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Ratchet Design

derfmeister

Plastic
Joined
Apr 11, 2009
Location
England, Wiltshire
Hi all,

Essentially i'm looking to design a new ratchet based on the one below. The new one must click 3 times in 1 rotation, thus 120 degrees each. Obviously this requires a re-design of the pawl, cog, arm, etc.

I was wondering if the design principles are summarised somewhere, or if someone can summarise them for me. Obviously I see alot of mathematical parameters which I could figure out with perhaps 5 hours of time + pen & paper. But, I was wondering if you know of any general guidelines.

Thanks!

cyclone.gif
 
Consider using a one-way bearing (Sprague clutch?) instead of any ratcheting mechanism. It won't force 120 degree travel but is usually a lot easier to design in and maintain in use.
 
First of all you have the amount of linear travel available. If it is the same as the animation.. Then consider the distance from the axle (axis) for the crankpin..
With the same fixed stroke, you need to shorten the crank..
Basically the lenght of the stroke, and the distance of the crank.

And, since you are trying to move a little more than 120 degees with the crank, ( since you need to account for the rachet catch and release) make sure that the get clost the 1:00 O'clock position on the upstroke, and has the maximum amount to swing all the way down to the 5 O'clock position.
At the extremes there is a reduced mechanical advantage for the lever.

Also, with your present layaout, the link rod (and cylinder) may need to be pushed to the left, because of the required short ratchet lever.

Pertty simple really. ( I think, so far)
 
If you can get one the right size a one way roller bearing style clutch is a lot easier to design and almost certain to be reliable. Basically design consists of settling on the sizes and arranging rocking lever stops. Looks like your requirement needs to seal at one extreme of the stroke. If so definately go for a roller clutch as this will poermit the odd degree or so of under or over travel using the seal as a stop. Ratchets need to be adjusted.

Clive
 
IMO 3T3D really meant that you need to stroke to 12:30 and 5:30 to account for clearance for the dogs at each end of the stroke. ??? At least that was what I was gunna say - but saw that he had somewhat addressed this issue on the way down.

It would seem to be VERY gutless at either end of that stroke - let alone the reduction in mech advantage of a shorter throw on the arm. (Going bigger or longer on piston? There are pills for that. ;) )

Very impressed with the GIF. It actually accounts for a slot in the pawl mech that aint even visible.



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Think Snow Eh!
Ox
 
It would seem to be VERY gutless at either end of that stroke - let alone the reduction in mech advantage of a shorter throw on the arm.

Agree - I'm thinking that you'd have to build a compound lever mechanism or else involve gears to get a 120° movement with any reasonable leverage.
 
Ox, that is what I tried to explain. 1:00 to 5:00 is just the advance.. Plus you need ratchet clearance.
I explain about as well as you spell.

Like someone suggested, maybe a rack and pinion is one possibility.

And fo course the other obvious possibility is to have a double ratchet, and stroke it in both directions. A little more complication of course.
But there is more than one reason why three strokes to a rotation is not a common design.
 
An existing planetary gearset could be employed to amplify the degrees of rotation, to feasible in just one stroke.

Many examples exist, from bicycle hub gearing to gears from a discarded automatic transmission and larger, depending on your application.

Machining a ratchet wheel in which to press the ring gear, then locking the sun gear and taking rotation from the planet carrier, would provide same rotation, while locking the planet carrier and taking rotation from the sun gear could provide reversed rotation if useful.

This with the additional advantage of compactness, not needing to expand the housing envelope, except for the increased diameter of the rotating member.

Ratios in the neighborhood of of 3:1 are common, requiring only a very feasible 40° of ring gear rotation in your case.

Precise 120° advancement might be achieved by attention to the pawl-set diameter on some existing ratios, within reason. Easily provided with dedicated gearsets.

In any case, a Sprague clutch would probably be required to counter back-rotation of the driven shaft during periods of pawl disengagement, though this could not provide a never-wavering 3 pulses to one complete rotation, (exactly 120° degrees)

For this, a specific number of teeth in an opposed "holding" pawl/notched wheel could provide the precision, any multiple of 3 in this case.

Bob
 








 
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