Mainspring Rate vs. Strain Screw Length

[labnoti]

We know that the mainspring must push the hammer with sufficient force to strike the primer with the firing pin to reliably ignite the primer.

How would you characterize the difference between a heavier rate spring and a shorter strain screw versus a lighter rate spring and a longer strain screw?

Grant Cunningham pointed out one important difference: the arch of the spring will be different and may interfere with the grip screw.

If a flat spring has a linear rate and we add some amount of preload with the strain screw, the spring will start compressing beyond the strain screw's preload along that line of rate in pounds per inch.

If another flat spring has a lower linear rate than the first, we will need to add more preload with the strain screw to achieve the minimum amount of force for reliable ignition (all other things being equal). Although the cocking motion will begin to compress the mainspring at a point farther along its line of rate because of the greater preload, the force to fully compress it (to the point where the sear releases) will increase less over the same distance as the higher rate spring. The peak force or the area under the force/distance line will not necessarily be less, but the force will increase less over the distance.

Why might it be more desirable to achieve reliable ignition one way or the other?

 

[gotboostvr]

I'd take the lighter spring with a fully tightened mainspring screw.

I know in my guns I get miss fires from light strikes very quickly when adjusting that screw without much gain. There's not much wiggle room there.

 

[earlthegoat2]

One could get a sacrificial strain screw and file it down a few strokes at a time while testing for reliable ignition.

When ignition is no longer reliable, mic the screw and with a new one size it a few thousandths longer.

Or just play with a bunch of different hammer springs until you find a combination of spring tension and strain screw pre load that works best for you.

 

[labnoti]

It is understood that the lighter spring requires more preload on the spring via the strain screw, and the heavier spring requires less.

Reliability isn't the distinction of either unless they are improperly adjusted such as with insufficient strain screw length for the mainspring rate. Finding a sufficient spring rate and strain screw length combination is not the difficulty. As earlthegoat2 mentioned, it can be discovered by backing out the strain screw until it starts to misfire and then measuring and shortening it to a length at which it reliably ignites. The strain screw can be loosened to temporarily test less preload, but should be ground to the desired length and fully tightened for continued use.

The question is which reliable combination works better given both have been adjusted to be reliable? A shorter strainscrew and greater spring rate? or a longer strain screw and less spring rate?

 

[WestKentucky]

My vote goes for the weaker spring with longer screw. My impression is that the items are functionally interchangeable but with slightly different characteristics. As springs wear they slowly lose power, and I would think that a weaker spring with a boatload of pre-tension would suffer less than a heavier spring simply because the travel is what has preloaded it and (arbitrary numbers here) the one compressed 1/32” has very little wiggle room before it becomes unreliable due to motion inside the frame whereas the one with 9/32 compression has a whole lot more to lose before the spring comes unloaded and able to move around.

may sound dumb. May be dumb. It’s after midnight and I can’t go to sleep.

 

[243winxb]

After you fine tune a revolver, just changing brands or lot number of primer may make the gun unreliable. Years ago i fine tuned my M28. Worked fine till i changed primers.

A test for a Smith and Wesson revolver, model 28, 357mag., may work on other guns also?
Gun empty.
Dryfire gun and hold trigger fully to rear.
Pull back hammer with thumb.
Hook a weight around the hammer (for example 3 1/2 LB minimum weight for 357).
The hammer must not move rearward when the gun is lifted.
The hammer should lift 3 1/2 lbs without going into the cocked position.