Fun day with bearings

[tbaker2500]

My Series I PCNC1100 sat for the last couple of days while I was sick. I go out today to run a part, oil it up and ref all. Yikes! The X axis is howling/growling. It's moving just fine, but whenever is gets up to a rapid move speed, I got a bad howling. My immediate thought was the ball screw was ground up. After very quick troubleshooting, I am hoping that I hear the sound from the axis motor. After loosing the coupling, I am pleased to hear that the noise follow the motor.

I tear the motor apart, and find that the rear bearing is completely shot. I ordered a new one from McMaster for $9, and it looks like I'll make out cheap on this.

So here's what I found out: The electrical connection box on the end of the motor covers up a a hole, used to push out the motor shaft. (I'm glad it was there!) But, there is not a watertight seal between the electrical box and the motor housing. Enough water leaked in and rusted out the bearing. Thankfully, everything else is OK.

When I reassemble the motor, I plan to seal all the joints to keep this from happening again. It is interesting that this motor is the only one with coolant actively running over it.

Well that was my fun story for the day.

[mayhugh1]

I might be wrong about this (flame suit on) but I don't think you can pull the armature out of a stepper, make a repair, and then replace it without the proper equipment for re-magnetizing the "permanent" rare earth magnets that are commonly used. It's my understanding that the air gaps in these motors are very tiny to take advantage of the keeper effect of the electromagnetic core for the rare earth magnets to maintain their high flux density over time. The magnets are "charged" after assembly of the motor; and, after that, if the magnets are separated from their keepers for more than several seconds the magnets lose a significant amount of strength and the motor looses a significant amount of torque. - Terry

[tbaker2500]

Could be! I know not. I guess I'll find out. :-)

[tbaker2500]

Looked it up, and it appears that there will be torque loss. I'll find out if there is enough for me to notice. In the meantime, all you folks who've upgraded to Series 3 motors, feel like offloading a Series 1 motor?

[tbaker2500]

So I let some of the magnetic fields fall out of the motor when I opened it up. After sweeping them off the floor and tossing them (you never can force those dislodged fields back in place without fancy equipment), I decided to install the new bearing and try out the motor anyway.
Initial testing shows that it doesn't lose a step when pushing really, really hard against me. So the strength of the system is still well above the force I can exert on it. After running some parts, it hadn't lost a step either. Next step is to get back to running aluminum parts with heavy roughing cuts.

I guess the question now, is with a potentially weaker motor, where is it most likely to fail? During chatter? At high speeds, at low speeds?

I guess I'll just get it back in service and keep an eye on it.

Oh, it's nice to have the machine back!

[zephyr9900]

So I let some of the magnetic fields fall out of the motor when I opened it up.

That is much more of a factor with older Alnico magnets. Newer neodymium-based magnets are much more stable when removed from their magnetic circuits.
Randy

[photomankc]

Answer to your question is - Whenever losing steps will completely ruin the maximum amount of time and effort in the part.

I'm not sure there is an easy way to determine if and how much torque it lost since you need to have tested it prior to the repair. The torque is lowest when the motor is moving at higher feed rates so my guess is that it would be most likely to be a problem when the cutting tool is moving fast and the chip-load is high or during rapids.

[TXFred]

I guess the question now, is with a potentially weaker motor, where is it most likely to fail? During chatter? At high speeds, at low speeds?

Steppers lose power as they go faster, so your motor should be weakest during a rapid move. Set up a cutting operation where you can take a heavy cut at full rapid, and see if the stepper can handle it.

Frederic

[tbaker2500]

Thanks. Will try. So far no problems.

[tikka308]

I should have a series 1 stepper i can probably send your way if needed

[tbaker2500]

Update on the motor. I've been running the machine now with pretty heavy cuts, and no noticeable lost positioning. So the motor may have lost torque, but not enough for me to notice in aluminum.