Tapering mainspring

[J.D.]

I'm making a coupla mainsprings for an old Pesersoli Bess. The working leg on the factory springs do not taper in thickness, but do taper in width about .045 " in 2 7/8" of length.

The working leg of the two factory springs straighten out nicely when at full bent, however, the middle of the working leg makes contact with the end of the non-working leg, so I can't accurately asses the natural bend in the two springs, at full bent.

So, using 1/8 inch thick spring stock, for a 1/2 inch wide spring, how much taper do ya'll suggest that I put in the thickness of that working leg for a snappy spring?

Thanks,
J.D.

[rich pierce]

It's not easy to predict how much a spring needs to be tapered.  It takes practice and experience.  Reductions in thickness weaken the spring by the cube- a spring half as thick has 1/8th the strength of the full thickness spring.  Reductions in width work as a straight proportion.  I make bows (archery) and have learned by experience that the ends of a bow (a wooden spring) can be tapered to about 3/4 thickness, and quite a bit by width to ensure the ends of the limbs are also "working", because the leverage on them is much less than closer to the "handle".  To match that to a mainspring, you want that buggy whip taper from the bend to the tip and similar reduction in width till it is just as wide as the tumbler at the hook.  I'd go conservative and remove stock later as needed to ensure the whole thing is working.  A  lot depends on getting the right curvature to begin with so it is possible to end up with a stright arm when at full cock.

[J.D.]

Thanks Rich, I've made a coupla bows too and learned that a reduction in thickness equals 8X reduction in weight, while a 1/8 reduction, for example, in width equals a 1/8 reduction in weight. So I got that.

These springs are thicker and slightly wider than the factory springs, so I do know that I will need to taper the thickness and width more than the existing springs, to get a snappy spring.

The springs currently have a .010 taper in thickness, and a .035 taper in width. No bevels on the legs, as yet. Will wait until the bend is made, then bevel, fit and file to get the correct bend.

The mention of bevels begs the question, are the bevels really necessary, or just a means of reducing the weight of the bottom leg to get more snap?

I can understand the bevel, on the non-working leg, to provide clearance for the barrel, but what purpose does the bevel serve on the working leg.

Thanks and God Bless,
J.D.

[rich pierce]

JD,
I have no idea what the bevels are for on the working arm of a mainspring.  I am guessing  that they are for style.  Some things are.  Flintriflesmith, Jim Chambers, and others understand these things far better and maybe they'll notice and chime in.

Not sure I agree that you'll get a better (you used the term "snappier") spring if you taper it more and narrow it more near the tumbler and will do most of the work there unless you make some other change.  Like a bow, perhaps a spring can be no stronger than the weakest effective working segment of a limb,  given that the leverage changes along the length.  The worst spring I ever had was on a large, very early styled commercial lock that has been available since the 1980s.   The spring does most of its bending near the tumbler and actually curves upward under full cock load.  It was not a fast lock in my experience and those springs broke like crazy.  It was practically impossible to get a mainspring vise to hold the tension already in the spring.

On bows, we know that ones with narrow and thinner tips shoot faster because of reduced mass, but on a bow, the mass of the limbs exceeds that of the string and arrow.  On a lock, the mass of the tumbler, cock, and flint dwarf that of the spring, so I am not sure a small reduction in mass at the tip of the spring will matter much. I mainly narrow or taper a mainspring to get the whole arm working progressively, and to arc a spring so that it becomes straight when under full cock.  It is tricky business as "preload" matters as well, just like a recurve bow, etc.

[D. Taylor Sapergia]

I think you need to start with stock that is a lot thicker than 1/8 especially for a musket lock.  I would suggest 1/4" material.  When you heat the metal red and bend it, it will use a lot of that steel forming the bend, and you won't end up with enough steel at the bend.
Here's a picture of TRS's 1st model BB lock spring.  Notice that there is lots of steel at the bend, and this ensures that only the leaves are doing the work.




Now, here's a picture of an Italian repro BB lock and the mainspring.  It has been formed from material that looks about 1/8" thick.  In order to get the power out of the steel, it has to be preloaded a great deal.  This is the spring on the plate but without the tumbler, and then out of the lock altogether.

[J.D.]

Thanks Rich and Taylor.

Rich,
I didn't express what I was trying to say very well. I am after a progressively working leg of the spring, kinda like the working tapers on a fly rod. So we are on  the same page. The additional taper mentioned in my last post is due to thicker and wider material than in the factory springs.

Taylor,
The  spring on the lower lock is the spitting image of the factory spring I want to replace, but yours has more preload. The factory springs measure less than 1/8 inch thick, by .450 " wide. The thick one mics .110 thick. The spring stock mics .130, so I thought that would work, but I think I understand why thicker material would be desirable for a better spring. My 1/8 " spring stock is nearly exhausted, so the next order will be for thicker material.

Thanks, for your help. This thread has been very informative.
J.D.

P.S. In a search of the old archives, I did find a thread on the purpose of the bevels on a mainspring, begun by yours truly. Ain't that a bite. I must be getting senile.

The link,if anyone is interested.
http://americanlongrifles.org/old_board/index.php?topic=7675.0

[westbj2]

"So, using 1/8 inch thick spring stock, for a 1/2 inch wide spring, how much taper do ya'll suggest that I put in the thickness of that working leg for a snappy spring?"

As mentioned, I think you are in trouble using 1/8" material....a bit thin.  As to the taper.  Initally shape the spring with mild just noticable taper on both width and thickness.  Heat treat it as normal then fit it to the lock.   By cocking the lock you will be able to see how the spring reacts to pressures exerted.  At this point you can file the spring carefully to achieve the correct balance/function.
Jim Westberg

[Dphariss]

It can be a lot thinner at the working end. Leaving the bend and bearing surface heavier of course.
A good forged spring can be tapered to about 1/2 its thickness or maybe less at the working end.
Leaving the spring thick eats up powder moving the springs weight.

The taper in width is determined by the clearance needed to clear the barrel/lock inlet when the lock is at full cock.
Dan

[J.D.]

Thanks again for everyone's help. As a perfectionist, I tend to obsess over the most minute details, so I do appreciate everyone's help.

Thanks, and God bless,
J.D.