Hi hellovn,
The advice you're getting here is good. You should realize that we can't tell you what the loads are on your machine without a tremendous amount of detail, and I doubt you'll find this forum adequate to discuss all those details - there's just too much detail. Instead, I'd suggest the following overview, and if you have more specific questions you can try asking them as they come up.
In any analysis of a machine like this, perhaps the most simple way of determining loads and stresses on the machine are to break down the various loads individually. For loads imposed by the rotating shafts and gears, start at the electric motor. That motor puts out a torque which is resisted primarily by the cutting tool. So between the motor and tool, the torque creates forces on the bearings inside this box. Assume these are in static equilibrium and then do a static analysis on the loads to determine these forces. For example, the torque produced by the electric motor produces a force on the belt (and subsequent torque) that drives the shaft. That force has to be reacted by the bearings on that shaft and in turn, that shaft has forces on it that are produced by the reaction of gears or pullys on the shaft. Do a static analysis of those forces to determine the loads and direction of loads on the case. You don't need to concern yourself with shaft RPM to determine loads on the case. You need to reduce the entire system that is imposing loads on this gearbox to a set of static loads that are imposed on the various bearing pads and mounting locations of parts on this case.
And don't forget weight. You will need to determine loads imposed by both weight and motor torque on the mounting locations for the case. All of those loads need to be examined. There should be a variety of cases depending on gearing inside the case, so you should be looking at a variety of different load cases.
After you've done the stress analysis, someone needs to evaluate it. It has to be evaluated with various considerations in mind, such as fatigue, material strength, resulting deflections, etc...
If your job is only to do the FEA on the case, then perhaps someone else can provide those forces for you. Normally, a single 'project' engineer is responsible for an overall project, and they may or may not be responsible for providing the load cases to someone that does an FEA analysis of those loads. It would help considerably to explain what your responsibility is and how you're working with others in your company. In fact, I wonder why someone else at your company isn't helping to explain all these things to you. Have you talked to your supervisor? Why are you looking for help on a message board?
Finally, note that as previously mentioned, the reaction loads on the case aren't the only things that need analysis. The shafts need to be looked at, bearings, gears, etc... All of the loads on the various parts should fall out of the static analysis. In addition, the bearings need to be looked at for clearances, thermal movement of shafts, proper alignment, etc... All your fasteners need to be looked at for proper preload, shear stress in threads, forces on them, etc... Lubrication... fatigue of parts ... economics ... casting manufacturability... There's a whole host of things that need analysis. Where does your responsibility start/end and what are you really trying to get out of this discussion here?
