Servo question.

[Finnsllc 917]

Ok so i was having issues with a servo used in a buggy. I first thought that the steering arms and part could be cathing on somthing or binding. So as its an inexpensive servo i took it apart after realizing it was def the servo. There were broken teeth on inside. As its a steering servo i rotated the gears 180 and hooked it back up to the car with out the top and bottom of the case reinstalled. What i noticed was that when the reciever and car were turned on the servo was spinning or constantly running. I grabbed the controller and went into the trim and adjusted it until the servo stopped running and was still. I had to adjust to LEFT 31. im trying to understand what im dealing with. Meaning, if i hadent taken the servo apart or saw this does that mean that the servo constantly was pushing in on direction? Hmmm 

[Finnsllc 917]

Ok so as i have explored this more. I cant reverse the gear due to the stops on the big gear. So i grabbed another servo,Brand new, and opened it up. I again plugged it into the car and powered up. This servo is adjusted LEFT 21 in order to stop the constant rotation. 

[Jonathon Gillham 4605]

Thats not uncommon. I think it seems bad but its really because back in the 80s and 90s there was a little trim tab that you moved nd it probably had about 10 clicks each side, now its digital thing with a scale of 100. So your 21 is actually a very small movement in the old money.

I don't remember needing as much steering trim BITD but then my servos were Sanwa or Futaba so good quality. Now I have a lot of cheap ones with far better specs but I suspect they're not as accurate. I don't seem to have the same problem in my race cars with Savox servos.

[Finnsllc 917]

Its an interesting phenomenon, I had a servo issue. I took the bottom and top casing off. With that removal, when i hooked up the power, the servo just spun and spun. I then adjusted the trim to stop the servo from spinning.  the point being that if i hadnt taken the servo apart, and then hooked it up while it was apart i would never had known the servo was "under load" the entire time. So by adjusting the trim by looking at the physical gears inside the servo i found that to be an incredibly interesting find. as i have always wondered and tried to adjust the steering, perhaps this is a piece of the puzzle 

[sosidge 624]

Obviously removing the top and bottom case is not normal running conditions. And having broken gears in a servo is not normal either.

But if all the gears are installed correctly and all the electronics are positioned properly in the case, the servo shouldn't run constantly.

There is a potentiometer connected to the output gear. That gives the feedback to tell the servo what position it is in. The input signal tells the servo what position it needs to be in. If there is a difference, the motor will run to move the servo to the correct position. And the position is checked again. Thousands of times a second on a digital servo.

So if your servo runs constantly, and you replaced the gears, there is (most likely) a problem with the potentiometer. These aren't usually repairable. Confirming your original finding - it's broken.

[El Gecko 1343]

5 hours ago, sosidge said:

Obviously removing the top and bottom case is not normal running conditions. And having broken gears in a servo is not normal either.

But if all the gears are installed correctly and all the electronics are positioned properly in the case, the servo shouldn't run constantly.

There is a potentiometer connected to the output gear. That gives the feedback to tell the servo what position it is in. The input signal tells the servo what position it needs to be in. If there is a difference, the motor will run to move the servo to the correct position. And the position is checked again. Thousands of times a second on a digital servo.

So if your servo runs constantly, and you replaced the gears, there is (most likely) a problem with the potentiometer. These aren't usually repairable. Confirming your original finding - it's broken.

This is a good point. The case has built-in stops for the pot, to tell the pot when it has reached the end of the travel, one way or the other. If the case is not installed, the pot will not know where to stop, and the motor will just keep spinning. It's best to only test servos with all gears and the case fully installed.

[sosidge 624]

1 hour ago, El Gecko said:

This is a good point. The case has built-in stops for the pot, to tell the pot when it has reached the end of the travel, one way or the other. If the case is not installed, the pot will not know where to stop, and the motor will just keep spinning. It's best to only test servos with all gears and the case fully installed.

The pot isn't actually reliant on the stops in the case, they are just there for physical safety. A servo with a full set of gears should operate as normal even if the top of the case is off. Of course, if the case is off, a significant bit of the structure that keeps the gears meshed is missing, so they may not stay meshed for long, and that will encourage the motor to keep spinning, because it will no longer be succeeding in moving the output gear

You can get winch style servos which have full rotation and basically run off a switch but I doubt that is the kind of servo the original poster has.

[El Gecko 1343]

19 hours ago, sosidge said:

The pot isn't actually reliant on the stops in the case, they are just there for physical safety. A servo with a full set of gears should operate as normal even if the top of the case is off. Of course, if the case is off, a significant bit of the structure that keeps the gears meshed is missing, so they may not stay meshed for long, and that will encourage the motor to keep spinning, because it will no longer be succeeding in moving the output gear

You can get winch style servos which have full rotation and basically run off a switch but I doubt that is the kind of servo the original poster has.

Right, my mistake. But I'd still suggest testing servos fully assembled for the other reasons mentioned

[Wooders28 4722]

One of the reasons I stopped buying cheap ,plastic geared servos, it became expensive!

I'm trying to upgrade all mine to Savox, but some have Alturn Race Servos or the cheaper Core RC 9016MG servos, but all metal geared.