Interesting hammer spring findings

[Racer377]

Prompted by another thread, I started looking into how spring rate and preload are affected by cutting coils. I pulled out my 13lb and 15lb springs from CZ custom and started measuring in order to plug them into a little calculator I made in excel.

Wire diameter is the same at .0405. The 13lb is shorter and clearly has a coil cut.

Gd4/[8nD3] cz 13 cz 15

k = constant, pounds of load per inch of deflection 15.91747503 15.34899378

G = modulus of rigidity of spring material, pounds per square inch 11500000 11500000

d = wire diameter, inches 0.0405 0.0405

n = number of active coils 27 28

D = mean coil diameter 0.208 0.208

Free length 2.415 2.59

Compressed length at full cock 1.6 1.6

x=preload -0.815 -0.99

F=-(Gd^4/(8nD^3))x 12.97274215 15.19550384

Moral of the story, if you need a 13lb and can't find one, you can make it out of a 15lb.

Edited June 4, 2012 by Racer377

[CeeZer]

Before you cut and tune - smooth out main spring strut and anything that would affect compression/decompression of the spring!!

When I tune main springs - I measure trigger pull SA and DA a few times each. I make first cut depending on numbers and my experience. Then - measure/cut/measure/cut... usually in 1/4 coil. If gun has to be reliable with as many ammo makes etc - there are lowest ## too as well as at least 100rds live fire test.

[PaperScraper]

Cutting a 15 lb spring shouldn't change the coil resistance. It will be a 15 lb spring without pre-load so your getting an exact 15 lbs of resistance.

It might end up being the same resistance as a full length 13 lb before you have cut the 13 lb down and removed it's pre-load.

This is one of the reasons for the extended firing pin, when you cut a 13 lb down and remove the pre-load you end up with light primer hits at times without the extended firing pin.

CZ pistols are some of the simplest pistols to tune and being the design seems to have taken inspiration from the Sig P210 there is no cuestion it is one of the most accurate out of the box semi-auto pistols on the planet.

Edited June 5, 2012 by PaperScraper

[motosapiens]

Cutting a 15 lb spring shouldn't change the coil resistance.

But it does. Actually, cutting coils *increases* the spring rate, all other things being equal (which of course, they aren't, particularly preload).

[PaperScraper]

Its been a while since I had to design any coil spring projects in school for Mech Design. But if a full coil is rated at a single rate, it would carry through the same weight all the way through. The Pre load actually adds weight to it if I remember correctly. Which is why the travel and compression and weight of truck springs are changed when lift lugs are turned in between the coils to gain lift. The ride goes to hell and it changes the full dynamic of the suspension.

[CocoBolo]

I use an 8.5# spring, got it at Cajun Gun Works. They also carry a 10.5.