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Large Hitec servos - an anomoly?

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Old 08-21-2005, 04:06 AM
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XJet
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Default Large Hitec servos - an anomoly?

I've been looking for an affordable servo for some large aircraft I'm building.

These models don't need the precision or light-weight of expensive digital or titanium-geared servos but they do need plenty of torque.

Looking at Hitec's offerings I'm left somewhat perplexed.

On the one hand we have a servo that would seem to offer exactly what we're looking for, in terms of torque (343 oz/in) and speed (0.14s @ 6V): the HS805BB "Mega Power" servo -- but it's got nylon gears!

On the other hand we have the HS705MG which has metal gears and so (you'd think) would be stronger with more torque -- but it's not. It only offers 66% the power (201 oz/in) and is slower (0.20s @ 6V).

Now everyone knows that nylon gears are weaker than metal -- so why not use metal gears in the more powerful servo???

Then, just to confuse matters further, there's the HS765HB which offers the same torque as the 705MG but has Karbonite gears. But hang on, we're told that Karbonite geared servos are not for use in aircraft weighing more than 12lbs -- so why even bother with one that spits out 201 oz/in of torque then because *no* 12lb plane I know of needs that much torque.

Even the HS755HB 1/4 scale servo (also Karbonite geared) becomes a bit of a paradox -- since few (in fact probably *no*) 1/4 scale planes weigh 12 lbs or less and thus qualify as being suitable for Karbonite-geared servos.

Forget the "standard size" servos (like the HS5945 -- too weak) and the xxxxTG servos (too expensive) because they'er also (believe it or not) too physically small for easy use in this application.

Although these airframes are large, they are not expensive, so using $115 servos would be a waste of money when a good old fasioned conventional 1/4 scale servo should do the trick.

Can someone make a recommendation for a $50 servo that has at least 200 oz/in of torque and which will not bust its gears out when used in a larger model with a gasoline engine?

And can someone explain why a manufacturer's *most* powerful servo has nylon gears rather than metail or those made from a material that is allegedly four times stronger than nylon?

I'm not knocking Hitec's servos, I've got dozens that work just fine and it's hard to beat them for value -- I'm just a little perplexed by the seeming anomolies in their 1/4 - large-scale range.
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Old 08-21-2005, 08:46 AM
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Default RE: Large Hitec servos - an anomoly?

If the servo has 343in. oz. of torque with nylon gears then it must be able to put out that torque without stripping the gears. The gears may be thicker,deeper than the metal gears.
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Old 08-21-2005, 11:05 AM
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Default RE: Large Hitec servos - an anomoly?

The 805 is used by many for GS applications and has been for many years. The gear-train is big and beefy compared to standard size servos (more gear mass). I don't recall any gear failures in aircraft. Why use metal if nylon gets the job done?

You might consider the 5745 jumbo digital, metal geared. I have several in GS models and they work great. These can be had for around $65.00 if I recall.
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Old 08-21-2005, 07:11 PM
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Default RE: Large Hitec servos - an anomoly?

Nylon gears have a much tighter mesh over time. That is why some of the larger servos use them I suppose.
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