Standard or Micro servo to use?

[boater76]

I have an 18 inch hydro and was thinking of using a 9g micro servo with 1.5/1.8kg tq. Think that would be enough for it?

Ken

[NativePaul]

No, Yes, Maybe.

I use 9g size servos in my 17" cat but I did try 2 that didn't last long before I found 1 that seems up to it, all had about the same quoted torque and had enough control authority but not all are man enough for the task, my first was a cirrus that stripped a tooth from one of the gears, my next was a Hitec zebra metal geared unit that burned out either the motor or ESC and I am now using a Saturn S70D 7g metal geared digital unit which seems very good and has lasted about 5 times as long as the other 2 combined so far, for less then the total price of the other 2.

I think a standard servo is way too big for an 18" hull and 9g may be too small, but that is what I planned for when I built my boat, all I have space for and can be made to work.

Servo fits in the faux exhaust with the lugs cut off and a little bit of filing on the cases for the hitec and saturn, and yeah i know its kinda funny looking but it's the minimal bodywork I could fit round my gear.

[Diegoboy]

The Saturn S70D is a 1.1kg/cm servo.

[boater76]

What do you use to mount your servo?

[Diegoboy]

I use these.

[detox]

The Aquacraft UL-1 uses a very basic standerd servo (VERY little torgue). It works verywell without any problems.


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[Fluid]

Standard servos have plenty of torque - around 44 oz/in (3.2 kg/cm) and are generally adequate for models weighing under 6 lbs. Micro servos have around 17 oz/in (1.2 kg.cm) of torque and are adequate only for small, slow models. I tried a micro in my two-pound 50 mph SAW rigger and it would not hold the rudder straight....


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[NativePaul]

Yeah 1.1kg at 4.8v/1.3kg at 6v, running off what I assume to be a 5v BEC built into the ESC the other two were both 1.2kg at some undisclosed voltage, 1.2kg seems pretty typical for a 9g servo.

I waterproof servos with silicon grease around the output gear and electronic grade silicone sealant between the case joints and under the screws when putting it back together then roughen the bottom of the servo and mount with 15min epoxy, its light, easy and strong.

I would think an 18" hydro would be closer to one pound than two, small but not necessarily slow, its not been timed yet but my Oncilla cat sure doesn't hang around on its 4800kv with 3s.

9g is marginal as can attest by my attrition rate and may or may not be enough depending on the size, weight, speed, size of rudder, hull/turn fin design and rudder throw/turn radius required, at 40-50g a standard size servo is rather a large burden on a little boat and may be hard to fit in. Mini servos at about 18g have the same torque as cheap std servos at around 2.2 kg, have much stronger gears than micros material for material, and not taking up that much more room than a micro will fit in places way too small for a std servo, they could be your answer.

[Diegoboy]

Standard servos have plenty of torque - around 44 oz/in (3.2 kg/cm) and are generally adequate for models weighing under 6 lbs. Micro servos have around 17 oz/in (1.2 kg.cm) of torque and are adequate only for small, slow models. I tried a micro in my two-pound 50 mph SAW rigger and it would not hold the rudder straight....


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I have a serious question about servo spec conversions.

Lets start with any servo. Like this Hitec HS-5055MG = Stall Torque (4.8V): 18.05 oz/in or (1.3kg.cm) How would one get that conversion?

I get 18.05oz converted to kg = 0.512kg
or
1.3kg converted to oz = 45.85oz

So lets go a step farther and add the inches to centimeter converstion...

You take 18.05oz/in converted to kg/cm = 0.201kg/cm
or
1.3kg/cm converted to oz/in = 116.47oz/in

None of which match the stated specs. Can anyone explain how they got those numbers please?

[Diegoboy]

...44 oz/in (3.2 kg/cm)....

Fluid, How did you get that?

[Fluid]

Straight off the website.

http://www.servocity.com/html/s148_standard_precision.html

But if you want the math:

44 oz/in = 2.75#/2.54cm = 1.25 kg/2.54 cm = 3.18kg/cm

Round off to 3.2 kg/cm. This is torque, not proportions.




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[Diegoboy]

Thanks Fluid!