Shaft - Hub Press Fit Calculator

When I started to work in things related with mechanisms, materials, engineering details, transmissions...etc I was struck by the torque and resistance capacity of these type of press fit joints. 

Nobody told me, by this time, how well should be controlled the dimensions of the parts that create the joint. Even small tolerances expand considerably the difference between min and max torque capacity.

I hope the list of suggestions I am writing here help you to design your joint:

• If your joint rotates and you can add a key in your joint, just do it, It is a safety device that will help you to sleep better.
• Double check the expansion of the external part, the hub. Only in the case your surrounding parts are too close to the hub.
• Provide generous chamfer at the hub and shaft entries.In general a 20 degree angle is enough. During the assembly, try to avoid an edge contact between parts, so look at the interference sequence and verify that chamfer ends do not coincide by the time you start to press. Sometimes a difference in the chamfer angle helps.
• Tolerance ranges capable and reliable for a standard grinding are around ±0.013, Eventhough you can use a tight values like ±0.008 but probably your cpk would be far away from 1.33 unless you pay so much for the parts.
• Take care with thinner parts. On shafts, the inner diemater can end as an oval. If you press fit so much a bearing cup, the rolling ellements could find areas with less clearance than others and that would generate a speed reduction in your rolling elements, sliding, heat and infant failures. In the other hand, a thin hub, will copy so well the shaft geometry that any size imperfection will end at the outside of the hub. Also the torque capacity will be very poor and depending on the material, the elastic curve of the hub can easily be overpassed and finally yields to a permament deformation.
• Use if possible a retainer compound,
• Assemblymag.com
• Loctite Retainers
• If possible, use the same material for hub and for shaft. If not possible, at least both would have the same elasticity module. In case using a different material have a look at the part with the lowest module, this one will limit the capacity and the geometry of the joint.
• To obtain better results and to avoid excessive wear during the assembly, due to the friction between surfaces a hub preheat or a shaft temperature reduction is needed in some cases. Also that effect will help the assembly process o reduce the force needed to join the parts. Caution must be taken to avoid preheat at temperatures that could temper the parts and reduce its surface hardness. In general, I do not accept prehteats that exceeds 180 ºC.

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.