XJet

I've just come from a day of jet-flying where a guy had to give up testing a new plane after one of his rudder servos (a Hitec mini-digital) stripped its gears.

The gears stripped because the servo was stalled after inadvertantly being given too much throw.

Now excuse me but it seems totally stupid to design a servo that is capable of self-destructing if stalled.

What is the damned point in producing servos with xxx oz-ins of torque when the geartrain shatters at just xx oz-ins?

Why on earth can't the people who design these servos make sure the gear-train is up to the task -- or derate the torque accordingly?

Given the choice between a servo that might stall under a heavy load - thus be unable to move the control surface any futher -- or one that simple granades its geartrain under the same conditions -- I'd always opt for the former -- there's a much better chance your plane will life to fly another day.

There seems to be an inordinate amount of focus on making and marketing servos that are supposed to have so much torque that , if the geartrains didn't self-destruct before stalling the motor, they'd probably rip the mounting lugs right off the cases.

Twenty years ago, the average RC servo was amazingly weak -- but the *only* time you broke a gear was if you had a really bad crash. These days, it's not at all uncommon (at least with some Hitec servos) to hear of people who strip gears while their models are still on the bench -- or seemingly for no reason at all.

Can someone please whack these servo designers upside their heads with a length of clue-by-four.

Tell them that yes, we want torque -- but we also want servos that can withstand their own power without wrecking themselves.

ZAGNUT

My Feedback: (1)

problem is that people want low weight along with high torque....something about eating cake comes to mind



dave

lord lucan

Xjet

....Yes ...agree with you here ..totally...its this ..." More wants More " philosophy that lurks all around us ...More horsepower.. quicker servo's , higher torque figures...Radios with a trillion functions ..all parameters become populated with values far in excess of what is actually required...including any " Safety " margins you oblige yourself with..frequently we've seen huge engnes hauling round big beautiful aiplanes..but ...when the fan at the front stops...the thing comes down on you like a manhole cover [:-]...enough is plenty...Dave.

Checklst

I agree but it is usually a marketing decision telling the engineers what the public wants not what best for the model.

It's all about size, high torque requires a LARGE service area to dissipate the load, so self destruction does not occur on the bench or in the air but Marketing says most consumers won't buy large servos like the old ¼ scale a few years ago.

Theirs and old saying “be careful what you ask for they might just give it to you “and they did small servos and (low cost) that don’t really hold up to the strain

zonker123

My Feedback: (21)

Maybe check linkage throw before hooking them to the control surface?
Mini servos are light for a reason, not a lot of mass there to absorb punishment.

3DFanatic

My Feedback: (5)

I'm with Zonker, why would you use a mini servo on a jet? They're not designed to take that kind of abuse...

ifixairplanes

My Feedback: (9)

ROFLMAO!!! Wow, i am going to use that one!

sean

XJet

What it was fitted to is irrelevant really -- it could just as easily have stalled in a parkflier -- but stalling it would still have resulted in the geartrain being stripped -- which is a bad design fault.

Remember -- this happened before the bird had even been taxied at any speed let alone flown.

NavyE6FE

Did the servo stall because the flight control hit a mechanical stop?? By that I mean did the rudder bind against the vertical fin or structure of some sort. If it did, how do you know that the torque the servo was putting out wasn't more than the servo was rated at. A friend of mine lost a plane because the servo throw was more than the control surface move. So he dialed in some dual rate so the servo wouldn't throw so far. While flying, he forgot about the reason for the lo rate and went to hi and the control locked up.

Flypaper 2

A Mr. Lin, who owns GWS and a model flying fanatic like us, asked the consumers what improvements they wanted. One thing was stronger servo gears, so he did just that. Compared to Futaba micros, the gears are twice as thick. I've stripped a few flying combat or a really bad crash with oversize ailerons.Maybe a few other manufacturers need to be fliers too.

XJet

Yes, the rudder surface was driven beyond its normal operating range after a gyro was fitted and it caused it to stall against the elevator.

This may well have exceeded the "rated torque" of the servo but my point is that a decent servo won't trash its geartrain when this happens.

Hitec should know better than to make servos with this weakness since anyone who's ever forgotten to turn on their transmitter before turning on a Hitec FM receiver will know that it's *very* easy to accidentally drive one of their servos beyond its regular operating range. I've heard of a few people who have done this and stripped gears -- that's an outrageously bad design.

Similarly, anyone using one of these Hitec mini-digital servos with *any* of Hitec's PPM receivers always runs the risk that getting hit by even a very brief burst of interference could drive a servo to its limit, thus stripping the gears. In such a case, a craft that would otherwise most likely have simply twiched badly for a moment will almost certainly end up crashing through catastrophic servo-failure.

As I said -- the *best* solution is to ensure that the servo either has sufficiently strong gears to handle all of the power it is capable of producing, or that a degree of torque-limiting is built into the servo amp (not too difficult to do) such that when it is stalled, only the rated (safe) torque is generated.

modeltronics

My Feedback: (27)

Quote ............Similarly, anyone using one of these Hitec mini-digital servos with *any* of Hitec's PPM receivers always runs the risk that getting hit by even a very brief burst of interference could drive a servo to its limit, thus stripping the gears. In such a case, a craft that would otherwise most likely have simply twiched badly for a moment will almost certainly end up crashing through catastrophic servo-failure.


If you have the HiTec digitals programed not to go to the limits then the interferance would not be able to drive them to the point where they fail.

Quote.....This may well have exceeded the "rated torque" of the servo but my point is that a decent servo won't trash its geartrain when this happens.


If the grears didn't fail in your servo then the electronics in the servo amp would fail as it tried to go where it was told to go. The servo will draw a lot of current if it is stalled and burn up if the gears didnt fail. I would want the gears to fail before the servo amp failed. Gears are much easier to replace.

Quote......As I said -- the *best* solution is to ensure that the servo either has sufficiently strong gears to handle all of the power it is capable of producing, or that a degree of torque-limiting is built into the servo amp (not too difficult to do) such that when it is stalled, only the rated (safe) torque is generated.


I'm sure they will make anything you want to buy in quantity and pay for.

Pete

famousdave

My Feedback: (61)

"The gears stripped because the servo was stalled after inadvertantly being given too much throw"

Maybe instead of designing servos with maximum flexibility and volume (throw) allowing us to have enough common sense to not program a servo for more than 125% of its max... Hitec should instead assume we are all stupid and go back 15 years and make servos that have fixed volume and weak motors.... we might as well all go back to analog radios too, who needs computers right - we might overprogram something....

Hmmm... I'll take the former and take the time to ensure units are programmed correctly thank you...

Power + Stupidity = something broken. You are lucky it was only a servo and not a surface or linkage (or worse)..
A mistake was made - LIVE WITH IT and don't blame Hitec.

I just love those who bash a manufacturer for something they did!! It is a good laugh though!

DP

XJet

But how would you feel if your car was designed so that flooring the gas would strip the gears in the transmission?

I guess it could be said that it's not a problem because you shouldn't floor the gas :-)

dshepard

But you floored the gas while the car had the parking brake on and had the trailer hitch tied to a tree.

Forgues Research

My Feedback: (7)

Did you ever think for second that this was a one off gear breakage. You seem to make a big deal out of a situation that might have some other reason to have happened.

When I was trouble shooting Robots in industry, first time problem meant raising the red flag, second time around, meant action to be taken.

Roger

famousdave

My Feedback: (61)

AMEN!!

XJet

Well there's a noisy minority which claim to have had nothing but trouble with Hitec servos and this includes a subset that ***** about weak gear-trains -- I was simply floating the suggestion that perhaps the gear train on some of their servos is too weak to match the power generated by the motor.

Personally I've had no problems with Hitec servos and I've now got 16 of them - ranging from the little HS81 micro servo through some HS425BBs, HS700's and a few digitals. I've had *no* problems with mine -- but I know quite a few HS81 owners who have and there seem to be some using certain models of digitals that also have.

I'm not saying that one should be able to abuse these servos with impunity-- just that there's *no* reason why a good servo can't be designed not to self-destruct when operated against an immovable force. Adding over-current or torque-limiting circuitry to servo systems is not rocket science and with some digitals costing close to the $90-$100 mark you'd think it's a little feature that would be nice to include.

Mind you, from a commercial perspective -- there's probably good money in selling new gearsets :-)

50+AirYears

If the servo jammed and stripped before it went to full throw, it obviously exceeded it's rated torque . Believe it or not, this could also be a safety measure. When an electrical motor is stalled, it's internal resistance goes to almost nothing. The currrent draw on the battery goes up enourmously. This can drain the flite pack battery quickly. That's why it's important when setting up any moving controls to get them as free as possible. Any jamming or sticking slows the servo motor, upping it's current draw. Enough use of a sticking control could drain a battery pack in one or two flights. In an extreme case, such as possible with a 5 cell pack, a prolonged jammed servo could heat the circuit board or wiring high enough to start a fire. Stripping the gear enables the servo motor to run free and minimize current draw. If the servo goes to it's internal stops, the circuitry inside has already limited the available current. If the servo is stopped by a jammed surface before it runs into it's stops, there is little or no current limiting. the circuitry tries to run the servo to the selecte position. I'd recommend that you be glad it happened on the ground, not in the air. Change the servo, and make sure it can't be overloaded again. The problem was in the control surface, not the servo.

XJet

It wasn't my plane on which this happened so I'm not too worried :-)