Question on cross-sectional tapering of mainspring working arm

[rich pierce]

Most mainsprings start as rectangular spring stock. But finished mainsprings often have edge bevels and are not truly rectangular in cross section through the working arm. The edge against the lock plate is pretty square and full, and the working arm tapers toward the exposed edge. Is this decorative or functional? And if functional, what are the tapers and bevels doing, mechanically?

I’m not talking about lengthwise taper. I get that part - I’ve made bows.

[smylee grouch]

I just assumed that outside edge was shaped so to help keep it away from the barrel.  silly??? 

[JTR]

Sorry, I was thinking taper and not bevel. 

[smart dog]

Hi Rich,
All I can do is mostly speculate.  First, based on stress tests done by Chris Laubach, the working leaf actually is the entire lower leaf, the bend, and the upper leaf to the peg.  In fact there is a local peak in stress right at that peg.  Before bending the spring, the flat bar can be easily thinned lengthwise to affect strength.  However, after the spring is bent and the hook or claw formed, adjusting strength my lengthwise thinning is not that easy.  It is only after bending, shaping, hardening and tempering the spring that you know how strong it will actually be.  If too strong, the easiest adjustment would be to grind a bevel on the outside of the bottom and top leaves.  I don't believe it has anything to do with avoiding the barrel.  Lock makers solve that problem by having a long stud on the upper leaf so the leaf sits low in the lock plate.  Moreover, even springs made that way usually have bevels ground into the outside edges.  Here are 3 original locks by Grice (1770s), Field (1820s), and Mortimer (1785) showing the long stud, low position on the plate but still with bevels.
 

[rich pierce]

Thanks, Dave. I have zero engineering aptitude so am apt to make crazy speculations. I was wondering if having the portion that is against the plate stronger than the outside somehow prevented any possible torque of the spring away from the lock plate. In other words keep it snugged up.

[Bob Roller]

I just assumed that outside edge was shaped so to help keep it away from the barrel.  silly??? 

Not silly at all.I made these springs with the end of the upper limb turned up to give needed clearance
for the barrel.I used 1/8" 1075 and after the strip is ready,I put it in a clamping fixture that established the taper on the
lower limb and then a cut of .050 behind the area where the claw goes and start the cut.I ised a 3/8 end mill,stub type wit
5 or 7 cutting edges and the precalculated angle was cut and stopped before the area of the bend was reached.A smooth cut file
with one safe edge then established a radiused taper then lighltly draw filed to remove any cross cut marks that could later make
a built in fracture line and a broken mainspring.THEN it was forged after determining where the bend should be.After the spring cools
then the outer side is ground and that was on the flat side of a fast white wheel to taper upper and lower limbs to the same angle.
Then the pin that goes thru the plate is done with a safe edge square file or a hollow milling cutter.The milling machine is again used
to make a semi finshed taper in the upper limb and then filed  and draw filed along its length.I bevel the outer edge after the spring
is opened  and tempers.The installation is established before the spring is opened and the hole drilled for the pin.I used the linked style
springs after 1960 in all my locks.Springs are a skill set of its own and totally different from any other lock part.I tried to copy as close
as possible,the springs in fine English locks.All of this reflects years of doing this kind of work and IF quality material is used a good out
come is attainable.I hope this is helpful.
Bob Roller

[D. Taylor Sapergia]

Rich:  I suspect that the tapering, beveling, etc of a mainspring is first, functional for creating a mainspring that has appropriate strength, and secondly for aesthetics and fashion.
As in making a bow, thinning a limb's thickness reduces it's strength about 8 times faster than reducing its width.  But still, we find bowyers trapizoiding bow limbs to tiller them, and I suspect that bevelling on the edge of the spring, does the same thing.
In the end, the spring maker wants to create a spring that has no arc when it is at full bent, and works over it's entire length, even as Chris Laubach has said, over the bend and up the upper leaf to the pin/tit.

[Bob Roller]

Rich:  I suspect that the tapering, beveling, etc of a mainspring is first, functional for creating a mainspring that has appropriate strength, and secondly for aesthetics and fashion.
As in making a bow, thinning a limb's thickness reduces it's strength about 8 times faster than reducing its width.  But still, we find bowyers trapizoiding bow limbs to tiller them, and I suspect that bevelling on the edge of the spring, does the same thing.
In the end, the spring maker wants to create a spring that has no arc when it is at full bent, and works over it's entire length, even as Chris Laubach has said, over the bend and up the upper leaf to the pin/tit.

The semi military Whitworth I had for more than 10 years had a superb 4 pin Brazier lock and the mainspring was thin and arced upward at full cock and when removed it was a job to get it back in the lock.Lock speed like a modern bolt action rifle and holding the trigger so the hammer could not lock and then moving the hammer thru the firing cycle all that was felt was that "oily" mainspring.During my time of lock making I never tried to copy that spring even with the consistent,uniform material we have now.Beveling the limbs after tempering does no harm and gives a finished look to the spring.Spring making is a separate skill and a much needed one in making locks and the only cast springs I used were frizzen springs but they are not compressed but do take a hit when the lockis fired.
Bob Roller