I love it! Too bad the motor doesn't have the guts. You may need to go to a starter motor to get it to run at 12 v. The mechanical design is really creative, & the movie with sound is a great idea.
Ed
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This pump - it's one of those ones you step on, right? Does it still pump if you tip it over and lay it on its side, so that the pedal moves in the horizontal plane? Then lay the whole assembly down. :-)
Cheers! Rich
On Thu, 25 Aug 2005 20:01:17 +0100, Terry Pinnell wrote: ...
Just under 10 cm, total just under 20, the full travel.
Long enough so it doesn't bind - what's an inch, 25 cm? OK, 50 cm.
When the pump is fully compressed, C will be directly above D. Put the motor such that points B and A are directly above point C/D when it's in the full-down position. Then, something is telling me that the wheel in your diagram should turn counter-clockwise, because when it's at the top, C will be displaced to the left a little, and so you want the top of the wheel to be going to the left - going the other way it looks almost like it would bind, i.e., the mechanical advantage would be going against you. (IOW, you don't want angle
0CB to become any more obtuse. ;-) )Good Luck! Rich
Thanks (for both replies).
I assume you saw the results of my earlier approach, using an ex car windscreen motor, repeatedly pulling and reversing? Worked fine - before attaching to the tyre valve!
But as soon as I connected pump valve to tyre, it stalled about half way through first depression (19A).
OK, that was what prompted the alternative idea of using my mains drill motor, which is not reversible; hence the crank idea.
I improvised a robust wooden 'frame' for the mains drill, but before proceeding with the challenge of the overall frame and crank I decided to test the motor for adequate torque. It's an old drill, so doesn't have built-in speed control. But with the power-controller I built myself I was able to get it down to a slow enough speed. I improvised the pulley method as earlier, and by being quick with the start/stop button, I was able to switch it off as soon as the pedal was fully depressed. But failure again! It got a bit further, but still couldn't cut the mustard. Astonishing how much force is needed to compress a tyre at around 30psi. (I'll figure out a way to measure it one day; guessing about 100 lb when I'm virtually standing on the pedal?)
So - thanks for the calculations, which I'll file for possible future use, but I'm beaten! As you probably saw from my reply to Ed near start of thread, this little fun project stemmed from my automatic cigar socket pump breaking. I'm off to get a new one .
-- Terry, West Sussex, UK
Come on Terry, use a bit of science! I haven't spotted what size pump you are using, but most of the tyre pumps I've seen have been about
70-80mm diameter. Guess at 3 inches. Area = 7 square inches. Force needed to compress to 30psi = 30*7 = 210 pounds. But that's directly on the pump shaft. There's also a lever effect. Guess about 4 to 1 (measure it) gives 210/4 or about 50 pounds on the pedal. Just pie are square and a bit of sums.Work out the torque of your motor (i.e. add load till it stalls- a spring balance perhaps?), and rearrange the lever (or better, add gearing) to make the torque enough to get the pressure you need.
Paul Burke
Saw the movie. Awesome! I'd have never thought to use a string, but I didn't know it was that style of pump. It has a mechanical advantage over just squeezing the piston part, but I was going to suggest (until you said you're abandoning the thing to just go buy a frickin' pump ;-) ) take the pump apart, and just mount the piston/cylinder by that pivot at the end, and just connect the piston rod to your crank.
But, as you've said, you need more torque. With your existing motors, you'd need to gear them down some or take shorter strokes.
Oh, well.
Have Fun! Rich
Your drill likely has enough power, but it's at the wrong speed. Slowing the motor is of no help - you need to gear it down or use pulleys, either of which results in increased torque at the lower speed. Trouble is, it becomes a mechanical monster if you have to use pulleys, while your initial mechanical design was magnificent. And unless you have a gear box with a big ratio, getting it down to reasonable RPM/increasing the torque with gears could get expensive.
And the damn thing looks and sounds so great in your movie!
Ed
Thanks, Ed. Yes, gearing the mains motor further down (it is already on the lower of its two speed settings 900/2400) would deliver more torque. But, as you say, at a considerable price in complexity - especially as I have no skills or serious tools for metal-working. (And my junk box has no appropriate gears or chains. Gear drives never feature in my projects for that reason!)
Also, I recall that my standard triac-based controller circuit claimed to preserve torque. Crude tests seem to bear that out. For example, I can just about grasp the chuck and stall the motor at a *range* of speeds. BTW, I did try to actually *measure* the torque, but it was too clumsy and time-consuming. The only spring balance I had allowed a max of a mere 7lb. I arranged the motor to pull that at a variety of leverages, using an L-shaped part in its chuck and extending the 'pulling arm' length with a foot length of 1"x1" soft wood drilled at intervals to take the pulling cord. The higher speed tests were at some risk to life and limb .
I briefly considered another gearing idea (from whatever minimum speed delivers close to max torque). This was to extend the pedal to say 4 times its length, and apply the pulley to that. But, apart from requiring a long and sturdy base platform, that could mean taking an hour or so to raise 4 tyres to required pressure!
BTW, I speculated up-thread that the pump needed some critical minimum pedal speed downward to work properly. But I've since found no evidence to support that. Theoretically, it would seem you *could* operate it at glacial speeds and the pressure would continue to rise. Maybe unavoidable leakage would be the limiting factor?
-- Terry Pinnell Hobbyist, West Sussex, UK
Here's the problem with that:
Steady torque==========> can just about grasp the chuck and stall the motor at a *range* of
I think we need a "motor guy" to tell us how to do it. I sure don't know - but I would have enjoyed looking at your setup. :-)
I would also speculate that it needs some minimum speed. If it doesn't, then you could try this:
| | | | | | | | |___________________ | | | | |___________________| | | | | | ^Bracket o o __ | |^nut o o | | | | o o | | | | o o| | | | o | | | | o| | | | | | | | | Rod | Pump | _ |< coupling | | | | | | < motor shaft --- --- | | | Motor | | | ---------
It's a linear actuator, made as drscribed below
Drawn is a 5/8" diameter threaded rod with a nut and a bracket that moves up or down as the motor turns. The torque increase and speed reduction factor equals TPI - the number of revolutions of the rod required to move the nut one inch. For example, your drill would move the bracket at 2.5 inches per second (at 2400 RPM) with a 16 TPI threaded rod, and with 16 times the torque. I use it in my window opener/closer. My motor torque rating is
30 oz-in, and it raises and lowers the (double hung) window just fine, and at an acceptable speed. You'd need a bearing mounted to something at the top of the threaded rod to prevent lateral motion of the rod. I used a small piece of aquarium hose with hose clamps for the coupling. Hardware store parts Vs a real linear actuator/acme screw present a much larger loss due to friction (your torque increase is lower than the TPI factor due to the loss) but the cost is low.An hour for 4 tires? Well, my little 12V pump is faster than that, but not by much. I estimate I need about
10 minutes per tire if they are real low (say 15 lbs instead of 28).Ed
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