Conversion of rotation to reciprocating motion

[hanson]

Hi all!
I am going to build a mechanism that can convert a rotational motion in one sense to a linear reciprocating motion.

One mechanism comes to my mind is the piston motion, but it doesn't really suit my appliation since what I need is a linear motion of constant velocity while the piston motion gives me a SHM.

Anyone know any mechanisms that suit my applications?

 

[Cyrus]

You are not going to get constant velocity from anything reciprocating.

 

[Danger]

There is a system that gives constant speed during the stroke, then stops and reverses very rapidly. I don't even know if it has an official name or not, but I'm talking about the thing they use as the line guide on an open-face fishing reel. It's like a double-acting lead-screw that a carrier runs on. A larger version was used to run the print head back and forth in old Anchor cash tills, and maybe other dot-matrix printers.

 

[moo]

A cam with a spring return can be designed to give a near constant rate of linear reciprocal motion, but if there is a load or high RPMs involved you may need a grooved track (that works like a cam) to both push and pull.

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

 

[Danger]

Thanks for reminding me, moo; I believe that the unit in the printer was referred to as a 'drum cam'.

 

[moo]

Great minds think alike Danger.

Not sure what my excuse is though...

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

 

[Danger]

I think that you got it right the first time.
I just thought of a decent way to describe the thing that I'm talking about without resorting to a sketch (although I'll do that if necessary). It's like a drill bit, but with the flutes cut both clockwise and counter-clockwise, crossing each other. The intersections are essentially just diamond-shaped areas of raised metal (or plastic). It turns constantly in one direction. The carrier (or follower, I guess) travels down one set of flutes, then gets picked up by the other one at the end and goes the other way.

 

[moo]

Good mental illustration.

Does it use an elongated (blade shaped?) follower? If not, what keeps it in the proper groove all the way to the end?

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

 

[Danger]

Hmmm... good question. I don't know. The follower is wrapped entirely around the shaft, so I can't see what's inside. If I can find the stupid things (any of them; I have 2 tills and a busted reel), I'll try to take one apart. They're not made to be disassembled though, so I'm not sure that I can do it without wrecking the thing.
I do know that if you reverse the direction of rotation, the follower backs up.

 

[moo]

Hmmm... with two (or more) followers, the grooves could be offset so followers don't cross intersections at the same time, or perhaps the followers themselves might be offset. Either way would allow round followers to be used.

Lol, do you follow?

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

 

[Danger]

Aaaarrrrrrrggggghhhhh...

 

[Danger]

Alright, moo... despite your last comment, I haven't abandoned you.
I ran across a coincidental stroke of luck just now. Stopped at my mother's place to collect some parts for my Hallowe'en outfit, and ran across part of a printer that uses the same system. This one, however, isn't enclosed. I can't quite see the follower, but the ends of the cam are accessable. It's actually a single groove. When it hits the end, it curves over and goes straight around the circumference of the cylinder for 180 degrees, then ramps back up and goes the other way.

 

[moo]

Ah, the single groove explains a lot then.

This sort of crap is not welcome here.

Uh... what?

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

 

[Danger]

Uh... what?

You tuned in right at the worst time. I was responding to a spammer who laid down a full-screen advertisement about some scam. That post was deleted, so I edited in an explanation of that in my response, which has also subsequently been deleted. It had nothing to do with the topic of this thread.
Incidentally, the only reason that I can't access the follower on this unit is that it's fastened in with miniature Phillips screws. Of course, the stupid things stripped out as soon as I tried to undo them (with the proper size screwdriver, I might add). I hate those stinkin' things.
Anyhow, I got to turn in now. Catch you tomorrow.

 

[moo]

Lol, ok. Cya

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

 

[banerjeerupak]

A piston already does convert the rotational motion into linear motion. Only problem is the constant power to be achieved. So what new are you doing?

 

[NedW]

Check out:
http://www.flying-pig.co.uk/mechanisms/pages/gearreciprocate.html

using a gear with over half the teeth missing and two parallell toothed tracks.

 

[Danger]

a gear with over half the teeth missing and two parallell toothed tracks.

Hmmm... I thought that I responded to this last night, but I guess I forgot to hit the 'submit' button.
That's a neat solution, for sure. I have a question about it, though. What sorts of application are this suitable for? I can see several obvious ones, but I'm wondering about at what point the shock load during reversal will cause undue wear on the teeth. For a heavy reciprocating mass, you'd have to use some pretty hefty gears.