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Author
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Topic: new screw design or change gear ratio??
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shawn8791
Senior Member Posts: 11
From:brazil, IN, USA
Registered: Oct 2006
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posted February 06, 2009 08:39 PM
  
I am experiencing problems with an 8" two stage extruder processing LDPE and LLDPE printed fluff with a ram stuffer. The extruder was sold to us 18 years ago processing 3500 lbs/hr, we now are averaging 1250 lbs. Yes, the screw has .060" wear, and the barrel is in good shape except for a section of .050" wear in the first stage metering area. The real problem seems to be the torque. The 700hp motor is always at current limit and the screw remains around 30rpm. Would you suggest a new screw design, or a screw rebuild and a lower gear ratio for the drive? We are still running PEs, but now additives like metallacines, and other concentrates are added. Any thoughts or suggestion are appreciated. ------------------
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Tom C
Moderator Posts: 917
From:Brodheadsville, PA USA
Registered: Jun 2001
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posted February 06, 2009 09:44 PM
I've seen this problem when heaters were burned out or thermocouples were in the wrong place, like installed in a heater.
Also there could be something wrong with the drive.0.050" in the first transition will reduce your output depending on the material viscosity. However it will not increase power draw unless you are really galling the screw and barrel. I am available to come to your site and sort this out. ------------------
Best Regards, Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
shawn8791
Senior Member Posts: 11
From:brazil, IN, USA
Registered: Oct 2006
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posted February 08, 2009 06:03 PM
Tom,The real problem seems to be the material changes over the years. These materials require more torque. You're ideas are good, but have already been investigated. Our concern is that we have already spoke to several screw design engineers, and get to completely different solutions. One is to redesign the screw to require less torque, the other is to rebiuld the screw and change the gear ration to provide more torque. Both companies are very reputable and have seen the process, we are looking for a third opinion. IP: Logged |
Tom C
Moderator Posts: 917
From:Brodheadsville, PA USA
Registered: Jun 2001
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posted February 08, 2009 07:30 PM
Go to my calculator at www.extrusiontechnicalservices.com I believe you will find that the calculator results are similar to what your current rate is. A lot depends on the MI of the material you are working with, but typical high power consumption at 1250 lbs/hr is 175 HP. If your extruder is designed in a conventional manner, then maximum screw RPMs would be about 70-80 and the power available at 30 RPMs is about 275 HP. In this case you should have enough power. If your material MI is very low, or you maximum screw RPMs (gearing) are higher then you might run out of power. Another potential issue has to do with some LLDPE resins which do not shear thin and can consume higher than expected power. In the method I use; computer models are employed to determine exactly where the power is being used in the screw, and then the new screw is designed away from that. Do you have a grooved throat or is it possible the feed throat is worn out?
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Best Regards, Tom Cunningham
www.ExtrusionTechnicalServices.com [This message has been edited by Tom C (edited February 08, 2009).] IP: Logged |
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