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  1. #1
    Moderator aeajr's Avatar
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    Let's talk about servos - Whatever is on your mind

    Let's talk about servos - Whatever is on your mind
    When I was a new pilot I was confused about servos, how they were used, how they were mounted and how to pick them. So let's start a discussion about servos for all the new pilots.

    Let's talk about types, sizes, how to pick them, etc., whatever you like.

    Here are some general comments to get things started.

    Servos are rated by:

    Size - Length, width and depth
    Strength - Measured in inch/ounces of torque
    Speed - Measured in degrees per second
    Weight - Ounces or grams (28 grams to the ounce)

    For illustrations I will use Hitec's servo line but my comments apply to other brands as well.

    Hitec web site -
    http://www.hitecrcd.com/


    Bearings

    Some servos have no bearings. The plastic case essentially holds the shaft and supports it against pressure in use. The Hitec HS-55, HS-81 and HS-82MG servos would be good examples of servos with no bearings. They are very inexpensive and work quite well when new, but over time they wear and tend to lose their centering ability. However for light duty sport use they are fine.

    I have many planes with these and similar servos of other brands. However today I generally buy servos that have some kind of bearings. These hold center better over time and move more smoothly over time.


    For example a Hitec HS-85 BB is about the same size but a little more expensive than a Hitec HS-81 or 82. If I were putting a new sport plane/glider together I would chose an HS-85 over an 81/82 because the HS-85 has bearings to support the shaft.


    Another example would be the HS-55 which does not have bearings. The Hitec HS-45 and HS-65 have bearings and can be used in similar situations. Partially because of the bearings they should center better and run smoother over a longer useful life.


    Analog vs. Digital Servos

    There are standard or analog servos and and there are digital servos. I will use Hitec brand again as examples but I also use JR, Airtroncis, Spektrum, Futaba and other brands.

    Analog servos are typically your lower priced servos. They work well in most sport applications.

    Digitals would be my preferred choice for any form of a competition plane or high speed planes.

    For me the main benefit of digitals is more precise centering. They can also tend to hold position better under force, but they will pull a lot of power to do it.


    If you are flying at 150 mph you don't want those control surfaces being blown back because the servo can't hold position. And in any kind of competition you want those servos to go to the correct center or offset EVERY TIME and hold there solidly. Small variations can be a real issue in competition.

    Some brands, like Hitec, have basically the same servo in analog and digital. For example:

    Hitec HS-85MG - Analog
    http://www.hitecrcd.com/products/ana...i/hs-85bb.html

    Hitec HS-5085MG - Digital
    http://www.hitecrcd.com/products/dig...hs-5085mg.html

    They are the same size, same spline, same ball bearings, same gears, but have different control boards and maybe a different motor. You can swap one for the other in your plane and they will fit.

    In my competition planes is it all digital servos with bearings. I use Hitec, JR and Airtronics digitals. I have to have solid centering in these ships.


    Spare Servos

    I always keep spare servos in my field box. If I am going to an away contest I will have the servos that match the digitals in the plane along with me. But some of my servos cost $50 or more, so keeping lots of spares spares in the box, hopeful that I will never need them, can be expensive.

    I still have some of the Hitec HS 81/82 and HS-55s. I at least one of each in my field tool box as emergency spares. They are cheap spares to get me through the day at the home field and work well enough and can cover a wide range of applications. Best of all they are low cost so they do not represent a big investment to just toss in the tool box for unexpected field repairs.

    Just this past weekend I had a servo strip in my Radian. I pulled out an HS-81 and in 20 minutes I was back in the air. They will keep me flying if I need one at the field. And if a friend is in need I can offer them a servo without breaking their bank or mine.



    So, what do you know about servos that you would like to share?


    What questions do you have about servos?

    Let the discussion begin!


    Edit: 10-2012 - Iadded a servo calculator spreadsheet to this post. You can use it to estimate what size servos you need for most applications. Isuggest at least a30%+ margin over anything recommended by the calculator to insure you have what you need in terms of servo strength.

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  2. #2
    Saburo Sakai's Avatar
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    RE: Let's talk about servos - Whatever is on your mind

    First, thank you for starting thisthread on these fascinating little powerhouses!
    In a Spektrum transmitter manual under "servo precautions" it states: "never lubricate servo motors or gears".
    I live on the Gulf Coast where salty air eventuallypenetrates through the walls of our houses. Despite taking precautions to protect my birds,corrosion on all things metal is inevitable. What's your take on a drop of LPS-2 on the shaft of the servo? One bird of mine, a microT-28 PZ, took a brief salt water bath in the bay courtesy of a gust and my ineptitude. I dried her, sprayed a light blast of water-repelling LPS-1 on the circuit board, wiring and anything metal, and put a drop of LPS-2 on the prop motor shaft and the servo motor shafts andthey've performed flawlessly for a total time now of maybe a half hour of actual operational "go" time. By the way, these tiny micro-servo motorsare something altogether brilliant. They must use nanobots to make them! So tiny!
    I apply any lubricant sparingly and would stop short of having any motor swimming in lubricant, of course. If there were some preventative shield that prevented the salt ion from bonding in the first place, that'd be great-a teflon barrier or something.Then again, there's the wiring and circuit boards. Thoughts? Again, thank you for this thread venue! Godspeed!
    There are Leaders, followers, and Troublemakers!
    -Capt. J.L.D., U.S.Army (ret.), my dad.

  3. #3
    Moderator aeajr's Avatar
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    RE: Let's talk about servos - Whatever is on your mind

    Normally I don't apply any lubricant to servos as they come lubricated from the factory.

    However dropping things in salt water changes everything.  I have done that with slope gliders.

    1) Wash EVERYTHING in clear water and dry it completely.  If the servos were submerged I would open them up to be sure no water got in. If it did, wash it as everything else. A blow dryer on low is great to get water out of places you don't want it.

    2) I then wash again in alcohol to help get the water out.  After it dries I again use a blow dryer on low.  Then I let it air dry for 2 days to be sure.

    3) I have used electronics spray, intended for the purpose, to treat circuit boards, receivers and such AFTER things have dried.  If I open the servos I relube them with servo grease and put them back together.

    Usually the servos, receiver and such are fine.  I have had batteries corrode despite my best efforts.

    I have never dropped a motor in salt water but I would give it the same treatment as everything else.  Wash it, dry it, alcohol, dry and then air dry for days to be sure.   Then run it till it gets very warm and listen to it carefully to be sure there is no grit or other residue in the motor.

    After you have done your best - put it all back together and fly.
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  4. #4
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    RE: Let's talk about servos - Whatever is on your mind

    My pet peeve.. Manufacturers of kits and Arfs that put in their specs for the proper servo as "mini" or "micro" or "standard". If you look at all the available servos the torque ratings can overlap by quite a bit. The size of the servo is irrelevant as compared to torque.

    I just wish they would give the required torque needed to make finding a servo much easier.

  5. #5
    Moderator aeajr's Avatar
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    RE: Let's talk about servos - Whatever is on your mind



    As a General guide for such designations.  These would be approximate strength I would use for torque for these designations

    Standard Servo - 40 ounce/inch - Example, Hitec HS-311

    Mini  Servo - 25 ounce/inch - GWS Naro Max BB

    Micro servo - 15 ounce/inch - Hitec HS-55/65

    Sub  micro servo - 7 ounce/inch - GWS Pico servo


    These are not industry standards, they are my own interpertation of what these terms would mean in terms of servo strength. 

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    RE: Let's talk about servos - Whatever is on your mind

    Servo are a pet issue of mine.
    Why do people use such big ones!
    It is quite common to use a servo with a torque rating that could easily lift the whole weight of the plane.

    I doubt if a full size 3D aerobatic pilot could lift even a tenth of the planes weight with one arm!
    Even a fully loaded B29 (120,000 lbs) still only had one man arm power but to be fair it was considered hard work to fly.

    On this basis even a 10lb 150mph plane should not actually need more than micro servos.

    I am not for one minute suggesting such a dramatic step but with a bit of sound engineering and aerodynamics it is perfectly possible to use smaller servos which can easily be installed close its surface, thus improving mechanical efficiency and saving weight. There are also advantages in reduced current draw, lighter servo cables, smaller batteries etc.

    Rant over!


  7. #7
    Moderator aeajr's Avatar
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    RE: Let's talk about servos - Whatever is on your mind

    A servo torque calculator is posted on the first page of this discussion if you care to look at it.

    It occurs to me that if I hook up my wattmeter I should be able to read the draw of my servos.   When I get a chance I will try that.

    If someone else had the time, please give this a try.  Remember that at rest the strain on the servo should be extremely low.  It is the air pressure against the surface that creates the load so use your hand to create some light pressue and watch the numbers.
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  8. #8
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    RE: Let's talk about servos - Whatever is on your mind

    I used an ammeter to measure system draw without the motor on. That seemed to work well. With a 6-channel micro receiver, two HXT900s, and an 18 amp ESC, I measured about 40ma with the servos at rest. The total load would increase to about 150ma when one servo was exercised. However, I didn't load the control surfaces during this measurement, so the numbers should probably be bit higher in flight. That said, I don't think my servos are straining much to power the elevator or ailerons. They are technological marvels.
    Tim

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    RE: Let's talk about servos - Whatever is on your mind

    It has taken me some time to learn that you do not need to go way over board on the servos. I used to use a standard servo when a micro would do.

    I also always go over board on the Rx pack. I like the 5 cell 6v packs. Lets the servos work to their best.

    I have given up on the plastic 9 gram servos. They just can not take the abuse they get in small foam planes. I have found the the EXI digital metal gear 9 gram servos work great. Plus they are $7.70 each. I use these when ever I need a 9 gram servo.

    For standard sized servos I use the SG 5010's They are less then $5.00 each and work great. I have almost 40 in the fleet now. Two helicopters setup with them and a load of planes. I have 5 of them in a 60 sized trainer i put an electric motor in. The ESC has a 3 amp BEC and I have not had an issue with the power.

    I flew the "OLD" Airtronics 94102 servos for a long time. They have been abused over and over. To save a buck or two I would order the BB conversions for them. The 94122 was the BB version and sold for $22.00. I could get the 94201 servo and a BB conversion for $15.00 a servo. So when I setup a plane with 5 or 6 servos, that was like 2 free servos .

    I do like all of the good information in this thread. I may have to make an adapter for my watt meter and see what the servo draw under load.

    Buzz.

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    RE: Let's talk about servos - Whatever is on your mind

    I'm using this booster tab on the oversized rudder on my old Zoombi. Servo is a HXT500. Works very well taking the load off the servo.
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    Gord
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  11. #11

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    RE: Let's talk about servos - Whatever is on your mind

    I am trying to find out if any one has used these servo's?

    https://www.leaderhobby.com/product....=9394001225424

    https://www.leaderhobby.com/product....=9394001225425

    I may order a couple to see if they are any good.

    Buzz.

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    RE: Let's talk about servos - Whatever is on your mind

    As I peruse the net looking at more and more servo's, I find there are a lot of servos rated at extreme torque. I amt getting ready to do an electric conversion on an oldie but goodie. It is a Model Tech Formula 3D. It is a 60 to 90 sized 3D plane so I am looking for high torque servos. I know I can order the Futaba, JR, Airtronics but at over $100.00 a servo, I am looking for a more modestly priced servo. I also have a Ripmax 1/4 scale Pitts S2A I will be converting so I need the same servos for it.

    I have found the PZ servos from Leaders Hobby for $23.00 each and Hobbypartz has these new Solar servos for $23.00 each.


    https://www.leaderhobby.com/product....=9394001225425

    http://www.hobbypartz.com/33p-solarservo-d772.html these are rated at 4.8v 305 ounce, 6.0v 368, 7.4v 417 ounce. I wonder how close they really are to those ratings?


    Buzz.

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    RE: Let's talk about servos - Whatever is on your mind

    So order two of each and test them!

    A concern can be stall current. I really don't think that you will need all of that torque. I'd likely route the servo power such that the receiver does not see the current.

    The Mfrs manual for the Model Tech Formula 3D is on the web. It calls for HiTec servos. The particular one had a bad rep at one time.
    Using the servo referenced in the manual for specs, you can find quite a few alternatives.
    The manual spec'd servos that are quite a bit faster than those you were looking at, and have much less torque.

  14. #14

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    RE: Let's talk about servos - Whatever is on your mind

    I know I am in the very small group, but I have not had much luck with Hitec over the years. It has been either really great or really bad, no in between.

    I have servos I can use, the SG5010's will be more then strong enough for the Formula 3D. They put out 160 ounces at 6v. I have these in 4 planes now without an issue. They center great, plenty of power and do not cost much.

    http://www.hobbypartz.com/kahaoubrmo14.html

    I was hoping to see if any one out there had used the servos. I have a few planes I could use the extra power of those servo's.


    Buzz.

  15. #15
    Moderator aeajr's Avatar
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    RE: Let's talk about servos - Whatever is on your mind

    Sorry I can't help. Most of my experience has been with mini and micro servos for small electrics and gliders.

    My overall experience with Hitec servos has been pretty good.
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    RE: Let's talk about servos - Whatever is on your mind

    "never lubricate servo motors or gears"

    I'd have to say that as general advice that's sort of true, but very small amounts of a proper lubricant in the right places can do wonders.

    Plastic gears may not "like" lubricating oils or light grease. The plastic may become brittle. There are "dry" "food grade" lubricants available.
    Unsealed potentiometers may fail if contaminated by dry or wet lubricants.
    Oils can thicken with age and even get "gummy". Oils can also attract dust and grit, causing wear out much sooner than expected.

    Brushed motors don't generally "like" lubricants on the brushes, although there are specialized products made for
    model racing car and electric train motor brushes. The products tend to allow the brushes to seat properly, and remove brush residue from the commutator.


  17. #17
    Moderator aeajr's Avatar
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    RE: Let's talk about servos - Whatever is on your mind

    I use servo grease whenever I rebuild servos. 
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    RE: Let's talk about servos - Whatever is on your mind

    Same here. Use synthetic grease like white lithium as it won't hurt the plastic gears.
    Gord
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  19. #19
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    RE: Let's talk about servos - Whatever is on your mind


    I still am using the first made HS-55s in planes. Planes come & go. The 55s last.

    Had 1 bad one. a buzzer.

    74 years.[X(]

  20. #20
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    RE: Let's talk about servos - Whatever is on your mind

    I added a servo calculator spreadsheet to the first post, for anyone who is interested.  I did not write it, but the numbers seem reasonable.  When it doubt, this might be useful in sizing your servos.
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  21. #21
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    RE: Let's talk about servos - Whatever is on your mind

    Analog vs Digital

    Analog servos only produce full torque when nearing their commanded position. Digital servos produce full torque at all times, thats why they hold better and stay centered or stay where you tell them to go during high stress manuevers.

    While the servo gets its position update at 50hz from the reciever, the analog servo updates the motor position at that rate.

    With a digital servo, the digital amplifier updates the servo motor at 270 to 300 times per second which is why its always producing full torque (and the reason they buzz at rest with even the weight of a control surface on them). The downside of course is that this motor updating consumes power so they use more Mah than standard servos.
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    RE: Let's talk about servos - Whatever is on your mind


    ORIGINAL: Quorneng

    Servo are a pet issue of mine.
    Why do people use such big ones!
    It is quite common to use a servo with a torque rating that could easily lift the whole weight of the plane.

    I doubt if a full size 3D aerobatic pilot could lift even a tenth of the planes weight with one arm!
    Even a fully loaded B29 (120,000 lbs) still only had one man arm power but to be fair it was considered hard work to fly.

    On this basis even a 10lb 150mph plane should not actually need more than micro servos.

    I am not for one minute suggesting such a dramatic step but with a bit of sound engineering and aerodynamics it is perfectly possible to use smaller servos which can easily be installed close its surface, thus improving mechanical efficiency and saving weight. There are also advantages in reduced current draw, lighter servo cables, smaller batteries etc.

    Rant over!

    Full size A/C with mechanical control linkage often use various methods to reduce the force needed to move the control surfaces.
    (Tabs that move, offset hinge points, parts of the control surface ahead of the hinge point and so forth.)
    Such things are less common on models. Control surfaces are also often oversized on models.


  23. #23

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    RE: Let's talk about servos - Whatever is on your mind

    I use booster tabs on a few planes. These are on a 1/4 scale Corby Starlett on the ails and rudder. The other is on a foamy Zoomby and a few othars with an oversized rudder. Both were tried with and without to see the difference. In all cases it was like putting an oversized servo in them with much more speed. I don't know why they aren't more prevalent in the bigger 3D planes where they're using two and three servos in the ails. Could save a lot of weight in servos and batts.

    Gord.
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    Gord
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  24. #24

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    RE: Let's talk about servos - Whatever is on your mind

    Well I just received my order for (5) solar D772 high torque servos. I will be setting up a test rig to see how well they center and just over all how well they work.

    For less then $18.00, these good for the budget if they work well.

    http://www.hobbypartz.com/33p-solarservo-d772.html


    Buzz.

  25. #25
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    RE: Let's talk about servos - Whatever is on your mind

    Those are huge servos. What plane are they for? At 2.2 oz each 6 of those weigh more than some of my planes.

    I look forward to your report.
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