Motor cycling between limits

Unless you use something like Solidworks (tm) Toolbox.

"Suddenly cam designs are easy. A friendly dialog box allows you to design even the most challenging cams in seconds. Simply select the follower type,then specify motions and the cam blank. In a mouse click, you?re done."

Best regards, Spehro Pefhany

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Ages ago, we sold some programmable temperature controllers wot used a cam to set the temperature vs. time profile for batch processes using a big flat disk (much like an old-fashioned circular chart recorder in reverse). It was marked with arcs that described the motion of the follower (spring-loaded roller follower attached to a pot shaft) so that you could cut the disk to create the profile. I'm sure there were some rules as to how fast the temperature could rise to avoid putting too much stress on the mechanism. ;-)

Best regards, Spehro Pefhany

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See crank & rod, and lever...

follower->pawl cam-> ratchet wheel

Yes, I know what a crank is, but I'm looking for calculation details. Given the geometry of the 'target' I specified:

- What diameter should the crank be?

- How long should the connecting arm be?

- In what position should crank axis be placed relative to the point moved?

- What setup is needed to avoid lock-up in the event that crank 'extremes' do not correspond to those for target.

And there may be other variables I'm as yet unaware of.

I could probably do this stuff when I was about 8 and had my Meccano Set #7a at hand .

--
Terry Pinnell
Hobbyist, West Sussex, UK

Except for the LUMPY wheel I think you're OK ;-)

A longer connecting rod will smooth out the thumpety-thumpety.

If you make the wheel diameter equal the required stroke you can run in one direction only.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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I read in sci.electronics.design that Spehro Pefhany wrote (in ) about 'Motor cycling between limits', on Thu, 25 Aug 2005:

That in itself requires secret knowledge. When CAN'T you use an offset castor roller? (;-)

Cam blank? That's what you're trying to find.

Done for?

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Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
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Maybe you should buy one of those plug-into-the-ciggy-lighter pumps? I have one in the trunk of the wife's car.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I read in sci.electronics.design that Spehro Pefhany wrote (in ) about 'Motor cycling between limits', on Thu, 25 Aug 2005:

Yes: it's rather easy to design a cam that works if the shaft rotates one way but jams solid if it rotates the other way.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Geometry isn't the problem. The pedal does not have to be in continuous contact with the cam. Just set the geometry such that the high lobes of the cam push the pedal all the way down, and the low lobes allow the pedal to return to its at rest position.

Torque and speed are the problem. Speed is relatively easy - just pick a figure that roughly translates to the speed you would normally use the foot pump. What you have to figure out after that is the torque required at the cam shaft. In rough figures, that's the maximum downard pressure required on the foot pump pedal times the cam radius at the high point. The motor torque, times whatever mechanical advantage you get between the motor and the cam, needs to exceed the torque required at the cam shaft.

Or you can just use a windshield wiper motor assembly, as has been suggested. It would probably work. You might need to bolt a small - say 3" diameter - circle on the end of the arm that the motor drives. The circle contacts the pedal as the arm travels. See the "drawing" below. Ed

Circle ---------------- | | =======||========|| Pump | ==| |===============||== ||---------------- | | Arm ^ || ---- ---- Bolt o | | | | | | | | | | | | | | | Motor |

/\\ ----------- || | | Rotation | | * * * | ---------------------------------* * | ||------------ | | -- * *|P|| PUMP | | | | Arm * 0 *|E||------------ | | -- * *|D|| | ---------------------------------* * |A|| | | * * * |L|| | | |0 | ----------- -- Motor

Slightly less than the maximum travel of the pedal.

Long enough to connect the crank to the pedal (probably a little longer than the crank diameter)

Centered over the point where the connecting arm is connected to the pedal.

Ensure that the pedal does not hit any hard stops during the crank revolution.

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Peter Bennett, VE7CEI  
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Thanks, appreciate the time you put into that.

I'll have a go at the cam idea in a few days. Right now I'm close to getting the 'reversible' cord-based approach working.

The reason I was asking about geometry was because I had a crank & connecting rod in mind. My ASCII doesn't approach your level, but this was the general idea:

------------ / \\ / \\ / \\ / \\ . Wheel (!) . | | | o | | A | | | ' B ' \\ o ,´ \\ |/ -------------,'. | '. Connecting rod (straight!) | '. o C Moving pedal ,,´ ,-´ -´´ distance moved approx. 20 cm ,-´ ,-´ ~ 30 deg o´-------------------o D Base of foot-pump

The solid wheel rotates around axis A and a straight connecting rod BC joins it to the 'target' point, namely the top edge of the foot-pump pedal, with B and C free to swivel. When the wheel rotates clockwise, C is required to be pushed completely to D and then return to its highest position, then repeating the cycle continuously.

So given only that CD = 20 cm (roughly a 30 degree arc) the questions are:

- What should length AB be? (Note: instead of a solid wheel, it could of course be a single rod, rotating around A.)

- How long should BC be?

- What setup is needed, if any? IOW, to get proper operation and avoid 'lock-up', what must the relative positions of wheel and pump be?

--
Terry Pinnell
Hobbyist, West Sussex, UK

Thanks. Several problems I envisage if I ever get around to trying to implement this. Not least: heavy motor and wheel (or crank arm) would have to be at least 2 ft above ground. Or I suppose I could rotate everything 90 deg and have the pump base at right angles to ground.

(My 'ground' when checking tyre pressure for either of our two cars is a fairly steeply sloping front drive.)

--
Terry Pinnell
Hobbyist, West Sussex, UK

See start of thread...

--
Terry Pinnell
Hobbyist, West Sussex, UK

Your question was about how to deliver a back and forth movement from a continuously rotating motor. You didn't say anything about any calculation details.

I would get rid of that pedal ""target"" and its cheap linkage. And you don't want to calculate anything- just use graph paper, scale drawing, and measure. You might just get a stiff spring, calibrate, and measure the load you will be dealing with. You can make a decent drive from door closer parts- efficient design is not an issue here.

Great!

The trick is to make the lengths almost irrelevant in terms of angles. Here's how:

Linkage arm ----- / _T_ \\ ______ | / \\ | ^ | | | | | | | | | | | | | | |This dimension is not critical, but must be at | | | | |least the height above the pedal with A at its | | | | |lowest point, + 2x the radius of the wheel to | | | | |which pivot point A is mounted | | | | | | | | | v v | \\ A / | ------ This is the most important dimension ~ --- ~ This dimension = height of pivot point | | A to pivot point O plus travel of pedal | O | ------ when pivot point A is at its lowest and \\ ______ / ^the pedal is not depressed.

The AB dimension in your diagram must equal the travel of the pedal, and the center of the drive wheel must be directly above the pedal pivot point when the pedal is fully depressed.

Pivot point A rides in a slot in the linkage arm. When the wheel turns A to its lowest, it pushes the pedal down all the way. As the wheel continues to rotate, point A can slide up in the linkage arm. The pump pushes the pedal to its rest position, so the linkage arm does not need to pull it up. Thus the length of the slot, AT, is not critical, as long as it exceeds the dimension from point A, at the top of its travel to the pivot point O, which is connected to the pedal.

The next trick is to make the dimension AO, which I called the most important, irrelevant. It also makes the AB (your diagram) dimension less critical, as long as that dimension is a little bit - say 1/2 inch - greater than the travel of the pedal. AO was set as the distance from point A to point O, plus the pedal travel, when A was at its lowest. But - when you make the slot, make the AO dimension a few inches less. (the slot will be a few inches longer) Then do this: Assemble everything, move the wheel so point A is at its lowest. Clamp the pedal down as if was being stepped on. Clamp a plate to the linkage arm such that the plate is

1/4" below the underside of pivot point A. Drill holes through the plate and linkage arm, and uses screws & cap nuts to mount it to the arm. The net effect is that the pedal will be driven to within 1/4" of its full travel, and no dimension is critical. Essentially, you are making and adjustable linkage arm.

The pivot points can be made as a "sandwhich" || | || | capnutwasher|bus||ing|washer===< bolt | || | || arm The bushings pass through the linkage arm at A and O. (There's an h between "bus" and "ing" in the diagram, hidden by the thickness of the arm)

Ed

Thanks. Looks straightforward enough - guess I was making heavy weather of it.

Looks as if it would need a large, strong and rather *high* construction.

--
Terry Pinnell
Hobbyist, West Sussex, UK

Great stuff - thanks! Another one for my Projects Pending list .

--
Terry Pinnell
Hobbyist, West Sussex, UK

The good news: I finished the reversible cord-based approach lash-up, and it worked fine - before attaching to the tyre valve:

(The red Pifco lamp serves here as power supply for the electronics. At this stage I decided to keep that separate from the heavy duty 12V car supply. The lamp has 4 D-type Nicads and I had added external connection points for 4.8V, 3.6V, 2.4V and 1.2V at the time I was testing my ex-screwdriver motor for curtain control. The relays for this pump project are just about satisfied with 4.8V.)

The bad news: As I'd feared, the windscreen wiper motor wasn't man enough for the job. As soon as I connected pump valve to tyre, it stalled about half way through first depression (about 19A).

I may have a go with my mains drill when time permits.

Thanks for all the help.

--
Terry Pinnell
Hobbyist, West Sussex, UK