Some soft materials will significantly flow out under torque pressure. With those materials the torque has to be repeated after sometime. With hard metals that factor is negligible. Ramon -----Original Message----- From: TechNet [mailto:[log in to unmask]]On Behalf Of Phil Nutting Sent: Friday, May 06, 2005 11:49 AM To: [log in to unmask] Subject: Re: [TN] Torque Dave, This is a good application for the "Square Cone Screw". For "un-torquing" hardware, we need to keep in mind that the screw is under tension and there is a certain amount of lubrication on new parts (not necessarily oil, just clean parts). For hardware that has been set for some time there will be some amount of oxidization which will impead the loosening of the screw. In one assembly we believe our inability to loosen flat head screws that were in countersunk holes in FR4 was because the board had been submerged in oil and the weave had expanded slightly. I believe it may also be because the exposed fibers of the FR4 dug into the screw and acted like a one way lock. Another thing to remember is that the torque of the screw does not equate to the clamping pressure on the device. The Power Devices web site FAQ suggests that after 20 pounds of clamping force there is little to be gained in the way of thermal transfer by adding more force. Phil -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Roach, David Sent: Friday, May 06, 2005 11:30 AM To: [log in to unmask] Subject: Re: [TN] Torque In our case, we have a few different transistors that all take #4-40 screws. The transistor manufacturers specify a torque to get the right heat transfer to the heat sink. On some of our transistors, they specify 7 to 9 in-oz of torque. On others, they specify 4 to 6 in-oz. I don't understand the reason for the difference, though it probably has something to do with the transistor package material and design. In some cases, we have had problems with screws "loosening" when the PCB goes through wave solder, so we have to re-torque them afterwards. One screw in particular that "loosens" goes through the bottom of the PCB, through the center of a large bridge rectifier, and into a large heat sink, about an inch cube of aluminum. I suspect that the FR4 relaxes slightly, and possibly the bridge rectifier case as well, so the screw is loose after wave solder. Generally, there are industry standards for tightening torques for different screw sizes. These probably vary depending on the grade (strength) of the fasteners. There should be a table in the Machinery's Handbook (http://www.amazon.com/exec/obidos/ASIN/0831127007/qid=1115393316/sr=2-1 /ref =pd_bbs_b_2_1/002-3342572-5276022) or Mark's Standard Handbook for Mechanical Engineers (http://www.amazon.com/exec/obidos/ASIN/0070049971/qid=1115393358/sr=2-1 /ref =pd_bbs_b_2_1/002-3342572-5276022). I don't have either of these handy right now so I can't look up the tables. Dave Roach Trane GCC 4833 White Bear Parkway St. Paul, Minnesota 55110 651-407-4294 -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor G. Hernandez Sent: Friday, May 06, 2005 10:10 AM To: [log in to unmask] Subject: Re: [TN] Torque Folks, I appreciate all the input the TORQUE inquiry has generated. I will be a waiting on the consensus of HIGHR or LOWER torque for LOSSENINF. Is that a table for recommended torque give dimensions of the hardware working with. Victor, -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Blomberg, Rainer (FL51) Sent: Friday, May 06, 2005 9:59 AM To: [log in to unmask] Subject: Re: [TN] Torque Ramon, You must be working with old rusty bolts to get more torque to remove. This does not happen with clean, freshly torqued hardware. Think of the screw/thread as two ramps sliding together and you can visualize the physics involved. Loosening involves overcoming friction under the head and against the threads. When loosening, you are unloading the forces holding the threads together...this is less friction, less torque than driving the threads together. I will try your experiment, but my experience is less torque to loosen. ----Rainer -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon Sent: Friday, May 06, 2005 10:23 AM To: [log in to unmask] Subject: Re: [TN] Torque Hi Victor: The untorqueing force will be higher. When the hardware is torqued, it starts from being loose and as it is tighten it gets hot due to friction and the heat makes the molecules from both surfaces soft so it slides. Once the hardware is cold, the initial force to break the cold bond has to be higher. This is ignoring the oxidation between the adjacent materials due to time and moisture. Some torque tools work in either direction. Find one of them and set the tool to a torque in the midrange and tighten a bolt. Reverse the tool and try to release it and you will see that it takes more torque. Make sure that your hand is placed in the same area on the tool and the pull is perpendicular to the bar. Regards, Ramon -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor G. Hernandez Sent: Thursday, May 05, 2005 5:25 PM To: [log in to unmask] Subject: [TN] Torque Fellow TechNetters: Can someone share their experience with the topic of TORQUE. How is the torque valve of hardware differ from the value of Un-torqueing that same hardware. Is the torque valve higher than the un torque value or vice versa. Power FETs can be damaged when the plastic package exhibits a hole where the bolt is inserted. Not the typical TO220 or TO66 plastic style. Victor, --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject 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Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send 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