Gear Question

A six leg geneva cross mechanism will stop dead six times per revolution, while the driver rotates six times.... Not exactly a 'gear' though...

Brian W

Excellent - I've reviewed, and a six-sided geneva wheel will do as I originally requested.

HOWEVER - I am trying to convert a rotary motion that slows 6 times per revolution into a continuous speed, and a geneva wheel can't be the driving gear, it seems.

Any help?

How smooth do you want? At the cheap end, fit a drive shaft with a boss on which a radiator hose will fit. use a driven shaft and boss at the other end of the hose (about as long as its diameter), and use a BIG fly wheel on the driven shaft. That can take out most of the harmonic frequencies. If the motion is one way, could substitute a good spring like a valve spring for the hose drive.

Brian W

Understood - needs to be morepredictable/precise. 6-sided gear X2 maybe?

Being used in a project where (very simplified) a spinning triagle slows when a poit reaches 12 oclock, and when a flat side faces 12 oclock

Thank you for your input, by the way

Hmmm - more predictable. Gears don't have to be smooth drives. A pin wheel driving a slotted wheel provides variable speed to the driven gear. But now, if you want 'precise', you need to specifiy the form of the input velocity curve. And how close to uniform speed you want the output shaft to hold...

Brian W

We use a CAD package to create a 3D model and drawings for our gears (turbomachinery applications) - helical gears, spur gears, internal ring gears. Ultimately, it still comes down to calucalations to get all of the parameters correct, then it goes into the 3D model. However, I believe the real trick is manufacturing - making good gears is a real talent.

Could you use something from this page instead of rolling your own?

Or this? I think, though I'm not sure, these can be ordered with a stepper and an intermediate reducer... Ned Simmons

So the rack is curved? Then presumably it's a not a constant radius, otherwise it'd be a gear? If the load is light I think you could cut your own worm to mate with the rack. Think of a modified acme thread with included angle between the thread flanks to match the rack's pressure angle. (Is it a coincidence the acme thread is 29 degrees = 2*

14.5 degrees?)

Another thing that comes to mind is the technique of cutting a worm wheel to mate with a thread by using a tap as a hob. I've heard of amateur astronomers doing this to make large worm wheels for telescope mounts, and as a way to make a gear to mate with the threading leadscrew on a lathe in order to make or repair a threading dial. It seems to me that it'd be possible (but not easy) to do the same to cut your rack, in which case the worm would not need to be skewed by its helix angle.

Ned Simmons

Dear Smitty Two:

Do you have an estimate of the "curved path"? Four-bar mechanisms are pretty good at generating curved paths from simple linear actuators...

David A. Smith

It STILL sounds like a lead screw + nut app.

Fred

It may work well enough for a prototype, but the tooth forms are not really compatible so I'm not surprised that it made the gear mfr cringe.

Even though you don't want to make your own worm gear, it may be instructive to look at the process of generating a gear with the tap-as- hob trick.

I do think, as I suggested before, that it would be relatively easy to cut a worm to mate with the rack that would be an improvement over using the stock worm. Whether it's worth the trouble for your prototype is another matter - at least it's a potential fall-back position if the stock worm proves unsatisfactory.

Ned Simmons

How about something like this with a rack type track and a servo-motor and gear in place of one of the bearings?