Home‎ > ‎

Update 3/14/2014

posted Mar 15, 2014, 11:12 AM by Danny Park
So, attempting to keep most of the design points in tact while bringing the design back into the realm of reality, I've changed the design somewhat. I've added a boost stage to compensate for the lack of diameter inside the buffer tube, as the swash is limited to ~1" diameter and the most you might be able to get is 1" of travel... assumedly... the first design was around 60% swash angle, which bothered me because it seems like the piston feet won't be very stable. I'm convinced that going beyond 45 degrees causes some issue, but I'm not sure what yet, so for the first second attempt, I've reduced the swash angle to something more reasonable like 30 degrees and added another stage. The stages, ignoring compressibility and other effects like that, should behave like gearing for compression ratios... So if the first stage has a 15:1 ratio (I'll call it boost or low pressure stage), will raise the pressure from atmospheric (~15psi) to ~225. The second stage, let's say 5:1, will raise the pressure again from 225 to 1125psi for a final compression ratio of ~75:1. Quick stress analysis gives me ~1500psi as the maximum  pressure I can get with a 1" tube with a 1/32" wall thickness (inventor gives me a safety factor of 1.3 with 6061... probably T6) but by the time I start back up, I'll hopefully be able to afford "real" machines that will laugh at steel rather than struggle with aluminum... 

I don't believe that this axial piston design puts much load on the motor (but I could be wrong), so my current design point for the motor is ~15,000rpm. My motor has a 2370rpm/kv rating, which puts me beyond 25k unloaded, so maybe 20-22.5krpm isn't that far outside of the realm of possibility, assuming that I can sink the heat into the receiver, casing, and main reservoir.

Because there is an increase in pressure, I'm considering removing the second pressure sensor or moving it somewhere else where feedback might be useful. After the initial testing and curve fitting, I believe we'll be able to extrapolate the pressure from the torque/current draw of the motor, or I could monitor it a little more directly if I were to put in a rpm sensor... As long as I have a reasonably accurate (I believe my sensors are +/-5psi at the moment, but with the design change, I'm guessing I can get it down to +/- 1-2psi as I'll need to drop the pressure anyways and the accuracy of the sensor is proportional to the full scale value... that and the arduino micro I plan on using only has a 12bit ADC... 

The current iteration has a ~1"x3" DxH cylindrical reservoir which can store 18 shots before it is empty at 800psi... and takes 4.46s to fill and can sustain an embarassing 4rps rof... if I increase the rpm of the motor to 20k, that increases the rof to roughly 6rps, and that is with the motor running continously... I have the torque code somewhere that I can run for the latest iteration, but I'm guessing full auto is going to be a big power drain (I think last time I estimated something like 1 shot per mah, and that was with more optimistic numbers). What I think I want to explore next is replacing the outer barrel (since I've already taken it upon myself to build the "loader" into the hopup unit and have the barrel secured into the hopup (maybe threads or a bayonet lug), I'm considering also replacing the outer barrel and having a sort of tubular (ID/OD) main reservoir with the barrel running through it and then have the compressor in the stock. 

The ROF is largely limited by the "constant-on" compression/stroke and rpm. The compression is limited by the number of stages if we set the swash angle at 30 degrees (or ~.5" stroke) and the piston diameter, which is limited by the diameter of the buffer tube (I'm also considering replacing the buffer tube with a motor if I can move the reservoir to the front, so I can add more stages... maybe a total of 3-4... The RPM is going to be limited by the W-Hr of the battery and by heat dissipation... 

So... I guess we're back to whether or not we're willing to give up on full auto for now... I keep feeling like if I spend another day or week trying to come up with a scheme to get a workable rof (~15rps) but, aside from manufacturing and weather issues... it's beginning to hold back the finalization of the second iteration prototype... (I sort of gave up on the first one when I found I couldn't hold parts to within a few thou...) So that's where we are now...
Comments