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All Bolts/Nuts when installed properly require proper Torque. Keep in mind that torque is NOT Bolt Tension or Clamping Force. Torque is an indication of the required Bolt Tension/Clamping Force when used with a calibrated Torque Wrench. Torque of a fastener in a given application is usually specified by the design engineers. Many fasteners require a direct torque value. Many of the bolts/nuts on our Cummins are of this type. However, other bolts in specific applications require "TTY" Torque to Yield Fasteners.
Torque to Yield Fasteners are used in high stress areas such as Head Bolts, Connecting Rods and Crankshafts as a brief example. Torque to Yield are required to achieve the specific preload. The process of torquing TTY Bolts is called 'Torque to Angle'. Torque to Angle is used in assemblies in the Cummins Engines.
TTY Bolts are designed to stretch slightly when installed. This process allows for more even loading and allows the bolts to hold torque better. Cummins allows the re-use of TTY bolts as long as they are not stretched past a Cummins Spec Limit. As an example, Cummins sells a tool to measure the length of head bolts ( Cummins PN# 3823921 ' Cap Screw Length Gauge'). This gauge will quickly tell you if the bolt has been stretched to much past yield. If the bolt does not pass the Gauge Test, it should be discarded.
When TTY bolts are installed, they are tightened to a specific torque in a given sequence, then tightened to an additional amount that is measured in degrees of rotation. This final twist in degrees of rotation provides the stretch to bring the bolt to its yield point and creates the needed elastic clamping force. The stretch is only a few thousands of an inch. Re-using TTY bolts will cause the bolt to stretch further. This is why Cummins has the Cap Screw Length Gauge and gives the Maximum length spec. A bolt that is stretched past the design limits will no longer hold the same torque load as before and it can fail.
If your doing Engine work on your Cummins, a Torque Angle Gauge is absolutely required to bring the TTY 'Torque to Yield' bolts to spec. The Cummins Torque Angle Gauge is rather expensive, much more economical versions are available at better auto parts suppliers. Fel Pro (The gasket people) make a cheap plastic Torque Angle Gauge that will work just fine.
Fel Pro calls it the Torque Angle Indicator # TRQ-1.
Also, Torquing a TTY bolt to a given torque will not give the same Clamping Force that the Torque to Angle does. As an example; Cummins, in the shop manual for many fasteners give the correct Torque to Angle sequence, if the Torque to Angle was not so critically important, they would just have listed to torque to xxx ft lbs and be done with it.
Proper use of fasteners is critical in what we are doing. A few weeks ago, a friend proudly told me he used all new Grade 8 fasteners on his entire truck project. Well, this is not correct. Different applications require different Grade Fasteners. Grade 8 can not automatically blessed as being best all around. Grade 5 may be the best choice in some applications.
The formula for determining torque is Torque= k x d x f. May I suggest for additional reading, to learn how the 'K' part of the equation is so critical to you, go to the following web site. This web site is the Fastenal Corporation Site. You want to down load the Fastenal Technical Reference Guide 2000 PDF file 801KB. This 54 page document will tell you 'everything' you will ever need to know about fasteners, torque control, re-use of bolts, choosing the proper grade fastener for a given job and much more. Its a wealth of information. The URL is;
http://www.fastenal.com/web/resourcesAction.ex?action=DocLib
Sorry to be so lengthy, but this is important to understand when rebuilding our engines.
Paul
Torque to Yield Fasteners are used in high stress areas such as Head Bolts, Connecting Rods and Crankshafts as a brief example. Torque to Yield are required to achieve the specific preload. The process of torquing TTY Bolts is called 'Torque to Angle'. Torque to Angle is used in assemblies in the Cummins Engines.
TTY Bolts are designed to stretch slightly when installed. This process allows for more even loading and allows the bolts to hold torque better. Cummins allows the re-use of TTY bolts as long as they are not stretched past a Cummins Spec Limit. As an example, Cummins sells a tool to measure the length of head bolts ( Cummins PN# 3823921 ' Cap Screw Length Gauge'). This gauge will quickly tell you if the bolt has been stretched to much past yield. If the bolt does not pass the Gauge Test, it should be discarded.
When TTY bolts are installed, they are tightened to a specific torque in a given sequence, then tightened to an additional amount that is measured in degrees of rotation. This final twist in degrees of rotation provides the stretch to bring the bolt to its yield point and creates the needed elastic clamping force. The stretch is only a few thousands of an inch. Re-using TTY bolts will cause the bolt to stretch further. This is why Cummins has the Cap Screw Length Gauge and gives the Maximum length spec. A bolt that is stretched past the design limits will no longer hold the same torque load as before and it can fail.
If your doing Engine work on your Cummins, a Torque Angle Gauge is absolutely required to bring the TTY 'Torque to Yield' bolts to spec. The Cummins Torque Angle Gauge is rather expensive, much more economical versions are available at better auto parts suppliers. Fel Pro (The gasket people) make a cheap plastic Torque Angle Gauge that will work just fine.
Fel Pro calls it the Torque Angle Indicator # TRQ-1.
Also, Torquing a TTY bolt to a given torque will not give the same Clamping Force that the Torque to Angle does. As an example; Cummins, in the shop manual for many fasteners give the correct Torque to Angle sequence, if the Torque to Angle was not so critically important, they would just have listed to torque to xxx ft lbs and be done with it.
Proper use of fasteners is critical in what we are doing. A few weeks ago, a friend proudly told me he used all new Grade 8 fasteners on his entire truck project. Well, this is not correct. Different applications require different Grade Fasteners. Grade 8 can not automatically blessed as being best all around. Grade 5 may be the best choice in some applications.
The formula for determining torque is Torque= k x d x f. May I suggest for additional reading, to learn how the 'K' part of the equation is so critical to you, go to the following web site. This web site is the Fastenal Corporation Site. You want to down load the Fastenal Technical Reference Guide 2000 PDF file 801KB. This 54 page document will tell you 'everything' you will ever need to know about fasteners, torque control, re-use of bolts, choosing the proper grade fastener for a given job and much more. Its a wealth of information. The URL is;
http://www.fastenal.com/web/resourcesAction.ex?action=DocLib
Sorry to be so lengthy, but this is important to understand when rebuilding our engines.
Paul
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