Main shaft rocking in bearings

[Anthony Oxlade]

Hi guys,

I've been having several problems with high vibrations since upgrading to TT and i am trying to tackle each source in turn. I am currently inspecting my head and shaft assembly. I recently replaced all three main shaft bearings and the main shaft. I have noticed that the main shaft can be rocked back and forth slightly. The movement at the top of the shaft is very small, but the rocking can be felt easily. I can see and feel that the shaft is moving relative to the bearing races. The main shaft measures 9.95 mm diameter.

I don't know if this play is the cause of my problems but i would like to know if it is typical or not? My vbar is reporting high vibrations at the frequency of the head (2225 rpm) and i wonder if my loose shaft is the cause (spectrum measured in a hover attached)

Another possible symptom is that my swash driver has suddenly started rotating by several degrees during flight. I had to straighten it several times today, despite being clamped as tightly as possible.


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[Jean-Luc Bolduc]

Don't think the small rocking is the cause. You vibs don't look high on the dark green color path.

[Anthony Oxlade]

Thanks, i'm just trying to rule out possible causes. My new silverline sensor is exhibiting slow and intermittent CCW drift and the heli just doesn't feel right at the moment. The repeated swash driver movement is also worrying me.

I think the amplitude of the peak at the head frequency is a symptom of something out of balance but i need to check on a new head to be sure.

[stevehof]

Thanks, i'm just trying to rule out possible causes. My new silverline sensor is exhibiting slow and intermittent CCW drift and the heli just doesn't feel right at the moment. The repeated swash driver movement is also worrying me.

I think the amplitude of the peak at the head frequency is a symptom of something out of balance but i need to check on a new head to be sure.

Did you mount your Sliverline with the supplied thin tape? They don't like being mounted with Futaba or or original Vbar foam. CCW drift is usually caused by not having enough counter torque pitch in the tail blades when the servo is centered. Make sure you have 4 to 6 degrees of right rudder after the Vbar initializes. Did you check to make sure your swash rotates and pivots freely? If I remember correctly a few ships had inner race swash ball screws that were a tad too long and when tightened they bound the swash up a bit.

[Anthony Oxlade]

Yes i'm using the supplied thin tape. I have set the tail blades to have ~5 degrees of counter torque when the servo is centred, and checked this in rate mode. One or two other people have observed the same CCW intermittent drift and have had no luck fixing it so far, despite expert help http://rc.runryder.com/helicopter/t700690p4/

The swash rotates smoothly when it is flat, i checked that none of the balls were binding the swash. I haven't checked by hand how it rotates at large deflections but i'll do this. I find it really strange that there is enough force to move the swash driver clamp when it is fully tight. I've never seen this before.

[stevehof]

OK, well, maybe try 7 degrees... My Silverline also had CCW drift but I just kept adding pitch and it went away. I do think I'm beyond 5 degrees at initialization.

Something has to be tugging on that swash driver. Binding in or at the swash, I would think, would be the first place to look. If you haven't already, check for binding at full throws in Vbar software swash section and adjust your cyclic ring (in the expert tab) if necessary to remove any binding.

[Anthony Oxlade]

I will try adding even more pitch and see what happens. However I've never had any drift sensitivity to the tail blade angle at centre servo before. It makes me think something else is amiss.

I've stripped the head and the swash feels perfect. I'll try replacing the swash driver next, the swash is already a new one.

[Jean-Luc Bolduc]

I have 2 Silvelines on my E6 and E7 and I have zero issues. 2150hs on E7 and 2300 on E6, so there must some other reason for the drift. Have you tried different tapes or checking with strapping the sensor with velcro.
And all agreed that the swash driver should not move. That unit clamps the shaft really well when tightened correctly.
Tried other mains and tail blades ? perhaps a slight imbalance on mains.

[Anthony Oxlade]

4cyclic, could you please let me know what your vbar vibration numbers are on the bench or in the hover? I would like to know what mine should be at for comparison because then maybe i can stop troubleshooting vibrations.

Maybe another set of main blades would be a good idea. I'll check the balance on my current ones when i get home.

Any ideas what could cause a swash driver to keep moving despite being clamped extremely tightly?

[Hendo67]

Your swash driver issue may be related to your bearing "rocking" issue. Your main shaft is .005" undersized, but I don't know what the tolerance is. Should be closer than that I would guess though.

[Anthony Oxlade]

Is everyone else's main shaft tight in the bearings? I have three shafts here and all measure 9.95 mm on my vernier calipers. I checked several other heli's and all were tight (L400, 600EFL, 550EFL). No other e7's at my club unfortunately

[stevehof]

Is everyone else's main shaft tight in the bearings? I have three shafts here and all measure 9.95 mm on my vernier calipers. I checked several other heli's and all were tight (L400, 600EFL, 550EFL). No other e7's at my club unfortunately

Funny thing is my 600 EFL pro's main shafts rocked back and forth horribly. Just as my 500 ESP's shafts did. All my E7's are solid as a rock.

I measured my two new spare main shafts with my Starrett micrometer at exactly .393 inch, which would be 9.982 mm. Even my cheap Chinese digital measure them at 9.98 mm. So, if yours are really 9.95 mm they are .002 inch undersize. My understanding of acceptable shaft size tolerance for this application would be about .0005 inch less than the bearing's ID (10mm), about .0127 mm. IF your measurements are accurate, you have three undersize main shafts. That would be very weird indeed. However, there's not much else that can explain the rocking??

If you don't mind spending a few more bucks, I'd order and try another main shaft. In any case, it's hard to believe that is causing the Silverline to wander. You might also try Futaba foam firmly strapped down. That worked for me as well as the firm tape to eliminate wandering with my Silverline and it might help absorb some of the courser vibrations.

[Anthony Oxlade]

Thanks for the measurements! I will take a decent micrometer home tonight and check all three shafts. I already have two more on order, as well as several main shaft bearings.

When my e7 was brand new i don't remember any main shaft play. It was my favourite helicopter and all these issues are getting me down. Still, i'm not giving up. I will have it flying perfectly again one day.

I have a complete new head on the way just for comparison. I'm also going to switch back to belt drive to see if this affects the rudder drift.

[stevehof]

Have you tried different sensor mounting tape yet. You have a lot of expensive stuff ordered when a 20 cent piece of foam tape and small velcro strap might be the answer. Have you thoroughly checked and or rebuilt the tail thrust bearings and control hardware yet?

[Anthony Oxlade]

Hi,

Yes i have tried different tape and strap combinations. My micrometer indicates that all three shafts measured at their centres are 9.98 +/- 0.005 mm.

The tail was rebuilt with new hub and bearings during the first round of vibration issues which turned out to be a bent hub. The main shaft and all three bearings were also replaced. If you look at my spectrum the vibration levels at the tail frequency are quite low compared to the head frequency.

My gut feeling is that the vbar sensor is faulty, however i wanted to see what other people's vibrations levels are like since this might indicate whether i need to troubleshoot my mechanics or not.