Ozmodeller

Guys

Have been running the JR version of the Graupner DS8911 http://www.servodatabase.com/servo/graupner/ds-8911 in my Reaction for only about twenty flights or so and have found that three of them have developed an unacceptable amount of slop in the gear train. Has anybody had the same experience with these servos and is there a fix for them as in fitting a gear train from a different servo or something else, not very happy with these servos so far?

Thanks
Peter

invertmast

My Feedback: (23)

pretty much all of my JR servo's develop a pretty significant amount of slop after use. I've even replaced the gear trains with new and they still have a good bit of slop. None have ever posed a problem in flight before or after the gear train swap.

FlyinTiger

My Feedback: (45)

I've switched to Futaba BLS 157s. Nice and tight after 100 flights.

DiscoWings

My Feedback: (76)

aren't the 8911's supposed to have super steel gears?

leopaul

I have 2 almost new, they have too much slop that i removed them and i use it only for front wheel and on my big flap of the boomerang sprint...I was too affraid of flutter at high speed...

DiscoWings

My Feedback: (76)

Are they 8911HV or just 8911? We dont' have the 8911 in the U.S. I think they call the 8711 as the 8911 in Asia.

Dave Wilshere

Metal gears expand once the servo warms up, many manufactures set the mesh so tight cold they tighten in use, poor centering or high current use is the result. I use exclusively JR servos in 100% of my jets and large models. Never seen flutter on any model, never will due to gear back lash. Mechanical set up on linkages has way more influence. Many of the worlds best pilots use JR servos, there is good reason.

PaulD

My Feedback: (39)

Jeez Dave,

I've got a ton of respect for your experience but what you wrote makes me go hmmmmmmmm......

With an engineering background and working for a company that makes/designs/tests transmissions I struggle to understand how there would be enough thermal expansion with the limited motion a gear sees in a servo under typical use. This would especially apply to jets where the actual servo movements in normal flight are so small and there's no vibration to generate load the same way you would see with a gas powered model. Also, while I realize that most jets servos are mounted inside the wing out of the airflow, the air velocity should provide some cooling to the gear train. Is there that much heat generated by the motor and electronics that would make its way into the gear train?

What I did find with not just JR but other brands of servos was the gear slop present did not come from backlash in the gears as much as the bushings for the gear posts were "egg shaped " allowing the post/pin to rock from side to side as the loads on the servo reversed. Why the servos have such small diameter posts/pins to support the gears is beyond me. Yes, I know that some servos have metal bushings to support the pins but even these will oval out over time.

I do agree that I have never seen flutter caused by slop in the gear train of a JR servo and there are so many other areas on an airframe that could flex and cause problems with the most common problem being just plain poor installation techniques.

PaulD

Helijet

Agreed Paul. I think that many modelers forget that there is also a constant load on the flight surfaces. Its much more important to have good holding power. There is nothing worse than pulling hard in a turn and the nose continuing to drop!

Dave Wilshere

Paul. The servos are not just used in Jets, they get burried in wings on 3-D aeroplanes, 3-D choppers and they waggle a lot, in very hot countries.
I've run tests on a bench with constant movement (no load) and can get a metal cased servo to 45+ degrees easily. The gears don't generate the heat. You are right that in a jet we make small movements and fly fast, but again I have installations where the servos are mounted say above a tail pipe in the fin and on a hot day you would be surprised how hot they are running.
You are right about the pins and wear, its a big thing in petrol models, I change cases more than gear sets.

Dave

Its the old game of many scenarios

raron455

My Feedback: (38)

I have a couple large 3D planes also (40% extra 30%slick etc..) , and I have heard plenty of complaints about that particular JR servo, as well as the Hitec 7955 developing slop too.
Dont know why,,Just saying,, can read the complaints all day on flying giants forum.

chuckk2

There were some JR servos that had a slop problem last year. turned out that they were missing a bearing.

speedracerntrixie

My Feedback: (29)

I had lots of gear wear issues with 8611A servos. Those were my last JR servo purchase. In this particular servo the final output gear is aluminum and is driven by a steel gear. Recipe for disaster. Icing on the cake was the Horizon rep telling me gear wear was because of improper set up. He was pretty shocked to learn that those servos were pulled from a 40% Extra that at the time was the national IMAC points leader for the advanced class. He then agreed to rebuild my servos at no charge but we still had a long discussion about driving an aluminum gear with a steel one.

Jim Cattanach

Nearly all my gear slop issues have been due to the holes in top part of the servo plastic case wearing where the pins locate.
They should all have metal bushings in my opinion.