Mntflyby

Need info on how to reduce the lash between the crown gear and the tail drive gear. Sliding the tail gear forward is not enough.

rotordoc1

First check to insure that your mainshaft has not slipped down a bit. If it has, you will find that the head will move up and down slightly by pulling up and pushing down on the head while on the bench.

If the play is the result of wear, then you can get a better mesh by installing 5m x 7m x 0.5mm spacers between the front bearing in the front transmission housing and the tail drive gear.

Mntflyby

Have checked the main shaft as you speak of. No free play up and down. This problem showed up when I had a tail strike and stripped the crown gear. Replaced the gear, that's when I discovered the excessive clearance between the gears. It may have been that way with the old gear and that's why it stripped. It was only a slight tail strike. Have since changed the crown gear again thinking it may have been the wrong gear. Have since determined all three gears are the same. Have tried shifting the main shaft bearing blocks up. I do not think shimming the tail driveshaft bearing will help because I can remove the set screws on the tail drive gear and slide it way forward of where it should be and the clearance between the gears is still too wide. Have tried a complete spare tail boom unit on it with no difference. It appears the crown gear is setting too low for some reason. I notice Century shows shims for the sprage. I do not know what they are for. Thanks for the help. I am going to give it another look over tonight.

rotordoc1

As you can see in the Predator Gasser NX manual, on page 36, the hub that the crown gear is attached to, is pinned solidly to the mainshaft using a pin that is held on center by two grub screws on each end. This means the gear mesh between the pinion and crown gear is directly related to the vertical location of the mainshaft and the horizontal location of the front drive pinion. I am sure you will find the answer by insuring that both of these items are in their proper position. If there is no up/down play in your mainshaft installation, then I would suspect the poor mesh is a result of an improper installation of the pinion gear assembly into the front transmission.

Mntflyby

Finally solved the gear mesh issue. The gear set was setting too low, increasing the distance between the crown gear and the tail drive gear. Two reasons for this: There is suppose to be a shim between the two parts of the one way sprage,it was missing. Also the bottom bearing retaining coller was on upside down. It has a raised area on one side. This raised side goes away from the bearing. This pulled the main shaft along with the gear towards the bottom too far.

rotordoc1

Good to hear. As I suspected, something was not assembled properly.

Mntflyby

Yes, that's what happens when you buy something that someone else puts together. I may have turned the bearing coller over during the process of working on the gear but I never had the sprage apart, so I have been flying it like that all along.

rotordoc1

And many times these people who have problems because of their improper assembly, go on various heli forums like RR and HF and trash talk the product.

Your opening question of "Need info on how to reduce the lash between the crown gear and the tail drive gear. Sliding the tail gear forward is not enough" was the proper way to introduce a problem.

Mntflyby

Yes and believe it or not the gears make less noise now. Thanks for the comment.