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Why not a slow speed retract servo?

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Simon Chaddock05/12/2010 20:41:35
5611 forum posts
2977 photos
The retract cycle in any full size plane takes quite a few seconds.
For a model a scale retract speed will be a bit quicker but a dedicated retract servo like the HS-75BB only takes less than a second for full travel.
You could use a proportional servo and a speed reducer but why not gear down a low power servo so that it has a travel time of say a couple of seconds?
A micro 3.9g servo generates 0.7 kgcm torque and a travel time of 0.2 sec/60 degrees.
What if you geared its motor down a further 20:1?
14 kgcm of torque (twice that of HS-75BB), a 2 second travel time and only a 250mA full load current draw.
Of course with gears and bearings to handle the torque it would end up a bit bigger & heavier but not that much.
An interesting thought.
Eric Bray05/12/2010 21:07:50
6600 forum posts
2 photos
Use a servo-slow plug-in gizmo. That saves you having a special servo just for retracts!
Simon Chaddock05/12/2010 22:36:16
5611 forum posts
2977 photos
Agreed but the HS-75BB I quoted is a specific retract servo and as it has a "bang-bang" non proportional action it cannot be slowed down with a plug in gizmo.
Really my observation was if you are going to design a specific retract servo why not make it slower acting. It will be lighter and perhaps more important draw less current for any given torque. 
Frank Skilbeck05/12/2010 22:43:01
4602 forum posts
101 photos
The new type of electrical all in one screw jack retracts are quite slow, I can see these catching on now they are being supplied in reasonable numbers.
Brian Parker05/12/2010 22:43:58
538 forum posts
Small Stepper motor?
Danny Fenton05/12/2010 23:17:05
9317 forum posts
4125 photos
I use standard servos but fairly meaty (this pair below are supposed to be around 6kg each). In that way you can slow them down via the transmitter (if yours will do it) otherwise a servo slow as Eric says.
I don't know about anyone else but I don't think I could modify a retract servo in the way that Simon suggests, though it is of course feasible.
The other benefit of using a std servo is you can adjust the end stops fairly easily.
I do like the look of the electric units however.

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