SpudFiles

I have a blow forward bolt pneumatic and the bolt was sticking forward (my poor machining, blame it on the 65y/o lathe) I have yet to lubricate the breach and slide as although I will would prefer for it to operate correctly without.

That's the background, my question is I was using one spring to return bolt when it was sticking, I have now added another identical in series with the first, I have checked I still have full movement without them binding and there is now maybe a preload of half to one spring.

Will this effectively increase the spring rate?

Bolt appears to now return although that is just using my finger to push it forward and not the much greater force the air will hit it with.

Thanks in advance..

For a constant length of bolt travel, in short no.
But, the pre load you applied will increase the amount of force quite drastically, which is probably where the extra force you can feel comes from, though from what I can tell that should be enough for your desired function.

Springs undergo a change in length proportional to the force that they undergo, and resist with a force proportional to that, force is constant throughout the length of the two springs.
So for two of the same type of spring if you pull the bolt back onto the springs distance x, both will contract by length 0.5x, and both shall therefore give back a force of 0.5kx.
So added together the force from both springs would be kx, or the amount of force you would have got back from just one spring of the same spring rate.

Thanks for the explanation, hopefully they will act as required if not I will add a bit of silicone grease, if not back on the spring hunt which is easier said than done due to lack of technical requirements and the lack of choice on ebay. Else I may try the original spring now I have a brass poston.

Thanks.

If the worst comes to the worst you can always make your own springs, along with having the benefits of being completely tailored to requirements, it would probably turn out cheaper, albeit a rather fiddly process to set up.
I think JSR has posted some good links about spring making in the recent past if you want to fish for them, if not a quick Google returns loads of results.