Explaining Free flight / Hammer gap

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ackuric

ackuric

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Mar 10, 2017
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Colorado, United States
Free flight / hammer gap is when there is a nominal distance (ideally at least 2/3 distance of valve lift) between the face of the hammer that strikes the valve stem when the rifle is left uncocked, and the valve stem itself. Why is it important to have this in place in modern pcp's you ask? To combat hammer bounce, and to allow the use of softer springs and heavier hammers without concern for further inducing more hammer bounce..




Why/when does hammer bounce occur? Why? Because the hammer follows the valve stem upon opening, without the continued hammer KE/momentum the valve's poppet would seize lifting off the seat. Once the hammer energy is diminished, the airs KE and Momentum (like an air spring) plus the force against the valve stem shut the valve very abruptly, and while doing so returning some energy to the hammer in its reversed direction, back to the hammer spring, if theres enough energy the hammer will then preload the spring and rebound back to the valve stem, potentially wasting sips of air...When it occurs is when there is not enough gap or no gap, and/or too much pressure drop or not enough force holding the valve shut after the valves cycles, or when too soft a spring is used with too heavy of a hammer.



How does Free flight / Hammer gap work?

Distance created between valve stem and hammer face when uncocked by use of either a short spring that is not preloaded, or a preloaded spring on a guide, this technique allows 2 things. Distance to create time to allow regulator refills and full valve closure / seating prior to being struck again, and unsprung distance for hammer to travel to allow for energy dissipation upon its return to strike the valve an undesirable 2nd or 3rd time. 



When you use the value of 2/3rd valve lift, this requires the hammer to travel 133% of the valve lift distance prior to striking the valve stem again. So say my valve lift's max flow is roughly .06" (most pcp's don't need more than .08"~), then I would set my gap distance to roughly .04" which the hammer has to traverse twice to sequentially strike the valve stem, for a total distance of .08"..meaning by the time the hammer begins its .08" journey, the valve should be near fully seated, the regulator should be refilling, and the hammer should be continually dissipating its energy until its rebounded, and hopefully by that time, with not enough energy to reopen the valve to any degree costing any air loss.



HTH explain Free flight / Hammer gap to those who haven't pondered it. 

-Matt
 
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With Acturic's indulgence may I offer this Historical short spring research I did back in 2005.

1546859155_4443172495c333293144a06.59643560_Excalibre hammer apart_zps8scz0wvo.jpg
1546859351_10686910095c333357c0a486.32126652_Excalibre Hammer module 05 jpg_zps6qillhzz.jpg


Here are some "short spring" hammer modulesI modified in 2005 for my RWS Excalibre/FX 2000.

The first pic #1 shows three hammer modules with hammer stems protruding with three different free flight gaps and one with an external light spring also designed to counteract any secondary positive hammer return " bounce". I made and tried many springs before getting the free flight gaps somewhat correlated to different power levels to give efficiencies down to 9+ bar-cc/fpe which is still a solid benchmark today.

Pic #2 shows the relationships between the module shells, the hammer rods and heads (the rod screws into the head as seen in pic #3), and in some units I incorporated an inner weak spring and external "o"ring as an auxiliary buffer to further cancel hammer bounce.

Pic #3 shows the rest of the valve, valve stem in relation to the hammer; and the adjustable hammer stem which screws into the hammer head. In the background , past the screwdriver, you see the module shell end- on with the hammer bolt projecting to our left.

In later years all these strategies have been explored by others and in some cases marketed.

Of interest here is that the hammer cocks on bolt closure, forcing the module forward after trigger engagement to compress the internal spring. The shorter springs enabled the system to operate just as the OP has described above and to eliminate hammer bounce or repetitive valve openings that waste air.

Fredrik Axelsson marketed these rifles under various names, as Tarantulas in USA. It was his vision in incorporating the adjustable hammer stem to allow for adjustment of spring preload, with the original full length springs, that allowed my short hammer springs a further fine adjustment of the free flight gap.

Pic #4 shows a range of free flight hammer gaps from the front end of the modules

1546861369_4870771785c333b398b8084.66731885_Excalibre Hammer module 02jpg_zpslezgpviw.jpg


Below: some of the experimental springs made from various gauges of piano wire; and the whole unit aligned.

1546862452_5347787745c333f74443334.11886785_20160130_112237.jpg


It was all trial and error but the results were quite spectacular at the time. 

The rifles are pretty. They were the first to shoot sub inch 100 yd groups from 2002 forwards. This one has shot over 70 such groups. It is unregulated .

Note that all Fredrik's FX rifles since this one have free flight - a more recent descriptive term - hammers including the present Crown etc.

With thanks to the OP for your indulgence of this little bit of relevant history.

Please expand the rifle pic to show what was being made back in 2000. Is't it pretty. .... . Kind regards, Harry inOZ.

1546862999_2738143055c3341972b5097.60376124_Excalibre upright_zpsh9j9pw1s.jpg



 
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To add, a short stiff spring additionally aids by having a higher spring rating which absorbs energy better than one without a gap that has the hammer preloaded against the stem. Where as a spring guide allows you to load a long light spring and create a similar effect as the short stiff spring without having to increase spring rating too much plus being able to control spring preload without effecting the ideal gap between valve stem and hammer face.



-Matt 
 
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All good points Matt; and clearly stated for those who may also be motivated to experiment.

Another note re-the Excalibre/Tarantula: In the pic with the screwdriver one can see how the HS preload was enabled from outside the rifle. The nail, shown protruding from the module, goes in through the rifle casing and inserts into the hammer head to hold it located while the screwdriver is inserted through the back of the action. Thus, with the hammer head located, the hammer rod can be screwed in or out to either (1): vary the preload on a full length spring, or (2): vary the free flight gap, which is more relevant to the OP.

Again, in relation to your explanation of the use of lighter rating springs, one pic shows a very long weak spring, which without the short heavy spring, was an attempt to exploit the valid point you make above. I did conclude that the shorter reverse-wound inner spring matched to the strong short outer spring seemed to have an advantage in slightly reducing velocity spread. But the biggest factor in that regard was the facility on the rifle to also externally vary the transfer port cross-sectional area with an adjustable interference screw. Not only does that allow for another velocity/power tuning interaction with hammer preload adjustments, but it can slightly reduce the difference in velocity between the shots of highest velocity and those of lower velocity thus reducing the spread. 

Matt I do appreciate what I have read of your posts and responses by way of explaining some of the more interesting dynamics related to our rifles. Accurate explanations can lead to improved understanding and education which in turn can motivate more people to get involved in the evolution/refinement of the systems. Fredrik Axelsson has been at the forefront of innovation since those early rifles. His "peephole"in the receivers of rifles, over the last eight or so years, has enabled us to visually see the free flight gap between the hammer and valve stem, thus enabling a tuning refinement of that gap which you have highlighted in this OP.

Best regards, Harry.

Ha ha, those were only a few of the bucket of springs I wound! 
 
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So what are the effects of say too much gap ?

Does the hammer slow down too much going forward ?
Click to expand...



Too much gap can cause more erratic shifts in velocity, especially when varying the degree the rifle is pointed at being up or down, since gravity will have more of an effect on the unsprung distance the hammer travels. That is why I recommend roughly 1/2 valve lift to keep the distance minimal. And yes gap does 'slow' the hammer even when moving to the valve for its first trip and requires additional valve spring preload, increasing gap slightly is another method of fine tuning for the perfect fps and or efficiency.



Yrrah, you're certainly right about the adjustable restriction of porting, which can be accomplished quite a few ways, which allows you to tune as efficiently close to the plateau as possible. Being marginally far away from the plateau (2-3%) means your hammer strikes should be very consistent and the window of variance is much smaller than if you were tuned well below 5% of the plateau of a given port/pressure/barrel distance ect. I eliminated the marauder velocity screw adjuster and sealed it shut for maximum air flow at all times, which while at my low powered tunes does slightly increase the ES(still well within reason @ 1.5-2.5%), the marauder's velocity screw is lack luster and doesn't have static repeatable settings built in, and I am not an ES chaser for low power tunes, because IMO if I am shooting long range where es really matters I'll be 2-3% within my plateau with ~1% es. Although I do respect FX and other companies that have static prebuilt settings for their adjuster that make tuning or swapping calibers that much easier. I've thought about designing something similar to what FX does for my marauder valve many times, but to cut my es in half for a tune that doesn't really need it seems like a lot of hassle so I always discard the idea. If I had an elegantly designed system in place I would certainly keep it however. I am not even sure the old marauder valve restriction screw would even do much for my current .23" ports. Lol.
 
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