February 15, 2007

          Tucson Model Yacht Club
                Tucson, Arizona

Builders Corner

FULLY PROPORTIONAL SAIL DRIVE

Do you want an inexpensive fully proportional sail drive, then read on. This idea came to me after talking to Gene Hart who told me about a magazine article he read detailing a mechanical proportional sail drive. It sounded rather complicated and I must admit I didn't fully understand the details but new the principle. Over the next few days it kept bugging me and I felt sure an idea was being formed in the back of my mind. So one evening while tinkering around in the workshop, thinking its about time I cleaned this place up but not having the resolve to do it, I rummaged around in one of my many odds and ends boxes, found a couple of micro switches, in another box I came up with a piece of aluminum and in yet another box found a 1/4" collar and some nylon bushings, and some lengths of wire. Well I felt sure I had the materials now how to assemble them to achieve the end result. After some time and numerous pieces of aluminum, cut, filed, drilled and thrown in the scrap box, (never throw anything away) I came up with what I think is the answer. Quite simple really, surprise someone hadn't thought of it before.

So to the details: Figure 1 shows the completed assemble. As you can see from the drawing in figure 2, it consists of a cam secured to the sail drive output shaft. A switch plate which is freely mounted to the shaft. An extended servo arm and a short connecting rod.

The cam is cut to allow the two switches mounted on the plate to be normally open. A small movement of the switch plate in either direction will cause one of the switches to make. This will start the motor, driving the output shaft and the cam until the switch drops of the high side of the cam and stops the motor. Moving the plate in the other direction has the same effect in reverse.

I'm not going to give a detailed layout for the switch plate or cam because it will depend on what size of micro switch is used but hear are a few construction tips.

1: The larger you make the cam the more sensitive the response will be, I made this cam about 1" outside diameter. To secure the collar to the cam I drilled two holes through the cam and collar and bolted them together with two number 80 screws. See figure 3

2: The switch plate should be made just large enough to mount the two roller type micro switches, Radio Shack has the ideal size. The pivot hole in the plate is fitted with a nylon bushing (Ace Hardware) to allow free movement on the shaft. The position of the switches is important, I suggest making slotted holes so the switches can be adjusted more accurately. A little dab of light grease will ensure smooth operation. See figure 2

3: The servo extension arm is made from a piece of light aluminum and screwed to a servo arm. The connecting rod is secured to the switch plate via one of the switch screws and the other end is connected to the extension arm.

The length of the arm will depend on the position of servo and the sail drive.

4: Finally the wiring, I used a 4 pin male connector mounted to the switch plate and wired from the switches to the connector with insulated stranded wire as per the wiring diagram in figure 4. I used a very flexible wire to connect from the 4 pin female connector to the motor and battery.

When you first operate this assembly if the motor does not follow the movement of the switch plate, reverse motor wires or the battery wires.

Well that's all their is to it. kiss works again.