Crosman Mk-177 low-volume HPA / Pump Hybrid.( an experiment )

[AirCats]

<starting this thread to document the small-reservoir-HPA / pump hybrid expriment I did on a MK-177. Maybe someone else will find the info useful as well>

Modifications:
*The MK valve body was drilled/tapped to 1/16" NPT; a 1/16"-> 1/8" NPT adapter (mcmaster part # 9171k231, max psi @72F is 4000) was installed; a Ninja Micro-bore hose was then connected to the adapter.
*the valve body is drilled/tapped to M4x0.7 and bolted to the main tube.

*A striker block was installed behind the valve body to limit the max possible valve-lift to about 2mm (I don't want it to dump all the air in one shot). Per my calculation, the max necessary valve-lift is around 1mm, so 2mm is more than enough. The block is bolted to the main tube by a M4x0.7 screw.

The ninja hose is coupled to a Hill pump (MK3) via a male/male foster coupler. All pressure is contained within the hose and the valve, the maintube is not pressurized.

Total reservoir volume of the system: ~9cc
-- MK valve internal volume about 3cc;
--the Ninja micro-bore hose is ~50" in length, ID is ~0.075" (according to Ninja) ; the Hill pump hose (also micro-bore) is ~30", assuming both micro-bores have the same ID, the hose volume is ~5.7cc; 
--plus the extra volume in the adapter and the male/male coupler


Pressurizing the system:
-- using the Hill pump, it took ~ 8.5 pumps to go from 500 psi to 1500 psi;
-- if using the MK177's internal pump system, the pressure goes up around 50 psi every 10 pumps; once it gets to ~800 psi, the efficiency goes even lower, up only about 30 psi per 10 pump. (the MK177 pump cup was lubed with Hill pump grease. Because the MK now has the potential of going tethered-HPA, I don't want to use any oil in it)

FPS & PSI:
below is a typical shot string, using 7.2gr Daisy HP;
-- Stock striker, striker spring, valve and valve spring.
-- pressure was read off the Hill pumps gauge; fps was from a ProChrony. 
-- elevation ~5000ft, temp ~70F; bolt handle was pushed forward to make sure it doesn't get blown back during the shot.
1500psi start-->  824fps
1350psi --> 783fps
1180psi --> 733fps
1050psi --> 690fps
950psi --> 623fps
800psi --> 578fps
680psi --> 525fps
580psi --> 457fps
500psi --> 417fps
400psi --> 362fps
350psi end

-- the fps drops 40~50fps every shot, the system doesn't seem to self-regulate. Maybe the striker is too heavy, or the valve spring too stiff... or the total reservoir volume too small.


 

[K.O.]

Takes ~22 pumps but I get 805+ fps with 14.3 with a modded valve MK pump; before I made it into a MK1322 I cheated(heated  valve) and was able to hit 1024 fps with 7.9 grain CPHP with it still a MK177... a little over 900 fps without heat... I think modded aprox valve size is .24ci...with a pump swept volume very close to that of a 2100.

Not really sure what amount of psi it builds but ... about 1300-1500.  I can with a valve extension get about .31-.34ci  I think...

cool to see what can be done with 4 times the volume... and have been thinking of how to do a Millennium pumper style external tank...

That's the good thing about the valve nice and thick back by the exhaust poppet...

[AirCats]

fps with higher psi:
bolt forced forward during the shot (to prevent it from being blown back):
2000 psi start -->920 fps --> 1700psi
2000 psi start -->912 fps --> 1750psi
2000 psi start -->913 fps --> 1750psi
2000 psi start -->917 fps --> 1720psi

(the micro-bore hoses are rated to 4500psi and the NPT adapter 4000psi, the weak link is the valve body. since the crosman valve in a flat-topped setup can see 2000 psi, I suppose it is ok to try this pressure. I will not go any higher)

[AirCats]

the MK HPA is more efficient than my 760/M4-177 HPA, when pressure is higher than 1000psi.

(760/M4-177 hpa experiment data can be found here:

http://www.gatewaytoairguns.org/GTA/index.php?topic=104369.0 )

[rsterne]

Part of the lack of self-regulation may be the block you put in to limit the valve lift.... In a typical unregulated PCP the lift increases as the pressure drops, that is what causes the valve to self-regulate.... The maximum amount of lift will vary, depending on pressure and hammer strike, but typically it may be as much as 1/3 of the throat diameter at the low pressure end of the usable shot string.... In almost every case, over a 4% ES string, the lift is about twice as great at the end as it is at the beginning....

Bob

[AirCats]

the problem probably has more to do with the weight of the MK striker.

In the M4/760 hpa experiments (link in the post above),  configuration-a and -b don't have striker-block and they don't self-regulate; config-C has striker block in place, it seems to self-regulate from 1800~1400 psi.

The MK striker weights about twice as much as the 760 striker. It will be difficult to reduce the MK striker weight from ~ 46g to ~26g.

[rsterne]

reduce the preload, then....

Bob

[AirCats]

then the lock-time might be too slow....