Get another 2G from your SCX RX motors
Posted: Mon Aug 25, 2008 12:26 pm
I have recently acquired a couple of new (second hand) SCX models and in pursuit of some extra revs I carried out a simple trick that all us locals should now be using. That is the process of 'skewing' the brush holders in the direction of rotation. This retards the comm. timing and in doing so increases the motor rpm. There is a slight trade off, with torque being marginally reduced, but SCX RX type motors have torque to spare on high current power supplies (5 amps plus)! Skewing is carried out by firstly, removing the motor from the model. Then, using a pair of small jumper leads with crocodile clips attached, run the motor flat out in its normal direction of rotation. Next with a small pair of pointed pliers, grip the brush holder (while the motor is running) and 'skew' the holders in the direction of rotation one at a time. You will noticeably hear the engine rpm increase and when released the revs will subside. The trick is to bend the brush holders sufficiently past the optimum point so when they are released and they spring back; it is to the optimum position. This has to be done carefully as over exuberance will see the edge of the brush holder dig into the comm. material (says the voice of experience). Do this to both holders alternately until the motor revs no higher.
Nothing new here I hear you say. Maybe not to us old lags but a good few young people could benefit from re timing an SCX motor. But wait! That's not all!! Listen on. Whilst having reset one motor on the weekend, I was still a bit disappointed by its persistence to vibrate quite pronouncedly (is that a word??) So, being ever vigilant and thinking outside of the square and wanting to push the envelope and a whole host of other clichés, I thought to my self, 'If only I could balance an arm without stripping a motor’, or at least go some way towards it,? Well, I kid you not, what happened next was a revelation.
At this stage, Paul Stevens, avert your gaze! You already have too many of my secrets!!
Being a 'back woods' kind of boy (many years since from the heart of rural London) I have always had a little pocket sharpening stone which over the years has been used to sharpen pen knives, darts, modelling knives and a host of other pointy and sharp objects. It fits comfortably in the palm of the hand and is of concise rectangular dimensions, being long and thin and flat. I noticed on the errant motor that the armature segments were a bit higgledy piggledy as if the centres where eccentric. I wondered? If I ran the motor and carefully held the flat narrow end through the opening in the motor, would it grind it flat, or to a better shape at least. So, holding the fast running motor in one hand, I gently applied the stone to the armature segments. Did my ears deceive me, did the motor speed up?? Surely not! I tried again and there was no doubt, the motor gained some useable rpm! Looking at the stationary arm I could see that the stone was taking the high spots off the poles. More on one pole than the others.
I persisted, occasionally dunking the running motor in a bowl of water to remove the filings. Sure enough, application after application, the motor was getting faster and faster!! And the poles were getting shinier and shinier and best of all!! The motor was getting smoother and smoother!! Huh! The end result was a motor with silky smooth speed and along with the comm. timing it must have gained considerable rpm. Its emitted scream was now higher than a good Scaley motor and almost at NC5 pitch. So what has happened?
The theory as I see it.
Firstly, the armature was obviously out of balance. This was caused by the poor alignment of poorly stamped armature segments. The vibration in itself was enough to noticeably reduce top revs. The process of grinding off the pole pieces aided its balance and allowed a degree of rpm increase but it is my belief that the overall motor speed was enhanced by increasing the air gap between the magnets and the poles. DC motor dynamics dictate that a close magnet gap or 'shimmed' magnets will increase motor torque but reduce motor rpm, this is a fact. Motors with increased torque are better able to use lower gear ratios.
ie. standard motor 'X' uses 3 : 1 final drive ratio for model weighing 'Y' grams for optimum performance 'Z'.
but
shimmed motor (X + a) uses 2.7 : 1 final drive ratio for model weighing 'Y' grams for optimum performance (Z + b)
Where 'a' = closer air gap and 'b' = model speed increase.
So what has this to do with the price of spaghetti? Simple. By increasing the air gap I have reduced effective flux density acting on the armature (magnetic flux is governed by the law of the inverse square). This will result in a small decrease in torque but will induce an increase in rpm! The SCX long arm RX motors already have heaps of torque when run on at least 1.5 amps supply and a slight loss is neither here nor there. To some degree it is beneficial as surplus torque will have been expended as wheel spin anyhow. But the still usable torque, coupled with the increase in RPM, should (WILL) cause the model to be faster to the tune of the rpm increase. In turn the model will
A. Be a lot quicker without too much reduction in acceleration and
B. Be a lot smoother to drive through and out of corners as the sudden torque and consequent whiplash effect have been reduced.
Possibly theory but all a bit 1984 (sorry kids, a George Orwell novel). I have officially named it the "Wix configuration" or 'Wixation' (add that to your spell checker) for short, a name to be used on all future occasions.
So, firstly, if you race in South East Queensland, please don't tell anyone about this, especially Paul, and secondly, if you do try this out, you aren't allowed to race it against me!!
Nothing new here I hear you say. Maybe not to us old lags but a good few young people could benefit from re timing an SCX motor. But wait! That's not all!! Listen on. Whilst having reset one motor on the weekend, I was still a bit disappointed by its persistence to vibrate quite pronouncedly (is that a word??) So, being ever vigilant and thinking outside of the square and wanting to push the envelope and a whole host of other clichés, I thought to my self, 'If only I could balance an arm without stripping a motor’, or at least go some way towards it,? Well, I kid you not, what happened next was a revelation.
At this stage, Paul Stevens, avert your gaze! You already have too many of my secrets!!
Being a 'back woods' kind of boy (many years since from the heart of rural London) I have always had a little pocket sharpening stone which over the years has been used to sharpen pen knives, darts, modelling knives and a host of other pointy and sharp objects. It fits comfortably in the palm of the hand and is of concise rectangular dimensions, being long and thin and flat. I noticed on the errant motor that the armature segments were a bit higgledy piggledy as if the centres where eccentric. I wondered? If I ran the motor and carefully held the flat narrow end through the opening in the motor, would it grind it flat, or to a better shape at least. So, holding the fast running motor in one hand, I gently applied the stone to the armature segments. Did my ears deceive me, did the motor speed up?? Surely not! I tried again and there was no doubt, the motor gained some useable rpm! Looking at the stationary arm I could see that the stone was taking the high spots off the poles. More on one pole than the others.
I persisted, occasionally dunking the running motor in a bowl of water to remove the filings. Sure enough, application after application, the motor was getting faster and faster!! And the poles were getting shinier and shinier and best of all!! The motor was getting smoother and smoother!! Huh! The end result was a motor with silky smooth speed and along with the comm. timing it must have gained considerable rpm. Its emitted scream was now higher than a good Scaley motor and almost at NC5 pitch. So what has happened?
The theory as I see it.
Firstly, the armature was obviously out of balance. This was caused by the poor alignment of poorly stamped armature segments. The vibration in itself was enough to noticeably reduce top revs. The process of grinding off the pole pieces aided its balance and allowed a degree of rpm increase but it is my belief that the overall motor speed was enhanced by increasing the air gap between the magnets and the poles. DC motor dynamics dictate that a close magnet gap or 'shimmed' magnets will increase motor torque but reduce motor rpm, this is a fact. Motors with increased torque are better able to use lower gear ratios.
ie. standard motor 'X' uses 3 : 1 final drive ratio for model weighing 'Y' grams for optimum performance 'Z'.
but
shimmed motor (X + a) uses 2.7 : 1 final drive ratio for model weighing 'Y' grams for optimum performance (Z + b)
Where 'a' = closer air gap and 'b' = model speed increase.
So what has this to do with the price of spaghetti? Simple. By increasing the air gap I have reduced effective flux density acting on the armature (magnetic flux is governed by the law of the inverse square). This will result in a small decrease in torque but will induce an increase in rpm! The SCX long arm RX motors already have heaps of torque when run on at least 1.5 amps supply and a slight loss is neither here nor there. To some degree it is beneficial as surplus torque will have been expended as wheel spin anyhow. But the still usable torque, coupled with the increase in RPM, should (WILL) cause the model to be faster to the tune of the rpm increase. In turn the model will
A. Be a lot quicker without too much reduction in acceleration and
B. Be a lot smoother to drive through and out of corners as the sudden torque and consequent whiplash effect have been reduced.
Possibly theory but all a bit 1984 (sorry kids, a George Orwell novel). I have officially named it the "Wix configuration" or 'Wixation' (add that to your spell checker) for short, a name to be used on all future occasions.
So, firstly, if you race in South East Queensland, please don't tell anyone about this, especially Paul, and secondly, if you do try this out, you aren't allowed to race it against me!!