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Tail Drive Shims...Who Needs ‘Em?

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  • Tail Drive Shims...Who Needs ‘Em?

    I do, that’s who. Being a relative heli newbie I just assembled these Robans and PhoenixTech 600 assuming “they are supposed to be that way”, but never liked the tail rotor play I ended up with. Thinking I was headed toward trimming the torque tube length down a few mm’s to tighten things up, but hating that prospect, I spent some time investigating what was really causing all that slop. I’ll try to make this short.
    Basically, the torque shaft can “float” inside both bevel drive gears. Enough clearance was engineered between the shaft and gear teeth to allow a light slip fit. There is no stop built inside the bevel drive gears, additionally, so the torque shaft’s precise depth inside each drive gear is not a critical measurement. The torque tube length, as furnished, does impact the insertion depth of the torque shaft in each gear.
    I replaced the plastic tail drive gears in my PT 600ESP with metal ones and in so doing experimented with those paper thin shims I’ve noticed in these kits. First, I added shims to the front tail drive gear until there was no fore and aft play between the drive gear and housing while the 2 bevel gears were meshed. Viewing from the side, it actually appears the 2 bevel gears are pushed together too far. Holding the autorotation gear firm, I twisted the torque shaft & discovered the only slop present now was the amount of play designed into the torque shaft teeth inside the gear shank. Not much. I repeated the same procedure with the tail rotor drive and discovered it took as many shims as the front to eliminate the gear endplay inside the housing. Now, the tail blades’ slop is mostly eliminated but for the small aforementioned clearance between torque shaft & gear shank teeth.
    Hopefully, this will help other newbies that, like me, gotta learn things the hard way.

  • #2
    Just realized I neglected to mention some additional observations. The front and tail bevel drive gears “float” in their housings also, supported by 2 bearings in each housing. Only mechanism holding the gears in the housings is the friction fit with their respective 90* driven bevel gear. While holding the 90* driven gear with two fingers, if you can slide the bevel drive gear in and out of the housing with your other hand adding shims will eliminate that endplay & reduce slop. Adding too many shims will become obvious when rotation becomes tight. Likewise, the 90* tail shaft bevel gear may need to be shimmed to eliminate side play. As stated previously, the only tail blade rotational slop that remains is from the torque shaft to gear shank tooth clearance.