Flange Coupling Calculation.

Quick and fast way to determine the torque capacity of a flange coupling,
Calculation is based on the combination between bolt tension and the coefficient of friction between surfaces.

This calculation do not consider, in any case, that bolts could work under shear load therefore all torque must be transfer between the surfaces.
It was observed in many cases, that once any of the bolts that belong to the pattern, starts to work under shear load, the torque distribution changes within the bolt pattern. 
Bolt will start to work also under combined bending loads which at the end will lead into a premature failure.

The tightening torque is determined using an "utilization factor" of 0.8.












































                                                                Flange Coupling Calculation

Additional Information about Flange Couplings:

Lessons Learned:

• A bolt that is not properly tightened can become loose after a short period of time
• If the fasteners are loose, they are subjected to alternating forces and may fail through fatigue.
• Few bolts could work only in tension, ans some of them work also in shear. Do not overload the flange to avoid premature failures.
• If it is possible, tight always the nut and not the bolt.
• Replace locknuts after some installations. Five or six is a good number.
• Try to keep the oil or grease out of the flange contact as well as from the bolt or nut area.
• Assembly instructions:
• Tight the bolt in pairs crosswise, looking for the opposite one each time.
  During all of the following steps, keep any gap between flanges even all around the circumference, and nuts made up approximately the same amount on each end of the bolt.
  • First time around just snug the nuts with a hand wrench.
  • Second time around tighten the nuts firmly with the same wrench.
  • Third time around apply approximately 25% recommended torque.
  • Fourth time apply approximately 75% of recommended torque.
  • Fifth time around, apply 100% of recommended torque.
  • Continue tightening nuts all around until nuts do not move under 100% recommended torque.
  • If possible, re-torque after 24 hours. Most of any bolt preload loss occurs within 24

Fastener Torque Calculation

Developed for designers and engineers that look for more information about the fastener than only the torque value.

This tool uses the concept of "Utilization Factor", that varies from 0.5 to 0.8. This factor is linked to the type of work the fastener is going to do in the joint. From just a quick clamp, no structural, no risk joint to a design where the fastener perfomance is critical.

Here are some screenshots of the application:

 

  Fastener Torque Calculation

    Additional information about joints and fasteners:

• Metric Grade
• Bolt Science
• Futek
• RoyMech
• Metric to SAE socket conversion
• MIT Open Course

   Lessons learned:

• Provide enough clearance for the wrench tools to access to the head of the bolt.
• Be sure that bolt can be assembled and dismounted, specially attention with long bolts that may be replaced without tier down all the transmission,
• Try to standarize all bolt types and sizes in the gearbox, specially in areas where the bolt is working in the same way. (housing calmping bolts for example)
• Do not mix same bolt sizes with different grades in one gearbox position.
• In castings, be sure that bolt head is not contacting an angled surface. Try to clean up always the casting surface, specially in aluminum castings.
• In aluminum, use roll form threads.
• When specifying the hole dimensions, leave at least 3 times the pitch size of gap between the tap drill depth and the thread depth. One way to specify this type of geometry can be done in this way: 
• Speficy the tap drill Max depth and specify the thread Min depth.
• A whaser under the bolt head can promote a better pressure and contact distribution. 
• Find the right location tolerances when defining the joint holes. Keep in mind always the rule of Clearance = Tolerance. An acceptable position value for threaded holes might be between 0.6 - 0.8. The MMC of the bolt is always its Outside diameter. From those values you may find the hole through diameter and its tolerance.
• If your calculations consider the bolt to be dry and without oil, make sure that you are receiveing this from your supplier.
• When the shank diameter of a bolt is less than the thread diameter thus allowing a radiused thread runout which reduces stress concentration - beneficial in fatigue applications.
• Distance between bolts to clamp the gearbox housings toghether should be around at 6.5  - 7 times the bolt diameter. (Example, M12 bolt diameter, distance between bolts should be around 78 mm and 84 mm)
• Avoid curved geometry between bolts in gearbox housings. Always think on designing for the min distace between bolts.
• If you suspect fatigue, a rule of thumb is to calculate the number of cycles a ferrous metal part has been exposed to before it failed:
• Failure at less than 10,000,000 cycles:  suspect fatigue.
• Failure at over 100,000,000 cycles: fatigue is not likely.
• It only takes 3600 rpm motor only 46 hours to produce 10,000,000 cycles.
• A 75 Rpm mixer shaft coupling bolt failing after 2 ½ years has over 100,000,000 cycles.
• From Fatigue Failure in Bolted Joint
• When joining materials with strengths lower than that of the bolt strength, a washer must be used to avoid bearing failure at the bolt/nut-material interface. In this case a washer should be used under both the nut and head of the bolt.
• The larger the washer diameter the thicker and stiffer the washer should be. It is also apparent that the strength of the washer must be at least equal to the strength of the bolt.