Oil pressing shrink fit

Is the interference fit expected to transmit all the torque?

Plenty of examples in machine design hand books, and require assuming interface friction.
I do not recall if it is assumed the friction is modified by residual oil.

A tapered fit requires the hub to be driven up the tapered shaft far enough to achieve the interference.
Requires Machining real nice matching tapers with controlled "gage points" to set the axial position of the hub on the shaft.
That is >>> WAY <<< more tricky machining and QA.

With a tapered shaft there will be a specific distance the gear needs to be driven on to the taper. If documentation is not available, this distance needs to be measured before disassembly.

With straight fit the gear sits against a shoulder on the shaft. Depending on the size of the gear, I would not likely bother with the hydraulic installation. Just heat it up and drop it on. Save the hydraulics for removal. Apply load on the gear during cooling as they tend to walk a little bit as they cool.

There is no keyway, correct? I have come across hydraulic fits with keys and it's impossible to use hydraulics due to leakage.

Hesam267
Please advise on the diameter of the interference fit. How much interference is required
Use standard formulas for thermal expansion.
Then calculate the amount of clearance.

Heat the gear -50 degrees of tempering temp.
Shrink the shaft in liquid nitrogen -370 f.
Use those temperature for you analysis.
Depending on the size of the diameter, if it is small. One has to be quick during assembly. And may require additional hydraulic press. Larger gears and shafts are more forgiving. But it would not hurt to give a press to make sure it is seated flat.

Small shafts and gear with a thermal press fit is and are a pita

There are tables with the different fit types along with ID and OD dimensions.
I have a really good one in the back of my old drafting textbook.
Take a look at the Machinery Handbook.
We have a 4" output shaft that we do.
We set an inside spring calliper to 0.010" over the shaft OD, then heat the gear until the callipers go in the pinion bore, then drop the pinion on.