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Discussion Starter #1
Ok i have read that if you change out the bobbed hammer on the USP Compact for a spurred hammer you could replace the hammer spring with a weaker hammer spring. The reasoning was that the spurred hammer weighs more so it hits harder.


The bobbed hammer weighs 'w' and need to go this velocity 'v' to hit with this force 'f' strong enough to ignite the primer.

The spurred hammer weighs more than the bobbed hammer. The velocity of the spurred hammer would have to be less to equal the same force.

Here is my problem, it would make take more force to move the spurred hammer to the correct velocity to get the right amount of force, right? So why would you replace it with a weaker spring?

I'm confused.

Thanks for your help guys.
 

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Ok i have read that if you change out the bobbed hammer on the USP Compact for a spurred hammer you could replace the hammer spring with a weaker hammer spring. The reasoning was that the spurred hammer weighs more so it hits harder.


The bobbed hammer weighs 'w' and need to go this velocity 'v' to hit with this force 'f' strong enough to ignite the primer.

The spurred hammer weighs more than the bobbed hammer. The velocity of the spurred hammer would have to be less to equal the same force.

Here is my problem, it would make take more force to move the spurred hammer to the correct velocity to get the right amount of force, right? So why would you replace it with a weaker spring?

I'm confused.

Thanks for your help guys.
Ignoring for the moment whether you can, or whether it's a good idea, here's one possible reason why, based on physics like you asked -

It's called the moment of inertia. Where weight is distributed on a rotating object can greatly affect it's moment of inertia, which determines how quickly it can be accelerated by a given force.

Here's the best example I can think of, and it is actually with a 100% weight difference from the light one to the heavy one -

Pick up an 8 lb sledge hammer by the handle, and hold it out horizontally from you. Now, as quickly as you can, swing it from your left side to your right side. Now take a 16 lb sledge hammer, but this time by the head, with the handle sticking out. Now swing it again. It will accelerate much faster and with much less force, even though it weighs twice as much.

As for your hammer question, it may either be incorrect information, or it might be true. Moment of inertia is just one possibility as to why.
 

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Although this is totally possible good luck trying to find someone to do it. I have been wanting it done for awhile now and can not find anyone that will. There is a p10/p8 model that are available but they are not sole in the u.s. I don't understand why? What is the big deal with switching out hammers. The gunsmith in my town does not even like putting nite sites on HK's. Weird!
 

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Discussion Starter #4
I'm not worried about finding the parts or doing it. Its appears to be fairly commonplace on the site. The fullsize and compact hammers are interchangeable (within reason). HK will install them if you want to send it to them.
 

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the relationship between angular momentum and angular velocity, torque and angular acceleration is what will actually determine whether or not the spring replacement is required. In terms of moment of inertia I believe this notion is referred to as or at least solely depend on the MOI tensor, which is a complicated mathmatical representation of all of the moments on the various points on the object, which in this case is the hammer. I digress but anywho the relationship between all of these things will determine whether the added weight will warrant a weaker spring, thus reducing the torque and eventual angular acceleration on the hammer resulting in the proper amount of force applied to the firing pin. At the very least it should slow down the lock time ;-) never a good thing for those of us who flinch.
 

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Discussion Starter #6
Well I have something to look forward to figuring out when I go back to college for mechanical engineering...
 

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the relationship between angular momentum and angular velocity, torque and angular acceleration is what will actually determine whether or not the spring replacement is required. In terms of moment of inertia I believe this notion is referred to as or at least solely depend on the MOI tensor, which is a complicated mathmatical representation of all of the moments on the various points on the object, which in this case is the hammer. I digress but anywho the relationship between all of these things will determine whether the added weight will warrant a weaker spring, thus reducing the torque and eventual angular acceleration on the hammer resulting in the proper amount of force applied to the firing pin. At the very least it should slow down the lock time ;-) never a good thing for those of us who flinch.
Ok, now this has just gotten silly. Angular momentum, angular velocity, and angular acceleration are all functions of two things - the MOI of the hammer, and the torque applied by the spring. But since you brought it up - an MOI tensor is used when the object's center of rotation cannot be determined, and it is a summation of all MOI's about any point in space, but normally about the center of mass.

But since in the case of a hammer, the axis of rotation is known, so the scalar form of the MOI equation is used, (I=m*r*r). There is no need to calculate the tensor MOI.
 

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Sorry...it's been a while since statics and rotational dynamics. Since the spurred hammer shape is irregular and assymetrical, wouldn't a tensor analysis yield a more accurate result. Although I totally agree finding the center of mass and using scalar analysis would be a helluva lot easier and get the job done. Oh it doesnt matter since it can be represented two dimensionally does it....(in regards to the tensor idea). Yes I know...I know this is sick but hey..guns are physics.
 

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Discussion Starter #9
So I guess i could always fix the gun facing upwards next to a ruler, place a pole .45in in diameter that is fairly lightweight, down the bore and pull the trigger. See how far the pole goes up (hence the ruler). Change out the hammer and spring and see if the pole goes the same vertical distance?

It is very rudamentry but seems to be a valid test.
 

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From looking at the two, if the spur part is hard rubber all the way through the mass difference is negligible in the grand scheme of things.

If it dosent light off the round with the lighter spring and bobbed hammer it wont do it reliably with the spurred one either.
 

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There are plenty of people on here that have done it. do a search for some of big_chris's work and you may uncover some answers. the hammers are perfectly interchangable but the spring I am unsure about. Seriously poke around with the search, if its been done it's probably on this forums history.
 

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I would suggest leaving the factory hammer spring in as is from the factory.

A friend who tried to be a gun smith once and a while tried doing a "trigger job" on my buddy's USP 45. This "trigger job" was just changing the hammer spring for a lighter one. The trigger was smoother. But her started having a lot of type one malfunctions.

You know, when you get a click when you were expecting a bang?
 
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