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| Author | Topic: screw breakage |
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felix Senior Member Posts: 34 |
posted February 06, 2004 08:59 AM
   
Hi folks, I have a customer here whose 2.5" 18:1 grooved feed screw has broken twice in under a year. I have some ideas as to the cause, but wanted to know some of your ideas. It breaks near the feed section. IP: Logged |
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Bob Cunningham Senior Member Posts: 81 |
posted February 06, 2004 09:45 AM
Cold starts are a biggie. There could always be clearance issues, etc., especially in grooved feed. I like to make sure that you won't have too much torsional sheer stress, which is a function of HP / gear ratio / root diameter / material. Is this screw made from 4340? 4140? Another material? If you can give the HP, ratio, root diameter, and material, I'd be happy to plug some numbers for you. But be sure to double-check that the screw OD is proper for the ID of the grooved section as well. -Bob Cunningham IP: Logged |
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louis33 Senior Member Posts: 79 |
posted February 06, 2004 11:06 AM
First, in what section did it break? What was the material? What is the start up procedure? What is the shut down procedure? Afterthose are answered, perhaps you can trace it to a specific operator. IP: Logged |
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Tom C Moderator Posts: 314 |
posted February 06, 2004 11:52 AM
Felix, The extruder drive may be able to provide more torque than the screw can handle. It is quite usual that a groove feed machine will require a large amount of torque. Usually a grooved extruder will have a screw that is cut shallow in the feed section. If the screw is cut deep it may not have enough strength to deal with the supplied torque. Additionally as grooved feed extrduers are capable of developing very high pressures in the feed area it is possible that a deep cut screw deflects in that area, work hardens, cracks, and then breaks. Tom C IP: Logged |
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Steve H Moderator Posts: 299 |
posted February 06, 2004 01:42 PM
Felix Louis, Tom and Bob's thoughts all should be checked first, if you aren't dealing with an external cause, my .02C would be: Hit us with the numbers and let's see what everyone comes up with Steve H IP: Logged |
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felix Senior Member Posts: 34 |
posted February 06, 2004 05:55 PM
Thanks for the input, the feed channel depth is 0.164 (grooved section)This corresponds to a root diam of 2.226". Flight is under pitch by 20% in the feed section (lead is 2"), after the feed there is an increased pitch. The screw base material is 4140 HT, with colmonoy 56 hard facing. The extruder HP is 60. The screw did not break at start up or shut down, it broke under 'normal operation'. The fracture point was at the end of the grooved section, 4 flights down from the shank end. The barrel and grooved section were relatively new. I think this answers all the questions. I have heard that shorter L/D extruders are sometimes harder on the screw? IP: Logged |
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Steve H Moderator Posts: 299 |
posted February 06, 2004 07:11 PM
Felix Sounds like the screw broke at the point where a grooved feed section extruder develops its maximum pressure. What material is the machine running? Steve H IP: Logged |
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felix Senior Member Posts: 34 |
posted February 07, 2004 12:36 AM
Steve, The screw is running 0.03 MI HMWHDPE. Felix IP: Logged |
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louis33 Senior Member Posts: 79 |
posted February 09, 2004 10:54 AM
So it is breaking at the poubnt where the grooved section ends and the "increased" lead begins? What are the amp guage or pressure readings when the machine is running "normally"? It's too early yet top say that it is a design problem, but I'm leaning that way IP: Logged |
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felix Senior Member Posts: 34 |
posted February 09, 2004 01:11 PM
quote: The machine was running for over 6 months without problem. The pressure I estimate was around 5-6000 psi (no press gage). The amps were at 60% of max, RPM was 60 appx. Does that help? IP: Logged |
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Bob Cunningham Senior Member Posts: 81 |
posted February 10, 2004 11:57 AM
What is the gear ratio of the machine? -Bob Cunningham IP: Logged |
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2beeps Moderator Posts: 71 |
posted February 10, 2004 07:41 PM
Felix, A 0.03 MI HMWHDPE would ( I think) have a VERY high melt viscosity---hence a high torque on your screw. If the screw is running Skip IP: Logged |
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louis33 Senior Member Posts: 79 |
posted February 11, 2004 08:36 AM
If as you say the machine was up and running for over six months without a problem, the only possible cause that I can see is a malfunction in a controller or heater, or possibly metal contamination that found its way into the machine. If you did not find any grooving on the screw, you can probably rule out the metal. The six months without a problem has me confused though. In the orignal post you say that it has broken twice in under a year. Does that mean that it runs for several months and then breaks? If that is the case, I am leaning towards start-up procedures that stress the screw. IP: Logged |
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Steve H Moderator Posts: 299 |
posted February 11, 2004 01:57 PM
Felix, I'm concerned that you don't have a working pressure gauge on this machine. Using the screw motor amps to some how estimate pressures being developed in a single screw extruder simply doesn't work. When I first started in this game, I worked for a man who believed that you could do this. The line in question used to blow its over pressure brass nuts regularly, the amps wouldn't change a jot. Then we fitted melt pressure transducers, and it was like going from night to day The screw breakage is occuring exactly where I would expect it to on a grooved feed machine. Steve IP: Logged |
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felix Senior Member Posts: 34 |
posted February 11, 2004 03:08 PM
The real cause of the breakage may never be known. The total reduction from motor to gear box is 14:1. When I say that it broke twice, I mean that it runs for a few months then breaks, it's happened twice. I am thinking that this extruder has a lot of torque, and if some variable changes (screw speed, or heater band temp) then the viscous HD will cause an excessive load on the screw, especially if it is a high output screw design as this is the case. The replacement screw is being made of a stronger material, namely 4340 HT. IP: Logged |
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louis33 Senior Member Posts: 79 |
posted February 11, 2004 05:11 PM
Ahh But a willow tree bends in the wind . . . I will stick with the faulty instrument theory. The only way to break the screw is for the front to stop turning while the back continues to do so. Pressure alone should not cause that. Higher tensile steel may solve the problem. Without looking it up, can someone chime in with the PSI difference between the two? IP: Logged |
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Steve H Moderator Posts: 299 |
posted February 11, 2004 06:31 PM
quote: Ah Grass hopper  read this link, esp the intro http://www.madisongroup.com/Publications/Reviewed/SPE-Grooved.pdf Steve IP: Logged |
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felix Senior Member Posts: 34 |
posted February 11, 2004 08:24 PM
quote: Steve, I would be concerned too. I am a supplier, don't work there. However, you would be surprised how many old lines I have seen that don't have the following instruments in operation because they are broken, or were never there to begin with: pressure gage, melt temp gage, rpm counter, rupture disk. This line didn't have any of these items. I calculate the output by measuring the line speed, film thicnkess and layflat. Then I can calculate the specific output of the screw by counting the rpms on the screw. I agree with Louis that there may have been a sensor malfunction that caused a drop in temperature, but it's not possible to assertain that. Also possible that someone increased the screw speed too fast, and jolted the screw with the corresponding change in output. Yes I know, the line needs more instrumentation, and it is an antique. It still makes good film though, inspite of all its down falls. The air ring is hilarious, very retro. IP: Logged |
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Steve H Moderator Posts: 299 |
posted February 12, 2004 02:15 PM
Hi Felix There's nothing wrong with old equipment, many a fine tune played on an old fiddle a) Operating extrusion equipment without working pressure instrumentation, is endangering the safety of the workers who have to use that equipment b) While the line is working, minimal instrumentation is fine. But when things aren't going well, trouble shooting is imposible c) With instrumentation, better product and more of that better product is going to come off that line I realise that you know this Felix, but these facts need to be rammed home to anyone who owns, manages, supervises or works with extrusion equipment. I'll get off my soapbox now. Steve IP: Logged |
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felix Senior Member Posts: 34 |
posted February 12, 2004 05:38 PM
Thank God that the operator had the presense of mind to turn the extruder off when it broke. They heard a loud pop and lost the bubble, the section of the screw that was still connected to the gear box kept rotating. It continuied to pump pellets because having broken downstream of the grooves, there was still a short flighted section of 4 diameters. The pumping of pellets into the extruder cause the broken screw to move forward, into the adaptor section. There must have been tremendous pressure build up because the screw tip was rammed so hard against the inner diameter adaptor that the tip had to be removed with a hydraulic wrench. The most dangerous situation is that there was no rupture disc, or it was blocked. This will have to be addressed, because it could become lethal is someone is standing near the extruder when the barrel is blown off the die. The pressurized hot molten plastic would come out like shrapnel. IP: Logged |
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louis33 Senior Member Posts: 79 |
posted February 13, 2004 08:34 AM
Good link Steve. My point goes to the fact that this has not been an on going problem, rather something that happened twice in the previous 12 months. New Operator(s)? IP: Logged |
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Steve H Moderator Posts: 299 |
posted February 13, 2004 09:16 AM
Hi Louis From Felix's first post, I concluded that he had eliminated the posible outside causes: Operator error (cold start etc) Electro/Mechanical (faulty heaters/instrumentation etc) And was focusing on the design aspects of the screw. It's a short beastie at 18L/D, I wonder if it's a constant compression, varying pitch jobbie? Steve H IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 5 |
posted February 22, 2004 09:01 PM
Significant stress can occur in the screw as a result of radial thermal gradients and associated thermal stresses. Since the machine runs ok for some months, thermal stresses can occur to put the total stress over the fatique limit and cause breakage after some time. Most likely scenario is hot flight tip and relatively cold screw root. See "Heat Transfer in Extruder Screws," SPE Antec 2000 Conference Proceedings, pp 83-88. Another think to check is barrel straightness. Often difficult to do under operation thermal conditions. Check barrel mounts to be sure the barrel is not constrained so as to bend when hot. IP: Logged |
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zabielski Senior Member Posts: 299 |
posted February 23, 2004 04:53 AM
Relative to rupture discs........ They case harden over time, and should be replaced annually. This is very often overlooked and does relate to safety. I would like to see a red flag attached to each new one to warn of this. Rupture discs are made of sintered metal and they simply harden with heat and time. IP: Logged |
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griffex Senior Member Posts: 21 |
posted April 03, 2004 02:44 AM
The topic is already over a month old but maybe Felix will see this, and certainly some of the others. 1. No-one mentioned rear barrel condition -- in this case, the intense water cooling that is needed to get the most bite from the system. Change that temp and feed rate responds, too. 2. If the factory isn't well instrumented, it probably isn't checking raw material either. Maybe they get an occasional batch of resin that is 0.02 MI instead of 0.03 (which is a huge difference in an already very viscous resin). 3. Is the shaft stressed too much? We have been dealing with the numbers in another post, so I thought I'd apply them, especially since grooved systems run more slowly and the torque is higher. If you know amps and volts in a DC system, you can get the HP directly. Failing that, it can be estimated from the lb/hr output which you can measure on line; divide by 4 to get HP. The factor may even be lower, but start there. For example, if you are running 150 lb/hr you may be drawing 37.5 HP. Using the formulas in the post, a 2.226" shaft that is transmitting 37.5 HP at 60 rpm is under a shear stress of around 18,000 psi, which doesn't sound high enough to break the screw. Maybe the screw is made in several pieces, and the real diameter at the end of the grooved section is less than the root. Interesting problem. If the new screw is substantially stronger, OK -- but if you want a further advantage, preheat the feed. A gear pump would help, too, as it would take over some of the resistance of the head, but if I had the money for that, I'd put in the instruments first. Hope this helps Allan Griff IP: Logged |
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felix Senior Member Posts: 34 |
posted April 05, 2004 10:14 PM
Allan, I definately agree that a sudden change in viscosity would cause excessive load on the screw. The change in groove cooling is also a very good possibility. Thanks for sharing! Not much to report on this problem except that the new high output screw went in and production is back to normal. Felix IP: Logged |
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