Pretty cool rig, but again.... why not just buy in spec springs and change them out periodically???
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Use a trusted source?
Agreed. The confusion comes from that nobody cares to ask for a precise definition. I tried and got different answers from different people who were supposed to know (spring manufacturers). When pressed, they all ended saying "I'm not sure". Finally I got a good answer from an old timer. The spring's poundage is product of its Hook's constant (lbf/inch) and its natural length. Or it is 100x of the pound force produced for each percent change in length.
The carbine spring in your example
Lo = 10.7", K=1.53 lbf/inch
K*Lo=16.3lbf, or it is a 16 pound spring per this definition.
With that calculation actually becomes even simpler. Again for the same carbine spring, as per the forces given, the install length is 6.8", and the compressed length is 3.2".
1.53*(10.7-6.8)=6 lbf
1.53*(10.7-3.2)=11.5 lbf
Or
16.3*((10.7-6.8)/10.7)=6 lbf
16.3*((10.7-3.2)/10.7)=11.5 lbf
Note that the poundage is independent of the spring's natural length, unlike K, which is handy for calculating shortened spring.
2 coils off 38 coils. The new natural length is
36/38*10.7=10.1"
16.3*(1-6.8/10.1)=5.4 lbf
16.3*(1-3.2/10.1)=11.1 lbf
You can arrive at the same figures with the K method.
The force is weaker, but not really alarming. Actually in a quest to make a "gas assisted straight pull action" I cut the spring of an AR -10 pretty short. No problem, not that anyone should try though.
Does solid height mean compressing spring till it coil binds(cannot be compressed anymore)? According to the old timer who explained spring poundage to me, one should avoid that in use. It could pass the spring's limit and it could take a permanent set. There should be limiter in design that keeps this from happening.
-TL
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NOTE: When counting coils, only count active coils.
The action spring has closed ends, so the last coil on either end doesn't count - 36 active coils.
Solid height (or solid length) is simply, the number of coils (active and inactive) times the wire diameter, it is as short as the spring can physically be (action spring = 2.74"). If you want your spring to last a long time, you should stay at 130% of the solid length for maximum service defection. For the action spring, this would be 3.56", so its design is a little over-compressed. (Just about all gun recoil and magazine springs are. That's why they have relatively short lives, compared to, say car suspension springs.)
This is starting to remind me of the grip screw spec thread.