Screeching Locos?

[davefoxx]

I just find it odd that it only makes the noise at a very specific speed range.

Probably some sort of harmonic resonance.  Next time try a inverse tachyon pulse.   

Seriously, I don't have an ultrasonic cleaner, either.  Usually I just tear them down, scrape the goo out, and use a sparing application of 70% isopropyl alcohol and Q-tips or paper towel to clean the mechanism and parts.  Don't forget to polish the points where electrical continuity is maintained through a physical connection, e.g., those brass contact strips, while you have the locomotive apart.  In just a few minutes, they'll run like new again for years, as Randy pointed out.

DFF

[CBQ Fan]

Probably some sort of harmonic resonance.  Next time try a inverse tachyon pulse.   

Seriously, I don't have an ultrasonic cleaner, either.  Usually I just tear them down, scrape the goo out, and use a sparing application of 70% isopropyl alcohol and Q-tips or paper towel to clean the mechanism and parts.  Don't forget to polish the points where electrical continuity is maintained through a physical connection, e.g., those brass contact strips, while you have the locomotive apart.  In just a few minutes, they'll run like new again for years, as Randy pointed out.

DFF

I did use a lot 70% iso and paper towels. I need to pick up a box of Qtips for the train room. I had to use a small flat head screwdriver to scrape a lot of it off. I remember buying these units cheap on eBay for the shells as the mechanisms were listed as in need of help, but the shells were like new.

[Steveruger45]

I just find it odd that it only makes the noise at a very specific speed range.

I wondered that too when I had the same issues.  Although I can’t prove it, I have a hypothesis that it is related to the thrust of the motor moving the rotor/shafts in an axial direction relative to the motor bearings such that at a certain speed a certain spot is reached where the friction squeal occurs.   

[CBQ Fan]

I wondered that too when I had the same issues.  Although I can’t prove it, I have a hypothesis that it is related to the thrust of the motor moving the rotor/shafts in an axial direction relative to the motor bearings such that at a certain speed a certain spot is reached where the friction squeal occurs.

Sounds official enough for me!

[peteski]

I just find it odd that it only makes the noise at a very specific speed range.

Resonance?  There is some play between the shaft's and the opening in the bearing. Without lubrication to dampen the vibration, the rotating shaft starts oscillating inside the bearing at certain RPMs. At least that seems like a viable explanation. 

[TimF]

I found his youtube channel, very interesting and helpful, especially for beginners

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The first video shows the "screeching" phenomenom

Tim

[mmagliaro]

My experience with screeching motors, even when it's just the motor on the work bench, is that they do tend to do it only at certain speeds.   The typically don't screech at startup and low speed.  They hit a speed in the middle somewhere that screeches like mad, and if you keep speeding up the motor, they stop screeching above some RPM.   As to why?  Never thought about it.  But that, to me, is the typical behavior until you oil them.
Which is why with those Rivarossi motors I've been rebuilding, I run them for a couple of hours in both directions, and make sure that they don't make any weird noises at any speed before I send them out.  I don't want any surprises caused by a bearing I thought was oiled, but wasn't (or wasn't oiled enough).