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As i said in another post, i have a driveline vibration that i was tryin to figure out. Well, i took the flex shaft out and ran the motor, still had a vibration. Then i took the coupler out and the vibration went about 90% away, a little buzz from the motor but i would guess thats normal. Ok, so i used an Octura 5mm to .187 coupler, the only 5mm to 3/16 listed on the OSE site. I would like to try another coupler, are there any other brands or better couplers than the Octura out there?? Or is this something i'll have to live with? I don't think the boat is gonna rattle apart, but i wonder if it's gonna hurt the motor bearings.
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does it vibrate when under load? Also, it is not good to run your motor out of the water. Brushless motors were meant to operate under a load. Is there a flat ground on the output shaft of the motor? -
Seems like the vibration is not as bad when under load, but you can hear it coming on plane tho while the prop is cavitating. Maybe it's not as bad as i think it is. No there is not a flat ground on the motor shaft. I didn't hold the motor WOT or anything out of the water, i just goosed the the throttle a few time while lookin around to see where the vibration could be from...you don't think i hurt the motor at all do ya?? The motor did not even begin to get warm. I maybe ran the motor a total of 25 seconds, just blipping the throttle.Comment
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If you can, try to grind a flat on the shaft for the set screw in the coupler. Motors probably fine. I've heard horror stories about motors getting wrecked from running without load. I'd just keep it to a minimum. Is your prop balanced? What prop is it?Comment
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Yes, prop balanced. I've had 3 different props on it too, tho the only prop i've had in the water so far is an X440. Temps looked ok from my first 2 runs (130* highest on the ESC metal caps) so i just put an X442 that i'm gonna try over the weekend. Good to know about the motor, i won't be runnin it on the stand any more.Comment
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I have had many Octura styles in my lathe lately modifying them for 78 wire and all I can say is they don't come close to my set screw style for being true. Not sure they are that far off but the larger the cable the worse they get. Make sure you check it at the motor end for slop.Comment
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I have had two 5mm-.187 couplers from Octura that I could not use. The last one I got was perfect. Perhaps they could do a bit more quality control?
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If the coupling is causing a vibration, then there are two possibilities:
1. The hole for the motor is not drilled in line with the coupling, causing the coupling to sit at an angle when mounted to the motor - this is unlikely to be the case.
2. The hole is too big for the motor shaft, meaning the coupling is offset from the motor shaft centreline as the set screws are tightened.
Every single motor I have checked with a micrometer has a motor shaft that is undersized to some degree. For example, nominal 4mm shafts micing at 3.97 or 3.98, a 5mm at 4.96 or 4.98. This is regardless of whether the motor is a "top dollar" German or a dirt cheap Chinese. The only motors that have shafts that are almost the claimed size are - surprisingly - the cheapest 700BB motors with a chrome plated shaft.
Now, I don't know how Octura make their couplings - I would imagine they have them made by CNC, and they probably make a lot of them in one go, perhaps a thousand at a time or more. Obviously, bulk manufacture means they cannot produce couplings to suit all the dozens of different *actual* sizes of motor shaft - that would simply not be practical. So, they have to make a coupling that fits most motors, in other words, an average size. Problem is, there isn't an average size, not even between motors from the same manufacturer.
Couplings need to be removable, so the hole tolerance must be sufficient to allow the coupling to be removed from the shaft by hand - using an interference fit would not be acceptable.
So, you could have a situation where you have a motor shaft that is significantly undersized from the stated diameter. When you tighten the set screw, the coupling will be moved off centre and will vibrate when spun up.
The coupling needs to be a good, snug sliding fit on the motor shaft, with no perceptible side-to-side wobble. If motor shaft diameters vary over a range of say 0.030mm, it is very difficult to make a "one size fits all". 0.030mm might not sound much, but at 30k rpm+, it's a huge imbalance.
The problem is the motors: if a motor with a 5mm shaft had a shaft that was actually 5mm, life would be easy. I can only assume that the motor manufacturers order their shafts deliberately ground undersized in order to make them a sliding fit in the ball-race bearings - does anyone know whether this is the case?
PaulComment
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I just got 2 of these MUCH better quality and the reverse collet stablizes the cable before the tube.
kintecracing.com/Accessories.htmlComment
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See it....find the photos.....sketch it it....build it........with woodComment
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Yep...nothing consistant is what I find as well. It is tough fitting couplers, etc to some of these motors.
You also have the issue of the stub and cable not being perfectly aligned which can send the vibes and motion up the cable.
Octuras couplers are made by a screw making machining company.
Tom is pretty anal about his products so it only takes a call to inform him so he can jump on his vendor about it. At least you can call and they will send another and expect the bad one back for testing.
When you build these couplers you have to allow for hardening and plating. You actually have to make them over size and expect plating to take up another .005ish for fit. Some plating can end up thicker at times or spots.Comment
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I had the same prob with the stock sv coupler. Put a octura coupler on and no more vibrationComment
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Well it looks like this is a fairly common problem, i guess i'll start by tryin one of those Kintecracing.com couplers first and go from there. Trouble is, if like "paulwilliams" says if the motor shaft is under sized then really will any coupler seat right on it? Also, quick question, while i was looking the Kintec Racing site, i see they have a thrust bearing uprade that mounts between the coupler and motor, are they worth installing? Thanks for all the help!Comment
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I say EVERY motor needs a thrust bearing on it. Why not take that load off your rear plate and bearing and shims? You can eat those shims away very easily and then you can have more problems...like metal to metal and then hot bearing and enbell and then bearing goes and then the arm cocks and then windings short and then...well you get the picture. :)
Yes, $6.50 is well worth it.Comment
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