Mechanical Engineering Dorks...

[Froggystyle]

Need a hand here.
I am trying to reduce the ratio on a control by one third. The Marine Machine controller we are using is designed for an outdrive and has a 3" stroke for the cable. Jet drives have a 2" stroke. I am trying to convert using a bellcrank.
Logic has basically told me that if I run an arm that is 2/3 the length of the top arm on the bottom, I will have a 1/3 reduction in throw. For some reason none of the smart people I know believe this to be true or not true. It is stumping them.
Bottom line is this... I am going to make a bellcrank arm that is the exact same pivot length as the MM control. This should get me an exact replication of the movement and remove any included angle from the question. I am not asking for how long the arm needs to be... I am interested if there is a ratio for reduction like you have on gears. I know that the number of teeth, which is essentially the diameter of the gear is how you reduce the ratio, and that it is a linear difference. I.E. a 4" gear working on a 2" gear will spin the 2" gear twice for every revolution???
Does this work the same way? Is this even right? I have been burning the candle at both ends for quite some time now and am admittedly not thinking straight, but want to build this linkage without a bunch of trial and error.
Any help???

[phebus]

Can't you just cut some popsicle sticks to see how length effects throw (ie., 1/2 length = 1/2 throw)?

[Froggystyle]

Can't you just cut some popsicle sticks to see how length effects throw (ie., 1/2 length = 1/2 throw)?
Yeah, but now I want to know if there is a ratio, or if I am smoking crack.
Bottom line is, I want to do this once, have a template for future use and know WHY it is x length. There has to be a rule somewhere....

[Froggystyle]

I am seeing if a changed title will help my views...

[Eliminator89]

Froggystyle, your intuition is correct. The length of the throw is directly related to the distance from the pivot point. The lever will travel X number of degrees. The farther out on the arm that you make the connection, the greater the throw. The calculation is length times the sine of the angle traveled. If you want to do it in ratio, the formula is travel above the pivot / distance from pivot = travel below the pivot / distance below the pivot. If you know how much travel you want, you can calculate how far you should be from the pivot. I hope that this helps.

[phebus]

Thanks, I was just going to get off this thing, and now you have me researching the mechaical advantage of a lever in relation to throw. If you were talking about a pulley system, and how a change in mechanical advantage effects length of pull I could tell you the effects.

[mike37]

on a 4" bar you would need to have you second point at 4-5/8" from the pivot point

[phebus]

Thanks Eliminator89, by the time I pecked out my response, you came up with the answer. Now I can get off. :sleeping:

[mike37]

man you can draw this shit out in like 30 seconds

[phebus]

man you can draw this shit out in like 30 seconds
I can't draw and post at the same time!! Geeez. :D

[Froggystyle]

So, if I was to have an arm that is 3" from the axis/pivot... what would my lower arm length be to reduce my 3" stroke controller (3" side of the arm) to 2"?
BTW... 90 degrees to pull of lever will be "neutral". I will be going 1.5" each direction for forward and reverse. Basically, what will make that motion into 1" each direction?
Thanks again folks. Phebus, E89 and Mike, send your addresses to me at wes@tridentboats.com along with your shirt size and I will get some off to you for the help.

[mike37]

So, if I was to have an arm that is 3" from the axis/pivot... what would my lower arm length be to reduce my 3" stroke controller (3" side of the arm) to 2"?
BTW... 90 degrees to pull of lever will be "neutral". I will be going 1.5" each direction for forward and reverse. Basically, what will make that motion into 1" each direction?
Thanks again folks. Phebus, E89 and Mike, send your addresses to me at wes@tridentboats.com along with your shirt size and I will get some off to you for the help.
are you trying to revers the action at the same time as reduction
if not you need to have both cables on the same side of the pivot

[Froggystyle]

Froggystyle, your intuition is correct. The length of the throw is directly related to the distance from the pivot point. The lever will travel X number of degrees. The farther out on the arm that you make the connection, the greater the throw. The calculation is length times the sine of the angle traveled. If you want to do it in ratio, the formula is travel above the pivot / distance from pivot = travel below the pivot / distance below the pivot. If you know how much travel you want, you can calculate how far you should be from the pivot. I hope that this helps.
By this ratio, it looks like my travel above the pivot (3") divided by distance from pivot (3") or 3/3 or 1 equals travel below the pivot (2") and distance below the pivot (2") or 2/2 which equals 1"
By this ratio, I can keep the throw the same, but change the lever arm at this ratio and get the same result. For example 3/2=1.5, so I would need 2/x=1.5 for a ratio... in which case x would be 1.33... correct? 1.33 is roughly 2/3rds of 2, so my intuition is correct.
Who needs a damn degree anyway when you have hot boat!
Thanks again guys...

[Froggystyle]

are you trying to revers the action at the same time as reduction
if not you need to have both cables on the same side of the pivot
The Marine Machine controller is set up to either be a pull forward or push forward, depending on where you mount the cable, above or below the pivot. I am going to reverse the pull direction to do the bellcrank. I just wanted to go through the mental masturbation of actually engineering the arm length and watching it work right...
This would be a lot easier if I knew how to do pivots on Solidworks... ;)

[mike37]

The Marine Machine controller is set up to either be a pull forward or push forward, depending on where you mount the cable, above or below the pivot. I am going to reverse the pull direction to do the bellcrank. I just wanted to go through the mental masturbation of actually engineering the arm length and watching it work right...
This would be a lot easier if I knew how to do pivots on Solidworks... ;)
I would have them both on the same side
have a pivot point then your reduction point at 2" and you controller at 3"
every thing going the same direction that way