Air Gun Home :: View topic

Dukemeister · Member

Inspired by my own ramblings in another thread, I'm now contemplating what, how and why mainspring prelaod helps with a springer. If a mainspring is linear, the stored energy available for release to the piston shouldn't be affected by preload (unless the spring can extend longer than the preloaded length during firing). I read some brief posts elsewhere that preload tames recoil, that I can understand - and should quiet the shot cycle as well since the spring won't be flopping about. But springers, especially magnum springers have a lot of prelooad. So how does it work, exactly?

rsterne · Moderator

Preloading a mainspring increases the force on the piston.... If you have a 48 lb./in spring with no preload and your cocking arm moves it 6" the starting force is zero and the force fully cocked is 6x48 = 288 lbs....

If you have 2" of preload, the starting force is 96 lbs. and the force fully cocked is 384 lbs.... Pretty easy to figure which gun would shoot harder.... Mr. Green

Shimming the spring increases the preload.... but eventually the spring will go coil bound before the sear engages in the slot in the piston.... The most power for a given spring is just before you get to that point.... where the guns cocks 100% reliably every time.... and won't fire when bumped hard....

Bob

rsterne · Moderator

Same thing applies to the hammer spring on a PCP.... A rear velocity adjuster (RVA) increases the preload on the hammer spring.... The spring exerts more force on the hammer.... accelerating it faster.... it hits the valve with more momentum.... opens it further and/or longer.... lets more air out (and can open a valve at higher pressure).... and produces more velocity....

Bob

Dukemeister · Member

Thanks Bob, it does make sense. Piston pre-load improves the efficiency. I woke up this morning with an extra hour to waste, so I did some calculations on how much energy a spring can store. I don't know how much cocking stroke a magnum springer has, maybe 6"? Looking at the table below, it seems likely that to get the high fpe out of a magnum springer like the B28, there must be combustion going on. There just isn't enough energy stored in the spring alone.

The compressed L is the amount the spring is compressed in the cocking stroke. The force in the spring is L*k + any preload.

Anyhow, I'm sure the experienced have puzzled it all through already. Thanks to all who help my self-learning Laughing