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OLi

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Reply with quote  #16 
I had a single screw extruder first seizing the coupling, the only case Nspectr beat me..... and it just stopped due to that. It was a big spline coupling with one grease zerk so grease was not distributing. So 3 zerks solved that. Next a few years later the inner of the final gear that was the connection to output shaft was totally mashed. Likely due to a minor overload when plastic was "frozen" in the extruder at some start or short stop that went longer. Not proven to be that but likely. That is what happened in that machine for 30 years, last 10 years working ok. Oh I think it broke the axial bearing once also for unknown reason way back. Another case of that in another similar size machine was due to not turning on the lube.... So check coupling, check operator if possible? Otherwise, add strain gauges and telemetry for torque measurement. It could be easier to monitor power consumption of the motor? With normal size I doubt the motor have power enough to break the shaft, they turned up the drive, added bigger motor? Normally they are DC drives here, swapped for VFD or more modern and turned up the amps?
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VibGuy~5

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Reply with quote  #17 

I’d look at the ratio before I’s say whether the motor could provide enough torque to crack the shaft. We used to assemble triple reduction gearboxes that had ratios of 31,000/1 (something like that, 1 revolution every 30 mins). The motor driving it was 0.18 kW, or just about 2 light bulbs.

It something jammed in the drive, it certainly would crack things, but usually it was the second reduction drive pins –(cyclo type gear). Like a chain, the weakest link goes first. I would have thought the gears would go before the shaft, so maybe something else going on. The crack at 45 degrees looks like a torsion crack(?). What dia is the shaft and what’s the motor power and speed?

OLi

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Reply with quote  #18 
In those I see that work on plastic motor is typically old DC drive 400-600RPM at least less than 1000RPM and extruder is less than 100RPM more like 40-60 RPM but it is only those I have seen.
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Danny Harvey

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Reply with quote  #19 
They just changed from a DC motor to an AC induction motor with a vfd.

It is 250 hp, 6 pole usually operated at 1062 rpm. That is direct coupled to a jackshaft which has a v-belt sheave. The driven sheave is mounted on the gearbox input shaft and drives it at about 385 rpm. Then it passes through a couple more sets of gears (all with assy phase problems) and shafts to get the 276 rpm, co-rotational output shafts. Those are connected by the splined couplings (more like a short section of shaft with w/ 6 tooth internal spline cut through). I would guess that the major diameter of the splined shaft is about 60 mm. I don't know what the shaft material is other than to say it is some sort of hard alloy.

I believe that the corotational shafts make it so that one screw loads the other making the torque load higher. The way they break confirms that.

In thinking about the change to the AC VFD, I recalled that at one time, this didn't have a jackshaft connected to the motor.  It had an Autogard torque limiter mounted directly on the motor shaft.  At some time, they must have burned up a motor bearing because it appears to have been modified move the motor back and add the jackshaft.  Unfortunately, that ultimately lead to the DC motors eventual demise because it left the NU318 bearing skidding for lack of a radial load.

Torque limiting sounds like something they may want to consider given their recent history.  I would expect that with a brand new VFD they would be able to accomplish that. If not, they still have an Autogard in the store room.
VibGuy~5

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Reply with quote  #20 

A torque limiter where and at what setting is the question? We’ve replaced couplings before with torque limiters (mechanical and electrical). Some have worked well, but only when the gearbox ratio is not high and the load is close to the limiter. Others haven’t worked at all, as by the time the overload torque had worked its way back to the motor, the component it was supposed to protect had already failed. A better option would be to have the limiter closer to your gearbox, but by now the torque is huge, the limiter is huge and you probably don’t have room!!

Our 31,000:1 ratio gearbox could produce about 34,000 Nm of torque. Did the gearbox ever see that?-Not a chance!! The cost of a torque limiter to protect your machinery from that torque? A number with many zeros (I would guess)

John from PA

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Reply with quote  #21 
Quote:
Originally Posted by Danny Harvey
They just changed from a DC motor to an AC induction motor with a vfd.

It is 250 hp, 6 pole usually operated at 1062 rpm. That is direct coupled to a jackshaft which has a v-belt sheave. The driven sheave is mounted on the gearbox input shaft and drives it at about 385 rpm. Then it passes through a couple more sets of gears (all with assy phase problems) and shafts to get the 276 rpm, co-rotational output shafts. Those are connected by the splined couplings (more like a short section of shaft with w/ 6 tooth internal spline cut through). I would guess that the major diameter of the splined shaft is about 60 mm. I don't know what the shaft material is other than to say it is some sort of hard alloy.


It would be interesting to know the timeline of the v-belt relative to the gearbox failures.  IMO, with a v-belt, I think the root cause is something internal to the gearbox.  The v-belts would break or slip at the front end.  Gearboxes have a durability rating, the point at whch the teeth flanks can deliver a finite number of cycles before they wear out.  But they also have strength rating, the torque at which a tooth would tear away from the root.  That number is often 4X to 5X of the motor rating and generally is in line with the torsion the shaft can carry without any issues.
RustyCas

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Reply with quote  #22 
Does this gearbox "sound" (audibly) worse than the vibration data would suggest it should?  Did the failures start after switching to the VFD?  If so, the speed control is likely tuned too tight, causing torsional oscillation resulting in a high-cycle fatigue failure as opposed to an excessive torque situation.  Unless the plastic has to be delivered at a very precise rate, the speed control should be tuned pretty loose (as opposed to "tight" -- I don't know the VFD terminology).  I've seen this a number of times.
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Danny Harvey

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Reply with quote  #23 
John,

Here's the history as I know it. The v-belt drive was original equipment. The jackshaft was added and the torque limiter removed at least 10 years ago.  The extruder was moved from it's original location where it extruded a powder to the new location where it is extruding plastic in 2012.  2013 is when the first gearbox shaft broke. A very clean break right at one of the gears. The real cause is unknown.

Earlier this year (when I started this thread, we picked up on a ticking at reducer secondary shaft speed which upon inspection turned out to be interference with the casing due to axial displacement of either the gear or the entire shaft assembly.  My recommendation was to check the adjustment on the thrust bearings but they really aren't equipped or trained to do that so nothing happened.

Somewhere along the line, they replaced the motor and that is about the time they started breaking things. I suppose that if they replaced the v-belt drive also, the old one could have been serving as a torque limiter by slipping. I don't think they did but I'll check it out.

Now, the ticking in the secondary shaft of the reducer is back but they have also broken two sets of extruder screws and shafts. 

Rusty,

I'm told it does. I'll pass that suggestion along because this thing is likely to have received that kind of attention by engineering. I collected vibration data on all the materials they process at various feed rates during the trials. They were indeed being very precise about the feed rate.

Thanks again




Danny Harvey

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Reply with quote  #24 

Rusty,

By "too tight" do you mean like + or - 1 rpm as opposed to 5 rpm? And would the nature of the failure still be a spiral fracture?

RustyCas

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Reply with quote  #25 
I've never actually gotten to see the damage (broken shafts) so I can't speak to what they looked like (I probably wouldn't understand what I was looking at anyway).  As to how the drive is tuned, I know very little about drives.  But part of the equation (I think) is not only how closely the rpm is held to a setpoint, but how fast the "reaction time" is to a deviation, and does this lead to overshoot in the opposite direction?  This can be done electronically at dizzying speeds, more than us mechanical guys can even relate to.  And the effect on all those mechanical components, with all that mass and inertia, can be devastating. 

And then there is the possibility of a defective drive or SCR or whatever they are using these days.  It is so easy for the "sparkys" to create circuits and devices that do amazing things, but which the physical world just can't keep up with.

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VibGuy~5

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Reply with quote  #26 

This would be the PID loop. Multiple sensor parameters can be used as an input to get a desirable output (for us a pressure or flow rate). In the VFD, the ramp rate can be set which would be rpm/s (or max rpm/s that you can accelerate or decelerate). For drives with lots of inertia, you’d want that ramp rate to be low, but then you might find you are overshooting on your PID loop. This is where we find our automation guys spending lots of time trying to ‘tune’ the system. A skill that comes with practice.

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