Blue Bird servos at Aloft Hobbies

[John in Boston]

Kinda! But with a more airworthy design as a start point.
This one will be for slope soaring, so I've modified things to suit. In short, a different airfoil, stronger skin, centerline wing join.
And our buddy Nate even made me some foam cores for the project!
I mentioned it here on the "Show Us What You Are Working ON" thread under the ALOFT FORUMS page in the discussion forum.
Simply click on my link there to view my photos and captions. I began the build while on an extended stay away on Martha's Vineyard, so I had to make a workboard while I was there and gather up materials and tools "on the road".

 

[Konrad]

Just learned that there is a problem with Aloft's ad copy. The small BMS-101xxx does NOT have ball bearings. Also please be aware of the voltage limits.

Now there are two types (actually 3) of BMS-101.
There is the BMS-101AMG (Analog Metal Geared) never used these.
There is the BMS-101DMG low voltage 3.6v to 6.0V ( I use these in my single cell lipo application, note only rated down to 4.8v by Blue Bird)
And then there is the BMS-101HV 7.4V rating, but BB says safe to 8.5v (I've used these with two LiFe cells).

Blue Bird

alofthobbies.com

Edit: This has been addressed

 

[Konrad]

Wayne, in post #3 you indicated that you are in contact with Blue Bird.

I just learned and verified by direct measurement that the output shaft journal clearance is excessive with the little plastic BMS-101DMG servo. I measured a clearance of around 0.08mm. I was expecting to see a clearance of about 0.02mm. By my math that is about 4 time more than "I" think is required.

Can we learn what is the design clearance? It looks like this would be a simple fix in the machining of the output shaft. With the way this servo is assembled I see no reason for this excess clearance.

This excessive clearance is wrecking havoc on gear backlash and positioning errors with the feedback pot.

All the best,
Konrad

 

[Konrad]

I looked at this in more detail. I think I've isolated the reason for apparent difference in perceived performance.

With a sample size of 5 BMS_101DMG the bearing clearance seem to be the same. (Same production run with all undersized journals). But there is some variance in the height of the output shaft. I attribute this to the tolerance as a result of pressing the aluminum features on to the shaft.

the longer gear assemblies are hitting the axial retention features before hitting the journals (bearing). This would account for what appears to be a good tight servo. But since this axial contact area is so small it will quickly wear allowing the shaft journal to make full contact with the case bore. This is the design feature to hold the radial loads. The Axial lip is not designed to hold radial loads and will wear out very quickly.

Blue Bird needs to adjust their machining of the shaft journal to take up this excessive clearance. 0.08mm plus is far too much clearance!

 

[Wayne]

OK - I have forwarded this off to BlueBird. I suggested if they make these changes it would be a good idea to label them as version 2 servos.