R/C servo design life, Wrenno etc.

BrendanP

Senior Member
Can anyone give me any advice re. the design life of good quality, brand name servos?

Are servo failures in service common? Has anyone had servos just die on them in normal use for no explanable reason?

How tolerant are servos of vibration and heat? I would think that a R/C aircraft would have a lot of vibration running through the airframe.
 

leftyretro

New Member
I've used many R/C servo in models and have some experiance. Servos can fail. But the biggest variable is the quality of the specific servo. Most of the major servo manufactures offer different servo models with different options, metal gears Vs plastic, etc. So it kind of comes down to you get what you pay for and matching the application to the quality of servo required is needed. One might need to know the mechanical torque required, the travel lenght and how many strokes per unit time the servo has to deal with. Buy the quality you need but don't buy more then you need.

Lefty
 

Michael 2727

Senior Member
So it kind of comes down to you get what you pay for and matching the application to the quality of servo required is needed.
If your servo just dies on you under normal light duty conditions I would be looking for dry
solder joints etc. On the other hand if under heavy loads I would be looking for a burned
out motor or driver stage. And if you force the servo to move un-powered you run the risk
of stripping teeth off plastic gears (looks at my dead one in the box :( )
A good idea would be to stick to one brand then if you do have any failures you can use
the expired ones for spare parts :)
 
What do you plan to do with the servos

I have a lot of experience with servos (building RC airplanes, cars and boats).
They came in three different categories
Core motors with bushings or ball bearings (cheapest)
Coreless motors with analog control, all these seem to have ball bearings
Coreless motors with digital controls (the most expensive)
They are happy with 4.8V and run very well vith 6V
Manufacturers list the specs with the following data
  • Torque
  • Speed (time to rotate 60 Degrees)
  • Weight of the servo
  • External dimensions
The three largest brands are Futaba, JR, and Sanwa (Airtronics in USA) but there are many more.
If you tell me how you plan to use the servos I may be able to give you some more info.
Andres
 

BrendanP

Senior Member
Thanks to everyones response.

I'm using JR 579 servos for throttle and choke control on a 5.5 HP 4 stroke Honda electric start motor. I was told these servos have steel gears and ball bearings.

I'm considering changing them for purpose made linear actuators. At the moment the servos are mounted on a plate attached to the motor. I was thinking of isolating them from the motors vibration by using push/pull cable to transmit the action from the servo to the throttle and choke levers. That way the servo could be sitting in a enclosure protected from heat, fuel spills etc.

The servos are cheap (er) and easy to control via picaxe.
 

leftyretro

New Member
Thanks to everyones response.

I'm using JR 579 servos for throttle and choke control on a 5.5 HP 4 stroke Honda electric start motor. I was told these servos have steel gears and ball bearings.

I'm considering changing them for purpose made linear actuators. At the moment the servos are mounted on a plate attached to the motor. I was thinking of isolating them from the motors vibration by using push/pull cable to transmit the action from the servo to the throttle and choke levers. That way the servo could be sitting in a enclosure protected from heat, fuel spills etc.

The servos are cheap (er) and easy to control via picaxe.

A concern of using a R/C type servo is that there be a mechanical fail-safe mechanism. Be sure there is a strong enough throttle return spring that can 'over power' the servo in the advent of a control or servo failure.

Good luck
Lefty
 

Wrenow

Senior Member
BrendanP,

To answer your question, the hobby servos generally last pretty well in their intended and spec'd use. As already pointed out, there are a wide range of specifications available to meet specific needs. In aircraft and cars, the servo is generally mounted to the frame, not the motor, and many use a mounting system with a rubber grommet at the screws to further reduce vibration, so there is some isolation.

Mounting it directly to an industrial engine, I would want some protection from both vibration and heat. Mounting it remotely, and operating the throttle through a sheathed cable sounds like a good plan.

Servos can take an amazing amount of abuse, sometimes, but remember that the position sensor is a common (often carbon or film) pot, mounted directly to the drive spline under the servo arm. Keep that in mind when determining how to thermally or vibration isolate them.

Sounds like an interesting use, but failsafes are a really good idea.

Wreno
 
The specifications on the JR 579 Servo are:
  • Metal Gear
  • Torque at with 4.8V: 8.3kg/cm
  • Speed with 4.8V: 0.23s/60deg
  • Dimensions: 32.5x19x38.5mm
  • Weight: 48grams
  • Single Ball Bearing
Remote installation (pull/pull) will satisfy the need for protection from heat and vibration but it will increase the strain on the output shaft. Having a single ball bearing means that the other end of the shaft is supported by a bushing.
 

BrendanP

Senior Member
Thanks again for the input guys.

Marmitas, I take it then I should look for a dual ball bearing servo rather than single?
 

Wrenow

Senior Member
The specifications on the JR 579 Servo are:
  • Metal Gear
  • Torque at with 4.8V: 8.3kg/cm
  • Speed with 4.8V: 0.23s/60deg
  • Dimensions: 32.5x19x38.5mm
  • Weight: 48grams
  • Single Ball Bearing
Remote installation (pull/pull) will satisfy the need for protection from heat and vibration but it will increase the strain on the output shaft. Having a single ball bearing means that the other end of the shaft is supported by a bushing.
To be more precise, the other end of the shaft is usually the pot. It may or may not have a second ballbearing support on the pot shaft.

Wreno
 

BrendanP

Senior Member
Wreno et. al., where can I get the female socket, in PCB mount, that the male plug on the end of the servo cable plugs into?

Can these connectors, both male and female, be bought loose so cables can be made up in various lenghts?
 

BeanieBots

Moderator
When used properly (ie within spec) I find hobby servos will give many years of reliable use. Ignoring mechanical failure caused by over-stressing during crashes etc, the most common failure mode is wearout of the position pot feedback. This results in 'jerky' movements.
A hobby servo that has started to exhibit this failure can be restored by a spray of perfume on the track. The alchohol cleans and the scented oils lubricate. (makes them smell nice too).
I'd only ever use a 'recon' one in land based models though.
 

Wrenow

Senior Member
Wreno et. al., where can I get the female socket, in PCB mount, that the male plug on the end of the servo cable plugs into?

Can these connectors, both male and female, be bought loose so cables can be made up in various lenghts?
The female plug (yes you are using the right term for RC) is a simple dip header 3 pins long available almost everywhere that sells electonic bits in long lengths that you clip to the number of pins you need, both in single and double rows, straight and angled. They have no shrouds, of course, and are considered male connectors in electronics.

Yes, the servo connectors are available in DIY kits from R/C stores. If you do not have one locally that stocks them, Tower Hobbies does http://www2.towerhobbies.com/cgi-bin/WTI0098p?&N=N&F=AIRM1072&L=EMOM0015 &S1=&S2=&S3=&S4=&C=MHAEMO&V=&I=&G=2&U=Y&M=MH&P=7, (as well as pre-assembled pigtails). Parallax has a cable, http://www.parallax.com/detail.asp?product_id=805-00012 that is (in R/C terms) M/M with a F/F adapter that is pretty much the same thing. A couple of these in the box is a really good plan. The cable is also an adapter for the Vex Robotics servos and motors, which use the opposite polarity of plug from the rest of the R/C community.

Hope that helps.

Wreno
 
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