Anyone taken a cheap servo motor apart?

[Northmount]

2 hours ago, AlZilla said:

One motor will drop down to 100 rpm, while another stops at 200.

May also depend on number of coils (magnetic poles).

 

On 7/18/2025 at 7:16 PM, AlZilla said:

The pulley doesn't wobble.

Did you measure with a dial gauge, or brace a pencil and bring it close to the pulley until it just touches some point on the side of the pulley rim while turning by hand, and also when running the motor?

8 hours ago, AlZilla said:

No, definitely not a belt. It makes a small winding/grinding sound with no belt. A belt makes it worse and more tension increases the noise.

Tends to say the bearing may be damaged (possible broken or cracked outer race or deformed balls), or if the bearing is supported by spokes vs. machined full face like the video shows and 1 or more spoke is broken.  This is definitely a mechanical issue (due to hearing and feeling grinding noise speed and belt tension changes).

Reminds me off a vibration analysis job on a large cooling water pump.  Driver was a steam turbine running around 7000 RPM.  The story came together in pieces.  You only get told what they think is relevant. 

The turbine had tripped on overspeed.  They rebuilt the governor and put it back together.  Now it had high vibration.  They had a spare turbine rotor in storage so swapped it out.  Still had high vibration. 

Then they called me.  I took vibration measurements on the turbine, gear reduction box, and the pump.  The pumps operating speed was about 500 rpm.  The vibration on the high speed pinion shaft was approaching 100 in/sec, fantastically high and dangerous.  (1 in/sec is bad.)  I got out of there as quickly as I could while capturing the necessary vibration data.  The vibration frequency spectrum showed peaks at the turbine speed and at the pump speed.  The highest peak was at the pump speed.  So I told them the pump / bull gear shaft was bent.  They replaced the gear box.  Still a problem.  Told them they needed to check the run out on the pump shaft.  Guess what, it was bent. 

Then comes the rest of the story.  A board (slat) had fallen off the cooling tower and was sucked into the pump.  Oh! that is interesting! 

The board had stalled the pump and broke up.  The turbine governor had opened up wide open to bring the speed back up.  As the board broke up, the turbine went overspeed and tripped before the governor could correct the speed.  The board bent the pump shaft before the board broke up.  The unit was about 1200 hp.  Pump suction was about 20" or so diameter.

[AlZilla]

10 hours ago, Northmount said:

Did you measure with a dial gauge, or brace a pencil and bring it close to the pulley until it just touches some point on the side of the pulley rim while turning by hand, and also when running the motor?

I didn't try it powered but I did hold a flat blade screw driver next to it and then slide it under. It doesn't show any wobble, at least at  hand spinning speeds. A runout gauge is not in my bag of tricks.

I might have to go tear it apart before the replacement arrives. I really don't want to run it anyway. No need of doing more damage. And I'm pretty curious. 

[dikman]

Definitely pull it apart, we're all interested now to see what the problem is!

[Cumberland Highpower]

On 7/20/2025 at 8:19 AM, AlZilla said:

I didn't try it powered but I did hold a flat blade screw driver next to it and then slide it under. It doesn't show any wobble, at least at  hand spinning speeds. A runout gauge is not in my bag of tricks.

I might have to go tear it apart before the replacement arrives. I really don't want to run it anyway. No need of doing more damage. And I'm pretty curious. 

In reality, other than testing a damaged pulley, that trick wouldn't do enough most likely to show what's bad.   Bearings are precision units (even cheap bearings) and you could have enough bearing damage or runout in the shaft to cause issues and not be able to tell with a screwdriver.  

A possible "possibility" is that maybe the shaft was "driven" backwards a little, enough to cause something to rub inside?  If that was the case would be an easy fix I'd think?

If it is a bad bearing, I'd almost gamble its the rear bearing rather than the front.

Edited Monday at 01:48 PM by Cumberland Highpower