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Air Gun Home Forum Index » Rifles » Thoughts on Hammer Travel Adjustment
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Thoughts on Hammer Travel Adjustment 
PostPosted: Sun Dec 02, 2012 9:43 pm Reply with quote
rsterne
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I have used an adjustable hammer only once, and that was on my Hayabusa.... The results were somewhat confusing and I didn't find it particularly useful.... I used an MRod adjusting screw threaded into my hammer, and what I found was that over quite a large range of adjustment it made virtually no difference to the velocity.... Only when extended a long ways did it start to reduce the velocity to a significant degree.... I though about why that was occurring, and realized that when you reduced the travel by extending the nose of the hammer, that since the adjuster was sitting on the valve stem when uncocked, the hammer actually moved back, increasing the preload on the spring.... Therefore, even though the travel was reducing, the hammer was seeing a higher average spring force, nearly cancelling the advantage of increasing the hammer travel....

I wanted to increase the travel on the hammer for my new Milleium Pumper project, so this morning I used Lloyd's Hammer Strike spreadsheet to investigate what was happening.... I used a Challenger hammer for the inputs, along with my heavy Disco hammer spring, and came up with the following graphs showing hammer Energy and Momentum.... The adjusting screw is 28 TPI, so each turn of the screw changes the hammer travel by 0.036", and it therefore changes the preload by the same amount.... That information is graphed as "Preload Varies with Stroke".... The second graph was generated with the same hammer travel distances, but is "Preload Constant", set at 10% of the free length.... That value is the same on both graphs when the stroke is 0.58" (the third dot counting from the left).... Both graphs have identical scales, so they are easy to compare, and the lines would cross at that third dot from the left....



The first thing to note is how little the hammer energy and momentum change with the normal setup.... I didn't look at extending the adjuster out a long ways, because I'm after more travel, not less.... but you can see a slight steepening of the trend on the left side as the travel decreases.... That would be consistent with the way my Hayabusa reacted to decreasing the hammer travel.... little change until you went quite a ways, and then decreasing velocity.... Next you will note the drastic difference in the second graph.... Once you hold the spring preload constant, the Energy and Momentum both increase linearly with increases in travel.... I estimated that the spring would become coil bound if the stroke increased beyond about 0.70", so the last (right hand) datapoint is omitted on the second graph....

Ponder this information while I do the next post about how I built a hammer where the travel is adjustable but the preload is constant at whatever you set using the RVA....

Bob

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PostPosted: Sun Dec 02, 2012 10:22 pm Reply with quote
rsterne
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I spent the afternoon building a hammer to put what I learned from Lloyd's spreadsheet into practice.... I needed a hammer that would fit into a Disco tube, but had no "waist" as I will be using an MRod trigger group.... I also wanted a bit more weight, so a straight cylindrical shape met all those criteria.... The length and diameter are the same as a 22XX / Disco hammer.... and since I was using a 22XX Crosman steel breech (with the forward breech screw location) I didn't need to relieve the top front portion of the hammer.... I wanted to be able to use the hammer spring from a QB 7X, which is 3/8" longer and slightly bigger in diameter than a Disco spring.... so I made the travel adjuster from a piece of 3/8"-24 NF bolt and threaded the drilled and tapped the inside of the hammer appropriately.... A spring guide was threaded into the travel adjuster, and moves with it.... The hammer spring sits against the back of the adjuster, so the preload doesn't change when you turn it.... What happens is that the front of the adjuster rests against the end of the valve stem, and the hammer moves back and forth over it as you change the adjustment setting.... Since the sear is in a fixed position, the further forward the front of the hammer the further it has to be pulled back to cock the gun.... You therefore have adjustable travel but constant spring preload.... Here are the parts, and the assembled hammer, which weighs 70 gr. including the guide, about 10 gr. heavier than a 22XX hammer....



In the photo of the assembly, the travel adjuster is protruding beyond flush about 0.1".... It can move back inside the hammer about 0.2" which increases the travel over stock.... Eventually the face of the hammer will hit the back of the valve, and that will be the effective limit of the available travel.... I shortened the threaded adjuster so that the hole in the hammer is deeper to accomodate the extra length of the QB spring without having to make a very long RVA....



The extended spring guide runs in a hole in the RVA screw.... It will be cut to length so that it protrudes through the screw only when the gun is cocked, acting as a cocking indicator.... The threaded hole in the top front of the hammer is for an 8-32 setscrew pressing on a small piece od Delrin which will act like a brake to hold the travel adjuster from moving.... There will be a screwdriver slot in the end of the guide to adjust the travel.... A standard Crosman cocking pin sits in the back hole on top of the hammer, but it is moved back 1/16" from the stock location so that it won't hit anything at the forward end of it's stroke....

I made a couple of other small mods while I was at it.... I lengthened the front of the cocking slot in the tube about 1/8" so that it is slightly further ahead than the front of the cocking slot in the Crosman steel breech.... I also shortened the back of the bolt 0.110" for the same reason.... The hammer face will now hit the back of the valve before the pin hits anything, and in fact I may turn off a small amount from the back of the valve to get the maximum possible travel possible.... Unless I lengthen the cocking slot in the breech, it (and the back of the valve) becomes the limiting factor on hammer travel....

I will likely just use the adjustment to find the maximum possible hammer travel where the hammer hits the back of the valve, and then set it so that it doesn't quite hit.... That will allow me to get the highest possible hammer Energy and Momentum with the least preload on the spring, and therefore the easiest cocking.... Once that setting is determined I will back off the RVA until I reach the final tune desired.... At least that is how I think the adjustments will be made.... At least now, increasing the hammer travel will have an instant, and definite, affect on the velocity....

Bob

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PostPosted: Wed Dec 12, 2012 10:48 pm Reply with quote
rsterne
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You never know what path you wil take until you get there.... Nothing could be more true of this adventure into airgunning Nirvana.... What started out as a new way to adjust the hammer travel has suddenly taken an abrupt right turn (or is that left?).... When I installed my new adjustable travel hammer into my Millenium Pumper I really didn't expect anything unusual to happen.... I figured that increasing the stroke would mean I could reduce the hammer spring preload, or maybe even fit a weaker spring, making the gun easier to cock.... and for the most part, that is exactly what happened.... However, when I adjusted the hammer for maximum travel (about 0.2" more than a stock Disco) and ALSO used a lot of hammer spring preload, I actually LOST velocity.... HMMMMMMMMMMMMM.... DOES NOT COMPUTE !!!

I intuitively figured the face of the hammer had to be hitting the valve body, limiting the lift, and that was causing this effect.... It required taking quite a few data points at different pressures and with different springs for this idea to gel in my mind so that I could wrap my brain around it (I'm slow sometimes!).... I can hear bstaley laughing, he already knows what's going on here.... It turns out that the valve stem in my Disco sticks out 0.30" from the back of the hammer.... At the maximum travel setting, the face of my hammer is recessed 0.20".... Therefore, when the outer face of the hammer hits the valve, the MOST it can lift the valve is 0.10" (the difference).... If I don't have enough hammer spring preload to lift the valve that far, you don't notice anything unusual, because the hammer face never hits the back of the valve.... However, at maximum preload, the hammer WANTS to lift the valve more than 0.10".... and IF it could, the velocity would increase (or at least not drop off).... Now for the "lightbulb moment".... When I recess the "travel" adjustment far enough, it is no longer a travel adjustment - it is a LIFT adjustment....

Lots of precedence here, I'll mention two that I am aware of.... One is the bstaley mod for the MRod, where he uses O-rings against the back of the valve to limit the hammer travel and turn the stroke adjuster into a valve lift adjuster.... I never really grasped this until he wrote up an excellent thread on how it works, which you can read here.... http://www.gatewaytoairguns.org/GTA/index.php/topic,37695.0.htm .... My compliments to bstaley on figuring this out and implementing it.... The idea (to paraphrase) of not affecting the first part of the shot string, while limiting the lift (and possibly also the hammer bounce) in the second half of the shot string are a perfect example of critical thinking followed by a simple solution, and I congratulate him for it....

The other place I have seen this used is in the Korean rifles which have a power wheel that adjusts the valve lift.... They use a metal block between the back of the valve and the hammer, which can be moved back and forth by a rotating adjuster on the side of the gun.... When you rotate the adjuster, it in turn rotates this block, which is threaded, and it's fore-and-aft movement changes how far the hammer can travel.... This, in turn, directly alters the lift of the valve.... less lift, less power.... There are probably other examples of this that I am not aware of, and if so, please accept my ignorance, I'm not trying to slight you in any way.... I'm going to post this and take a breath here.... lots to think about....

Bob

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PostPosted: Wed Dec 12, 2012 10:48 pm Reply with quote
rsterne
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OK, to continue.... Let's assume for a minute that my new style "travel adjuster" is now a "lift adjuster".... How is it different than the bstaley mod?.... Well, for one thing, it is recessed into the front of the hammer instead of the way the MRod (or Challenger) adjuster sticks out the front.... This INCREASES the available hammer travel instead of decreasing it.... Secondly, it has a spring guide attached to it.... How about the lloyd-ss "long stroke hammer"?.... Lloyd of AirGun Lab has been using hammers with a hollow front for quite a while to increase the stroke, a simple and effective method.... He takes that a step further by machining the back of the Disco valve to shorten it, allowing even more hammer travel.... I believe he leaves a central "stub" on which he mounts an O-ring to seal the stem, perhaps he will chime in with the details....

What I've done here is to combine several good ideas, none of which I can take credit for.... However, the result is a hammer for the Disco that first of all increases the stroke, secondly provides adjustability for the valve lift, and thirdly incorporates a spring guide / cocking indicator.... I believe I first saw that idea used by Ribbonstone although his simply sat in the hammer, held in place by the spring.... There is even a fourth possibility, and that is adding a cocking knob to the end of the spring guide (which Lloyd and I and others have already done).... That would additionally add mass to the hammer, and could be turned to adjust the lift.... Lloyd uses a captive O-ring on that shaft as a visual indicator of the valve lift, enabling direct measurement of that difficult to ascertain measurement....

Anyway, there you have it.... I'll be doing a bit more work on this over the next few days, trying to develop it and optimize it.... I am hopeful that the result will be a hammer that not only increases the potential FPE of the Disco, but provides easier tuneability and hopefully even an increase in efficiency and/or shot count....

Bob

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PostPosted: Wed Dec 12, 2012 10:49 pm Reply with quote
rsterne
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After writing the previous two posts, I went out into the shop, pulled the hammer out of my Millenium Pumper, and modified it slightly.... I removed 0.040" from the back end of the adjusting screw to allow one more turn of travel.... I can now recess the hammer 0.25" from the front face, reducing the maximum valve travel to 0.060" at the point the hammer face impacts the valve body.... I set the RVA to a whisker short of coil bind on the 2.00" x 0.045" hammer spring, maxed out the travel, and took a shot at 1800 psi, recording the velocity.... I repeated this, adjusting the position of the travel/lift adjuster.... I then decreased the hammer spring preload a turn at a time and did everything again.... Here are the results....



If you look at the curve for the hammer spring near coil bind (blue line, reading from right to left), you will see that as you increase the hammer travel (and hence the Energy and Momentum) the velocity increases and then plateaus.... This is the expected reaction to increasing hammer strike, as I've demonstrated many times on regulated guns (ie at constant pressure, in this case 1800 psi).... However, the last 2 turns of travel show something different happening.... At about 1 turn out, where the valve lift can no longer exceed about 0.10" without the hammer hitting the back of the valve, the velocity starts to decrease.... In that last turn of travel (1/24th of an inch = 0.042") it drops like a stone.... This is clear evidence that the hammer is hitting the valve, and that the loss of valve lift is killing the velocity.... At 2 turns out, we see the same pattern, with the velocity dropping off during the last turn of adjustment.... At 3 turns out, the dropoff doesn't occur until the lift is less than about 0.080" (1/2 turn out).... and at 4 turns out it doesn't occur at all, with the maximum velocity occurring at maximum hammer travel.... That tells me that at 850 fps, the valve is only opening 0.060" or less at 1800 psi....

Sooooooooooooooo.... the facts fit with the theory.... I can now adjust the valve lift providing that I am running lots of hammer spring preload.... There is one problem, however.... The adjustment, even using 24 TPI (0.042" per turn) is VERY finicky.... One turn (with the spring at coil bind) is over 250 fps.... Considering that, it remains to be seen if there is any practical application.... It will depend on whether or not I can achieve better efficiency running on the "left side" of the curves (where the velocity is determined by valve lift directly) compared to on the "right side" where the velocity is determined by spring preload (and it's effect on lift and dwell).... More experimenting is called for.... and the Millenium Pumper may not the the ideal candidate for that....

Bob

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Thoughts on Hammer Travel Adjustment 
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