Help with some calculations
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Help with some calculations
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Topic: Help with some calculations (Read 1046 times))
2K1TJ
Sharp Shooter
Posts: 796
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Real Name: Ashley
Help with some calculations
«
on:
August 19, 2017, 07:35:01 PM »
I just ordered 2 of the cheap Chinese hand pumps with plans of motorizing them and "boosting" them with 20 or so psi of air.
I plan to use a crank mechanism like most of the ones that I've seen done before. They will be pumped alternatingly, like bicycle pedal cranks.
I hope to use a DC treadmill motor and controller that can be found free/cheap on Craigslist. Most are rated at 2.5HP or more. I'm sure I'll need to gear it down with pulleys or sprockets for it to make enough power at the slow speed I plan to turn it. And being DC, I can fine-tune the speed as needed with a pot. Will this be enough to pump them?
I haven't recieved the pumps yet, so I don't know the maximum force needed at the highest pressure yet. Nor to I have the exact pump stroke. I measured my Benji the other day and it was around 16". I didn't have a scale handy so I couldn't measure the weight/force needed to pump it.
I know I've left a lot of unknown variables, but i was hoping somebody could point me in the right direction to get me started anyway.
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USA, GA
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Re: Help with some calculations
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Reply #1 on:
September 06, 2017, 01:20:56 AM »
A very fit human might put out 1 hp, so a 2.5 hp motor is plenty.
You can estimate how much force it will take to produce peak pressure, or you can measure it with a force gauge or weights. How much percentage of your weight does it take to produce full pressure? You can also calculate that force, if you know the final stage piston diameter:
Pressure = Force / Area
So, 3000 PSI = 94 lb / .0314 square inch
The equivalent diameter piston to reach 3000 PSI with 94 lb of force would be .2 inch
If you set up your mechanism to run the pump through its full stroke without crashing at the ends of the stroke, it should work. If the motor can't produce enough torque, it will stall and burn out. A sure-fire tell-tale...
It might help to arrange the mechanism so that the pump stroke slows at the ends. This will improve leverage to reduce peak motor torque, and it will help with pump efficiency.
Talking about fire, the pump seals may not be up to the job due to overheating. The replace them with a better material seal.
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Roadworthy
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Re: Help with some calculations
«
Reply #2 on:
September 06, 2017, 02:50:57 AM »
The 2.5 horsepower motors should be quite adequate to operate the pumps. Horsepower is derived from dray horses and there is no way a person could begin to approach it. In other words even a one horsepower motor would be plenty or even overkill. I suspect commercial pumps use a quarter horse or less.
Of greater concern is how fast you plan to pump and whether you plan to do so continuously until pressure is built. I believe there is a recommended duty cycle for hand pumps. I think it's like pump slowly and stop after fifty strokes to let things cool down.
My experience with trying to save too much money usually involves using far more time than I ever imagined. I wish you luck on your quest but try not to be too disappointed if it doesn't work out the first few times.
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rsterne
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Re: Help with some calculations
«
Reply #3 on:
September 06, 2017, 07:53:25 PM »
My Freedom 8 ShoeBox compressor uses a 1/3 HP motor, and will deliver air at 4500 psi at 8+ CF/hr.... The HP piston is only 1/8" diameter, with a 3" stroke.... In addition, it has a medium pressure stage that is 1/4" diameter with the same stroke, both compression strokes occurring at the same time.... The supply air is from a shop compressor, delivering 125 psi.... Judging by the temperature the motor runs, at 4500 psi output, I would guess a 1/4 HP motor would not be enough to run without overheating.... JMO....
Bob
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Re: Help with some calculations
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Reply #4 on:
September 06, 2017, 08:11:00 PM »
Look at this cheap 4500 PSI compressor. The low pressure piston has a small diameter piston "sprouting" from it. In that case, the small piston may be .25" in diameter, with a motor strong enough to drive it:
youtube.com/watch?v=7NdnItyJs-U&feature=youtu.be&t=18m15s
Back up the video to see the rest.
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2K1TJ
Sharp Shooter
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Real Name: Ashley
Re: Help with some calculations
«
Reply #5 on:
September 08, 2017, 07:36:49 PM »
Thanks guys. I'm fairly confident that this will work, but with the falling prices of compressors, I'm not sure it's worth the trouble. But the project was never really about saving money anyway, it was, to me, more about the challenge of making something from repurposed things.
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USA, GA
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Re: Help with some calculations
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Reply #6 on:
September 30, 2017, 08:24:04 PM »
See:
https://www.gatewaytoairguns.org/GTA/index.php?topic=132966.0
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Help with some calculations