Thread: lifespan of servos question

OK, my question is simply this; How long should we expect servos to last? Is there any rule of thumb about when it might be wise to change them even if they're still working?

Today I had a crash due to a servo failing. It's a JR DS811 and is on the aileron of a Raptor 50. I have had this Raptor for about a year and a half and bought it 2nd hand with these servos on it and up to now they have all worked fine and have given no hint of potential problems.

At the time of the failure I was just doing a gentle circuit when the heli suddenly began to roll on its own and I had to flip in some negative pitch to stay airborne and then hope it would roll back around upright again so I could set it down. I did manage to get it set down more or less on its skids but there was still a fair amount of damage.

Back in the pits everything was working except the aileron servo, it appears the motor in it has just decided to expire.

Are any of these servo testers able to spot a potential problem?

I don't change servos in helis or planks without need.
According my experience they don't break down if you use them also over a very long time especially in planks.
But be very sensible after any incident and watch the gears.

There is a fine line between infant mortality and end of life. Can you rely on a servo not to fail 10 mins out of the box.... Not really is there a chance it will fail in flight Definatly. so there is a period between the two that is the service life. The only way to test this is to excerize the servo to establish the life (assumedly in tens to hundreds of hours)

You will need a little jig and a new crappy servo say an S148. Set the servo up with a long arm on and set a pair of springs on the opposing ends of the arm to a point where each is partilly streched. Use a spring balance and try to see the applied force at end of travel. This needs to be a reasonable proportion of the rated load say between 50 and 75%.

Set up a servo excersizer and run on a mains power for as many days as it takes to stop working. Make an assumption that all servos will last the same ammount of time (they probably wont) and devide by 2 its what the medics do to establish LD50 (50% of the leathal dose) they should be working at that point in time its time to change them.

Its a reasoned argument that taken to the logical conclusion you could use on any servo type but beware there are vast differences in types and you need to group by type fro example brushless ones will have a longer MTBF than brusshed ones there in no mechanical contact in the motor conversly there is more electronics.

You could of course get MIL217 handbook and calculate the MTBF for all the servo parts and reach a theroectial Mean Time Between Failure based on the experiance of others for each componant within the servo and that includes all the electronics but it probably wont stop your heli crashing because conditions in a model heli are rarely 'Ground Benign' and these need definition in order that the calculation is valid...

Enjoy

Steve

Hi Sheryl,
Sorry to hear that your Raptor chewed the turf

Good servos should last for years in a plank but with the higher demands placed on a cyclic servo it's reasonable to expect that it'll be a cyclic one that gives out eventualy. Unfortunately, it's kind of unavoidable as in the case of your servo packing in and very hard to diagnose before the fact and take remedial action. Most electronic and mechanical devices on a heli are subjected to high loads and demands and it's fair to say that none are imune to failure. I guess the older a heli gets, the more likely it is that something will give out when you least expect it, so crashing isn't as bad as it seems, it's just an inconvenient and expensive way of being forced to replace all those old and worn parts

I don't use a servo tester myself but as far as i know, most are designed as a means of checking servo direction, centring, travel and speed without the need to have the servos hooked up to a spare reciever so i don't think they'll tell you much about the condition of a servo motor that you can't see by simply using the Tx and Rx to move the servo. There may be more advanced testers available somewhere but i haven't come accross one yet. I suppose those with knowledge of electronics could probably read the condition of the motor if they have the comparison of the readings from a new servo?

I had regular servo failures in the 450 with HS 65. The motors burn out - details aren't clear to me, they still move but lose their torque and get hot.
I've had two servo failures in the plastic elevator mount of the T-Rex 700: One Align DS 610 and one high-and-mighty JR 8915 (or 8717, can't remember).
The lesson here is, allow adequate cooling, especially in a flybarless 700.

My own "law of servo failure" is that the probability is inversely proportional to the size of the servo:
sub-micro servos will fail if you look at them sideways
micro servos do fail occasionally
fullsize servos usually don't fail

Unfortunately, all this doesn't help you too much
About three years ago I got really p*ssed off because after crashing once with HS 65 failure, the next servo failed soon after, in the same manner. Apparently the swash servos had either reached end-of-life simultaneously, or had gotten similar amounts of abuse from earlier "incidents".

After a crash, I move the servos with the radio and apply force with the other hand on the servo horn. If you can make it twitch erratically, the servo is broken.

I'm an electronic engineer and I've had a few servo failures since moving onto helis. I had none in the twenty years before. My problems have been due to vibration and inaccurate assembly of the servos. Servos are not designed to be screwed to the frame, they are meant to be mounted with rubber grommets. The ones that have failed with me have been due to vibration induced fatigue failures where the flexible internal wire meets the solid solder wicked up from the solder joint. The wire is usually free to move but the solder is fixed. If you take the servos apart you can check for this failure. The cure is to attach the wire to the pcb at this junction with a flexible silicone type glue, bathroom sealant. You will probably find blobs of this glue inside the servo, but unfortunately the assemblers are under too much time pressure to put the glue in the right place. The worst fault is the black wire that goes from the board to the motor. When this breaks the servo can move on its own and cause blade strikes. To be fair the blade strikes only happened twice with the servos that came with my Twister Storm and I did have bad vibration problems caused by the frequent contact between the heli and the ground.

Chris

I would have said that the servo spec was not up to the job JR DS811 Servo Specifications and Reviews if you also had one on the collective I am surprised it done the job asked of it, simple solution is to use decent servos, the ds811 would be ok in an old plank but not in a heli.............

Originally Posted by jwatts007

I would have said that the servo spec was not up to the job JR DS811 Servo Specifications and Reviews if you also had one on the collective I am surprised it done the job asked of it, simple solution is to use decent servos, the ds811 would be ok in an old plank but not in a heli.............

Hmmmm, I'm afraid you are incorrect (and no I don't have one on collective)

MacGregor Industries Modeller Support Centre - Frequently Asked Questions


The DS811 is basically the JR equivalent of the Futaba S3152. Granted it's not a very high torque or high speed servo
but it's not like it's a big no no to use in a heli for sport flying. I suppose about the only servo I could have used where
someone wouldn't come along and say "that's what you get for using a crap servo" is if it was a Futaba BLS or high end
JR.

I'll add that in the time that I've been flying I have only had one other crash due to a servo failing and that was with
a 450 that really did have a cheapy servo in it. In other helis I have Align, JR, Futaba, Hitec, AceRC, Acerlab, MKS
and Savox servos that have all worked perfectly.

6.0V:

66.0 oz-in (4.75 kg-cm) the spec for this servo is just not good enough for your Raptor 50, with a ecppm heli you may just get away with it, have had this conversation with fellow pilots before who are trying too save a few bob on there helis, this time you were lucky the servo gave out away from yourself and others, next time though there could be an accident, throw them away and go over spec and be safe.........................

Originally Posted by jwatts007

6.0V:

66.0 oz-in (4.75 kg-cm) the spec for this servo is just not good enough for your Raptor 50, with a ecppm heli you may just get away with it, have had this conversation with fellow pilots before who are trying too save a few bob on there helis, this time you were lucky the servo gave out away from yourself and others, next time though there could be an accident, throw them away and go over spec and be safe.........................

Well, I take your point but in this case the servo did not fail because it was over stressed, it was well within it's design limits at the time. In fact I was barely giving any aileron input at the moment it failed, just in a very gentle circuit. The cause was either a cumulative vibration issue or something else. So although I agree that generally speaking it's a good idea to have over spec servos, in this case the servo was not at all pushed too hard. (and if you're really convinced you're right you might want to correct MacGregor as they seem to have a difference of opinion).

Originally Posted by trillian

Well, I take your point but in this case the servo did not fail because it was over stressed, it was well within it's design limits at the time. In fact I was barely giving any aileron input at the moment it failed, just in a very gentle circuit. The cause was either a cumulative vibration issue or something else. So although I agree that generally speaking it's a good idea to have over spec servos, in this case the servo was not at all pushed too hard. (and if you're really convinced you're right you might want to correct MacGregor as they seem to have a difference of opinion).

1 The servo was over stressed, more so as the Raptor 50 is mcppm.
2 The servos are used at all times while in flight just to hold position.
3 MacGregor state that this servo is for use in a 30 sized machine.
Horizon Hobby: DS811 Advanced Sport Digital Servo by JR (JRPS811)

I wouldn't have used an 811 on a 50 size machine. It would be OK in a smaller model - up to 30 size maybe - but anything bigger would be pushing it.

I haven't changed the servos in my Vigor since I built it - and Lord knows how many years ago that was! But they were all decent spec in their day (4000s and the like) and have lasted well.

I do have some servos that are nearly 40 years old, and still working fine, but I wouldn't use them in a critical high load application!

Originally Posted by jwatts007

1 The servo was over stressed, more so as the Raptor 50 is mcppm.
2 The servos are used at all times while in flight just to hold position.
3 MacGregor state that this servo is for use in a 30 sized machine.
Horizon Hobby: DS811 Advanced Sport Digital Servo by JR (JRPS811)

Well, I took your point and stated my side politely. It really didn't need to go any further but if you want a tit-for-tat point scoring game with me then I shall now dismantle your argument completely.

Sorry but you are just plain wrong. The servo was NOT over stressed, please prove to me that it was and back up your position with something factual. You are quoting something from Horizon not MacGregor. MacGregor DO list the DS811 for use in 50 helis (they designate it for 'sport' use, which is what it was in my case). I have posted this already but will do again for your convenience;
MacGregor Industries Modeller Support Centre - Frequently Asked Questions

If you feel the servo was over stressed I suggest you find reliable data on how much torque is required to hold position on a flybarred 50 heli and then you can use this data in your letter to MacGregor. Otherwise you're just talking out your proverbial posterior.


May I suggest you begin your letter to McGregor something like this; "Dear MacGregor, I know you have been in business for something like 50 years and have been a long time distributor of JR radio equipment but I have been flying for about a year or so and I had a discussion with my mate about this and we think you should change your website to say something along the lines that the DS811 servo is only suitable for an old plank and shouldn't ever be put in a heli"

Originally Posted by trillian

Well, I took your point and stated my side politely. It really didn't need to go any further but if you want a tit-for-tat point scoring game with me then I shall now dismantle your argument completely.

Sorry but you are just plain wrong. The servo was NOT over stressed, please prove to me that it was and back up your position with something factual. You are quoting something from Horizon not MacGregor. MacGregor DO list the DS811 for use in 50 helis (they designate it for 'sport' use, which is what it was in my case). I have posted this already but will do again for your convenience;
MacGregor Industries Modeller Support Centre - Frequently Asked Questions

If you feel the servo was over stressed I suggest you find reliable data on how much torque is required to hold position on a flybarred 50 heli and then you can use this data in your letter to MacGregor. Otherwise you're just talking out your proverbial posterior.


May I suggest you begin your letter to McGregor something like this; "Dear MacGregor, I know you have been in business for something like 50 years and have been a long time distributor of JR radio equipment but I have been flying for about a year or so and I had a discussion with my mate about this and we think you should change your website to say something along the lines that the DS811 servo is only suitable for an old plank and shouldn't ever be put in a heli"

You illustrate the problem women have, in just not being able to understand that they are wrong, now listen, your servo was not good enough for the job it was asked to do, that is why it failed, simples, even a women should understand it, "but I very much doubt it", if you had invested more in some decent spec servos your helicopter would not have fallen from the sky and broke, and probably have saved you some money as well............

Originally Posted by jwatts007

You illustrate the problem women have, in just not being able to understand that they are wrong, now listen, your servo was not good enough for the job it was asked to do, that is why it failed, simples, even a women should understand it, "but I very much doubt it", if you had invested more in some decent spec servos your helicopter would not have fallen from the sky and broke, and probably have saved you some money as well............

Oh you are a character! Well I think I am beginning to grasp the concept now that you have worded it in such a way that the logic cannot be disputed.

How about if I replace it with an Align DS610? and I'll replace the elevator servo while I'm at it. Those would seem to be well up to the task at hand.