Interference Calculation

How did you calculate ? which equations ? have you considered the cup internal diameter ? Which are the hole dimensions ? Have you considered the chamfer effect? Which value for the friction has been considered for the computation?

Your interferences seem a bit high at 0.1mm (0.004") max and 0.066mm (0.0026") min. Shouldn't they be more like 0.025mm (0.001") max and 0.013mm (0.0005") min?

I am sorry for the first message which was written too quickly.

Now I made a check considering the geometric interference and the effect of roughness which reduces the "true" interference. the friction coef was in those computations 0.1 (usually it is 0.08...0.14 but I assumed it is lubricated so that I took a low value).

The forces are not -computed - in the range you mention 5 kg ≈ 50 N ( kg is according to ISO a mass unit). n the case of the lowest interference the computed force is 216 N and in the case of the highest 1440 N. I would be very care full with interference reduction since if the pressure after assembly is too low the joint is not any more tight and oil can come out. How ever I think you should modify the hole diameter and the precision class (H7 is too tight for such an assembly) even a H11 could be good provided that the nominal diameter is not any more 12 but higher.

It could be possible that the friction is lower but only 50N seem to me a very low force to press this cover. If you consider now the cover itself, the stress should stay under the elasticity limit for the steel in order to maintain the pressure on the housing wall.

And a last but not least remark is that the aluminum housing has a temperature expansion coefficient about 2x the one of steel so that this has as well to be considered. If the gear box work at 120°C for instance then the temperature changes with 100 °C and the 12 mm hole becomes 12.012 which reduces even more the interference and the sealing pressure.

5kg (50N) would be an extremely light press fit: a thumb can easily exert that much force.