I need ideas for a fixture design challenge

Reminds me of a ball suspended on a water spout at a fair ground, the water strikes the edge of the ball causing it to spin suspended in the air, you can vary the flow to increase the spin or stop, whereby the ball drops into mechanism at rest.

Regards JD.

Sounds to me like you want an old pinball machine. You can retrofit it to your needs and do simple spring calculations for the plunger assembly to get you the speed you need for the correct RPM's, assuming the friction is made high enough so the object doesnt spin without rolling. You wont need any of the bumpers, flappers, score keeping mechanisms (although, it would be cool if you could rig something analogue/mechanical to determine the objects velocity and could thus compute the RPM's).

Pneumatics?

I would use pneumatics as well.

Tube just slightly bigger than the golf ball but with a slot in one side just leaving enough meat in the tube wall to contain the ball all the way up. This height to be determined by testing. Air nozzle under the ball facing up but with a device as to give you adjustability in an angle slightly facing away from the centre of the ball.

Open nozzle and the ball will go up. This test determines hight of tube needed. Angle the air stream and the ball starts spinning. The dgree of angle determines how fast the ball spins. Stop airflow and the ball falls down.

have a blue peter moment out there and play with some stuff around you shed. Sounds a nice afternoon of experimenting to me.

Is the part free floating or is it spinning bettween centers?

That information was not given so I chose a design without a spindle or shaft thus making it free spinning.

The same design could be modified for a suspended item but then without the tube. Just angle the air more as it now only has to turn the item and not carry it as well. Less energy needed so flatter angle of air stream.

Wow. Neat ideas.

What I failed to mention is that the ball under test would be securely fastened such that it rotates about a particular axis through the center of the ball. Then the test is repeated such that it is spun about a second orthogonal axis. It can be removed an repositioned for the second test.

The constraint that no electricity or motors be used should be extended to preclude pnuematics as well. I want it to be a portable, standalone, manually powered (spring?) device.

It amazes me how you guys quickly exposed weaknesses in my requirements!

Thanks,

Dave

Ok I understand the bit about <do not use any of the above mentioned external power sources including electrical motors but not exluding anything else I might have forgoten to mention> but what about your definition of orthoganal axis?

The normal explanation for it is: "If two symmetrical planes intersect at right angles, the line in which they cut is called an axis of orthogonal symmetry". To me that means you need to explain yourself a bit further as we need to know what planes you have used or had in mind?

If you just mean "any other line along any other angle and redo the test" why not say so?

I will be back with my new offerings but would like to hear your reply to this as well

Yours sincerely,

the quick of the mark but lacking info engineer

You could try thumping it with your finger and/or magnets

I remember I used to make a toy from a large coat button and some string. The button must have 2 holes on the same pitch diameter and 180 deg appart. The string goes through one hole and back the other. Make it so it is looped and stretches to a maximum that is shorter than your arms can comfortably extend in front of you, i.e. you hold the string and you can stretch out to the max without any problems.

Then you put the button roughly in the middle and swing it round perpendicular to the string for some time, experiment tells you how long. When the string is sufficiently would up, you pull the string tight and apply force outwards. The string starts to unwind and the button starts spinning. When you are at the end of the strings winding, you let the string pull your hands in to rewind but in the opposite direction. when almost stopped you pull again. This can generate very high spinning speeds so be carefull, you know the modern approach, goggles, gloves and permissions in triplicate at least 3 days before commencing test.

Button weight is important as it gives you differnce in speeds. Length of string is important as it makes your muscles ache when to long and you move to fast if too short.

If you want a drawing you need to tell me how to get it up here, i'm good but not that good

I got the picture. I may have had a toy like that once. That may be useful. Thanks.

I'm also considering something like a 'nut on a bolt'. When spun it rotates until it hits a stop. This controls the duration by limiting the #of rotations. How I constrain the spin rate, I haven't figure out.

Regarding the 'orthogonal' term. If the Earth were my unit under test, and the first test's spin axis runs through the NS poles (or wherever it normally does), the second spin axis would need to intersect the equator.

- Dave

almost like the line to the sun at almost 90deg?

Yeah is that almost right?

instead of holding the bolt you can hold the nut. Wind a string around it with a weight and let go. Monitor the spin rate and correct either by changing the weight or by adding momentum on a sticky out bit to the bolt.

if a bolt does not give you the required spin rate try a ball screw. These can give you very low friction rates and I know, we work with them. Just don't let them shoot out of the nut as the balls will fall out and you can never get them back in, at least not with my patience

If I understand you need a device with a "mechanical" motor to spin a ball to a given rpm and for a given time. It has to be portable.

It could be possible to use the same system as in the mechanical "his master's voice" plate players.

A spring is loaded with a crankshaft and a mechanical regulator loads more or less a brake in order to maintain speed constant. The shaft can be connected to a threaded part which acts upon a coupling and disconnects the system after a number of revolutions since speed is constant the nr of revolutions is directly related to time.

Between the main shaft and the threaded one you could use a gear box so that the higher speed of the main shaft is reduced to the one you need for the "stop" control.

I think that for such a small inertia it could be better and less complicated to use a small electric motor powered by rechargeable batteries thus making it portable.

If you will ever decide to use compressed air it would be better to look at a conical tube with a series of slots at an angle so that the air will lift the ball and escaping through the slots will spin it at same time. The ball will float on air. The tube could have a fixed shaft as position index. The ball is placed on the fixed index and air is released, the ball floats and spins. For the time it is possible to use 2 air reservoirs, a main one from which you fill a smaller one from which the air escapes under the ball.

It will be more cumbersome then the solutions above but it is feasible. The main reservoir can be filled with a hand pump (specially designed for it).

I hope you can find some thing of interest in above suggestions.

Do you read ALL the posts?

The discussion started by stating that any form of electric motor was not allowed. Dave, the man who posted this thread, later than added that compressed air was not allowed either.

The other thing is your gearbox to reduce the spin rate....Dave stated he was looking for a spin rate of 7rps minimum and 14rps maximum. Seems you will have trouble getting there in the first place and a gearbox will not be needed.

1. spherical tea ball strainer large enough to contain the ball
2. connect a rubber band across the curvature of the strainer so that the two rubber band strands are somewhat apart from each other and there is enough end material to hook onto a hanger (see 4). This will cause the strainer to not over-rotate when it unwinds. you may have to experiment on size and thickness of rubber bands to get the right speed.
3. insert the ball
4. make a hanger to hook the rubber bands on
5. wind the ball assembly the appropriate turns
6. let it fly
7. open the strainer and reposition the ball
8. go to 5

Dear Case 491,

Do not think other people are stupid. According to a proverb from an other country then UK it is said that other mothers have also clever children. In fact I read and even with great attention the different constrains and comments. The reason I nevertheless less suggested the e-motor is in fact double : the system would be very simple to build up since every component is available of the shelf; the power requirements are very small so that a "normal" battery can be used. As you could notice I looked at different possibilities and since I did not know why an e-motor was not accepted I also suggested a totally mechanical solution and even looked at a possible pneumatic one.

I wanted as well give to the person who asked for ideas a solution according to his most limiting request as suggest an other which in the actual technological environment could become interesting.

I presume that you know that only limited persons do not change their mind and I estimated the one who asked for suggestions not to be one of them.

I am sorry to react with an important delay but I am very busy and do not have time as much as I would like for comments.

Is there a conclusion here? What is the schedule you are working with?

nick_name:

Thank you for the post. Your mechanical solution is kind of what I figured was required, although I could never describe it as you have (I'm an EE). Electric fields from a battery powered motor might be problematic for the device under test, plus the batteries would probably have to be replaced often. I don't want to use pnuematics for a variety of reasons. Lifting the ball in air also presents a problem as the spin axis of the ball must be controlled. Out of curiousity, what kinds of processes or mfg functions would a use pnuematics to lift and spin an object?

Jaguar:

Interesting. I think I understand your idea. A key issue that may make rubber bands problematic is repeatability and durability. It will be used a few hundred times in a day.

Jaberwalkee:

I'm fishing for ideas over the next month or two. It just seems like there must be a toy or device out there that I just haven't thought of. A few of these test fixtures may be shipped to a factory overseas, so I don't want it big, complex, or expensive.

Thanks for the ideas!

Dave