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Author Topic: Free Turning - increasing speed/duration at no cost  (Read 1293 times)

GG

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Free Turning - increasing speed/duration at no cost
« on: October 24, 2019, 05:14:53 pm »

You do not have to be in this hobby long before you encounter someone, possibly yourself?, who has a problem with poor duration or speed of an electrically powered model.  Another symptom might be a hot, maybe very hot indeed, motor!


Initial thoughts might be directed towards a bad combination of motor, propeller and battery.  But if this is not the obvious cause then the next thing I would do is to turn the propeller over with a gently applied finger tip, with the RC turned off of course!!!


It should be almost effortless to rotate the propeller and ideally you can feel the "magnetic compression" as the poles move past the magnets.  This works for both brushed and brushless motors.


Listening to the drive-line rotating might reveal a grinding/scraping noise, something anyone the the slightest mechanical empathy hates hearing, suggests a lack of lubrication and/or alignment.  Some prop shaft and tube assemblies claim they can be run with out any lubrication, but the simple steel shaft, brass tube with a couple of bearings at each end that I usually use do work better with lubrication.


It's worth noting that motors also have bearings which ought to be lubricated.  One of my pre-sailing checks its to use one of those needle type applicators to place a drop of oil on motor bearings.


The alignment of motor and prop shafts is another thing to check.  With a direct drive system, these two shafts should ideally be perfectly inline.  The coupling between the shafts ought not to have to accommodate any noticeable misalignment .  But, sometimes the model layout demands that these two shafts be slightly out of line, but it ought to be a minimum .


Which neatly brings me to the subject of poor couplings.  They ought to rotate smoothly but some can have a "wobble" which is a problem no matter how well the shafts are aligned.  Replacement seems to be the only answer in this case.


The final thing, which perhaps ought to have been checked for first of all, is something that many novices do, that is make the drive line "tight".  This is where the propeller, or more usually the lock-nut/washer, presses too firmly on the lower bearing in the prop tube. This may be due to the belief that there should be no gaps open for water to sneak into the tube?


My ideal set up is to be able to pull the washer away from the bearing but only by a barely perceptible amount, perhaps best described as you can feel the movement but its damn hard to see it.  When sailing ahead, the propeller thrust will be transmitted to the bearing.  Be careful not to leave too large a gap though since you might end up with the propeller thrust being transmitted all the way up to the rear bearing of the motor, something it is not designed for!


Just a little extra care in setting up a models drive-line can pay triple dividends of, higher speed, greater duration and a longer model life.
Glynn Guest   
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Martin [Admin]

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Re: Free Turning - increasing speed/duration at no cost
« Reply #1 on: October 24, 2019, 05:50:16 pm »


Good basic stuff Glynn!   :-))
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malcolmfrary

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Re: Free Turning - increasing speed/duration at no cost
« Reply #2 on: October 24, 2019, 06:48:18 pm »

As Glynn says, the motor is there to turn the shaft, not to absorb longitudinal thrust. 
The prop shaft does need that bit of "just perceptible" fore-and- aft play to avoid it becoming a brake. 
The inner end also needs restraining.  When applying reverse, you really don't want the pro to begin heading backwards.  Having the prop blades find the rudder is noisy, having the U/J slide off its splines is embarrassing.
Sometimes the U/J end (or its locking nut) becomes the bearing face, sometimes there is a pile of washers, but there needs to be a thrust washer arrangement at both inner and outer ends.
Lock nuts need to be done correctly - one that tightens up the end play in use is deeply annoying.
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DaveM

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Re: Free Turning - increasing speed/duration at no cost
« Reply #3 on: October 24, 2019, 07:20:49 pm »

All well and good, but what about using "end-play" with sensible couplings like this one?
« Last Edit: October 24, 2019, 07:24:23 pm by DaveM »
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Subculture

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Re: Free Turning - increasing speed/duration at no cost
« Reply #4 on: October 24, 2019, 09:59:43 pm »

Don't need to worry about it- you're using a ballraced motor- ball races work just fine against axial loads.

Martin [Admin]

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Re: Free Turning - increasing speed/duration at no cost
« Reply #5 on: October 24, 2019, 10:28:50 pm »


If you struggle with motor  / shaft alignment,.... like me, use a double knuckle UJ.   :-))











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malcolmfrary

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Re: Free Turning - increasing speed/duration at no cost
« Reply #6 on: October 25, 2019, 09:42:12 am »

All well and good, but what about using "end-play" with sensible couplings like this one?
Back when I was working for a living, "just perceptible" meant about 2 thou, you could hear it, but seeing it was difficult.  I imagine that the rubber part of the coupler shown could live with that.  Even if it was a rigid all metal coupler, the motor should still be working within limits in that the rotating bits should still be clearing whatever is at each end.  A couple of thousandths of an inch should leave the brushes running in the same tracks on the commutator, assuming that kind of motor.
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