This model is a modified model of Main-Gear-Box (see URL) created previously.
https://www.myminifactory.com/object/3d-print-main-gear-box-helicopter-driven-by-2-engines-75211
The modification purpose is for using the major components of the Helicopter-Power-Train model which driven by Turboshaft model (see URL) created recently.
https://www.myminifactory.com/object/3d-print-turboshaft-engine-with-radial-compressor-and-turbine-75203
About Helicopter-Power-Train model image, refer to the photo.

The following modifications are introduced in order to obtain the stable rotation.
1. Use Metal Bearings for all the rotation shafts.
2. Change Input Gear into Bevel Gear.
3. Provisions so that Main Rotor Head will be attached in the Future.

Assembly Manual (PDF format, total 13 pages)
The detail assembly manual including "Parts-List", "After printing treatment" and "Assembly procedure" are prepared based on "Standard Skill (Filing, Drilling, Tapping and painting)".

Download files includes;
-files: STL Files (24 items)
STL file name: "ws" of last 2 digits means "With Support" which special designed.

Purchase Parts Information;
- Bearing
Metal Bearings are used for every shafts. See Assembly Manual.
Estimate cost is about 6,000yen for 1 set of bearings in Japan.
-ABS Tube
See Assembly Manual.
-Screws and etc.
Micro screws M1.4x3.5L - 5 ea - Direct screw-in to 1 mm diameter hole (No nut),
Other size of Screws - See Assembly Manual.

Total Net Print Time: Approx. 35HR
- (Estimated as case of PLA, 0.4mm Nozzle, 0.2mm Layer Height, 40% infill and No raft and support)
Note: When at actual print, each parameter may be adjusted by your experience.

3D printing settings
Raft, Support, Layer Height, Infill: Depending on your experience.
But the parts having gear, small shaft and thin portion: 100% infill is recommended.
My models were printed by "idbox" using with 0.4 nozzle, 1.75 PLA.

I do hope your success!!

[Update]2021.7.5
Option model: "Freewheel (One-Way) Clutch" Object=176523 uploaded
See Figure as reference.