Throttle and choke servos

[Gobeil]

I have had 2 x 110oz HV MG servos fail after about 100 flights on an Extra 91". They are used on the throttle and choke, so about 5 to 6 inches behind the firewall and are linked to the engine with a 2-56 metal rod and nylon clevices. They were not "singing' in the min or max positions and the power is A123's.



Both servos do not respond to Tx commands unless I start rotating their arms and even in their new positions they do not stay there firmly.



I replaced both of them temporarily with other identical servos I have and all is well again.



Is this a common problem with some servos ?



Have I got the wrong servos ?



What are good and reliable servos for this application ?



Thanks

 

[Tech8]

I just had a 225MG fail in a 30cc plane. I think the engine vibrations cause the throttle servo to work overtime to maintain the position and it eventually burns out. I know you're using a much stronger servo than I was but I suspect the metal rod we are using transfer the vibes . I thinking of using sullivan gold-n-rod or similar nylon pushrod. The problem I am having is the throttle linkage is so short on this plane (like 3 inches) that by the time you add clevisis to the ends it is only an inch and a half of nylon. Maybe some others have input????

 

[Gobeil]

I took apart the first servo that failed and found that 1 winding out of 3 is burnt open. The second one has exactly the same symptoms, so it must be the motor again.



After some homework, I have come to the conclusion that one must give quite a bit of play at high throttle and at no choke. If the idling stop screw has been withdrawn and while the engine is idling, the throttle servo should not reach the minimum stop (screw or mechanism) and therefore will not be strained. The choke will only be high during a short time at start up, so even if it bumps against the max mechanism, it will result in very few shocks to the servo.

 

[orthobird]

i have had two JR HV 8911 fail on a 40%er, and on a 91" extra. one is a da 170, the other is a DA 60 size,

the airplane with DA 60 was 3D and was heavily abused. the 170 was used for strict imac.

i made a similar post, and i figured out what i was doing that was wrong.



i set my end points, lets say, trim all the way down, so that with throttle stick all the way down, there was no binding, like one click away from the buzzing.

on top stick, again, 1 click away from no buzzing.



now, i know, when the motor is idling, the butterfly is slightly open, so then, with the motor vibration, it probably never was binding.



however, on the high end, i am certain when i had it full throttle, the servo gears and all were being banged at about 6000 vibrations per minute or so!!



so i believe that was probably the cause of the failure.





i have now gone back in, and on top end, backed it up by 10 clicks or so.

interesting thing is that when the carburetor butterfly is open 80% or 100%, i do not believe there is any difference in RPM or performance.

 

[Gobeil]

That make sense. I have tried as well the same thing and did not notice any difference between 80/85 and 100% throttle.



How about a miniature bicycle cable and housing that is attached on one end to the servo and servo mount and on the other end to the carburetor and engine mount. The cable and housing could be looped on itself so that all vibrations are lost in the loop. A low torque but accurate standard servo could be used.

 

[orthobird]

i have seen some set ups where they leave the spring to work, and they use a cable to pull on the carb throttle lever. i believe this may be what you are describing?

 

[Gobeil]

Are you talking about a spring in line with a pushrod ?

 

[nebwake]

Buzzing isnt always the definitive factor in binding. Really you should use an amp meter to determine the load on the servo. Geometry also plays a HUGE role in keeping the servo from having issues. With good linear geometry and no binding there is no way you should have an issue with a servo on throttle if you use a high torque (200+ oz) servo.

 

[stangflyer]

Interesting thread. I'm curious though of the brand, (other than the JR) name of the servos that failed. Having upfront information is always good to have. I've been using HS 5665 mg's on my throttles. Have not had any issues with these servos.

 

[TazmanianDevil]

Try ball links on both ends of the rod it helps...

try to install the servo NOT on the same surface the engine is.

try to use servo protector mechanism...http://www.anticsonline.co.uk/2039_1_1238831.html