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,
- 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.
