Coupler balance

[blackcat26]

Does anybody drill a hole in their motor coupler 180deg from the set screw to offset its weight for balance. Seems like we zoom in on getting a prop perfect but neglect this.....Drive dog also or is this being too picky?

[sailr]

The small diameter of the coupling won't set up that much of a vibration although I have had the same question as you. You would also need to add a setscrew as well on the opposite side.

[detox]

I spin my couplers on motor just to see if there is too much vibration. Most all of the smaller Octura 5MM .150 cable types seem OK. The larger 5mm 3/16" cable collets may give you more problems with vibration especially on smaller 5mm shaft...I preffer using the larger 8mm NUE shaft option with the larger Octura 8mm 3/16" collets.

You will have more problems with cable resonance (cable vibrating inside stuffing tube). I spray contact adhessive on outside of teflon liner before sliding it into stuffing tube. This will prevent teflon liner from spinning inside of 1/4" brass tube. Some people bend stuffing tube S shaped to lessen the loud resonance sound. Use lots of lube also.

[Rex R]

I did once on a custom BRASS coupler. I didn't need to do more than a small dimple (as I recall, 1/8" dia. by 1/32"deep) that was all it needed.

[m4a1usr]

Does anybody drill a hole in their motor coupler 180deg from the set screw to offset its weight for balance. Seems like we zoom in on getting a prop perfect but neglect this.....Drive dog also or is this being too picky?

That is one of those questions that doesnt get addressed enough to satisfy inquiring minds. The answer for the coupler is because the slight weight imbalance is so approximate to the motors front bearing there is only a slight effect. Even the big 5mm x .187 couplers that have 2 set screws and they are at 90's, not offset. Because of the couplers proximity to the bearing and the fact that its a radialy loaded bearing (more then enough rating) imbalance has negligible effect.

Now the strut is a different story. For the drive dog, like the coupler, its close to the bearing. So even though few of us run an actual radial ball bearing its imbalance again is minor in effect. The prop is a different critter. We balance them carefully because its further out on the shaft, think distance plus leverage arm. The largest amount of force is at the back of a prop, i.e. greatest section of surface area for the blades. Not to mention we dont want to add to the drive dog imbalance. What can happen is harmonic vibration that changes thru the rpm range and if the prop vibration couples with the dog vibration you can second, third or even forth order harmonics. Essentialy that means that the effect can be compounded. And not at a linear rate.


John

[Rumdog]

I can't see the coupler being out of balance enough toeffect anything. The vibration coming from your flex or wire as the prop rips through the water is gonna have way more effect. as long as the broe for the motors/flex has minimal runout, you'll be just fine.

[blackcat26]

Thanks everybody! Just one of those things I've been wondering about. I didn't think it was a big deal but just wondering.

[sundog]

What I've come to realize is if the shaft fits the hole in the coupler without force then it has some clearance - and when the setscrew is tightened the shaft is forced to the other side - which is now out of balance (setscrew weight notwithstanding). With two setscrews at 180 degrees you can approach balance by keeping the shaft centered with judicial setscrew tightening. With three setscrews offset 120 degrees, you can obtain balance. I have had Jeff make me some custom 3-screw couplers to eliminate this problem in my jet drives - where even a small amount of imbalance is very noticeable, annoying and stressful on the whole powertrain.

Another twist to the original question: when you tighten a single setscrew onto a flat sided shaft will the weight of the screw make up for the metal ground off for the flat? Doesn't seem likely.