Sear and Hammer engagement theory

Well I may have answered some of my own question but would still like opinions.

Amontons' First Law: The force of friction is directly proportional to the applied load.

Amontons' Second Law: The force of friction is independent of the apparent area of contact.

Kinetic Friction...Surface roughness and contact area, however, do affect kinetic friction for micro- and nano-scale objects where surface area forces dominate inertial forces.

Now for the pressure applied to the sear as a result of the force exerted by the hammer spring and the surface area of the hammer hooks.
P = F/A where

P is the pressure
F is the applied force
A is the surface area where the force is applied
F/A is F divided by A

So with the above statements, it appears I was somewhat...somewhat correct in theory. For a given force exerted by the mainspring, the pressure exerted on the sear would depend on the surface area of the hammer hooks. It would be increased if the surface area is decreased, and vice versa.

So then using the above laws of friction it appears there shouldn't be a significant difference in friction and pull (Amontons 2nd law) but with Kinetic friction is appears it might affect the pull.??

But i believe this applies to dry friction and not so much to lubricated objects as the coefficient of friction goes down with lubrication. And with surfaces and forces are so small it may not change to a noticeable degree.

So where did that get me....nowhere

JHS, Let me start by explaining a single action "sear". Just like a pair of scissors (sears), it takes two parts to make them work. In revolvers, the SA sear is made up of a hammer notch and an extension of the trigger. With some SA pistols (ie a 1911), you still have a hammer notch but the other half is a separate part called a sear, which is really misnamed but who cares. DA sears are a totally different story and depend on the manufacturer's design.

The SA sear concept is simple ... move the trigger to the rear and the trigger extension or sear will move on the hammer's sear notch until it runs off the edge. At that time, the hammer will thrust forward under spring tension. There are two spring forces at work on the sear surface itself, one from the trigger and/or sear spring and the other from the hammer spring. These two forces combine to form "sear tension". If either is lowered, sear tension and trigger pull will be less. The "sear angle" has much more influence than the springs or the smoothness of the sear surfaces. In nearly all guns with hammers, the hammer's sear notch is cut slightly "under square". This means the sear notch works like a ramp so when you pull the trigger, the hammer actually cams back just a little before it releases. The amount of sear contact surface is very small but it does play a part in actual trigger pull but not what you might think. Any roughness on either the hammer notch surface or the sear (trigger extension) surface will cause "creep" (the feeling of movement). Also, the lighter the sear tension, the more creep you will feel. Example: if a trigger has an 8 lb pull, chances are a rough sear will not be noticed so the trigger will feel "crisp", even though it is heavy. If sear tension is decreased by using lighter springs, the pull will be lighter but creep may increase to a point where it gets very annoying.

The design of the sear or hammer notch has very little influence on sear friction because almost all sear tension comes from the springs. If you polish a sear notch and sear where there is virtually no friction created by roughness and compare it to an unpolished sear, the actual trigger pull weight probably won't change enough to measure; however, a smoother sear surfaces will reduce creep and will feel way smoother. So to answer your question, split sears or solid sears make virtually no difference in actual trigger pull.
Most experienced shooters will shoot a gun with an 8 lb pull just as well as a gun with a 3 lb pull. Although nearly all shooters prefer a light trigger pull (myself included), trigger pull weight really doesn't make much difference but creep sure does. If it feels like the gun is going to shoot three or four times before it actually does (due to creep), it really interrupts your concentration and adversely affects marksmanship.

While we are on the subject ... here's the difference between an amateur and a professional trigger job. Most amateurs will just install "reduce power" springs. A reduced power hammer spring will increase lock time (the time it takes for the hammer to drop after the sear releases). More lock time gives you more opportunity to move the muzzle off target. Hammer spring tension and lock time are proportional ... as an example: replacing a 23 lb hammer spring with a 19 lb hammer spring in a Ruger SA revolver lowers trigger pull by about 18% but increases the already long .075 second lock time to about .088 seconds. Reducing spring tension will indeed lower trigger pull but will increase creep and lock time so it's usually counter productive. Trigger springs do not affect lock time but do make creep more noticeable.

Some amateurs get brave and stone the hammer's sear notch and the trigger extension or sear ... sometimes ruining the hammer and/or trigger. They may not realize ... the scratches formed when you stone a sear must be in the same direction as movement or it doesn't accomplish much. With a sear or trigger extension, it's easy to do because the part is accessible. The sear notch in the hammer is the hard part because the stone movement is across the notch, not in the direction of sear movement. When scratches from stoning (or machine cutting from the factory) are across the sear, it increases creep; however by using a very fine stone, the scratches get very light. After fine stoning, polishing the sear notch with a muslin buffing wheel (in the direction of movement) removes nearly all scratches and reduces creep to nearly nothing. On some guns, such as a Ruger SA, the hammer's sear notch is way deeper than it needs to be. The deeper the notch, the more trigger travel you will get and of course the more feeling of movement. By removing about 30~40% of the sear notch, you still get positive cocking but way less trigger movement.

Saving the best for last ... I mentioned "sear angle" above. As it turns out, having to cam the hammer back with the trigger .... even slightly, increases both trigger pull and creep because it is working against the heavy hammer spring. You can cock a hammer on most any unaltered gun and actually see the hammer move back just a tad as you squeeze the trigger slowly. By increasing the sear angle where it is perfectly "square", the trigger doesn't have to work near as hard to release the sear. If you increase the sear notch angle too much, the hammer will "push off", won't stay cocked, or will result in a "hair trigger" ... very dangerous. By using a sear jig, you can adjust the sear angle just perfect where the hammer does not move forward during sear movement nor does it move back ... perfectly "square". This will minimize creep, lower trigger pull, yet provide a safe hammer cock that can not be pushed off.

I wish you could test fire some of my guns that I've done trigger jobs on. All have standard factory springs, polished sears, with squared sear notch angles. Trigger pull is typically about 3.5 lbs but when most people dry fire them, they swear it is much lighter. There's virtually no creep with short trigger travel so the sears break very crisp. This is the optimum condition for good marksmanship yet just as safe as a factory trigger.

As usual very informative, Thank you Iowegan. That gives me a lot to think about in my pursuit of a better trigger. I failed to discuss the trigger spring in my original post, so thank you for bringing that up. I understand the relationship between the sear angle, camming and benefits of having a neutral (square) or near neutral sear angle on trigger pull. My CZ has a stock hammer and it's sear/hammer hook angle gives a positive engagement (camming) of which I would like to eventually change. I don't notice any camming in my GP100 and I can't even see the interaction in my Mark III. As you indicated, even though the SA trigger weights are approximately the same for my CZ and GP, the GP feels much better.

I'm actually surprised that your mainsprings remained stock, very interesting. I have installed reduced mainsprings in my CZ (did notice more creep) and GP, which drastically lightened the DA pull. No light strikes and an easier pull. Now for the angles and polishing.

Well thanks again Iowegan, your wealth of knowledge helps yet another enthusiast.

I find that reducing the hammer notch to neutral increases the felt creep.

I keep all of my sears and hammers positive. I don't change any springs.

I get 2.5# to 3# trigger jobs and I know my trigger work is SAFE.

claytonfaulkner, The numbers for hammer spring tension can be found in the Brownell's catalog .... 23 lb for a standard factory Blackhawk, Super Blackhawk, or Single-Six (all SAs use the same exact spring except for the newer ones with a key lock and the Old Army). The Wolff reduced power hammer spring is rated at 19 lbs. If you do the math ... 19/23=.826, or about a 18% reduction in actual trigger pull as measured with a trigger pull tester. Just a note about springs .... spring technology isn't very exact so +or-10% is quite normal. In other words, you could get a factory gun with a hammer spring ranging from 20.7 to 25.3 lbs. Wolff is in the spring business so their springs tend to be more exact ... +or-5%. It is entirely possible to replace a factory spring that was on the low end of the spectrum (20.7) with a Wolff spring on the high end (20 lb) and not detect a difference in tension.

I just happened to have a spring tension gauge that measures actual spring tension. Of course you need to "load" the spring just like it is in the gun in order to get accurate measurements. As you may know, a hammer spring is compressed to about 1.65" when the hammer is forward and compressed to 1.4" when the hammer is cocked so the tension ratings are based on how much weight it takes to compress a hammer spring from 1.65" to 1.4". I have used this tension gauge many times on Ruger SAs and found most factory springs are pretty close to spec but a few fall way under and a few are well over the 23 lb rating. This is one of the many reasons why trigger pull in out-of-the-box guns can vary so much. The Wolff springs I have tested are usually within 1/4 lb of advertised weight, again some a bit over ... some a bit under.

Here's the drill with a factory NM Blackhawk hammer: The distance between the center of the axis (hammer pin) and the tip of the spur is 2". The distance between the hammer strut pocket and the axis is .33". Mechanically, a hammer is nothing more than a lever so you can compute the mechanical advantage/disadvantage quite easily; 2/.33=6. Because the hammer is designed for a 6:1 mechanical advantage, a 23 lb hammer spring will take about 1/6 as much force to cock or about 3.8 lbs. Of course this doesn't count the force required to rotate the cylinder, compress the internal hammer plunger, or any internal friction ... just spring tension only. Conversely, when the hammer thrusts forward, the lever turns into a mechanical disadvantage. The spring strut only moves .25" and the nose of the hammer moves 1.5" from fully cocked until the hammer hits the firing pin ... a reverse 1:6 ratio. As you can see, the mechanical disadvantage of the hammer makes for a very long lock time. Lock time is the biggest single reason why mastering marksmanship is so difficult with a SA revolver versus any other platform. Further ... the sear notch on a Blackhawk hammer is located almost the same distance from the axis as the strut pocket. This keeps the ratio very close so a difference in spring weight will also affect sear tension and lock time by about the same % as noted in the first paragraph.

This post brings back some memories of when I worked in a lab some 45 years ago. My memory has faded but I do remember doing lock time tests on a variety of firearms. The ones that stuck in my head were the worst; a Ruger Blackhawk (.075 seconds), a typical 1911 was .043 sec, S&W and Colt DAs were about .039 sec. The fastest was a Remington Mod 788 rifle with .010 sec. I can't tell you what I had for breakfast but for some dumb reason, these numbers stuck. Simply stated, the lab test was a similar to a phonograph cartridge taped to the frame of a gun. The cartridge was connected to a storage oscilloscope in the "time domain mode". When you pulled the trigger, the sear would cause a "click" that was displayed on the o'scope then when the hammer hit the firing pin, another much stronger click was displayed on the o'scope. With the trace in storage, you merely measured the amount of time between the clicks. Very accurate, +or- .001 sec ... considering the technology in the 60's. Today there's probably some whizzbang device that is way more accurate and reads direct on a digital display.

JHS, Sometimes you get lucky and get a decent sear cut .... smooth enough where you don't feel creep with lighter springs. Usually just the opposite is true, lighter trigger pull ... more creep, the same as Precision32 noted. If you do a "full job" by polishing the sear notch, re-cut to neutral, and a shortened notch, you can actually get very crisp SA trigger pulls under 1 lb for a Ruger SA ... way too light. I often bend the trigger spring on a SA to increase sear tension to get to the desired pull weight. I've even been known to replace the 23 lb hammer spring with a 26 lb spring from a Ruger Old Army (black powder revolver). This also increases sear tension and results in a faster lock time.

I have a stainless CZ-75B that I bought new. Out of the box, SA trigger pull was about 6 lbs with some creep. The hammer moved back notably when I pulled the trigger so I altered the sear notch a little. It is still a bit under square but nothing like before. Both the sear and the hammer were very rough cut so I did my fine ceramic stone trick then buffed them with 500 grit compound on a muslin buffing wheel. With stock springs it checks in at a crisp 4 lbs. I could go lighter but I'm concerned about it going "full auto" with a perfectly squared sear notch ... not worth the potential safety issues and besides ... it is very accurate even with me pulling the trigger.

This is a wonderful thread!

Iowegan, how or where does one go about finding such a lab job as the one you refer to?

Excellent thread, thanks OP and Iowegan!
I learned a lot.

Well, once again, thank you, Iowegan, for sharing your incredibly vast knowledge.

I've had a lot of trouble shooting my SBH accurately and it keeps getting worse as I get more frustrated. You have given me hope that it is worth more than a handsome paperweight.

I believe you have uncovered a horrible mistake I'm making when I shoot my SBH. I think I'm moving the revolver inappropriately during the lock time. I'm guessing that if I focus on holding the revolver steady after the trigger breaks, I will increase my accuracy significantly.

The only thing that would make this better would be a "HOW TO" with pictures.

Awesome read. Thanks for sharing.

Ahhh lock time... Directly related to hammer spring tension, hammer configuration, hammer arc length, and friction in the moving parts. All the data makes me wonder if there have been studies on accuracy of a weapon with different lock times?, of course in hand where the length of time may directly affect accuracy. Would the small differences in lock time really make a difference? I could see if the numbers were large (relatively), but we're talking about pretty small numbers and even smaller differences. Regardless this discussion does show there are considerations one must take when altering the trigger/hammer system of a weapon, some obvious and some not so obvious.

In regards to hammer/main springs, many of them may be [over] powered to ensure a certain degree of safety and reliability from the factory. Much in the same a stock recoil spring may be [over] powered so as to provide adequate tension for +P loads. With that said, it therefore may allow you some wiggle room to play with, just as sear and hammer-sear notch angle can be manipulated. Even more room if you reload for competition. Just my .02 cents.

Sorry for the drift in topics from the OP ... but it's all related. Let me make a profound statement .... "trigger pull is very over rated with all guns". There are many issues that have way more influence on accuracy than trigger pull.

When I owned my gunsmith shop, I had many customers bring guns in to be "accurized". My first question was: "is it the gun or is it you?" The typical answer was "I can't hit squat with this gun but I can shoot other guns quite well ... so it must be the gun." Early in my gunsmithing career, I did what the gun owner wanted but in more cases than not ... the owner brought the gun back with the same accuracy issues. This was especially true with single action revolvers but applied to other guns as well. After spending a couple hundred bucks for a full accuracy job, no doubt, the owner was not happy. Later I bought a Ransom Rest and had it mounted on a cement base at a nearby range. I would take the owner, the gun, and some of my "bench mark" ammo to the range for a Ransom Rest test. In most cases, the guns shot very acceptable groups so this helped eliminate costly accurizing issues. Yes, sometimes there was an issue with the gun but in nearly all cases, it was a matter of "shooter interface" or ammo. Ammo was easy to isolate because I would also test fire the guns with the owner's ammo.

Back at my shop, we would go through the three main shooter interface issues ... trigger pull, sights, and grips. The single most important shooter interface issue is grips. You must be able to hold the gun steady while aiming, when squeezing the trigger, and maintain a steady hold after you squeeze the trigger until you hear a bang. Failure to do so always results in bad marksmanship. Guess where all the bad habits happen like jerking the trigger, anticipating recoil (pulling the muzzle down), pulling the muzzle left or right while squeezing the trigger, or flinching ... yup, it all happens during lock time and go unnoticed by the shooter. Once a good fitting set of grips are installed and the shooter practices dry firing, most of these bad habits go away. Again, the hardest gun to master is a single action revolver ... mostly because of the long lock time but also because "plow handle" grips don't fit anyone very well. Sights are rarely an issue, however everyone has different vision so what might present a good sight picture for one person may be totally wrong for another shooter.

Tater, Here's some info that may help. The sight radius on handguns is typically 2" more than barrel length. Assuming a 6" barrel, each .009" of muzzle movement results in a full inch of POI at 25 yards. In other words, during lock time, if you move the muzzle just the thickness of a fingernail (about .030"), you will miss the bullseye by 3.3". The shorter the barrel, the more sight radius works against you. A lighter trigger pull helps hide the root cause but certainly doesn't fix the problem. Here's a drill that I highly recommend ... have a friend load your SBH with a random combination of live ammo and spent cases so you don't know if you will hear a click or a bang when you pull the trigger. When you shoot a live round, you can't see what the muzzle is doing but when your hammer drops on a spent case, I can almost bet your muzzle will take a swan dive because you are anticipating recoil. Trigger finger placement also makes a big difference. Most people put way too much finger on the trigger ... up to the first joint. If you are right handed, this will pull the muzzle to the left. By centering your finger print on the trigger, you will tend to pull it straight back. A little experimenting with dry firing will reveal where your trigger finger should be so the muzzle doesn't lurch to the side during lock time.

As I mentioned before, nearly everyone prefers a light smooth trigger pull but most people don't even consider grips. From what I've seen on this forum and first hand experience in my shop, most shooters are more concerned about how pretty their grips are versus how well they fit the hand. Yes, I'm guilty too ... I prefer nice walnut plow handles on a SA for looks but when I go shooting, those grips come off and a set of oversized finger groove grips go on. I've found my marksmanship skills improve tremendously with grips that fit because I have much better control during lock time.

JHS, Yes, there have been many "studies" on lock time and how it can affect accuracy. I know .075 seconds doesn't sound like much but it is more than enough time to move the muzzle way off target. It is not unusual to see a shooter put all 6 shots in the bullseye with a DA revolver (about half the lock time of a SA revolver) but spread groups to 5 or 6" with a SA revolver that has an equally light trigger pull. Without a doubt, the longer the lock time, the more your bad habits will show up and demo accuracy. Bottom line ... most shooters chase the wrong rainbow. Get some decent fitting grips and your factory trigger pull will not be much of an issue.

Here's one of my Blackhawks with a set of custom grips. These same grips will fit my Vaquero, Single-Sixes, and other Blackhawks. I also have a similar set of grips for my 7 1/2" Super Blackhawk.

Nice grips!

I just finished doing some light stoning and polishing on the trigger in my '76 10/22. Now it has zero creep and breaks like glass. It's still a little bit heavy, but that's the built-in safety factor. I have a WOLFF increased-power hammer spring, and reduced-power springs for the sear/disconnector and for the trigger reset. I do all my work with a Lansky polishing stone.

Great thread! Thanks Iowegan. This is why this is the best forum hands down. I bet you are glad I am not your neighbor, I would find every excuse to drop by for a visit and pick your brain.

How about this: Is there an 'optimum' amount of hammer-sear engagement in the SA? A 'usual' or generally-agreed-upon engagement distance that minimizes the opportunity for creep to be felt, yet maintains safety & reliability in the field?

JHS, Single action revolvers have very long and heavy hammers with a long hammer throw (distance the face of the hammer moves from cocked to fire). As such, it takes more travel time with normal springs. You could speed up a hammer's lock time with a heavier hammer spring but that would also increase trigger pull plus it makes the gun harder to cock. Lighter hammer spring are the opposite ... longer lock time, slightly lighter trigger pull. The long throw of the SA hammer is part of the design and can't be changed.

DA revolvers, such as a GP-100 have a much shorter hammer, a shorter hammer throw, and the hammer weighs less. These parameters make the hammer move a lot faster ... almost half as much lock time as a SA. In most DA revolvers, lock time is typically just a tad faster than in the SA mode. That's because the hammer doesn't move back quite as far in DA before it releases.

The trigger latch spring has no impact on hammer travel or lock time ... just trigger tension.

Hogue makes decent grips but the question is.... do they fit you? Because there is so much variation in shooter's hands, there's no way for anyone but yourself to know the answer. When grips fit you well, your scores go up. With my 4" GP-100, the old style Lett grips with the wood inserts will increase my scores quite a bit (in both DA and SA) compared to Hogue grips. The next guy may say just the opposite.

MZ5, I've never seen an actual specification. I think it would depend on several factors such as spring tension, angle of the sear notch, angle of the sear, etc.

I have been schooled that 0.015" sear engagement is adequate. Even at that small of engagement you should be able to see or feel the hammer cam back (over cock) as you squeeze the trigger.

On two stage trigger systems you can go a little less.