600 Trex 550E DFC Head Dampers

[Bruno]

Just read some horrible experiences with head/feathering shaft play causing DFC links to wear and fail.

Is someone able to clarify if/where you place a shim on the feathering shaft?

How much if any head/feathering shaft play is acceptable.

When I completed my build of the 550E DFC there was and still is a minimal amount of lateral & horizontal movement/play. Since the build, I have flown the heli with about 10 packs.

I just stepped away and checked the head. I do have vertical and horizontal (lateral) play. Seems to be the same amount of play I had right after the build. I checked either of my two stock Trex 450s......Zero head play.

Is this play due primarily/absolutely to damper wear? I am using the stock dampers.

I purchased the KBDD EXTREME Ed stiff dampers (neon lime) after reading other posts on damper wear.

I was going to fly today but now I will hold off until I learn more about acceptable tolerance with head/feathering shaft play/slop.

 

[pvolcko]

Shims would normally go between the blade grip and the center hub, on the damper side of the step washer. You need to be careful with this, though. You do not want to over-shim it because it will remove flex and damping necessary for the head system to work properly in flight. Basically, the head it going to "flap" in flight and if you over-shim it it will likely end up causing either pre-mature damper wear or even pre-mature damage/wear to the bearings in the blade grips due to too much pre-loading, which results in the extra damping forces being applied to the bearings. Too much play in the head can result in over flapping and damaging vibrations which can stress and cause DFC components to fail.

Personally, I would not screw with the shimming, I'd go the damper change route. Before doing so make some careful observations of the play in the stock head. Replace the damper and put it back together and see what play changed. It should make it more difficult to flex the dampers up and down (by rocking the blades up and down), but it should not cause the "in and out" play to change significantly. If you do feel that has changed to become looser, then go ahead and shim it some in order to get that play feeling similar to stock. If it tightened up too much, then you may need to remove or perhaps use thinner shimming washers than what was used in stock setup. In general there should be little to no in and out play, at least that's visually observable.

Often the in and out play is only observable by feel. Twist the blade grips (un-hook from swash), feel the resistance and any notchiness, and then pull outward on the grips (both at the same time) while doing the same twisting. If you feel the same resistance and notichiness then you may have too much pre-load due to over-shimming. Usually pulling outward and twisting the grips will result in the resistance and notichness going away (at least that's been my experience). If the head is overshimmed then when you pull outward the centers of radial bearings will not un-load, they will still be tightly compressed (skewed) by the dampers and shims. On the other hand, ff there is too much play you may actually notice a gap has visually formed between a grip and the hub or that there was little change in the feel between just twisting and pulling out and twisting. You may also be able to pull one of the grips and see the grips slide in that direction a bit.

Unfortunately all of this is a bit difficult to quantify without testing rigs and the like. It is a matter of feeling and judgement and observing changes in flight behaviors. Personally I'm not really able to judge these things due to flight characteristic changes yet, so I depend on stock setups to be correct and then monitoring changes in damper stiffness and play and whatnot to know when I need to replace them.

 

[Bruno]

Seems the 550E manual:

Comcast

fails to reference the inner spacer which is indicated in the instructions that come with the thrust bearing package H60001T-1 which indicate spacers on either side of the outer grip bearing.

I inserted the inner spacer of the outer grip bearing and that took up all the play.

However, I am not able to torque down the socket screw without it being too tight.

Instead, I have to adjust the torque of the socket screw to allow free bearing movement.

Is this typical?

- - - Updated - - -

Shims would normally go between the blade grip and the center hub, on the damper side of the step washer. You need to be careful with this, though. You do not want to over-shim it because it will remove flex and damping necessary for the head system to work properly in flight. Basically, the head it going to "flap" in flight and if you over-shim it it will likely end up causing either pre-mature damper wear or even pre-mature damage/wear to the bearings in the blade grips due to too much pre-loading, which results in the extra damping forces being applied to the bearings. Too much play in the head can result in over flapping and damaging vibrations which can stress and cause DFC components to fail.

Personally, I would not screw with the shimming, I'd go the damper change route. Before doing so make some careful observations of the play in the stock head. Replace the damper and put it back together and see what play changed. It should make it more difficult to flex the dampers up and down (by rocking the blades up and down), but it should not cause the "in and out" play to change significantly. If you do feel that has changed to become looser, then go ahead and shim it some in order to get that play feeling similar to stock. If it tightened up too much, then you may need to remove or perhaps use thinner shimming washers than what was used in stock setup. In general there should be little to no in and out play, at least that's visually observable.

Often the in and out play is only observable by feel. Twist the blade grips (un-hook from swash), feel the resistance and any notchiness, and then pull outward on the grips (both at the same time) while doing the same twisting. If you feel the same resistance and notichiness then you may have too much pre-load due to over-shimming. Usually pulling outward and twisting the grips will result in the resistance and notichness going away (at least that's been my experience). If the head is overshimmed then when you pull outward the centers of radial bearings will not un-load, they will still be tightly compressed (skewed) by the dampers and shims. On the other hand, ff there is too much play you may actually notice a gap has visually formed between a grip and the hub or that there was little change in the feel between just twisting and pulling out and twisting. You may also be able to pull one of the grips and see the grips slide in that direction a bit.

Unfortunately all of this is a bit difficult to quantify without testing rigs and the like. It is a matter of feeling and judgement and observing changes in flight behaviors. Personally I'm not really able to judge these things due to flight characteristic changes yet, so I depend on stock setups to be correct and then monitoring changes in damper stiffness and play and whatnot to know when I need to replace them.

Thanks for the elaborate explanation. Very helpful with confirming my issue. I seemed to have "visible" excessive movement in/out & up/down with the feathering shaft. Either movement concerns me with DFC link wear/failure. My post#3 addresses this.

 

[pvolcko]

I would make sure the screw on either end of the spindle shaft are snugged down tight. There is a lot of force on that screw and loctite needs to work with seating torque in order to keep it from backing out. If the screw is "loose", even with LT, it will very possibly work itself out and cause a costly, potentially dangerous crash.

In what way are the bearings not "free" when you snug them down? Like I said, they will have some preload on them causing the inner race of the radial bearings to skew and you may feel some "notch" as you rotate the grips. If you grab the grips and pull outward while twisting them that notchiness should lessen somewhat (it may be there still, but hopefully a bit less noticable). I had this on my 550X build and it caused me some concern. But after flying it a couple times things seated in and smoothed out. Fresh builds will have a little bit of "rub" to them, not to worry.

Sounds like you got the slop out of it with the spacers so I'd complete the build and give it a go. Go easy on the flight (low headspeed, just some simple circuit flying or even hovering) and then land and check things out to see if there is abnormally high temps on the servos and to check if thing seem smoother.

 

[Bruno]

It's too tight.....even with lateral pull on the grips. Seems I need thinner spacers. I just bought these ( Kyosho 8x10mm Shim Set 10pcs KYO96644 | eBay) to put between the damper and the spacer as suggested earlier. Ideally, the KBDD dampers will take up the slack otherwise I will try the spacers once I have either in my possession. Thank you for any and all feedback.