Lock mainspring

[varsity07840]

I recently replaced the mainspring on an original 1816 Springfield lock and it's overly strong, even for a musket. Am I correct that the proper way to lighten up the spring is to reduce the width? If so do I gring along the entire length of the spring?

Thanks,

Duane

[Long John]

The change in the force exerted by a spring is proportional to the change in spring width and proportional to the square of the change in thickness.  If you reduce the width to 1/2 of its original width you reduce the force by a factor of 2.  If you reduce the thickness to 1/2 of the original thickness you will reduce the force by a factor of 4.  I would not grind a spring I would draw file it, keeping the file strokes parallel with the long dimension of the spring.   The key is keeping everything smooth and changes in dimension gradual so flexural force is not concentrated in one portion of the spring.

Best Regards,

JMC

[Dave Dolliver]

No, John.  Stiffness of the spring is proportional to the CUBE of the thinkness.  the formula is 1/12 x Width x Thickness cubed.   So thickness is far more effective to change the strength.  Just be sure that any scratches go lengthwise of the spring so that no stress concentrations will occur.  Only the long leg of the spring that contacts the tumbler needs to be worked on probably,  but continually check the bend of the spring under load to make sure the bending is smooth.

Dave Dolliver

[Blacksmoke]

I agree with John, in regards to draw filing a main spring.    Also take care to dress the spring the intire length so that the flex will occur over a longer section of the leaf.  I am not a big fan of "cast" springs as there is no integral and longitudinal "grain" in them and after a period of normal use they will break.   I use old car or truck leaf springs from the "Model T" era, to forge my own springs out of.     
  Hugh Toenjes

[LRB]

   There is no reason a spring cannot be "ground" thinner. It simply cannot be overheated while doing so. As Jim Kibler pointed out in another recent post, the real problem with cast springs is the possibility of occlusions, or inclusions in the cast. longitudinal grain is helpful, and a plus, but not as significant as many may think. Steel grain is not long as a unit, but much like the shape of a grape, or the particles in flake board. A well cast spring, properly heat treated will last, and not sooner or later break, just because it was cast. The number one reason of broken cast, or all springs, is the lack of using a spring vise while fitting, or removing them, and deep scars left from shapeing, or re-shapeing them. Cast springs just don't break because they were cast, or ground on.