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| Author | Topic: Screw Torque Calculations |
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Tom C Moderator Posts: 313 |
posted March 18, 2004 06:16 PM
   
Does anyone have a good source of information on maximum screw torque calculations? I'm finding just a little information and it does not lead to the same answers. Some of the big issues seem to be in safety factors and tensile yeild strength. These number are all over the place, nevermind loss of strength at temperature. I've already seen the stuff from Spirex, one of the few sources. I can't verify their equations, or figure out what if any safety factors have been applied. Any help would be appreciated. I'll try and post a summary on my web site. Tom C IP: Logged |
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Bob Cunningham Senior Member Posts: 81 |
posted March 19, 2004 10:18 AM
Are you talking about the torque required to get an output of a certain material at a certain rate? Or do you mean just the highest torque you can apply to a screw before you break/fatigue the screw? -Bob Cunningham IP: Logged |
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Tom C Moderator Posts: 313 |
posted March 19, 2004 02:42 PM
Sorry, I'm looking at screw breakage torque. Tom C IP: Logged |
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Bob Cunningham Senior Member Posts: 81 |
posted March 19, 2004 03:59 PM
The forumla I use is (321,000 / root dia ^ 3) * (HP / gear-in-speed) (diameter in inches) That gives torsional sheer stress in PSI. Look up the material in question for the PSI for your material, we try to keep under 40,000 for 4340 to minimize fatigue etc. I think that formula depends on a 1750 RPM motor base speed, I've had that for so long that I forgot the particulars. -Bob Cunningham IP: Logged |
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louis33 Senior Member Posts: 79 |
posted March 19, 2004 04:17 PM
Tom Isn't that the same as the PSI rating of the screw material?
[This message has been edited by louis33 (edited March 19, 2004).] IP: Logged |
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Bob Cunningham Senior Member Posts: 81 |
posted March 23, 2004 01:41 PM
Tom, Was that anything close to what you were looking for? Or were you looking for something else? -Bob C. IP: Logged |
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TomBlack Moderator Posts: 39 |
posted March 23, 2004 07:47 PM
tom, i use something similar; (hp/rpm)63024(16)/(PI x rd^3)= shear stress hp/rpm is obviously the input torque and incorporates the gearing as rpm is the max available screw speed, 63024 is the conversion from ft-#'s to in-#'s and 16 is merely a constant. The rd is the feed rd given in inches and the shear stress is the shear stress of the material of construction in psi. Given that 4140 has a nominal tensile yield of 120,000 psi, then it's yield in shear is 60,000 (1/2 of the tensile yield). I've always looked for a 2:1 safety factor which means I'm looking for a calculated shear stress value which is something less than 30,000 psi. ------------------ www.ppsincorp.com IP: Logged |
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Bob Cunningham Senior Member Posts: 81 |
posted March 24, 2004 12:09 PM
Tom- Since 63024 * 16 / 3.14 = 321,141, I'd say our formulas work out pretty close! Nothing like a good double-check. -Bob C. IP: Logged |
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Tom C Moderator Posts: 313 |
posted March 24, 2004 05:08 PM
Thanks guys. I'm now at the "Plastics Encounter" in Charlotte. I'll have to wait until I get back to my office to follow up. Tom C. IP: Logged |
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griffex Senior Member Posts: 19 |
posted April 03, 2004 01:03 AM
Interesting stuff. I have been giving out a screw-snap chart since 1978, based on Spirex, never looked further. It is close to yours, a little more conservative as I used a little more safety factor. Thank you for the chance to compare. Here is a real-world problem: What is the shear stress in the feed zone of a 4.5" screw with 0.750" feed depth and 0.250" metering, running HDPE at 75 rpm? First we need to estimate HP drawn. We can do this by first calculating drag flow, 2.64 D^2Nhp of 782 lb/hr (p = melt density of 0.78). If we really get 85% of that, we have 665 lb/hr, and if we get 5 lb/hr per HP that calls for 133 HP. Applying the 321,000 formula, the stress in the screw in the feed zone (diam 3") is (321,000 x 133) / (27 x 75) = 21,083 psi, safely below any of the estimates you used above. Now I look at a worse-case scenario, where the viscosity is so low that we get only 4 lb/hr per HP, and the bite (inpush) is so good (optimum rear barrel temperature) that there is a pressure peak at the entry to the metering zone, and we get 100% of drag flow (overbite cancels out die resistance). Now the throughput is 782 and the HP is 196. Applying the 321,000 formula again, we get a shear stress of (321,000 x 196)/(27x75) = 30,990 psi, still safe as the flight adds some strength, too. However, this shows the effect of viscosity and rear-barrel temperature can be quite important, and with feed depths much deeper than 0.750" we may be in trouble. For example, if the channel depths are 0.900" and 0.300" the root is 2.7" in the feed, the drag flow is 938 lb/hr, and at 4 lb/HP the HP is 234.5 and the stress is 50,991 psi, which is above the 30-40K you noted as preferable, and pushing the 60,000 limit one of you quoted. Please note that the rpm and compression ratio of all these screws remain the same, which is a good illustration of why I don't like to see compression ratio mentioned as if it defined the "right" screw. Allan Griff IP: Logged |
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