Preloaded springs

[doug]

     This question could fit in several sections but I chose here because the principle applies to muzzle loading locks in general.
     I recently bought a Rigby Snider sporting rifle (cartridge) which uses essentially a muzzle loading lock.  The little link was broken which joins the tumbler to the mainspring and the mainspring, unrestrained extends about 1" or more below the bottom of the lock plate.  It seems like a massive amount of preloading of the spring and presumably contributed to the broken stirrup link.

     The question I suppose is twofold; first is just how far below the lock plate should the unrestrained mainspring extend without putting excessive strain on a stirrup link type linkage (think in terms of how much tension is needed for a percussion lock)    and

how reasonable is it to modify (reduce) the preload on a original mainspring on a quality gun to improve a perceived design flaw?

thanks Doug

[P.W.Berkuta]

I am surly not an "expert" on the subject but I have some "feelings" with respect to modifying original parts. I would leave the original spring alone and make a replacement to play with. As far as how much the end of the spring should extend below the bottom of the lock plate - that will depend on the strength of the spring, the power of the spring needed to fire the rifle, lock time, the strength / hardness of the steel that the lock and parts are made from. My observation of modern side locks is that the main spring is smaller and thinner than that of a muzzleloader so I think that the springs "leg" of the modern gun would need to have more travel to generate the power needed to fire the rifle. Just my opinion -----

[keweenaw]

First you need to decide if the spring in the lock is the original.  If it is an original Rigby it should be an elegantly made spring with gracefully tapering, buggy whip arms.  If it is at all crude with file marks, etc. it's probably not original.  Also if it is original you don't want to be messing with it as you are more likely to ruin it than to do the modification you want.  Modifying the spring by any large amount would require annealing, rebending, rehardening and retempering and you don't know what steel it is made from. 

Aside from all this, those links are the weakest piece of the lock.  They break more often than any other part of the lock  and one needs to replace them from time to time.  They are not fun to make.  The pins need to be exactly the correct size, the connecting plate needs to be as thick as will work in the tumber arm and they need to have both the correct geometry and length.

Tom

[doug]

      The spring is original without question.  Its strength seems puzzling because it appears to be several times stronger than necessary and I am fortunate/surprised that it did not break the bottom out of the stock when the link broke.
      The links themselves are not terribly difficult to make; I will turn the pin on one side with the lathe, then use a hollow end mill on the lathe to cut the opposite side.  To get the two sides to line up, I drill a hole in a piece of flat bar, using the mill, clamp the future link onto this with the pin in the hole then mill straight down with the hollow end mill which leaves a pin on the up side which is directly over the pin on the down side.  Saves an awful lot of set up time on the lathe
     My concern is that if the link broke the first time because of an overly strong spring, presumably it is at high risk the second time as well.  Trade off is that I don't want to modify original parts.

cheers Doug

[Stophel]

Somebody brought me a 53 Enfield a couple of years ago to work on.  The mainspring was thin and whippy, with a LOT of preload.  So much so, I had a real hard time getting the thing back on.

I'd say this was standard for 19th century English locks.

By the way, this lock was EXCEPTIONALLY well made.   

[westbj2]

Doug,
Most good english springs do show the pre-load or protrusion which may seem excessive when out of the lock and relaxed.  If you look at the spring closely, note that the limbs are nicely tapered both in width and thickness.  This tapering causes the spring to flex over its entire length right up to the crotch in front.  When at full cock, the lower limb will be as straight as an arrow.  
Best to assume there was some problem with the link and just make a new one.
I have an Alex Henry lock in the shop which requires a swivel head vice positioned in just the right spot in order to compress it because of the relaxed shape.
Jim Westberg

[FALout]

The pin likely broke because the hammer over-extended the  mainspring putting too much force and fulcrum on the pin

[dannybb55]

Make a super strong stirrup. Fix what broke, not what survived.

[doug]

Make a super strong stirrup. Fix what broke, not what survived.

      that is not really possible other than to the extent of good workmanship.  The dimensions of the stirrup are governed by the tumbler and the claw on the spring

cheers Doug

[Dphariss]

     This question could fit in several sections but I chose here because the principle applies to muzzle loading locks in general.
     I recently bought a Rigby Snider sporting rifle (cartridge) which uses essentially a muzzle loading lock.  The little link was broken which joins the tumbler to the mainspring and the mainspring, unrestrained extends about 1" or more below the bottom of the lock plate.  It seems like a massive amount of preloading of the spring and presumably contributed to the broken stirrup link.

     The question I suppose is twofold; first is just how far below the lock plate should the unrestrained mainspring extend without putting excessive strain on a stirrup link type linkage (think in terms of how much tension is needed for a percussion lock)    and

how reasonable is it to modify (reduce) the preload on a original mainspring on a quality gun to improve a perceived design flaw?

thanks Doug

The mainspring is probably just like its supposed to be. Links fail sometimes especially when this old.
The locks were designed for a "heavy first lifting" it allowed a lighter loading of the sear on full cock since the geometry allowed the leverage to change as the lock was brought to full cock.
In percussion locks the heavy first lifting produced higher pressure on the nipple which kept the hammer on the nipple better and increased nipple life as this reduced gas leakage and erosion.
The internals on the late best quality flint guns are identical and I suspect that the lighter sear pressure and the higher pressure as the cock moved down may have helped maintain cock speed as the flint scraped the frizzen.
In cartridge guns having positive pressure on the firing pin helps prevent primers from forcing the pin back which can result in shearing bits of primer into the firing pin hole of the breech block. This can cause reliability/accuracy problems. Mushy mainsprings also cause accuracy problems with cartridge guns
Anyway this is why there is a preload. ALL locks have some preload.

Dan

[The other DWS]

Those little links are the "weak link" in the system.
How did it break,  Did the link fail or did the pins shear?  It'd seem like the type of metal and the way it was heat treated would be real critical considering the forces and the impacts that it is subjected to.

wondering too if there might not be a way to use today's metallurgy to improve on them by making them of a tougher alloy or a better heat treat.

  As finely built as those original locks are I wonder if its possible that they were designed as a sacrificial part, like a sheer pin to prevent breakage of other parts.  It would have bee easy for the mfg to include a few extra parts in the original package sold.

[doug]

Those little links are the "weak link" in the system.
How did it break,  Did the link fail or did the pins shear? 

      No idea how it broke and I am extremely relieved that the spring did not break out the bottom of the lock inletting.  Not much wood and lots of power.  Logic says that the link must have been in there when I removed the lock although I did not see the pieces fall out when I removed the lock (@!*% it).  Tiny piece on a tiny pivot pin holding a powerful spring.
      Just to hijack my own thread and toss in some eye candy; I actually bought two guns, a trade gun and the Rigby although I only actually wanted the trade gun.  The seller essentially said you have to buy both or none.  The trade gun is virtually unfired; appears to be one of those that sat in a HBC backroom until the 1930s or so.  Interesting that the barrel was blued and yes I am positive the gun is original and not a replica or restock.  It had been in the owners family since 1947.  Lock is dated 1875.  Also interesting in that the frizzen is bridled at this late date.
      the percussion gun in the last photo is dated 1884.

cheers Doug

[FALout]

Very interesting piece you have there.  I can image that the Rigby is also a nice piece of history.

[Dr. Tim-Boone]

What do you suppose is the date on the trade gun??

[doug]

     the newest one, is by Parker Field and co and stamped 1875, their last year of production of trade guns.  The percussion gun is stamped Barnett and 1884 and the second flinter has  a home made lock but (forgotten makers name) appears to have been made around 1800 - 1810

cheers Doug