Quantcast

#4 shear pins

The Rocketry Forum

Help Support The Rocketry Forum:

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
Does anyone know how much force it takes to shear #4 or 4-40 nylon screws. This would be the ones from Mcmaster Carr. I believe it is 35 pounds to shear the #2 or 2-56 nylon screws. Not sure on the 4-40 pins.

I am working with a 1.64 airframe 5.0" in length. Estes tubing reinforced with aluminum tape, I don't know what the mil is on it but I do know it is now stronger than the stock tubing. I come up with 51.38 psi to shear 2, 4-40 pins using .28 grams of 4F BP. I haven't ever pressurized this small of airframe tubing before. Does this sound like to much pressure for this tubing?

If the tubing was one more inch in length I think I could use less pressure to shear the pins. Didn't have the option for that or to use 2-56 nylon screws.

The airframe is reinforced at the shear pin area with .005" brass shim stock and the tubing has been hardened with cyano. I am relatively sure it will get the chute out lol. I plan on testing it before putting it on the pad but I have no more tubing to replace it with if I blow it before the launch. I am sure it is going to be over a 250 pound pop. I could lower the pressure and just pressure fit the NC but what fun would that be :)
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
Thanks, Will and Bob I was hoping you got in there, I always like how you put info in there :)

I originally came up with a little over 73 psi to shear the pins but I tried working it down to reduce the pressure in hopes that it would not make the airframe tubing fail. I can see it is better to have a little more pressure to insure the pins shear. I am going to bring the pressure back up and my be reinforce the outside of the airframe and give it a test. First I will check out the link.
 

dixontj93060

Well-Known Member
Joined
Feb 20, 2009
Messages
12,882
Reaction score
19
Sandman,

Be careful on your units. It may take 73 pounds of force to shear a couple of nylon pins, but that's WAY different than 73. psi. The psi required within the airframe will be much lower and is dependent primarily on diameter of the frame.

-Tim

Thanks, Will and Bob I was hoping you got in there, I always like how you put info in there :)

I originally came up with a little over 73 psi to shear the pins but I tried working it down to reduce the pressure in hopes that it would not make the airframe tubing fail. I can see it is better to have a little more pressure to insure the pins shear. I am going to bring the pressure back up and my be reinforce the outside of the airframe and give it a test. First I will check out the link.
 

WillMarchant

Well-Known Member
TRF Supporter
Joined
Jan 15, 2009
Messages
2,323
Reaction score
54
What Tim said!!!

Figure out the area of bulkhead that your ejection charge is working against. For a 6 inch airframe it is pi*radius*radius or 3.14*3*3 which is about 28 square inches.

Use a BP calculator to estimate the pressure that will be generated by your motor or charge. A lot of people aim for 10 PSI. But you'll need to iterate the charge size in a minute...

So 28 square inches being acted on by 10 PSI is 280 pounds of force.

If the shear pin combination you've chosen needs about 100 pounds of shear force then you're sitting pretty.

Always leave yourself margin!!! And test test test on the ground!!!

How much margin is enough? That is the key question and I don't have a definitive answer...

You may notice that smaller airframes will generate smaller shear force and may need a higher PSI of gas. So you may want to change the shear pin combination or the gas charge to try and get a reasonable compromise.

How many shear pins are enough? I've used RockSim and looked at the deceleration right after motor burnout. You can then do the simple f=ma calculation to see what "pull" is being seen by the separation point(s) in your rocket. You want the shear force of your retention to be sufficiently large to prevent that drag separation.

I tend to ignore gas expansion in the compartments as I use vent holes. Maybe that's a mistake. It is probably OK for relatively low altitude flights.

Bob, did this sound right? I think we want to try and give people an idea of how to size their shear pin retention system and then how to adequately separate...
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
I did use a BP calculator for my figures but it is modeled for larger airframes. It just does not give you examples for the smaller tubes. I worked my way up in pressure until it would shear the pins. It seamed to have a range it would shear two 4-40 pins. I was wondering how accurate the data was, what I did not have was the force it would take to shear the pins. The data Bob put up supported the calculator that it is a range that they shear.

The smaller space will take more psi, pressure per inch to produce pound force against the NC to move it. Now increased by having to overcome the shear pins to move the NC. I can shear 2 pins at 51.38psi and .28 grams minimally via the calculator. Maximum before having enough pressure to shear 3 pins is 73.39psi (pressure) and 0.40 gr BP.

Pound force is a different measurement, I believe is the pounds exerted for the pin to shear mechanically. Math genius I am not but I am used to BP firearms using pound force more, pressure being less of a problem from heavy barrels. Rockets and thin airframes is more of a pressure issue if I am on the right track.

Bobs data as well as Will's link to, supported what I came up with the calculator. I was working on the lower end of the psi to shear the pins to keep the pressure down on the airframe. I decided it would be better to bring it up more to the middle of the range to insure the pins shear and be on the safe side. Airframe will have to take the heat lol.

Pound force for the complete pulse must be over 200 pounds, less actually used as the NC moves. This part is what I believe to be happening as I understand it. It was not until I started to figure out the charge for this small space that I realized the pressure was going to be so high and that I wanted to know where the shear pins failed. My first test I want the pins to fail and I will back off from there. I think the airframe will fail if I am just under the shear pressure. I am also more used to 2.6 to 6.0 airframes which need lower pressures.
 
Last edited:

dixontj93060

Well-Known Member
Joined
Feb 20, 2009
Messages
12,882
Reaction score
19
Here, try this. Best ejection charge calculator I have found by Chuck Pierce posted on the Aerocon Systems website: Charge Calculator.

Also here is an excellent article on Info Central explaining ejection charge sizing, airframe psi and pin shear force: Black Powder Usage.
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,351
Reaction score
26
I did use a BP calculator for my figures but it is modeled for larger airframes. It just does not give you examples for the smaller tubes. I worked my way up in pressure until it would shear the pins. It seamed to have a range it would shear two 4-40 pins. I was wondering how accurate the data was, what I did not have was the force it would take to shear the pins. The data Bob put up supported the calculator that it is a range that they shear.

The smaller space will take more psi, pressure per inch to produce pound force against the NC to move it. Now increased by having to overcome the shear pins to move the NC. I can shear 2 pins at 51.38psi and .28 grams minimally via the calculator. Maximum before having enough pressure to shear 3 pins is 73.39psi (pressure) and 0.40 gr BP.

Pound force is a different measurement, I believe is the pounds exerted for the pin to shear mechanically. Math genius I am not but I am used to BP firearms using pound force more, pressure being less of a problem from heavy barrels. Rockets and thin airframes is more of a pressure issue if I am on the right track.

Bobs data as well as Will's link to, supported what I came up with the calculator. I was working on the lower end of the psi to shear the pins to keep the pressure down on the airframe. I decided it would be better to bring it up more to the middle of the range to insure the pins shear and be on the safe side. Airframe will have to take the heat lol.

Pound force for the complete pulse must be over 200 pounds, less actually used as the NC moves. This part is what I believe to be happening as I understand it. It was not until I started to figure out the charge for this small space that I realized the pressure was going to be so high and that I wanted to know where the shear pins failed. My first test I want the pins to fail and I will back off from there. I think the airframe will fail if I am just under the shear pressure. I am also more used to 2.6 to 6.0 airframes which need lower pressures.
If I'm reading your post correctly, you're are messing up your units.

Pounds is a load or a force. Pressure is a load or force per unit area. They are not the same.

For example, let's take the case of (2) #4-40 nylon screws being used for shear pins. From my previous post, you have to develop at least 91 pounds of force to be certain that you will break the shear pins when the pyro charges fire.

The pressure required to develop this load depends on the airframe diameter (cross-sectional area.)

Pressure in PSI = Load in pounds / Area in square inches

or minimum PSI = pounds / 3.14 * (diameter in inches/2) ^2

In practice you probably want is 10 psi +/- 5 psi internal pressure for most rockets. While this may seem high, the reason you pin the airframe and the nose cone together is to prevent the pieces separating due to drag separation, internal pressurization as the rocket ascends, and in there case of a dual deployment rocket, to prevent an inertial main chute deployment when the apogee charges go off.

If you were to use such large shear pin on a 2" rocket, you need to develop 30 psi to break the shear pins. The formula is required load / cross-sectional area = 91 psi / 3.14 * (2 inches /2) ~ 30 psi. Too much pressure for a model rocket airframe IMO. If you needed shear pins for this small diameter (really high performance rocket) , you would go to (2) #2-56 nylon screws.

(2) #4-40 nylon screws are about right for a 4" airframe where you need ~7.5 psi to shear the pins and not enough for a 6" airframe where the pins will release at low ~3 psi.

To figure out how much BP is required, you need to know the volume you need to pressurize to ~10 -15 PSI. You can use the BP calculators mentioned previously to figure this out.

Hopefully this explains how shear pins work.

Bob
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
This is an experiment using a 1.64" airframe rocket and making it dual deploy. I only had 4-40 pins to use and it ended up I worked my way to figuring this out last on this project. My pressure and pound force calculations are coming from the calculator from your link. I do admit I need the calculator to come up with the charge size for this project. I do understand how shear pins work.

I usually work with the larger airframes and work with 10psi but the calculator will not work at 10psi on the 1.64 airframe. The psi had to be much higher to give me a charge size that would shear the pins. I should have gotten 2-56 pins but there was no time before the launch.

My thoughts are with that kind of pressure that the airframe might fail. So I reinforced it with aluminum tape and used brass shim stock to help shear the pins faster. I am going to ground test my charge with the 4-40 pins today and see what happens. Hopefully my reinforcing will help but I plan on replacing the tubing and using 2-56 shear pins.
 

dixontj93060

Well-Known Member
Joined
Feb 20, 2009
Messages
12,882
Reaction score
19
STAND BACK :bangbang:

This is an experiment using a 1.64" airframe rocket and making it dual deploy. I only had 4-40 pins to use and it ended up I worked my way to figuring this out last on this project. My pressure and pound force calculations are coming from the calculator from your link. I do admit I need the calculator to come up with the charge size for this project. I do understand how shear pins work.

I usually work with the larger airframes and work with 10psi but the calculator will not work at 10psi on the 1.64 airframe. The psi had to be much higher to give me a charge size that would shear the pins. I should have gotten 2-56 pins but there was no time before the launch.

My thoughts are with that kind of pressure that the airframe might fail. So I reinforced it with aluminum tape and used brass shim stock to help shear the pins faster. I am going to ground test my charge with the 4-40 pins today and see what happens. Hopefully my reinforcing will help but I plan on replacing the tubing and using 2-56 shear pins.
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
LMAO! hardly :) I will let you know Saturday, maybe with some pictures.
 
Last edited:

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,351
Reaction score
26
This is an experiment using a 1.64" airframe rocket and making it dual deploy. I only had 4-40 pins to use and it ended up I worked my way to figuring this out last on this project. My pressure and pound force calculations are coming from the calculator from your link. I do admit I need the calculator to come up with the charge size for this project. I do understand how shear pins work.
You need to pressurize a 1.6" airframe to minimum of 45 psi to insure that you will shear the (2) #4 screws. Do the BP claculations for 10 PSI and then use 4.5 times that amount of BP in your charge.

If you do this with a 1.6" airframe and use (2) #4-40 shear screws, make sure the video is rolling! I think you'll have lots of little pieces after the charge goes off.

Bob
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
Al, I am going to show up lol!

Bob, I think so too. It will be a confetti deployment :y:

George, Great and a good camera man will be there.

Here is a before picture. It's an Estes Stormcaster, stock length, HK45 Perfectflight altimeter and it will be going up on a F21W. Thirty inch main chute to be deployed at 300ft AGL. Maybe low enough to get a good shot of it deploying or destroying lol! Max altitude 2295.78ft, no streamer or chute for the apogee separation. Stability margin 2.04, pad weight ready to launch 10.0oz.

 
Last edited:

dixontj93060

Well-Known Member
Joined
Feb 20, 2009
Messages
12,882
Reaction score
19
With the light airframe components in an Estes kit, I think I would just drop back and finesse a friction fit. Just make the apogee separation adequately tight, and then the main separation marginally tighter. Then just enough BP to pop it. Will take more estimating, ground testing and "experienced vigorous shaking of the rocket", but I'd say that route has a higher probability of success.
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
With the light airframe components in an Estes kit, I think I would just drop back and finesse a friction fit. Just make the apogee separation adequately tight, and then the main separation marginally tighter. Then just enough BP to pop it. Will take more estimating, ground testing and "experienced vigorous shaking of the rocket", but I'd say that route has a higher probability of success.
That will be a safer route for sure and this late in the game is what I was going to do if there are kids at the launch. It is most likely to be in the 20's for temp and I was hoping just the hardcore rocket guy's show up lol. I already plan to replace the Main chute section so maybe the test will still be a go. I would like to see if my reinforcing made a difference. The inside of the tubing is covered with aluminum duct tape overlapping by .250" and a 1.750" band around the outside on the NC end. The NC shoulder only goes into the airframe less than 1 inch. The pressure should start bleeding off fast once the NC is out of the way but there is still a lot of chute to push out of the way also. I started working on this rocket almost 2 years ago and this was kind of race to the finish for this launch. I think it will be an awesome marginal field rocket once I put the smaller pins in and get the pressure down.
 

WillMarchant

Well-Known Member
TRF Supporter
Joined
Jan 15, 2009
Messages
2,323
Reaction score
54
You are going to ground test this before flying it, right?
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
You are going to ground test this before flying it, right?
Yeah, without the pins and a smaller charge. It works with 0.13 grams. On it's side it pops the NC and deploys the chute but the chute only comes out of the tube about 1/2 inch. Will be better without dragging on the blanket I think. I might pack the charge a little tighter.
 

bobkrech

Well-Known Member
Joined
Jan 20, 2009
Messages
8,351
Reaction score
26
Yeah, without the pins and a smaller charge. It works with 0.13 grams. On it's side it pops the NC and deploys the chute but the chute only comes out of the tube about 1/2 inch. Will be better without dragging on the blanket I think. I might pack the charge a little tighter.
That's cheating. You really need to use to pins. :hohoho:

Bob
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
Yeah, I agree with Bob. You should test it the way you intend to fly.
Count down to launch has already started so no time to test it with the pins. I don't want to have it fail at home if there is a safe way out of this. It's the only rocket I have for this launch.

Don't worry once I see how things look at the field I might be testing it anyway with the pins lol! I didn't do all that work for nothin ;)
 

WillMarchant

Well-Known Member
TRF Supporter
Joined
Jan 15, 2009
Messages
2,323
Reaction score
54
It would be a shame if your rocket failed to separate and someone was killed or injured or property damaged by a ballistic recovery. I'm trying to think how the lack of ground testing will sound when being explained to the police and insurance investigators.
 

DAllen

Well-Known Member
Joined
Jan 18, 2009
Messages
4,774
Reaction score
718
Count down to launch has already started so no time to test it with the pins. I don't want to have it fail at home if there is a safe way out of this. It's the only rocket I have for this launch.

Don't worry once I see how things look at the field I might be testing it anyway with the pins lol! I didn't do all that work for nothin ;)
Ruh-roh George.

This is not a good idea. As one who has had a good share of deployment failures I would far rather have a failure doing a ground test when the rocket is going ZERO miles and hour rather than when it is coming in ballistic at a BUHzillion miles and hour. Even with the drogue deployed on a relatively soft field, bad things do happen to airframes when the main doesn't deploy. I had a LOC Weasel do that at LDRS 27 and landed in the grass at the Kloudbuster's field and it still messed up my forward body tube.

-Dave
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
It would be a shame if your rocket failed to separate and someone was killed or injured or property damaged by a ballistic recovery. I'm trying to think how the lack of ground testing will sound when being explained to the police and insurance investigators.
As hap hazard as my statement may have sounded it was more of a joke. I always ground test, more just because I am picky that way. It would be more than a shame if someone got injured from neglecting such an important step. I just did not want to ruin the rocket with what we all thought on here was to much pressure for what I was trying to accomplish. I chose to friction fit the NC, an acceptable method but risky as you can't count on it to stay on during the apogee separation. Ground tested as previous post states with results.

Even when you do it by the numbers you should still worry that you payed enough attention to detail and have enough craftsmanship to get it right so your not dangerous. If your lacking on ether you will get your share of failures. To this I can say I have no shares as when I have doubts I ask other flier's in my club or in here as there seams to be a larger base of collective knowledge for trying different things. Sorry I gave you the impression I was being reckless. I hope we can meet on the field sometime and have a good time, I can promise you won't worry about my 10 oz. rockets :santa-smile:

How the flight went today, I did get to fly :happydeer: It was 23 degrees on the field and only one adult dragged a child out so he could fly :( So I did not even bother to think about the shear pins. I had to put more tape on the NC as it loosened up from cold expansion. I believe there is video of the launch and as chance would have it, I think the weight of the chute helped to deploy the main at altitude during apogee separation. I got to decorate the :tree: as several others did, I do think I got them on best distance into the woods though lol!
 

dixontj93060

Well-Known Member
Joined
Feb 20, 2009
Messages
12,882
Reaction score
19
Congrats on a successful flight. But sorry about the recovery. Were you able to extract it from the :tree:?
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
Congrats on a successful flight. But sorry about the recovery. Were you able to extract it from the :tree:?
It was our rocket clubs last launch of the season and we had a large storm coming down on us but the hardcore group of us always get together for a what seams like always an extreme launch lol. Usually the weather is to cold for all but the boldest of the clubs members. Surprisingly are crowd has grown for this event from past story's.

We had probably 20 people or more people show up for this launch and we might have had 4 or 5 rockets that we could not recover from the trees by the time we stopped. Mine was one that is still out there. This morning I am looking out my window at 18 inches of snow or better and it is still coming down hard. I am thinking about that rocket in the tree out there but I am glad I got to go launch one at our last launch, it was a great time and good friends. My fingers are still cold lol :happydeer:
 

sandmantoy

Well-Known Member
Joined
Jan 28, 2009
Messages
536
Reaction score
1
Got a few pictures of the Stormcaster before I set it free :cool: I will make another one and maybe move the electronics bay back a little to make the main chute bay a bit larger as well as get the 2-56 pins for it. I used a 30" drag queen for the main, it did not have a spill hole contributing to the extra drift at altitude. I think I will change that in the new one to an 18" chute with a spill hole in it too.


Nice boost shot of the F21W
 
Last edited:
Top