Written by Rob Turnbull Tuesday, 13 November 2007 00:00
This article covers replacing the Trex600n shaft drive gears.
The front and rear umbrella gears in my Trex 600n wore down to the point of causing vibration on the tail after about 150 flights - when checking for the cause of the vibration, an overly large amount of backlash on the gears told me to change them. Changing them takes a little while, but it is not too hard to do.
The first step is to remove the tailboom assembly from the helicopter.
With the tailboom assembly removed, we need to remove the tailboom case from the front end of the boom, and also the metal tail gearbox at the back end of the boom.
The rear gear assembly
The metal tail gearbox itself needs to be opened up. To do this, remove the two bolts on both sides of the assembly holding the side plates onto the main silver housing. Remove one of the bolts at the very rear of the assembly, leaving one in place to keep the spacer rod safe. Also, remove the vertical fin by undoing the two bolts that fix it to the main silver housing. Finally, remove the pinch bolt in the side of tail gearbox to allow it to be removed from the tailboom.
With the tail blade assembly removed, the tail gearbox should now come off the end of the boom. If it doesn't, it's likely that the shaft drive is locked into the umbrella gear. To unlock it you need to turn the shaft drive from the other end of the shaft (the front end) while holding the umbrella gear at the back end to stop it rotating, and pulling it at the same time.
Rotating the shaft drive clockwise and anti-clockwise while pulling the tail gearbox (and holding the gear still) will eventually get the drive to unlock itself and allow you to pull it off the boom.
With the tail gearbox removed, you can now remove and replace the umbrella gear. It is a push-fit gear and it's quite tight. I used a suitable sized hex driver to help push the existing gear out of the housing.
Once the new umbrella gear fits into the housing a little bit, it will go the rest of the way fairly easily. Make sure it seats all the way in!
The tail gearbox can now be refitted to the boom. When doing this, ensure you get the shaft drive keyed into the new umbrella gear correctly, then tighten the pinch bolt to secure the tail gearbox onto the boom.
In order to keep this article focussed on replacing the gears, please see the building the tail gearbox article for instruction on how to finish the rebuild of the tail gearbox.
The front gear assembly
The front drive gear assembly and the umbrella gear come as a set (as a pair, or as a bag of three pairs!) and it is wise to replace them both at the same time to keep wear on the gears as even as possible.
To change the front gears, you need to split the tailboom case apart - it doesn't need to come completely in half, but needs to be opened up enough to allow the removal of the front umbrella gear housing (if this can come out, the front drive gear assembly can too).
The umbrella gear case has mouldings on it's left and right, not visible when fitted, that hold it tightly in the tailboom case. See the picture below right for details.
With the tailboom case split open enough, you should be able to remove the front drive gear assembly and the umbrella gear case.
Remove the two screws in the side of the umbrella gear case. This will allow you to push the umbrella gear out of the housing. It is a tight push fit again so will require some force to push out.
With the old umbrella gear removed from the housing, fit the new umbrella gear by pressing it into place, or perhaps by gently tapping it in with something that won't cause any damage to the gear or the housing.
What I did here was to gently tap the case onto the umbrella gear until it was in as far as I wanted.
The front umbrella gear position in it's housing is key to a good gear mesh. If you don't get the umbrella gear pushed in far enough, the front mesh will either be too tight, or simply not fit back into the main housing together properly at all. Push it in too far and the mesh could be too loose and slip, which would likely result in a stripped gear.
As you can see from the image below (left), I set mine with approximately 1mm gap between the black plastic housing and back edge of the gear teeth. This meshed nicely with the main drive gear and is working very well after a good 50 flights or more since the refit (to date).
At this stage, you also need to remove the bearings from the old front drive gear assembly shaft and fit them onto the new replacement shaft.
Next, slot the new front drive gear assembly into the main gearbox housing, and then push the umbrella gear housing into place.
This is all done on one side of the main housing. This allows you to fine tune the gear mesh, if you don't quite get it right to start with, before closing the tailboom case back together again. Also, don't fit the screws into the umbrella gear housing yet to allow further adjustments of the umbrella gear position, should it be required.
Check that the gear mesh is not too tight by gently holding the gears in the housing and turning them. If they feel too tight, the umbrella gear might need pushing into its housing further. If they feel like they have too much backlash, the umbrella gear may have gone in too far.
Once you're happy that the mesh is good and the gears turn freely, but are not too loose, then it's time to fit the umbrella gear housing screws, using CA glue, and finally, close up the tailboom case.
Refit the boom into the tailboom case and slot the shaft drive into position - you might need to hold the tail assembly still while turning the front gears by hand to get the coupling in the umbrella gear to line up and allow the shaft drive to slot in. Ensure the boom is pushed all the way into the tailboom case and then fit and tighten the four boom clamp bolts.
Finally, fit the tailboom assembly back into the heli body, getting the four plastic lugs positioned into the cutouts in the side frames, and then refit all twelve screws (six per side) usinf CA glue, and the two tailboom brace bolts (one per side) using blue threadlock, to secure the whole thing in place.
Don't forget to clip the rudder pushrod back onto the bell crank!
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