DrPR wrote:
The problem with the servo arm mechanism is that to get a long travel you need a long arm, and the longer the arm the poorer the mechanical advantage (the more force you will need). And it requires a lot of space for the long arm.
A simpler mechanism is shown in the drawing. A cable is wound around the drum a few turns to prevent it from slipping, and is then run through a series of pulleys (tube rollers). The elevator is attached to the cable as shown so that both sides are going up (or down) at the same time.
A similar rig is located at both ends of the elevator, and both drums are on the same shaft. This ensures that all four corners of the elevator platform move up/down in the same direction and the same rate.
A single reversable motor drives the shaft. You can use a small high speed low-torque (low power) motor and gear it down to gain torque to turn the drum shaft.
The limit switches (single pole single throw push to break) interrupt motor power for the associated motor direction. If you use a bidirectional servo that senses stall to drive the drum shaft you may not need these limit switches.
Phil
Hi Phil and thanks for the suggestions!
Actually, this method you show was my second in the three methods I came up with, and I'll show you my logic tree so you can see how I came up with this method:
The first method that I and Mark came up with was to use miniature jack-screws in each guide rail with a captive bolt in the track, and with limit switches for stop points. The captive bolt would be attached to the guide arms on the elevator. But with that method you need precision, and very tiny jack screws and nuts, and those look prone to jam and fail, and you are lifting it at the weakest point with the least amount of attachment material.
Then I thought about the drum version you mentioned and I discounted it for these two reasons: First off as you pointed out you need limit switches or load sensing. I have load sening circuit in my Mercedes power antenna, and while it works okay in there, I do have a nice side business repairing them for people where they have failed, and once the hull and decks are sealed, there will be no access to get to them to do any repairs, so they have to work trouble free. And I don't see that system nearing 100% maintenance free.
With the system, I propose, yes it is a bit more bulky, b ut this is a LARGE carrier and the lower hull area is cavernous! I figure to need a 4" x 4" x 4" cube adjacent to each elevator opening just under the hanger deck, and there is plenty of room for even the largest of servo's. I calculate to need a 2 1/4" lift and so 1 to 1 the servo arm needs to travel that much, and by my calculations, I would at most need 3" x 3" x 3" so there is plenty of spare room down there. Also, the servo arm will only have to be 3" long to achieve a 2 1/4" pull at just over 90 degree travel, and I see those are readily available in several different materials, so I will not have to have one custom made, I can buy one off the shelf.
Let's talk about torque, and the amount of weight needing to be lifted. If I got a metal gear servo, they have really good generic ones with variable voltage:
http://www.hobbypartz.com/33p-solarservo-d227.html Check out that servo. It is only $10 ea. It runs on three different power settings and at 4.8v it still has 159 oz's of torque. It's very quick and smooth, and strong. I just weighed one of the 3D printed F-14's and it only weighs .5 oz. So if you doubled that weight to account for paint and weapons, say it tips the scale at 1 oz. Times two is 2 oz. The whole elevator I cannot imagine weighing more than 4 ox total. So that's 6 oz in all, but let's add 2 more oz for "Just in case weight" (Say adding a lot of weapons and crew guys. Still only 8 oz and the servo can lift 160 in low power mode, say we use 6V it can lift 180 oz! That's 22.5 times more than the elevator and cargo weigh. I will not lose that much with a 1.5" arm length. So I think that the servo can easily lift that much.
Next is the issue of the end of travel stops. With a programmable radio, I can set the travel speed, which we want slower, and the end point stop adjustments far more easily, than with having to use limit switches, and all that that entails. A couple of taps on the radio and I can easily fine tune the top and bottom stop points, but with limit switches, it may not ram hope at the correct point and I believe there is a lot more adjusting that would be needed. The servo arm method will have just two attach points, the holes at the end of the arm. Each cable set will enter a guide tube and go to its respective hang point, then over one pulley and down to the attach point on the elevator support beams.
I intend to make a full scale mock up once I get a little closer to see if it will indeed work, so we'll see. In the meantime, thanks so much for taking the time to make the suggestions and to make a very nice drawing! We'll see how it turns out.
Login
Register
FAQ
However it would probably still end up in a smaller lay-out (although size isn't your primary issue, it might come in handy). 
