Servo use/abuse can it hold....

Hi,
For a project i am looking at the use of a hobby servo, but i was wondering if the servo would hold after extended use, and maybe which kind servo i
should use. I have been told a few times (and read on the net) that r/c servo's wont last when used extensively.
Some data: Continous load = 12 N/cm, this load will be continous, 24/24. This force will be rotational, so no force from the side, other then the gears inside the servo (on the output shaft). Resolution used/required = 180 degrees in 100 steps ( precise ). Environment: Will be used in a device which will go all over the world. Expected conditons are like on a containership, so salty air might have influence, so does humidity, cold, warm, dry. Device wil not get wet.
Been thinking of using a water sealed, dual bearing, non-metal gear type, with 2 or 3 times the torque rating required.
Anyone has extended knowledge of servo's used in these conditions?
Thankx.
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No knowledge but will be very interested in the results! Keep us posted...
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No knowledge but will be very interested in the results....keep us posted!
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Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge: As far as I know, there has been no study/applications of a servo under the conditions you describe.
Since the servo will have a "continuous load" a digital servo MAY be your best bet for longevity since they are designed to provide their "holding power"/torque under a partial movement, where an analog servo provides it's torque at full movement.
The biggest problems are going to be:
1. All the servos I am aware of are not "water sealed"/water proof
2. When you are talking about electrical items, the salt air WILL HAVE a major influence with corrosion. The cold, warm and dry will have minimal impact on the servo.
I suggest experimenting with it and definitely post your findings. I am certain there are many of us who would be interested in the results.

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Thankx for the reply Ted.
An digital servo is what i'm aiming for, specificaly for the stat torgue, and the precision. I did see a water sealed servo, but i have to dig to find it again, i'm not sure though if it is really nescessary in the application. I have a very experienced source if it commes to salt air and electronics, you be suprised what the best solutions are.
I'll try to keep you guys updated.
Thankx.

wondering
inside
type,
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Mich wrote:

You haven't specified the required lifetime for the servo, but frankly, it wouldn't be hard to set up a test rig, perhaps with a larger load to accelerate failure.
What may be hard to imitate is salt ingress - for example, a hot servo when engulfed with salt water may suck in moisture as it suddenly cools.
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when
Thankx for the reply Poxy.
I'm looking at a lifespan of between 1.5 ~ 2 years. But the main thing is that the servo will not be running all the time, it maybe does a 180 degree turn 300 times in a year, so thats in 2 years only 600 times. Not too much i suppose. I did setup a testrig, with a simple Hitec HS-300 servo, which i let wind up a elastic band in two directions, to the point the servo couldn't pull it any further, controlled by a little microprocessor. I expected the servo to desintegrate in a hour or so, but it lasted days and days, and its still not faulty, or even making odd sounds at all. I think i let it run 3 days of 8 hours.
The servo will be stored in a closed box (not sure if i will ventilate), so i don't expect to many trouble with salt air, and since the thing won't be running like mad, i won't expect it to get hot at all.
M
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| I'm looking at a lifespan of between 1.5 ~ 2 years. | But the main thing is that the servo will not be running all the time, it | maybe does a 180 degree turn 300 times in a year, so thats in 2 years only | 600 times. Not too much i suppose.
Your original post suggested that it would be running all the time. I was picturing it fighting against a specific torque 24/7, with the torque trying to turn the servo one direction, and the servo trying to keep it from moving. In that case, a small servo would sit there buzzing, and a large one (or high torque anyways) might be able to stop the motor entirely because the gear train itself produces enough friction to not require the motor to stop things.
If the torque is only in effect while the servo is moving -- i.e. from friction -- then things get a _lot_ better.
| i don't expect to many trouble with salt air, and since the thing won't be | running like mad, i won't expect it to get hot at all.
Probably not.
--
Doug McLaren, snipped-for-privacy@frenzy.com Condense soup, not books!

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it
only
I'm sorry i wasn't clear enough. My bad.
Most of the time the servo will be sitting still, just holding. and indeed, if the gearing is sufficient, the holding off the motor itself wont be that big. Very small movement might be needed so now and then, so this is where the digital servo would come in handy.
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| I have been told a few times (and read on the net) that r/c servo's wont | last when used extensively.
I suspect that this is true. But have no personal experience with it.
| Some data: | Continous load = 12 N/cm, this load will be continous, 24/24.
Actually, the correct units would be 12 N*cm, not 12 N/cm. (torque force * distance.) I realize that this is little more than a nit-pick, but the difference may very well trip you up when you convert from the oz-inch figures that most servo manufacturers give in the US.
12 N*cm = 17 ounce*inch, which is pretty small. Even a standard $8 servo could easily handle this, but it's not meant to do it 24/7 so I don't know how long it would take to burn out. Probably not long.
Your best bet would be to get a servo that can create MUCH more torque than you need, perhaps one that's been modified for extreme torque and slow speeds? Using things like this --
http://www.servocity.com/html/robotzone_servos.html
sounds good. The gearing would reduce the load on the motor.
Digital servos can generally provide more torque than an equvilently sized standard servo, but I don't know if they'd stand up to constant use any better or any worse.
| This force will be rotational, so no force from the side, other then | the gears inside the servo (on the output shaft). Resolution | used/required = 180 degrees in 100 steps ( precise ). Environment: | Will be used in a device which will go all over the world. Expected | conditons are like on a containership, so salty air might have | influence, so does humidity, cold, warm, dry. Device wil not get | wet.
I'd seal every possible point that it could leak with silicone seal if at all possible. You say it won't get wet, but even salt air can be bad on anything electronic.
| Been thinking of using a water sealed, dual bearing, non-metal gear type, | with 2 or 3 times the torque rating required.
I'd go for 20x + the torque rating required. Our servos are meant to run for a few seconds at a time. Running for days at a time, I'll bet they wouldn't even last a single day (perhaps not even an hour), even running at 1/2 capacity.
--
Doug McLaren, snipped-for-privacy@frenzy.com
"Verbosity leads to unclear, inarticulate things." --Quayle
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Thankx for the reply Doug.

Thank you. If you start something, do it well. So if its N*cm, it will be N*cm.

You can read in my other reply about a test i did.

I have seen that, it is to big though for what i need it for. But then, there are servo's capable of 180 N*cm for good prices, so i suppose that should be sufficient.

Me neither, the main thing i worry about is the more sensitive circuit compared to the analog ones.

Yup, i have a source who has wide experiene with this, so i should be oke.

type,
Thankx again.
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I'm thinking a digital sail servo, but I've never heard of one. mk

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Another question comes to mind. That is how much space do you have? I
you go to Servocity.com, they list the Hitec standard servo using 160m operating at no load. I don't know how much current your continuou load would place on a servo, but using this as a consevation figure 160ma x 24hrs = 3.84 Ahr. Since you mention container shipping, imagine time frames of weeks if not months, now that would be a bi battery
-- sfsjki ----------------------------------------------------------------------- sfsjkid's Profile: http://www.rcgroups.com/forums/member.php?action=getinfo&userid870 View this thread: http://www.rcgroups.com/forums/showthread.php?threadid6053
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Hi sfsjkid, thankx for the reply. ( i wonder how your mother called you for diner??, just kiddin)
I gave the container shipping as a example, it will not be battery operated at all (thank god).
M
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| Another question comes to mind. That is how much space do you have? If | you go to Servocity.com, they list the Hitec standard servo using 160mA | operating at no load. I don't know how much current your continuous | load would place on a servo, but using this as a consevation figure, | 160ma x 24hrs = 3.84 Ahr. Since you mention container shipping, I | imagine time frames of weeks if not months, now that would be a big | battery!
http://servocity.com/html/hs-311_standard.html says --
Current Drain (4.8V): 7.4mA/idle and 160mA no load operating
`no load' in this case means that the servo is moving as fast as it can -- so unless you keep moving the sticks, it only stays like that for a fraction of a second.
So if the servo isn't moving at all, 7.4 mA. Not much.
If it's fighting a load, the current usage will be much more than 160 mA, at least while it's moving. Once it's done moving, if the load is still pushing back on it, what happens will depend on how much friction there is in the gears and motor. If little, it'll sit there buzzing, using lots of power and wearing out quickly. If a lot, it'll probably find a place where friction keeps it from moving, and the current usage will be 7.4 mA again.
Depending on the application, the best bet might be to remove the potentiometer and apply it directly to whatever is being moved rather than doing it in the servo if precision is required. But things start getting tricky ...
Either way, the robotzone servos look ideal. I wouldn't mess with a digital.
--
Doug McLaren, snipped-for-privacy@frenzy.com
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When I read the thread post, I'm wondering what will be the application?
If you are looking for actionning a device like a lock for a door or anything else, try to transpose the locking mecanism of a mechanical plane gear.
The servomotor is working only for unlocking moving locking. All the load at rest is supported by the locking mecanism. So there is less electrical load and thermal effect on the servo on the long term.
Hope to help you in your quest…
MLB
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Mich wrote:

180 degrees means retract servo I think.
Get the biggest you can, and seal it as best you can. Maybe slip a condom over it completely and fill with silica gel.
Resolution should be adequate.
Weak point will probably be the carbon (film) potentiometer - worth seeing if any are made with moulded carbon track or retrofitting your own 'upgrade'
Most servos die from either major trauma or pot tracks going scratchy leading to jitter and hesitation about usually the neutral point. If yours is not spending much time there this should help a lot.
Motors, gear trains and electronics should not be overly stressed if you operate well within limits, but the output sensing pot is always the weak link.
Yopu might consider an external feedback pot of considerable quality if the application allows it.

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wondering
inside
type,
Thank you for your reply.
I have noticed some servo's with 6 finger/track potmeters inside, i suppose those would be much better to use then the simple ones.
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RC sailboat servo used for the mainsheet control. It's at least water resistant has tremendous torque/holding power.
--
Tally Ho!
Ed
"Mich" < snipped-for-privacy@xs4all.nl> wrote in message
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I've seen Futaba servos used extensively on government UAV's. And to my knowledge, nothing abuses a servo more than an R/C helicopter. Take it for what it's worth!

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