Trigger design fundamentals


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Khornet
September 14, 2005, 11:51 AM
I am working on the development of a surgical instrument that requires a trigger mechanism to release a spring-loaded component. The design we have now has a lot of creep and pull (actually push) force variability. I want to know if anyone can suggest where I can find information on the fundamentals of trigger design.

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jondar
September 14, 2005, 12:05 PM
You might get a copy of "Bolt Action Rifles" by Frank de Haas. It's available, I got mine thru Amazon, around $10. Or Ebay even less. I bought it to adjust the trigger on my Pre-64 Model 70. Was detailed enough that even I could understand it. Hope this helps.

Zak Smith
September 14, 2005, 12:06 PM
Another idea would be to modify the device such that it accepts a regular Timney / Shilen trigger pack. No need to reinvent the (whole) wheel.

Gewehr98
September 14, 2005, 12:08 PM
Another idea would be to modify the device such that it accepts a regular Timney / Shilen trigger pack. No need to reinvent the (whole) wheel.

How many trips through an autoclave would that survive?

Zak Smith
September 14, 2005, 12:10 PM
Ok, then AI triggerpack? :neener:

How hot does an autoclave get?

Gewehr98
September 14, 2005, 01:09 PM
Steamed at 121°C for a minimum of 15 minutes, pressure 103kPa (15lbf/inē)

That'd be hell trying to maintain a no-lube trigger-quality sear engagement, using a non-ferrous and corrosion-resistant alloy. Not saying it couldn't be done, but I doubt it'd be cheap, and it wouldn't break like a Jewell. ;)

Jim K
September 14, 2005, 01:21 PM
How important is safety? If an accidental "firing" is no problem, you can make the engagement short and the release quite light. (My blood sugar test "gun" has plastic trigger and sear and a light "push", but if it goes off too soon, there is no problem, just re-cock and try again.)

Also, polish the engagement surfaces and use a true 90 degree angle on the "cocking piece". Most guns use over 90 degrees so that the sear/trigger actually has to push the hammer or cocking piece back against its spring. This is a matter of safety, but of course makes the trigger pull harder.

Let us assume that lubrication is not practical, so friction has be reduced in other ways. Ensure that the "trigger" spring is as free from drag is possible. If it is in a hole, make sure it doesn't touch the sides or if it is around a guide make sure contact is minimized, possibly by using a cruciform guide. If a stainless steel flat spring can be used, it would probably be better than a coil spring.

All FWIW,

Jim

Khornet
September 14, 2005, 01:53 PM
It does get autoclaved, but it is disposable, so it happens only once. The mating parts are plastic, the spring is a stainless steel coil. Basically, it's a spring-loaded cylindrical part ('bolt') that engages a trigger button. The button engages a notch in the bolt such that pushing the button in a simple, sliding motion that is at right angles to the bolt travel, disengages it from the bolt.

The interface geometry is: The notch on the bolt is a simple ramp, about 25 degrees. The trigger has a rib that engages the notch. The rib has a semi-circular cross section.

The device needs to be packaged, sterilized and shipped in the cocked position without accidental release (we all hate that...) Right now, it is too susceptible to that, and because we can't increase the spring force, I'm thinking we need to add a detent to the bolt notch.

Thanks for the book recommendation and for all the helpful comments.

brickeyee
September 14, 2005, 02:16 PM
Buy a solder sucker and rip it appart.

Gewehr98
September 14, 2005, 03:55 PM
Namely, the atropine injector in my chem warfare bag. Except for your trigger mechanism wouldn't be thigh-impact initiated. :eek:

http://www.sdcounty.ca.gov/deh/images/er_atropine.jpg

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