What makes a good lock design?

[northmn]

Here is some of may observations and input.  The small Siler has retained its reputation for being a "fast" lock since before I started this game in the 1970's.  I was always given the impression that the epitomy of locks were the late English with the stirrup on the main spring and the frizzen rollers and rain proof pans.  Maybe all the bells and whistles don't beat simplicity   Larry Pletchers timing studies showed that it may not possible or at least difficult to improve on the Siler.  I have another lock similar in size that Dixie used to sell called the Ashmore.  It had a different frizzen spring but worked great.  When the frizzen toe went over center it popped open beautifully.  Some of the larger early locks also work very well, with little problems but are slower becasue they are larger.  The most reliable flintlock I ever had was a repo Brown Bess.  You practically time the ignition time from trigger pull with a stop watch but it generally went boom.  The stirup on the mainsring is a weak point that can easily break (or you can lose the *#$^** stirup when disassembling like I recently did)  One lock had a roller in the frizzen that wanted to break at that point, a design weakness.  Is simple best?

DP

[Nate McKenzie]

Yes.  Especially if the geometry is right.

[Blacksmoke]

northmn:  There are a number of basic criteria that need to be in place for any flintlock to function properly and efficiently.  For me the "main" thing is the "main" spring.  It really is the life blood of the lock. At forward position it should register max. tension when pulling the cock towards half cock.  As it travels further to full cock position the tension should be at minimum-- measured in oz. not lbs.  This will do two things: first it make the cock travel faster as it is released from full cock notch and it means lesser tension on the nose of the sear giving you a light trigger pull.  In conjunction with the mainspring is the frizzen spring which the cock is now working against.  Pan closed ,the tension to open it should be 50% of that of the cock fully forward.  These are the two most important criteria for optimum lock performance in my opinion.  There are many more criteria which all have to work together for proper lock performance but I do not have enough room here to get into them all.         Hugh Toenjes

[northmn]

The idea behind the stirup, which was used a lot in percussions and the late English flintlocks, was to use a strong spring and a short spring snap.  I wonder if that principle wasn't good for percussions and maybe not giving a "push" needed for a flintlock. 

DP

[Acer Saccharum]

I ended up making my own tumbler, which was very educational, to say the least.

Here is the lock at full cock. Note that the toe of the main spring is at the hub of the tumbler shaft. This give the shooter tremendous leverage: pulling on the hammer at this point is very easy. At this position, you get speed, but no power. Also note that the tip of the tumbler arm contacts the inside of the spring hook, acting as a natural stop when I pull the hammer back to full cock. The pulling back is really easy once you get past half cock, and when at full it suddenly gets difficult when the tumbler arm hits the mainspring.


In this view, the tumbler is at half cock, so note that the nose of the mainspring is farther out on the cam nose of the tumbler. This is where the mainspring is really starting to lay on the power. It does this because the spring is farther away from the hub, so it acts as a longer lever, putting more of the spring's power to work.


Here, the spring is as far out on the cam as this geometry allows. It takes a lot of force to pull the hammer back when it is in the down position. At this lowest point of travel, the shoulder on the cock must contact the edge of the plate. The tumbler should be fitted to stop on the bridle as well.



As one gets into these locks, he realizes how complex such a 'simple' mechanism is. The parts are entirely interdependent; there is a minute difference between a smooth and fast functioning lock and a piece of junk. That difference is a combination of geometry, the precision of fits, and the quality of materials. Skimp on any one of the three elements, and you have a dismal sinkhole for your time and money.

Please appreciate the locks you can buy today. They are the best deal going since the age of the English and German import locks.

Acer

[rich pierce]

I will add that the interaction between flint, frizzen and frizzen spring is a complex one and is where some locks have problems.  The angle of the jaws as they approach the frizzen, the resistance of the frizzen, and the timing of the cam that flips the frizzen open can make or break a lock.

Even the position of the flint at rest matters.  I have a very fast and reliable tuned Durs Egg lock I love but the flint is right in front of the flash hole at rest and gets blasted every shot.  I may add some metal to the stop on the cock someday, or not.

[Larry Pletcher]

One thing I use to evaluate a lock is to see if the tumbler stops on the bridle at the same time the cock stops on the lock plate.   If both stop at the same time it may dampen vibration in the lock -- my speculation.  This may be more a mark of the lock tuner than some of the other items mentioned.
Regards,
Pletch

[doug]

      One thing that I ran into on a current project; the sear spring is in a bad position.  If you look at the photo, the fastening screw is too far forward which makes it difficult for the tumbler to clear the screw and stop on the bridle.  On the lower lock, it apparently broke the sear spring screw during the locks percussion phase.  Second with this spring is that the little tit that keeps it in place is too far aft leaving very little spring to actually flex combined with the back of the lock being a bit shorter than average, in my opinion.
      Also the unusually long arm on the tumbler and the long stirrup link means that the cock cannot rotate nearly as much as I would like.  Simply shortening the link does not work because the spring and stirrup bind on the tumbler arm.  I have since made a new tumbler with a slightly shorter arm and much shorter link and have gained at a guess another 20 degrees of rotation.
      Also the lower down the bottom arm of the mainspring starts, the more readily it will clear the shaft of the tumbler.  If the mainspring is relatively high up on the side plate, as the gun is cocked, the tip swings towards the center of the tumbler shaft.   If the spring is right at the bottom of the lockplate, that is very much less true.
    Just my opinions anyhow.

[Acer Saccharum]

......the more readily it will clear the shaft of the tumbler.  If the mainspring is relatively high up on the side plate, as the gun is cocked, the tip swings towards the center of the tumbler shaft.   If the spring is right at the bottom of the lockplate, that is very much less true.

That issue of the mainspring position is a tough one to design. I like the spring to stop before it hangs below the bottom plate. But when the hammer is drawn back to full cock, the spring tip can contact the tumbler shaft. It did on my lock above, so I ground a little recess on the shaft so the spring tip can travel in toward the center of the shaft without binding on it.

[northmn]

Getting some interesting input.  To get a couple of locks to spark, they were off of manufactured guns back when, I redrilled the pin hole on the mainspring so that they would have to compress more.  It did make them pull a little rougher, (if possible on that type of lock) and I wondered if the mainsprings would break, but it did help them spark.  The frizzen spring really is an over center operation such that when the toe of the frizzen gets to a point the spring should kick it open.  I should get hanier with a camera as I have a little Italian lock I bought years ago that does this beautifully.  The spring and position is perfect. I think the rollers on frizzens are almost just for show.

DP

[Dphariss]

Here is some of may observations and input.  The small Siler has retained its reputation for being a "fast" lock since before I started this game in the 1970's.  I was always given the impression that the epitomy of locks were the late English with the stirrup on the main spring and the frizzen rollers and rain proof pans.  Maybe all the bells and whistles don't beat simplicity   Larry Pletchers timing studies showed that it may not possible or at least difficult to improve on the Siler.  I have another lock similar in size that Dixie used to sell called the Ashmore.  It had a different frizzen spring but worked great.  When the frizzen toe went over center it popped open beautifully.  Some of the larger early locks also work very well, with little problems but are slower becasue they are larger.  The most reliable flintlock I ever had was a repo Brown Bess.  You practically time the ignition time from trigger pull with a stop watch but it generally went boom.  The stirup on the mainsring is a weak point that can easily break (or you can lose the *#$^** stirup when disassembling like I recently did)  One lock had a roller in the frizzen that wanted to break at that point, a design weakness.  Is simple best?

DP

The small Siler is a good lock but its not in the class with the late English *unless* someone works on it quite a bit then its close. But its still not the same. If there was no advantage the "high tech" locks and they were less reliable they would have not prospered. By 1830 if not before "roller" locks were being specified for the Upper Missouri trade rifles.

The stirrup type tumbler in nearly frictionless. The cock speed is very high IF THE MAINSPRING IS A FORGED ONE. Cast springs are heavy and tend to be muscle bound since they are thicker than they would be if forged, shaped and tempered right. But making a forged spring for a link type mainspring is tough today since the thin HAS to be just right to work with the link and tumbler. So casting is "safer".
In all locks the reduced pressure as the cock/hammer is drawn back puts less pressure on the sear nose at full cock. Less wear and a lighter release. This is also possible with the older style as Acer points out but it really shines in a properly laid out and assembled link type tumbler lock. On the percussion the "heavy first lifting" of the hammer helped keep the nipple sealed and prevented the cock being blown back.

The problem is that many modern locks are not properly made. They may have the right pieces but they may be laid out incorrectly. As Acer states EVERTHING works together. You get one thing off and its all less than optimum.
Most locks with weak springs can benefit from having the springs re-arched. Castings made from original locks often reproduce a somewhat collapsed main and/or frizzen spring.
Dan

[Acer Saccharum]

Dan, I never thought of a cast spring being 'muscle bound'.... or too heavy. Makes a lot of sense. The less mass that has to be moved, more of the available power goes into making sparks. Lighter hammer means quicker acceleration. Same with the spring. Quite probably the forged spring has more zip per gram than a cast.

Another thing which Rich Pierce mentions is the angle at which the flint strikes the frizzen, and the curvature of the frizzen, and the bevel of the flint all being vital geometry to producing good spark.

I suppose we will put up with cast springs, as forged one would put the price up to $180 or $200 a lock.

Anyone know what a 1790's lock would sell for today, adjusted for inflation, etc? ( the workman of the day didn't have health insurance costs like we do today, so that should be taken into consideration)