Pumpers - Design/Mod Discussion Thread

[rsterne]

BillG asked about starting a thread to discuss the design and modding of pumpers.... I agreed it was a timely topic, an offered to start the ball rolling with my three "Uber-Pumpers" (well actually 2 rifles with 24" barrels, and a carbine).... After returning to airguns after decades away (and never any building, just shooting, back from when I was a kid into the 70's) in 2007 when I retired, I spent the first while with springers and then started working with 1377 series pumpers in the form of the 2289 Carbine.... After initial experiments with porting, flat-topped piston and valve, etc. I started working on using pumps with larger swept volume to cut down on the number of pumps.... I first built a .177 cal based on a 2200 pump tube and linkage, which was developed to where it was shooting 7.8 gr. JSB Express at 991 fps (17 FPE) on 30 pumps.... I then progressed to a .22 cal based on a 1400 pump tube and linkage, and achieved 902 fps with 14.3 gr. JSB Express (26 FPE) on 22 pumps.... My final build was a .22 cal carbine based on a 2200 pump with a 14.6" barrel, and at maximum it achieved 765 fps (19 FPE) on 24 pumps.... It was subsequently detuned to retain air and now shoots two shots of 600 fps (11 FPE) from 19 pumps.... The 2200 pump has about 50% more swept volume than a 13XX pump, and the 1400 is basically double the 13XX.... so what the 1400 pump does on 10 pumps, the 2200 does on 14, and the 13XX on 20 pumps....

I'll try and give a summary of my work with these guns, but my records from those days aren't complete, and it stretches my memory a bit at times to remember every detail.... but I'll at least try and hit the hows, whys, and wherefores of what I did.... I hope this leads into a discussion of pumper design in general....

Bob

[JMJ in NC]

Great info and much appreciated, Bob.

I'd particularly like to read more about the 1400 pump set-up.

JMJ

[rsterne]

.177 Uber-Pumper.... Parts list:

Crosman 2100 Main Tube & Pump Linkage
Crosman 1760 Barrel (24")
Crosman 2289 Trigger Group & Pump Handle
Crosman #1399 Skeleton Stock, bolted on
Crosman Long Steel Breech & .177 Bolt
Custom Barrel Band, Breech Band, Flat Top Piston, and Hammer Spring Seat



I experimented with a 2200 valve (smallest volume), a 2289 valve with the long check valve, and one with the short check valve (biggest volume).... At 3 pumps, the biggest valve gave 498 fps, the smallest 527, and the other in between.... At 7 pumps, they were all about 700 fps.... At 10 pumps, the biggest valve was 784 fps, the smallest 765, and the other in between.... It was pretty clear what was happening.... When pressure was the most important thing (low pump numbers, only a tiny pulse of air available) the small valve came out on top.... When the total volume of air released by the valve is more important (when striving for maximum power) the larger valve worked better.... So rule number one of "pump and dump" style pumpers (where all the air is realeased every shot):

When you don't want to pump much, use a small valve.... When you want maximum power, volume will help, but more pumps will be necessary....

Bob

[rsterne]

After the above testing I settled on the 2289 valve with short check (the biggest volume) for future development.... I ported out the valve exhaust port slightly, mostly just rounding the inside corner.... Experience (and the advice of others) had taught me that in .177 you don't want to go hog wild, it is just as easy to lose performance as gain it by overdoing it with a Drelmel.... I drilled out the transfer port and barrel port a bit as well (records incomplete).... The most interesting result of this was not an increase in performance at a given number of pumps, but that the number of pumps before the valve retained air increased from 13 pumps to 15.... That in itself gained 20 fps.... However, 900 fps remained out of reach, so it was time to go for a heavier hammer spring.... 7.8 gr. JSB Express used thoughout the testing....

Hammer spring lessons: Springs used and results....

1377/2289 spring - Retains air at 15 pumps @ 850 fps - Max was 890 fps @ 20 pumps (570 fps 2nd shot)
2200 spring - Retains air at 26 pumps @ 940 fps - Max was 953 fps @ 35 pumps (950 fps 2nd shot)
Disco spring - Retains air at 31 pumps - Max was 991 fps @ 30 pumps (I didn't try more)

It was pretty obvious that if you didn't mind pumping (a lot) then you could get more velocity by fitting a heavier hammer spring.... Whether this is worth it (other than an interesting experiment) is something only YOU can decide.... So rule number two of "pump and dumpers"....

If you can't get the velocity you want without retained air.... fit a stiffer hammer spring.... but be prepared to pump a lot more for relatively small gains....

Bob

[robert w]

rsterne your like me ... an old alzhiemers ward reject haha tell us more

[rsterne]

With the lessons learned on my .177 Uber-Pumper, it was obvious I had to do a .22 cal version.... Logic told me I needed to pack more air (bigger pump) into a larger valve.... I was able to get a 1400 pump tube and linkage, and I decided that the simplest way to get more volume was to add an extension into the middle of an existing 2289 valve.... I made one from the back half of a valve, which increased the length by 0.60" and increased the volume from ~1.6cc to ~2.9cc.... I tried it with and without the extension, adjusting the piston length to compensate.... At anything but very low pump numbers, the larger volume paid dividends....





Although I achieved over 800 fps (14.3 gr JSB Express) in initial testing, I experienced melting of my Delrin F-T-P so those results are flawed.... Once I had the bugs worked out, I achieved the following results with a 2200 hammer spring (the Disco spring wasn't available then, and I sold the gun before testing it with one)....

1 pump - 250 fps
2 - 380
3 - 470
4 - 535
5 - 586
6 - 632
7 - 670
8 - 702
9 - 727
10 - 750
12 - 788
14 - 823
16 - 852
18 - 875
20 - 894 (450 fps 2nd shot)
22 - 902 (680 2nd)
25 - 804 (810 2nd)

While those are some pretty impressive numbers, let me tell you, this gun at 20 pumps was a WORKOUT.... It's a good thing I didn't have a Disco spring, I probably would have bent something pumping more than I did.... In later years I rebuilt a Crosman 140, using an O-ringed piston and boring out the valve and ports.... The best I achieved was 705 fps at 10 pumps, 800 at 16 pumps, and 845 at 20 pumps.... so my .22 Uber-Pumper had that beat by 50 fps across the board.... I think I pretty much maxed out what the 140/1400 pump can deliver.... Rule number three of the "pump and dump" school of hard knocks....

If you want a lot of power, be prepared to pump.... and pump.... and pump some more.... and the more you pump, the harder it gets....

Bob

[rsterne]

You knew I couldn't resist the temptation to find out what the 1400 based pump and the extended valve would do with a .177 barrel, didn't you?.... Well of course I had to get all silly and try it.... With the 7.8 gr. JSBs I got 1040 fps at 24 pumps.... with 6.9 gr. RWS Hobbies I was rewarded with 1130 fps and a nice loud CRACK as the pellet went supersonic.... KEWL, huh?....

Bob

[rsterne]

I then decided to put what I had learned from my two Uber-Pumper rifles into an "Uber-Carbine" for Grouse Hunting.... The basic idea was to get more power at less pumps than what you could do with a 2289.... I used a 2200 pump tube and linkage, and all the tricks I had learned previously, and came up with this....



Here are photos of the custom parts I made.... and the mods necessary to the top of the 2200 tube....



Here is the bottom of the 2200 tube.... and the modded trigger group where the breech band attaches....



The valve was shortened to just behind where the front screw hole was for the trigger attachment, and the O-ring groove recut.... A flat was machined on the bottom of the back of the valve to allow it to slide back further in the tube.... and a set-screw added for strength.... This necessitated moving the screw holes for the breech and trigger, and the transfer port, back in the tube.... These mods allowed a longer stroke to utilize as much of the 2200 tube as possible.... The adjustable flat-topped piston, steel pivot pin, and new metal front pivot block are self-explanatory.... The breech band was necessary to attach the front of the trigger group.... The gun was later fitted with a Disco hammer spring and a Rear Velocity Adjuster (RVA, or power adjuster), although those are not in the photos....

With the Disco hammer spring just shy of coil bind, I achieved a maximum of 765 fps at 24 pumps with just a puff of retained air.... At 30 pumps (the equivavlent of 45 pumps on a 13XX) I got two shots of 670 fps (14+ FPE).... By cranking back on the RVA, I detuned the gun to give me one shot of 600 fps at 10 pumps, or by using retained air and pumping to 19 pumps, two shots at the same velocity.... Here is a graph of the results....



It would be possible to tune the gun for other combinations as well, either as a single shot, or a two shot using retained air.... simply by readjusting the hammer spring preload with the RVA and using a different number of pumps.... One thing on the graph is extremely important.... Note how the velocity is the same on both curves below 13 pumps.... There is virtually no velocity gained by increasing the hammer spring preload until the gun starts retaining air.... This is rule number four of "pump and dumps"....

Unless the gun is retaining air, there is no advantage to increasing hammer spring preload.... All it does is make the gun harder to cock....

Bob

[robert w]

bob you did some fine work on the pumpers

[PakProtector]

hey-Hey!!!,
I am still pining for a two shot hunting pumper. I'd rather an aerobic pump cycle rather than a strength building one, so a long lever/small piston seems called for. 20 cal will suffice; figure about 24 FPE/shot...so I do understand the pumping 'thing'. Looks like about a 6 ci resevoir to charge. I would not mind retaining a significant amount of air; the second shot has got to go where the first one did( or, shame on me, didn't ).

I have read of the pump durability issues plauging the Indy, and suspect lower operating pressure can make that issue go away. That and reducing the heat input; no sense cooking stuff with the pumping. I am tempted to start with a cross between a Marauder and a Sheridan C. For hunting I may be able to get comfortable with 18-20 FPE too... I got an 18 FPE and 9 FPE shot out of my Blue C with 13 pumps...would rather the pumps were easier.
cheers,
Douglas

[rsterne]

24 FPE from a .20 cal will be tough, but may be do-able.... Something you need to start thinking about is the volume of HPA released, and when the valve closes relative to the position of the pellet in the barrel.... Once again, if that is over 50%, it's inefficient.... And what does that mean for our beloved pump-n-dumpers?.... Yep, you got it, they're inefficient.... Any air blasting out the barrel after the pellet has left the building is pumping you did for nothing....

Bob

[rsterne]

Here is a graph I did recently for my .22 Uber-Pumper.... The swept volume is 33cc per stroke (2 CI) so at 10 pumps, you have compressed 20 CI of air into the valve (less pump losses).... Since at 10 pumps, the gun produces 17.9 FPE, that works out to 0.89 FPE/CI based on input....



You can see how the velocity gradually flattens out even though you are still compressing the same amount of air with each pump.... Therefore, the efficeincy drops.... Again, this is for a "pump and dump" situation....

Bob

[PakProtector]

---Any air blasting out the barrel after the pellet has left the building is pumping you did for nothing....

you keep reminding me of the *MOST* incomvenient stuff...

So based on that graph, it looks like a PCP with an onboard pump is likely to have highest efficiency potential. Pump...fire, pump few more times, fire again. Maybe even pump..fire, fire, pump a bit to top off...and so on. IIRC I hit 1.2  FPE/Ci average with Marauder, it wouldn't suprise me if a narrow window would yeild 1.4 or so for a pair of shots.
cheers,
Douglas

[rsterne]

PCPs can run efficiencies up to nearly 2.0 FPE/CI if....

A. the pressure is high enough
B. the valve duration is short enough

I'm usually pretty happy with anything in the 1.2 FPE/CI area for guns with moderate power.... I have VERY little experience with retained air (air conserving) pumpers.... Basically, the bigger you make the valve the more pumps to have to put in to start to get the pressure up to what you desire.... However, the bigger the valve, the more shots you can get with minimal velocity difference.... After all, it's just a PCP with a small reservoir that you have the ability to top up with an onbard pump....

The limiting pressure you can run will be determined by the diameter of your pump and the length and strength of the linkage.... and how hard you want to work....

Bob

[Bill G]

Very good.  My interest would be in a rig that retained air.  I would like to fill it by way of tank and simply replinish after each shot to maintain consistency.  I tend to like to run the higher psi due to the lower volume used to get the higher velocity.  Higher psi= more efficient, right?  I think the kicker is, can a long handle and linkage give the kind of psi I would need to top of the reservoir to say 2700psi.  Goal wise, I'd like to be able to get 2 shots at 4% or less before recharge, primary shot and follow-up. Part 2 is I'd like 50ish fpe.

The real question is, are these pumps multi stage are single stage brute foce pumps?  It sounds like they are brute force.  The longer the lever and link the more mechanical advantage.  In theory I'd have to figure out what stroke and diameter I could manage physically. Also, as for the piston,  do they use o-rings in conjuction with the seals you'd find on the end of a springer piston?  I've never had the chance to view any of these parts.  I was given a Crosman pumper pistol. It has the original box and book ect.. I think it is a 136.  I'll look at it.  It si all brasswith a parker type finish.  The gun is really accurate for what it is, I've taken starling with it at 20yrds using a post or rail for support.  I can pump it 7X and it will still have enough air in it to barely eject the nexy pellet.  By the way it is .22cal  It was given to me about 15yrs ago and I may have put 50 shots through it the hole time I've had it.  Any way, it sparked the thought of a high power pumper in .25 or so as a low support field rifle.  As a kid I always enjoyed the versatility of my pumpers but was left wanting for power and range.  I'm big now and my toys need to match if you know where I'm coming from. 

So whats the deal with pump/piston and what are the limitations?

[rsterne]

I'm a neophyte in terms of pump linkage design.... Even the simple scissor linkage frys my brain when I try and figure out the mechanical advantage, which changes with every increment the lever moves, becoming virtually infinite just as it rocks over center and locks.... The problem is determining the highest load (ie greatest force) required to pump, which is proportional to:

A. the pressure inside the valve
B. the area of the piston (ie the square of the diameter)

I heard somewhere that the pumping force can be determined by "solving a simple quadriatic equation" but without the equation I'm at a loss.... I do have some practical advice I have gleaned from asking those who should know.... A Benjamin 39X is rated at 8 pumps.... If you push it "much past" that it gets hard to pump and eventually the linkage will buckle.... Nobody seems to know (or they're not telling) what the pressure in the valve is at 8 pumps (and to be fair that varies with headspace).... Tim at Mac1 airguns makes a billet linkage which is stronger and with better pivots, and he also makes a longer one, which I just purchased.... Nice piece of kit.... When I get around to building my next pumper it will have a pressure gauge onboard, and that will answer a lot of my questions.... From what I understand the absolute maximum you can get away with in a 39X is 1500 psi, above that not only is the brass tube questionable for safety margin, but the stock linkage is likely to fold.... The valve in a 39X indicates the tube measures 0.775" ID, which at 1500 psi means a load of 707 lbs. on the piston.... If you compare that to a 13XX tube that only measures 0.620", at the same pressure the force is only 453 lbs....

I have a spreadsheet which I use to try and estimate the pressure inside the valve at the end of each pump stroke.... The problem is that there is a very large "fudge factor" reducing the efficiency down so that it agrees with real world values (what scant information there is).... The one reliable piece of information I was given was that with a stock 13XX tube, linkage, and valve volume, and a properly adjusted flat-topped piston and valve, the pressure at 15 pumps is about 1300-1400 psi.... so it is around that pressure that a 13XX with stock springs and an F-T-P starts retaining air.... The "headspace", which is the volume between the piston and the check valve is critical for efficient pumping.... Even tiny increases REALLY hurt the efficiency.... Here is a pair of graphs showing the theoretical pressure in a 39X with different pumping efficiencies and different headspace....



The first graph shows that with 100% pumping efficiency, and a 0.5cc headspace, at 8 pumps you should have over 1000 psi.... but if the pumping efficiency is 70%, that will only be 800 psi.... Take that same 70% pump efficiency and reduce the headspace to 0.1cc, however, and the pressure should be about 1100 psi.... That is the primary reason a 13XX benefits sooooooooo much from a flat-topped piston.... It gets rid of headspace, and not only that, the rigidty of the piston gives an even further improvement over the stock rubber cup as the pressure increases....

BillG, you have some pretty lofty goals.... 50ish FPE from 2700 psi and 2 shots within 4%.... My first attempt at a .25 cal pumper will be based on a Disco, and I hope to get 30 FPE without too many pumps.... My original thoughts were for a pump-n-dump with a valve volume of about 5cc (a 39X valve is 4.2cc).... I'm currently planning on increasing the valve to ~6cc and I'll have the provision for an extra 20cc reservoir, for a total of ~26cc.... Right now, everything is a moving target....

Bob

[PakProtector]

Pumper arm math is less difficult than it looks. Whilst caffienating myself I thought a matrix would be the easiest...but it is all dependant on the pump arm angle. Going to take out paper and pencil after cutting grass and see what I can come up with using the Sheridan C linkage as an example. Back in school we used Matlab on a Trebuchet project. A yard sale provided an incomplete set of billiard balls... The movement of the bits was multi-degree of freedom and one eq'n per degree was required. Just the linkage to hang the weight from instead of fixing it to the short arm was a non-trivial exercise( that one improves efficiency) as does a 'shark fin' to keep the short end artificially long...good stuff. I think Excel will suffice.
cheers,
Douglas

[Remo]

All I can say is WOW!  The analysis is fascinating, but I think I may need to consume large quantities of adult beverages before venturing into the reading of pump arm mathematics.  Hope the Wife doesn't mind, its in the interest of science.

Eric

[airgunandy]

With a few simple tweaks to the valve, I get around 16 fpe out of my Benji 392 on just 8 pumps. It would seem to me that starting with something bigger than a 1377 would be a better way to go. You can buy a modified 392 from AoA that has air conserving technology in it that allows for something like 2 or 3 pumps between shots.

I'm a pumper fan through and through, but there are practical human limits to the technology of a multi-pump. The things I like best are the light weight and easy handling, like a vintage Blue Streak. To get more power than a Steroided 392 or Streak means building a bigger and heavier rifle. And more work to shoot it. Then we go down the same power hungry Magnum road that practically killed off the nice, easy to use spring piston rifles.
As far as I'm concerned, anything that needs more power than 10 to 20 fpe to be killed can be handled with a "real" gun. Why ruin a perfectly good airgun to do a job it doesn't have to do?     LOL

[PakProtector]

The magnum springer got difficult to shoot as well as carry. The magnum pumper will only get a bit more difficult to carry... And, like the mountain climbers say, 'because it's there'...
cheers,
Douglas