Mechanical Arm (December 5th, 2008)            Plans Here!!

Here's a mechanical arm I built for a class this semester. It uses syringes and rubber tubing to act as a hydraulic system. For the most part syringes work nicely, but filling them up so there's no trapped air is a pain. Since water is practically incompressible, the syringe plungers should move equally because both syringes are the same diameter. I should have changed the gripper syringe's diameter so its plunger had less movement but more power. Currently, half of its length is unused.

Hydraulic Animation
The general idea.

We had three weeks to design and build it, and then we were supposed to have a contest to see who could stack six wooden cubes into a pyramid the fastest. The contest never really materialized although I think mine would have come in second. A lot of other people's arms didn't really have much range of motion or were awkward to use.

I set my syringes up to act as first class levers while everyone else used a third class lever setup. Both have the same power, but I think it was easier to adjust mine to get a good range of motion. Our arms were required to have four degrees of freedom. Mine swivels at the base, moves at the shoulder and elbow, and has a gripper that's way too complicated.

The gripper has a total vertical displacement of 27 inches. It can reach 5 inches below its base, and a maximum of 22 inches above. The whole arm can rotate through about 35 degrees which is pretty good compared to my classmates.

It's not nearly as polished as I'd have liked it to have been, but the deadline kept getting closer. String and hot glue really saved the day :D

Homemade Mechanical Arm Using Syringes
The completed mechanical arm.

Demonstrating how the arm works.

Some closeups of the different joints.

Finally, six years after I made this arm I got around to making plans for it. Below are the flat patterns for the arm. You can just print them off at 100% scale and they will be the right size. I included a scale in imperial and metric at the bottom of each page so you can check to make sure it printed right.

Most of the people who email me for plans are not in the US, but I am, so the hole sizes are in inches. For those using the metric system, I'd drill the holes to suit whatever fasteners and dowel sizes are commonly available where you live.

Plans to build a Homemade Mechanical Arm Using Syringes
Click the above image to download the plans in .pdf format.

Gripper Detail
Here's how the gripper works. As the syringe's plunger moves out, the rubber band pulls the fingers shut. When you pull the plunger back the pink strings pull studs on the rear part of the fingers together, thus opening the other end. A cotter pin acts as a crude pulley because I was running out of room to route the string.

Arm Detail
This arm pivots on a 1/2" dowel. The two wooden washers in the first page of the plans go on the 1/2" shaft to space the arm properly from the next section.

Arm Detail
The syringe is mounted in a block of wood that pivots on two short screws through the side. As the arm rotates, the block of wood that holds the syringe needs to pivot as well. This is so the force required to pull the arm is always parallel with the axis of the syringe.

Arm Detail
The arm pivots on two little machine screws. They don't need to be that long, but they were all I could find at the time. I tried a number of different positions, and the holes I'm currently using are what's noted on the plans.

Arm Detail
The syringe is mounted in another block of wood that's free to pivot. A piece of coat hanger wire runs from the plunger to the 3/8" dowel to pivot the arm.

Arm Detail
The whole arm rotates on a 7/16" bolt that is held captive by the rectangular base. The nut should be loose enough to easily allow the arm to turn. It's also a good idea to put a washer underneath the arm to reduce the friction.
The syringe on the left has a piece of coat hanger wire fastened to the plunger which pushes and pulls against a wooden knob to rotate the arm.

Arm Detail
Here's how the syringes are set up. The pair on the left that rotates the whole arm isn't a good idea. The tubing makes such a sharp bend it tends to pinch and not work very well. You can see in the videos I have to adjust the tubing to get the arm to rotate.