Press fit MP35N into Ti

[mrcadman2u]

I am designing a part that is Titanium 6-4 and a shaft made of MP35N to be press fit into Ti part. The shaft is .625 and needs to be symmetrical in Ti part that is about 1.5 in long supports the shaft. The shaft is approx 7 inches long so needs to be pressed about 3.5 inches. The question that is debatable is what type of fit do I need to produce the desired result. Currently the hole is nominal .75 +.0005 -.0000 and the shaft is .75 +.0005 -.0000.

More info if you need it.

Thanks in advance.

Jerry L. Eden
Mechanical Designer

 

[metengr]

Several questions for the assembled part;
1. What is the service temperature and environment, is it constant or does it have a range?
2. Loading, does the assembled part see static load or cyclic (e.g., rotational) load in service? Impact or vibratory service? Bending or torsional loads?

 

[alexit]

For purpose of assembly a 1.5" long cone (0.5° included angle) is good for the press. If you can have a stepped shaft, you have .75 nominal one side, then cone (to .743"), then .740 nominal. Now you don't have to press 3.5"...

 

[metengr]

Before I would consider using a tapered fit design versus a standard interference fit (like 0.001"/inch of shaft diameter), you still need to consider the differences in the coefficient of expansion between the two dissimilar materials, loading and environment (corrosion concerns between two dissimilar materials).

The coefficient of expansion for the MP35N is reported to be 7.2 X E^-6 in/in/degF

for Ti6-4, the coefficient of expansion is reported to be 5.2 X E^-6 in/in/degF

 

[israelkk]

mrcadman2u

According to your posting the hole is nominal .75 +.0005 -.0000 and the shaft is .75 +.0005 -.0000 this can result in a slip fit. For example if the shaft will be 0.7500 (minimum tolerance) and the hole will be 0.7505 (maximum tolerance) you will have no press fiting at all.

You have to decide what force/torque the press fit should hold and then considering maximum temperatures you have to calculate the minimum desired press fit that will hold the loads even at maximum temperatures and worst case of toleraces. The problem will be what value of friction coefficient between the shaft and the hole to use.

Then you have to decide what safety factor to use.

In an aerospace gearbox design where one gear was pressed to a shaft we used the minimum friction reasonable coeficient and quite large safety factor.

Given all that you still need to actually test the assembly.