Possible problem with Fanuc AC servo motor "lock" (Z-axis)

[ions82]

Well, I thought that I had a stuck Z-axis as a result of all the dried up oil that is all over the little Kitamura MyCenter Zero that I've been working on. However, I now suspect that there is actually some sort of locking solenoid in the Z-axis servo motor that is keeping it from moving. It is currently up against the second step of a limit switch. However, I used the over-travel function (that allows you to jog the axis off the switch in either .001 or .0001 increments) in an effort to get it moving. I can hear the servo motor trying to turn the ball screw, but the control throws an error and cuts the power.

In exploring the issue, I tried turning the ball screw by hand. Judging by feel, a friend and I came to the conclusion that there was some sort of mechanical stop that was keeping it from turning. Upon inspecting the servo motor, we saw a little part that looked like it may've been a solenoid that locks the motor. We removed the plug and found that it has about 112V DC running to it when the control is on. There are two wires in the circuit, and the "solenoid" side had around 297 ohms of resistance (putting it at around 375mA). It seemed like those numbers would be about right for a small solenoid.

Anyway, we are now trying to find some more information about this. We'd like to avoid having to tear apart the entire top half of the machine, but we're not really sure if there is a better way to proceed from here. Does anyone have any experience with something like this? The servo is a 5S/3000 (red octagon cap). I sniffed around fleaBay and only found one with matching numbers. It was listed for over a grand, so I'm hoping I can just work with the one I've got. Does anyone have any pointers? If so, please let me know. I have spent the last two years trying to get a working CNC machine in my shop (currently 0 for 2). Please help!

 

[Pipes]

3rd connector would be brake

If the motor on your Z axis has 3 connectors, as in the picture below, then your assumption is correct it does have a brake.

Block the Z axis (even though it has a counter balance) and decouple the Z axis motor. With the motor removed it will split the difference on where the system is stuck.

Make sure you strap the motor down before trying to jog it as it may attempt to take leap from initial torque. If with the motor removed it can be jogged then the brake is releasing and there is something binding the axis.

If the motor does not move and you get the fault then the brake is indeed stuck. If you have a DC supply that you can use to actuate the brake then you can try freeing the brake by applying the voltage repeatedly while trying to move the motor shaft. You should see the brake voltage and current requirement somewhere on the label. If this does not work then you will have to either send the motor for repair or crack it open yourself and see if you can free up the mechanism.

 

[ions82]

Thank you for the input. I have a feeling that I will ,indeed, be required to remove the servo motor to get to the bottom of it. Unfortunately, there is virtually no room to work with up there. There is no access to the mechanism that couples the ball screw to the servo motor. So, I may end up trying to pull the whole thing out at once. It's also likely that I will have to remove the entire counterweight system just to get access to some of the bolts. It's gonna be a bear.

Anyway, the tag on the motor itself listed 107V as the only voltage. I got 112 VDC at the little cannon plug for the brake. So, I assume that it is the correct voltage, and the brake just isn't releasing. The machine sat for several years, so it wouldn't be a big surprise if something in there just got stuck in place. It'll be tough to get that motor out of there. I just hope that it doesn't need "professional" repair or replacement. I'm sure either option would be costly.

 

[Stuart Caruk]

The smaller connector is in fact a brake. Oddly, we have lots of experience manipulating these motors on our robots and Kitamura Horizontals with Fanuc drives.

In the past I've had robot arms move out of position in shipping making it impossible to get the Canastoga covers rolled past them on the trailers carrying the robot shipments, and I've had wreck in the toolchangers jamming the magazine chains solid. Getting things free'd up can be a royal major pita on disassembly, or... you can cheat.

On a robot with the same motor, if you pull the motor, the arm will drop uncontrollably. On a CNC machine, you can't spin the chain without sending power to the brake to release it.

I'd suspect your brake is stuck, or it isn't getting any power so you can roll the shaft around. If you want to check there is a simple trick you can use. Power off the machine and remove the 3 cables from the motor. remove the 4 bolts holding the motor to the mounting face so it would be free to rotate. Remove the grease fitting if it stops the motor housing from turning. The motor housing should be free to rotate now if you twist it around by hand. The brake should be still on, so spinning the housing will turn the ballscrew and cause the axis to move. If you can move the axis up and down, you can be fairly certain the issue is in the brake either not getting power to release, or it's stuck on from gunk and crap. It's quite possible if the machine is dirty that coolant has gotten into the area and dried, making it all stick like glue.

If the brake doesn't release, the motor should overload and trip out in short order, which is good, because it's protecting the motor.

Good luck,

Stu

 

[Pipes]

If no zero coupling .....

It would be very odd to have to remove the entire ballscrew assembly to remove the motor.

If you can't see and get access to a zero backlash coupling between the Z motor and ballscrew then it may just be a slip in key coupling. This is sometomes the case on motors mounted with their shafts down or mounted into a reducer.

Just loosen the bolts on the motor flange and see if it can be pulled or gently pried up. If it starts to move up you may be in the clear.

If it won't budge try ad get a picture to post as someone may recognize the coupling if they see it.

 

[3t3d]

Again to emphasize, if you loosen the motor mounting bolts, and the head is not blocked up, the motor will try to spin free as the head CrAsHeS into the table on top of your foot...

All in all it is a pretty simple contraption, a little thought will isolate/solve it pretty quick.

 

[scudzuki]

I think the OP stated in another thread that the head is counterweighted.
Still, I'd have it on a bottle jack to move it if necessary.

Joe

 

[ions82]

At this point, I have a feeling that the brake got some oil/coolant that dried out and is keeping it held in place. While we were investigating, we had to remove the four bolts that hold the servo motor to the coupler. We did this in an effort to access the area where the plug is (thinking that there would be a solenoid in there). The gasket on the little block that mounts the brake plug had one corner pushed down in. So, I suspect that some liquid made it's way in there and dried out.

Anyway, when we turned the motor 45 degrees or so, the Z-axis moved. So, we know that part isn't stuck. Also, the overload protection is working, too. It would trip if we tried to jog the Z more than .001". When we would manually try to turn the Z-axis ball screw by hand, we could hear/feel something making solid contact in both directions. We figured that the "brake" is actually more of a lock (as the machine also has a Z-axis lock setting.)

I am thinking that it's safe to say that the problem is in the braking portion of the motor. It appears to be getting the proper voltage. The tough part will be getting the thing out of there and all apart to find out! Accessing the coupling may require us to remove the entire counterweight system. It'll be a big PITA, but it will fantastic if I can actually get this machine running. I've been trying to start a business for 2-3 years now, and not having a working CNC machine is really the only thing holding me back at this point. It's not the newest or fanciest machine in the world, but it will be more than adequate for getting me started. Thank you for all the help thus far and for taking time out of your day to read my post!

 

[Pipes]

Try cleaning

Try to clean the brake area by removing the connector where you saw the possible coolant penetration, pull back on the wire enough to get some spray contact cleaner in there. Do not use too much as the excess will run doen into the motors front bearing and wash out any remaining grease.

The brake in these motors is normally a friction disk and coil pack as in the attached picture. Could also be a broken spring or chewed up post jamming up the works.

 

[mike_kilroy]

If your quick you could try a lazy man's fix: DISCONNECT THE 110VDC AND HIT IT WITH 220VDC FOR 1/2 SEC TO SEE IF THAT FREES IT.

Of course if you leave the 220vdc on it too long you will simply burn out the brake solenoid (coil) and ruin it totally. But you should be able to hit it with 220vdc momentarily a few times and that should free it once it moves once or twice....

Just for reference, we had Z axis 90vdc brakes on some Cincinnati large machines that would drop too far on power off failure and were dangerous to the operator. We supplied a special transformer that with a 150vdc output and 90vdc output, and we had a small 1 sec timer relay apply 150vdc initially then drop back to 90vdc for long term hold off. Allowed us to put beefier springs in the brake so it operated in 15msec instead of 50msec on power failure.

We today use brakes on a Navy project rated 24vdc. We have to apply 90vdc to pull them in fast enough then drop to 24vdc via plc program in 1 second. So hitting your 110vdc (really 90vdc probably) brake that pulls only 375ma with 180vdc (220) for a second should not hurt it. but the force to release will be increased by 4x (square of the voltage)! so this may be enough to brake it free. Once broke free, it should stay free if just dried coolant etc.

How to get 180vdc? BE CAREFUL! just go to radio shack, get a 1N4004 or equiv diode, put in in series with a 220v source to the brake terminals. BE CAREFUL! make sure nothing else is hooked up to the motor. Just these 2 wires, and do it with a momentary switch so you can click it on momentarily for like 1/2 sec then off again. After a couple clicks of this, if brake is going to free, you should be able to hear it clunk on/off when you flick the switch.

BE CAREFUL if you try this!

 

[ions82]

Wow. This is some fantastic information. Thank you for all of the input, tips, and advice. When I created this post, I certainly didn't expect to get many replies (let alone so much detail). I will get together with my friend that is helping me with the project and see how he thinks we should attack it. I very well might try the additional voltage trick. It doesn't sound TOO complicated, and I'm sure my friend (an electrical wizard) would have no problem helping me set it up.

Again, thank you for taking time to read through this post and offer help. This forum is absolutely invaluable. I'd be lost without PM!

 

[AlfaGTA]

Don't know about the Fanuc servos but on the Sieman's units the brake is polarity sensitive. If you connect it backwards it will hold tighter ,and not release. So be sure you have it right way around when connecting an outside power source.
Cheers Ross

 

[zurferjoe]

Thats one of my problems in a nutshell !! Well written sir ! I turn on the machine and one time looked at the indicator showing me the power being used and it was 50% even the thought he z motor was not turning ! So if can can figure out the passing over the reference point Im laughing !! Thanks again Harry

 

[zurferjoe]

Well, I thought that I had a stuck Z-axis as a result of all the dried up oil that is all over the little Kitamura MyCenter Zero that I've been working on. However, I now suspect that there is actually some sort of locking solenoid in the Z-axis servo motor that is keeping it from moving. It is currently up against the second step of a limit switch. However, I used the over-travel function (that allows you to jog the axis off the switch in either .001 or .0001 increments) in an effort to get it moving. I can hear the servo motor trying to turn the ball screw, but the control throws an error and cuts the power.

In exploring the issue, I tried turning the ball screw by hand. Judging by feel, a friend and I came to the conclusion that there was some sort of mechanical stop that was keeping it from turning. Upon inspecting the servo motor, we saw a little part that looked like it may've been a solenoid that locks the motor. We removed the plug and found that it has about 112V DC running to it when the control is on. There are two wires in the circuit, and the "solenoid" side had around 297 ohms of resistance (putting it at around 375mA). It seemed like those numbers would be about right for a small solenoid.

Anyway, we are now trying to find some more information about this. We'd like to avoid having to tear apart the entire top half of the machine, but we're not really sure if there is a better way to proceed from here. Does anyone have any experience with something like this? The servo is a 5S/3000 (red octagon cap). I sniffed around fleaBay and only found one with matching numbers. It was listed for over a grand, so I'm hoping I can just work with the one I've got. Does anyone have any pointers? If so, please let me know. I have spent the last two years trying to get a working CNC machine in my shop (currently 0 for 2). Please help!

If its like my Bridgeport , the screws where painted over at the back 12 very very very 4 mmm screws later coupling ! Thanks for the solenoid tip ! That looks like my next problem ! Happy Days all the bestz harry