electricpete
Electrical
- May 4, 2001
- 16,774
Machine Description: Outdoor vertical motor (700hp, 900rpm) driving a pump through a rigid coupling. Rated torque = 5250*700/900 ~ 4100 ft-lbf. Locked rotor torque is probably around 8000 ft-lbf.
What happened: Motor tripped during start on time overcurrent (51 relay). On-site inspection revealed motor would not rotate by hand, even after uncoupled. Sent to repair shop who provided attached photos showing a damaged anti-reverse-rotation device.
Overview of anti-reverse-rotation device operation: Slide 1 is an overview of the parts of the device. Principle of operation: During start, the vertical pins slide up the ramp on the stationary ratchet plate which pushes them into the holes in the rotating pin-holder where they are held I place against the side of the hole by centrifugal force (and friction). When the motor slows down, the pins fall down onto the stationary ratchet plate again but a portion also remains in the hole. If the motor rotates reverse then the pin is pushed backwards against the flat vertical portion of the ratchet plate preventing reverse rotation. By the way normal rotation is CW viewed from the top. There are 12 equally spaced ratchet ramps and 6 equally spaced pins (why are there 12 holes... there were originally 6 holes but during a previous refurbishment they were found damaged so 6 alternate holes were drilled at that time... that previous repair was associated with reverse rotation damage to the holes and the vertical faces of the ratchets found during proactive refurbishment which had never caused any problems with motor operation in the plant).
Inspection Findings: The photos show that one out of 6 pins is broken roughly in half (not much deformation, maybe a brittle failure?).One pin hole has a wallowed entry (on the side that would suggest that damage occurred while rotating forward with pin stuck on ramp)[revised to remove incorrect info]. One ramp is severely damaged. The vertical portion of another ramp is slightly damaged at the top.
The repair shop’s conclusion is that the pin bound in the hole due to rust and lack of clearance (0.005” clearance on a pin which is maybe 0.5” diameter and 1” long). They plan to increase clearance to 0.020”. They also suggested using stainless steel pins to address the rust, but as far as I can tell from the photos the rust is mostly on the pin holder rather than the pins (I’ll ask them about that). I asked if parts could be lightly coated in oil to reduce rust. They said lubrication is not typically used since it would attract/retain dirt/dust. I suggested some kind of dry powder lubricant and they said they’d consider it.
MAIN QUESTION: Based on the info above, do you have any ideas about the cause of the failure or what are the proper actions to prevent recurrence (I'm preparing for a phone call with the shop Friday 1/22 to finalize our repair plan).
My questions for the shop (not necessarily important for responders to this post). To prepare for my phone call I made a list of questions. This is not the main part of my post, I'm just listing it here because it's convenient for me. You can add questions or weigh in on my queestions if you like, but you don't have to read them (all the important stuff is above):
1 - Is there rust on the pins? (I don't see it). What can we do about rust in the pin holes?
2 - What is the silver pattern on bottom of the pin holder slide 2
3 - Slide 4 why does the brown rub pattern seem to be further out at a larger radius on the ramp and then closer in at a smaller radius on the flat?
4 - Is the rub mark on the non-damaged stationary ramps normal / expected outside of a failure?
5 - What about the slightly damaged vertical wall slide 5 at 10:00 position (two to the left of the obvious damaged ramp). It looks as if the pin was starting to fall and caught the top of the vertical wall and then pin pushed up out of the way. Is that normal?
6 - What materials are the pins, pin-holder, ratchet plate? Are any of these parts painted? (what explains the colors on the ratchet plate).
6a. Regarding pin material - Is it expected to have a brittle looking failure without much pin deformation
6b [new]Were the materials changed during the last repair? Is the stationary ratchet plate now harder such that more of the reverse impact is transferred to the pin?
7. How much clearance is between the top of the ratchet and bottom of the pin holder
8 What are exact dimensions of the pin (I said 0.5" x 1" but that's just my guess). And what is the height of the stationary ramp?
9. If we lined up the pin's plane of the failure with the bottom of the pin holder, where would the bottom of the pin be... resting on the bottom between ramps of resting in the middle of the ramp where the damage is or somewhere else.
10. are those clearances per side or diameteral.
11. You'd think the pin breaking would allow the motor to successfully start. Did the pin break before trip but still slowed it down enough to trip?
12. What is the radius of curvature of the bottom corners of those pins? Would a larger radius of curvature be beneficial to help the pin slide smoothly up the ramp during start?
13. Could the proposed larger clearance allow the pin to tilt enough to score the inside of the holes during starting? Maybe larger radius of curvature on top corner of the pins would help prevent that? And also radius of the hole entry?
miscellaneous note - Since the are exactly twice as many ratchets as pins, all six pins will contact at the same time (assuming dimensions are precise enough to permit them all to contact). Other designs have different odd combinations of pins and ratchets which means the pins wouldn’t all contact at the time (one would hit first to stop motion) but that type of staggered design has the advantage that it significantly limits the arc available for reverse windup before contact is made and is therefore probably a better approach in terms of stresses during stopping reverse rotation. But the distinction doesn't seem important because it appears the damage here occurred during forward rotation.
=====================================
(2B)+(2B)' ?
What happened: Motor tripped during start on time overcurrent (51 relay). On-site inspection revealed motor would not rotate by hand, even after uncoupled. Sent to repair shop who provided attached photos showing a damaged anti-reverse-rotation device.
Overview of anti-reverse-rotation device operation: Slide 1 is an overview of the parts of the device. Principle of operation: During start, the vertical pins slide up the ramp on the stationary ratchet plate which pushes them into the holes in the rotating pin-holder where they are held I place against the side of the hole by centrifugal force (and friction). When the motor slows down, the pins fall down onto the stationary ratchet plate again but a portion also remains in the hole. If the motor rotates reverse then the pin is pushed backwards against the flat vertical portion of the ratchet plate preventing reverse rotation. By the way normal rotation is CW viewed from the top. There are 12 equally spaced ratchet ramps and 6 equally spaced pins (why are there 12 holes... there were originally 6 holes but during a previous refurbishment they were found damaged so 6 alternate holes were drilled at that time... that previous repair was associated with reverse rotation damage to the holes and the vertical faces of the ratchets found during proactive refurbishment which had never caused any problems with motor operation in the plant).
Inspection Findings: The photos show that one out of 6 pins is broken roughly in half (not much deformation, maybe a brittle failure?).
The repair shop’s conclusion is that the pin bound in the hole due to rust and lack of clearance (0.005” clearance on a pin which is maybe 0.5” diameter and 1” long). They plan to increase clearance to 0.020”. They also suggested using stainless steel pins to address the rust, but as far as I can tell from the photos the rust is mostly on the pin holder rather than the pins (I’ll ask them about that). I asked if parts could be lightly coated in oil to reduce rust. They said lubrication is not typically used since it would attract/retain dirt/dust. I suggested some kind of dry powder lubricant and they said they’d consider it.
MAIN QUESTION: Based on the info above, do you have any ideas about the cause of the failure or what are the proper actions to prevent recurrence (I'm preparing for a phone call with the shop Friday 1/22 to finalize our repair plan).
My questions for the shop (not necessarily important for responders to this post). To prepare for my phone call I made a list of questions. This is not the main part of my post, I'm just listing it here because it's convenient for me. You can add questions or weigh in on my queestions if you like, but you don't have to read them (all the important stuff is above):
1 - Is there rust on the pins? (I don't see it). What can we do about rust in the pin holes?
2 - What is the silver pattern on bottom of the pin holder slide 2
3 - Slide 4 why does the brown rub pattern seem to be further out at a larger radius on the ramp and then closer in at a smaller radius on the flat?
4 - Is the rub mark on the non-damaged stationary ramps normal / expected outside of a failure?
5 - What about the slightly damaged vertical wall slide 5 at 10:00 position (two to the left of the obvious damaged ramp). It looks as if the pin was starting to fall and caught the top of the vertical wall and then pin pushed up out of the way. Is that normal?
6 - What materials are the pins, pin-holder, ratchet plate? Are any of these parts painted? (what explains the colors on the ratchet plate).
6a. Regarding pin material - Is it expected to have a brittle looking failure without much pin deformation
6b [new]Were the materials changed during the last repair? Is the stationary ratchet plate now harder such that more of the reverse impact is transferred to the pin?
7. How much clearance is between the top of the ratchet and bottom of the pin holder
8 What are exact dimensions of the pin (I said 0.5" x 1" but that's just my guess). And what is the height of the stationary ramp?
9. If we lined up the pin's plane of the failure with the bottom of the pin holder, where would the bottom of the pin be... resting on the bottom between ramps of resting in the middle of the ramp where the damage is or somewhere else.
10. are those clearances per side or diameteral.
11. You'd think the pin breaking would allow the motor to successfully start. Did the pin break before trip but still slowed it down enough to trip?
12. What is the radius of curvature of the bottom corners of those pins? Would a larger radius of curvature be beneficial to help the pin slide smoothly up the ramp during start?
13. Could the proposed larger clearance allow the pin to tilt enough to score the inside of the holes during starting? Maybe larger radius of curvature on top corner of the pins would help prevent that? And also radius of the hole entry?
miscellaneous note - Since the are exactly twice as many ratchets as pins, all six pins will contact at the same time (assuming dimensions are precise enough to permit them all to contact). Other designs have different odd combinations of pins and ratchets which means the pins wouldn’t all contact at the time (one would hit first to stop motion) but that type of staggered design has the advantage that it significantly limits the arc available for reverse windup before contact is made and is therefore probably a better approach in terms of stresses during stopping reverse rotation. But the distinction doesn't seem important because it appears the damage here occurred during forward rotation.
=====================================
(2B)+(2B)' ?
