Why on a Leitner-Poma lift does the e-brake and rollback brakes drop when there is a tower fault?
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Tower Faults
Started by Kicking Horse, Dec 15 2008 07:38 PM
14 replies to this topic
#3
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Posted 16 December 2008 - 07:12 AM
I suspect the following:
A normal stop generally involves the drive "powering down" to a stop - it could be either absorbing energy (regen) or providing it (any drive). Either way, the stopping distance will be consistent. If a deropement involves a grip entanglement with a sheave train, there is a chance that the drive might "drag" the carrier through it while attempting to provide the standard stopping distance - probably causing a worse mishap. It might not be necessary to set the emergency brake with a tower fault, but shutting off the drive is pretty important.
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A normal stop generally involves the drive "powering down" to a stop - it could be either absorbing energy (regen) or providing it (any drive). Either way, the stopping distance will be consistent. If a deropement involves a grip entanglement with a sheave train, there is a chance that the drive might "drag" the carrier through it while attempting to provide the standard stopping distance - probably causing a worse mishap. It might not be necessary to set the emergency brake with a tower fault, but shutting off the drive is pretty important.
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This post has been edited by Emax: 16 December 2008 - 07:35 AM
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#5
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Posted 16 December 2008 - 05:28 PM
Look at the ansi standard , they do not specify what kind of stop it needs to be ...... it just needs to stop. I prefer to have a brittle bar fault to be an e- stop and a rpd fault to be a normal stop . Different manufacturers will have different scenarios. An e-stop does not neccesarily mean a shorter stop. All an e- stop means is that the prime mover of the lift will be disengaged from the drive train via the contactor with the electric motor or killing the internal combustion engine and the E-brake providing the force to slow and stop the lift. If you have questions about what a certain lift will do in certain situations refer to the lift's as built wiring diagram , this will give a definative answer.
Normaly the roll back back brake will only apply in the following conditions .... the 24vdc power is off for more than the power off timed relay is set for ,system 1 or system 2 reverse rotation devices sense reverse rotation, or on some Poma's the rollback brake will set after the E-brake is commanded to(operated initiated , tower fault or numorous other causes) after the lift has stopped and after the timed relay has timed out as a redundant safety.( good idea)
Just feel good about the instalation if the lift stops when there is a tower fault. Period.
Normaly the roll back back brake will only apply in the following conditions .... the 24vdc power is off for more than the power off timed relay is set for ,system 1 or system 2 reverse rotation devices sense reverse rotation, or on some Poma's the rollback brake will set after the E-brake is commanded to(operated initiated , tower fault or numorous other causes) after the lift has stopped and after the timed relay has timed out as a redundant safety.( good idea)
Just feel good about the instalation if the lift stops when there is a tower fault. Period.
This post has been edited by aug: 16 December 2008 - 06:05 PM
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Posted 17 December 2008 - 05:45 PM
Kicking Horse, on Dec 17 2008, 05:42 PM, said:
Aug I do feel good that the lift stops, however I would of assumed that it should be a more controlled stop... no?
950fpm to 0fpm in 1.4 secs or 14ft.
950fpm to 0fpm in 1.4 secs or 14ft.
That would pitch everyone out of the chairs...
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#8
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Posted 17 December 2008 - 05:57 PM
Kicking Horse, on Dec 17 2008, 05:42 PM, said:
Aug I do feel good that the lift stops, however I would of assumed that it should be a more controlled stop... no?
950fpm to 0fpm in 1.4 secs or 14ft.
950fpm to 0fpm in 1.4 secs or 14ft.
Obviously you have your numbers wrong, that seems dangerously short.
"Maybe there is no Heaven. Or maybe this is all pure gibberish—a product of the demented imagination of a lazy drunken hillbilly with a heart full of hate who has found a way to live out where the real winds blow—to sleep late, have fun, get wild, drink whisky, and drive fast on empty streets with nothing in mind except falling in love and not getting arrested . . . Res ipsa loquitur (it speaks for it self). Let the good times roll." HT
#11
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Posted 17 December 2008 - 06:09 PM
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This post has been edited by aug: 17 December 2008 - 06:09 PM
"Maybe there is no Heaven. Or maybe this is all pure gibberish—a product of the demented imagination of a lazy drunken hillbilly with a heart full of hate who has found a way to live out where the real winds blow—to sleep late, have fun, get wild, drink whisky, and drive fast on empty streets with nothing in mind except falling in love and not getting arrested . . . Res ipsa loquitur (it speaks for it self). Let the good times roll." HT
#13
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Posted 19 December 2008 - 04:46 PM
The lifts I am familiar with that travel 1000' feet per minute stop with a normal regen stop when there is an RPD fault. Approx. 0.4 meters per second squared decel rate. I believe the thought behind this is if the RPD is adjusted correctly, the rope will still be on the rubber when the fault occurs. The lift comes to a controlled stop attempting to keep the rope on the rubber or back in the groove.
The older detaches we have that travel up to 1100' feet per minute stop with a service stop when there is a breakfork fault. Approx. 0.6 meters per second squared decel rate. In this case, the rope is not where it should be, the drive is disconnected like EMAX advises and brings the lift to a faster stop, but not so fast that unwanted line reaction might create a bigger problem.
Our newsiest pulse lift which travels 1000' per minute stops the lift when a breakfork fault occurs with a ramped e-stop which provides a quicker stop then a service stop. It also locks out the rams on the tension system.
The decel rate in the original post would be putting people on the ground in my opinion.
The older detaches we have that travel up to 1100' feet per minute stop with a service stop when there is a breakfork fault. Approx. 0.6 meters per second squared decel rate. In this case, the rope is not where it should be, the drive is disconnected like EMAX advises and brings the lift to a faster stop, but not so fast that unwanted line reaction might create a bigger problem.
Our newsiest pulse lift which travels 1000' per minute stops the lift when a breakfork fault occurs with a ramped e-stop which provides a quicker stop then a service stop. It also locks out the rams on the tension system.
The decel rate in the original post would be putting people on the ground in my opinion.
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#14
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Posted 02 January 2009 - 03:41 PM
Having some experience on the subject I can tell you that Emax (Hi Bud) is exactly correct on the reasoning behind this being an emergency stop. While a regenerative or pulsed brake stop would be more deterministic and repeatable the overriding concern that led to the selection of the emergency stop is the removal of driving torque should a grip become hung up. A regenerative drive WILL provide motoring torque in copious amounts in such a situation and potentially make a bad situation much much worse. An emergency stop is already configured to ensure that all driving torque is removed as quickly as possible so this was selected as the stop type for the line safeties.
As for your description of the stopping distance from speed, if the stop you describe occurred then it should be immediately reported to the BC authorities. It is well outside of any accepted standard that I know of and potentially very dangerous to the public. This has nothing directly to do with CPS/RPD faults rather it is a braking issue. Braking logic for the bull wheel brakes (emergency and rollback) for any emergency stop is that the emergency brake sets immediately on an e-stop and the roll back brake sets only after a 10-12 second timer times out, as a backup (I believe an earlier poster alluded to this), or on a deceleration fault (indicating the emergency brake has failed to properly decelerate the lift). It is important to note that a CPS e-stop is no different from an operator e-stop with respect to braking logic.
Even with both brakes setting simultaneously it is very difficult to produce the deceleration rate you describe so I would also check for other potential causes of the driving torque requirements increasing by significant amounts as this could also cause the lift to decelerate at an excessive rate once the driving torque is removed - bull wheel bearings, motor bearings, gearbox, terminal adjustments etc. should all be gone over. It is helpful to watch the armature current reading on the DC drive (torque on an AC) while running empty. If excessive amounts of current (or torque) are required to run an empty lift then there is a problem somewhere. Generally the "overhead" requirement to run a detachable lift of average size and at "reasonable" temperatures at full speed is 100 HP. I will leave the math to you but the running current/torque can be calculated from this. Alternatively, reference the original acceptance test for maximum current and compare it the amount being drawn on a busy day (if you are going to do this perform the next steps first).
Once this is done, and assuming that there is no problem found, ensure that both brakes are not setting simultaneously. Earlier posters are correct in that the only time these brakes should set simultaneously is when either an 'A-B' or cable tachometer rollback is detected. To test for a failure simply reset all safeties and watch the hydraulic unit as you pull the e-stop lanyard. The emergency brake pressure should drop immediately with the rollback pressure dropping 10-12 seconds later. No run is necessary. In the event that they are dropping simultaneously then you should immediately report this condition to LPOA service as this would cause twice the nominal braking force to be applied to the braking surface (assuming properly adjusted brakes) and needs to be repaired.
Failing simultaneous setting of the brakes, I would strongly suspect that your braking torque is set too high. You need to have a mechanic go over the brake set up with a focus on the emergency brake and the braking torque. Pay special attention to the e-brake restriction as it can cause problems if it is misadjusted (I doubt to the extent you are describing however). Of course, as mentioned, this would happen on any e-stop because the logic and hardware are common. Do you regularly see short e-stops on this lift?
If you've read this far you've probably realized that I gave the steps in order of increasing likelihood of being the root cause. i.e. in the sequence that takes the longest to get to the most likely culprit. I did this because, in my opinion, all these things should be gone over on a regular basis anyway and doing so will likely increase your knowledge of the system and your ability to monitor its proper operation...sorry about that.
As for your description of the stopping distance from speed, if the stop you describe occurred then it should be immediately reported to the BC authorities. It is well outside of any accepted standard that I know of and potentially very dangerous to the public. This has nothing directly to do with CPS/RPD faults rather it is a braking issue. Braking logic for the bull wheel brakes (emergency and rollback) for any emergency stop is that the emergency brake sets immediately on an e-stop and the roll back brake sets only after a 10-12 second timer times out, as a backup (I believe an earlier poster alluded to this), or on a deceleration fault (indicating the emergency brake has failed to properly decelerate the lift). It is important to note that a CPS e-stop is no different from an operator e-stop with respect to braking logic.
Even with both brakes setting simultaneously it is very difficult to produce the deceleration rate you describe so I would also check for other potential causes of the driving torque requirements increasing by significant amounts as this could also cause the lift to decelerate at an excessive rate once the driving torque is removed - bull wheel bearings, motor bearings, gearbox, terminal adjustments etc. should all be gone over. It is helpful to watch the armature current reading on the DC drive (torque on an AC) while running empty. If excessive amounts of current (or torque) are required to run an empty lift then there is a problem somewhere. Generally the "overhead" requirement to run a detachable lift of average size and at "reasonable" temperatures at full speed is 100 HP. I will leave the math to you but the running current/torque can be calculated from this. Alternatively, reference the original acceptance test for maximum current and compare it the amount being drawn on a busy day (if you are going to do this perform the next steps first).
Once this is done, and assuming that there is no problem found, ensure that both brakes are not setting simultaneously. Earlier posters are correct in that the only time these brakes should set simultaneously is when either an 'A-B' or cable tachometer rollback is detected. To test for a failure simply reset all safeties and watch the hydraulic unit as you pull the e-stop lanyard. The emergency brake pressure should drop immediately with the rollback pressure dropping 10-12 seconds later. No run is necessary. In the event that they are dropping simultaneously then you should immediately report this condition to LPOA service as this would cause twice the nominal braking force to be applied to the braking surface (assuming properly adjusted brakes) and needs to be repaired.
Failing simultaneous setting of the brakes, I would strongly suspect that your braking torque is set too high. You need to have a mechanic go over the brake set up with a focus on the emergency brake and the braking torque. Pay special attention to the e-brake restriction as it can cause problems if it is misadjusted (I doubt to the extent you are describing however). Of course, as mentioned, this would happen on any e-stop because the logic and hardware are common. Do you regularly see short e-stops on this lift?
If you've read this far you've probably realized that I gave the steps in order of increasing likelihood of being the root cause. i.e. in the sequence that takes the longest to get to the most likely culprit. I did this because, in my opinion, all these things should be gone over on a regular basis anyway and doing so will likely increase your knowledge of the system and your ability to monitor its proper operation...sorry about that.
#15
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Posted 16 July 2009 - 09:17 PM
Kicking Horse, on 15 December 2008 - 07:38 PM, said:
Why on a Leitner-Poma lift does the e-brake and rollback brakes drop when there is a tower fault?
read the print pretty simple really
the print will tell you what stop will be used
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