known issue, and certainly not unique to the Featherweight brand of switches. In fact, I brought this up during the insanity that went on when the magnetic and wifi switches were 'banned' from use with flight computers that controlled ejection charges. The argument was that mechanical switches were 'safer' than electronic versions, but as you, and many others, have discovered, they have a very common failure mode. As mentioned, there are many ways to solve it, including using a 3D printed mount, reusable thread tightener, burring the threads, etc.

Really, any brand of screw switch that does not have some sort of friction lock on the threads is free to turn from vibration and close during continuous vibration. Nothing new or unique to rocket screw switches.


Tony

example of a popular screw switch rotating from vibration:
Screw-switch-test.gif
 
Last edited:

jderimig

Sponsor
TRF Sponsor
Joined
Jan 23, 2009
Messages
4,173
Reaction score
1,970
One thought I have is to introduce a shunt for all of my charges so that if the altimeter does arm itself and fire itself due to doors opening/closing, it won't fire the charges.
A Shunt by itself will not prevent the charge from firing. My recommendation is NEVER transport a rocket with with a charge AND battery installed. Those two should always be brought together at the range.
 
Joined
Jan 18, 2009
Messages
7,962
Reaction score
508
Location
Stafford, VA
I understand the issues and I've never bought a screw switch. I have bought double sided circuit board and made my own. I always mount them so they can be unscrewed (OFF) so the screw head tightens against the body tube to keep it off. This prevents them from vibrating ON, just like tightening them down prevents them from vibrating OFF during flight.

This is definitely an issue, but one I think is easily mitigated once you are aware of it.
 

Tractionengines

Well-Known Member
Joined
Aug 29, 2020
Messages
461
Reaction score
351
Location
Northeast Ohio
Another option. A thin o-ring under the head of the screw. Big enough the head can't screw in far enough to close the circuit from vibration, but small enough the torque from a screwdriver can compress it and you can close the circuit.
I have not done this in rocketry AV bays, but other things. So YMMV.
 
Joined
Jun 30, 2015
Messages
1,465
Reaction score
1,681
Location
Australia
Travelling from home to the launch site with a connected battery and the only arming safety of a screw switch would seem worthy of any Darwin award that might be issued. Lets also mention that your vehicle insurance probably just got voided ( did you add in the optional transporting armed pyrotechnics premium) and you are definitely not covered by Tripoli insurance.
My 2c....
 
Last edited:

jbr

Well-Known Member
Joined
Nov 22, 2017
Messages
564
Reaction score
280
I have one rocket with 84 launches and use screw switches
I deform the threads so the screw is harder to turn and it stays where it is supposed to
 

Troy3003

Well-Known Member
Joined
Feb 12, 2021
Messages
272
Reaction score
197
Location
Shelbyville, KY
Steve at SMT has a screw switch setup that he uses in his bay designs that you can use independently along with his printed switch housing. You can remove the screw and the switch is spring loaded to the off position.
 

rharshberger

Well-Known Member
Joined
Oct 13, 2014
Messages
10,963
Reaction score
2,794
Location
Pasco, WA
Steve at SMT has a screw switch setup that he uses in his bay designs that you can use independently along with his printed switch housing. You can remove the screw and the switch is spring loaded to the off position.
other than being a bit bulky for some applications I really like SMT solution.
 

manixFan

Not a rocket scientist
Joined
Feb 15, 2009
Messages
2,519
Reaction score
1,690
Location
TX
I agree that energetics should not be prepped until at the field. But even that can still be an issue with screw switches. One of our club members got his rocket ready to go, put it in the back of his pickup for the trip out to the away pads, and drove off. When he got to the pads, he heard beeping coming from the back of his truck. One of his altimeters had been turned on by a screw switch vibrating closed during the short trip out to the pads.


Tony
 
Joined
Jan 18, 2009
Messages
7,962
Reaction score
508
Location
Stafford, VA
I use pull pins in series with screw switches. When you put the pins in, it's a second power switch that is OFF along with the screw switch. Besides the safer transport, you can turn off the power without the pull pins in place while prepping. The insert the pins and turn on the screw switches just before closing the rocket and putting the apogee shear pins in place.
 

Adrian A

Sponsor
TRF Sponsor
Joined
Jan 22, 2009
Messages
2,375
Reaction score
449
Another option (and what I do for Featherweight screw switches used for air starts) is to just back the screw all the way out and transport it separately. If you're worried about losing it, just tape it to the airframe which also acts like a remove before flight reminder.
 

Arpak

Well-Known Member
Joined
Nov 23, 2020
Messages
202
Reaction score
199
Location
San Antonio, TX
I use the PCB switches but with three caveats

1. I install longer screws and chew up the ends so they can't fall out
2. When being transported I unscrew them far enough that they push out into the body tube.
3. I only use them with eggtimer wifi devices (quantum, proton)

I think I'm not alone in these methods.
 
Top