Spring-loaded trigger stays in position after impact

[oogaboogaengineering]

TL;DR Summary

I'm designing a spring-loaded trigger.
A latch releases a striker using stored energy, but after impact the striker stays in place.
I want it to retract or bounce back automatically.
Looking for ideas to make that happen without adding complex parts.

(image1: before impact) (image2: after impact)
Hi everyone,
I'm working on a compact, spring-loaded trigger mechanism the system uses an "L-shaped" latch to hold back a tensioned spring. after releasing the latch, the spring force pushes a pin (or striker) downward to hit and activate a target surface.

The current problem I'm facing is after the impact, the striker stays in place, pressed against the surface. I want the striker to either bounce back or be retracted right after impact.
Any advice or mechanisms you've seen in similar spring-triggered systems would be greatly appreciated.
Thanks in advance!

 

[berkeman]

Welcome to PF.

TL;DR Summary: I'm designing a spring-loaded trigger.
A latch releases a striker using stored energy, but after impact the striker stays in place.
I want it to retract or bounce back automatically.
Looking for ideas to make that happen without adding complex parts.

I want the striker to either bounce back or be retracted right after impact.

That would seem pretty straightforward. What means have you found and considered so far?

 

[Baluncore]

The current problem I'm facing is after the impact, the striker stays in place, pressed against the surface. I want the striker to either bounce back or be retracted right after impact.
Any advice or mechanisms you've seen in similar spring-triggered systems would be greatly appreciated.

Most of the energy stored in the spring, will be lost during the impact.
What source of energy is available to rearm the trigger?

 

[oogaboogaengineering]

Welcome to PF.


That would seem pretty straightforward. What means have you found and considered so far?

I was thinking about using a return spring, like the kind found in hand tools (pliers, wire strippers, scissors, etc.).
In those tools, the spring helps reopen the handles after squeezing.
Maybe I could integrate a similar spring in my system to help bring the striker back to its resting position after impact.
Do you think that would work well in this kind of setup?

 

[oogaboogaengineering]

Most of the energy stored in the spring, will be lost during the impact.
What source of energy is available to rearm the trigger?

You’re right, most of the spring's energy is used up during the impact.
In my case, the goal isn't just to deliver a hit the striker needs to puncture a sealed surface.
So the motion has to be sharp and forceful, but afterward I want the mechanism to move back out of the way (instead of staying pressed against the surface).

The system is manually preloaded it's a one shot trigger that I arm beforehand. So I'm not trying to create a continuous firing or self resetting loop, just a quick strike followed by automatic clearance.

I’ve been thinking about adding a second low-force return spring, or using geometry that encourages recoil, but I’m open to any clever ideas.

 

[Baluncore]

Two springs are a possibility, but one spring that passes zero-length, just before striking, would withdraw after the strike, as part of a damped oscillation. The time to withdraw would be determined by the mass of the striker, and the strength of the spring.

The spring would then be pulled back from zero length, to rearm the trigger.

 

[oogaboogaengineering]

Two springs are a possibility, but one spring that passes zero-length, just before striking, would withdraw after the strike, as part of a damped oscillation. The time to withdraw would be determined by the mass of the striker, and the strength of the spring.

The spring would then be pulled back from zero length, to rearm the trigger.

One quick thing I was wondering in my setup, the spring and striker are physically one unit, so they move together.

Wouldn’t that mean the spring stops exactly at its neutral (0) point after impact? Since there’s no independent mass flying past or extra stored energy, I’m not sure if it would naturally pull the striker back without something else helping. I was also thinking of adding a second, lighter return spring just to make sure the striker reliably pulls back after puncturing the layer like a safety net.

Do you think that’s overkill, or could it actually help stabilize the motion? And if you were to add a return spring in a setup like this, where would you place it for the best effect?

 

[Tom.G]

Configure the spring so the neutral/relaxed position has the striker just clear of the target. The 'cocked' position would be as your current design with the striker further away from the target.

When triggered, the inertia of the striker will cause the striker to continue to the target. Then the spring will retract the striker to its neutral position.

You may have to add some weight to the striker and use a stronger spring to actually puncture the target.

Cheers,
Tom

p.s. Please let us know how you finally solve the problem. We like to learn too!

 

[Baluncore]

Wouldn’t that mean the spring stops exactly at its neutral (0) point after impact? Since there’s no independent mass flying past or extra stored energy, I’m not sure if it would naturally pull the striker back without something else helping.

How can it strike if it does not have mass with kinetic energy?
If it passes the neutral point before striking, then it will pull back after the strike. How far it will pull back, will be decided by how far past the neutral point, the strike occurs.

 

[oogaboogaengineering]

Configure the spring so the neutral/relaxed position has the striker just clear of the target. The 'cocked' position would be as your current design with the striker further away from the target.

When triggered, the inertia of the striker will cause the striker to continue to the target. Then the spring will retract the striker to its neutral position.

You may have to add some weight to the striker and use a stronger spring to actually puncture the target.

Cheers,
Tom

p.s. Please let us know how you finally solve the problem. We like to learn too!

How can it strike if it does not have mass with kinetic energy?
If it passes the neutral point before striking, then it will pull back after the strike. How far it will pull back, will be decided by how far past the neutral point, the strike occurs.

I think I understand the principle now. I’ve looked into lancet systems and realized that I should add weight to the striker, and also fix the spring at both ends the bottom to the system and the top to the striker so that the spring can recoil properly. I might need to use loosen spring too. Thanks a lot.

 

[Averagesupernova]

No matter what you will need some kind of wind-up mechanism. This may be very simple and need not provide any more energy than is enough for one retract cycle. The actuation of the trigger could be combined with charging this retract mechanism.

 

[Averagesupernova]

I got the point. I'm planning to make the string act like a pendulum, so after impact it should retract back to its natural position.

Your idea of making it act like a pendulum is ok, but it will oscillate. You need a ratchet that catches and holds once it gets returned to a certain point. But to release it needs to be pulled farther and at a certain point the ratchet is released and then allowed to spring forward into the target and the cycle repeats.
-
The way you've described it I've wondered if you are assuming it will always return to the position where it is ready for release again. This cannot happen. It implies perpetual motion. I maybe misinterpreted you.

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Okay, thread is reopened. Thanks all for your patience.

 

[Tom.G]

Gee, the "closed"-"open" sequence seem to imply an automated flagging of
p-e-r-p-e-t-u-a-l + m-o-t-i-o-n wording. (I hope this post doesn't trigger it.)

 

[oogaboogaengineering]

Your idea of making it act like a pendulum is ok, but it will oscillate. You need a ratchet that catches and holds once it gets returned to a certain point. But to release it needs to be pulled farther and at a certain point the ratchet is released and then allowed to spring forward into the target and the cycle repeats.
-
The way you've described it I've wondered if you are assuming it will always return to the position where it is ready for release again. This cannot happen. It implies perpetual motion. I maybe misinterpreted you.

You're right, adding a guide for the spring to minimize oscillation is something I had overlooked. Thanks a lot!