Build: Balance Beam Scale

[Handeman]

I didn't know where to put this. It's not a rocket, but I built it for rocketry and I figured if I wanted one, someone else probably did too. So anyway, here's my build thread on the balance beam scale I built. I actually went to Home Depot and spent about $5.50 on this. I bought the chain to hang the pans on and a two pack of bar magnets for the dampener.

The attached picture is the complete scale. The scale turned out even better then I had hoped. I don't have calibrated weights yet, guess I'll have to spend some money on those too. You can see the dime sized piece of paper on the right pan. That actually caused a 1/2 mark deflection in the pointer when it was added.

Most of the scale is made of pine I had laying around. The pans are the bottoms of 1 lb. coffee cans. I bought three feet of the chain that holds them. The aluminum is from the side channel of a shower door frame. There is a 1/2" by 1/8" piece of steel about 1.5" long that is used as the piviot point.

On the left side is a large nut attached. This is a counter weight to the aluminum piece that was attached to the right side of the beam as part of the dampener.

 

[Handeman]

Any balance beam scale is only as good as it's knife edge it balances on. What I used was a piece of steel with a groove filed in it. I used a large square file to create the groove and then used a small rat tail file to round out the groove.

What I used for a knive edge was two deck screws. The tips sit in the metal groove and every thing rotates around this. I'm not convinced this is the best method. It certainly works and is easy to make. A utility knive blade and a piece of notched C channel across it might have made a more durable edge. I was considering the channel that you attach to a wall and then put shelf brackets into, but I didn't have any of that and I did have the deck screws.

The first picture you can see the grooved piece of steel I epoxied to the wood. This is the base the beam pivots in. The second picture you can see the beam sitting on the screw points.

 

[Handeman]

There were several issues I learned about and got past while building this scale. First is that you have to have most of the weight of the balance beam below the pivot point. When I first assembled this, I added the threaded shaft and some nuts to be used to zero the scale. (see the first photo) At that point, the weight of the pans and chains were about the same as the wood beam and the rod and nuts. This caused the scale to be very hard to balance and the slightest amount of extra weight on one side or the other, and the beam would tilt completely one direction or the other. I did get it balanced and when I add a piece of paper smaller then a postage stamp, the beam would flop all the way to that side.

This problem was fixed by adding weight to the bottom of the pans. I this case, I epoxied a 3.5" x 1.5" x 3/16 piece of steel bar stock to the bottom of each pan. (see second photo) I suspect this is going to cause problems in the long term by causing the screw points to form dimples in the steel groove and possible induce inaccuracies in the scale. It would have been better to design the balance beam with the pivot near it's highest point so almost all if it's weight was below that point. Guess I save that for version 2.

 

[Handeman]

The aluminum plate on the right side of the balance beam is the magnetic dampener. The dampener works because of Lenz's Law. What this says is, when a conductive metal is moved through a magnetic field, a current is created. This current also creates a magnetic field in opposition to the field that created the current in the first place. These opposing fields are what causes the resistance to motion and dampens the movement of the balance beam. I this case you want to use a non-magnetic conductive metal like aluminum, copper, brass, etc. The metal moves between the two magnets as the beam moves and dampens the movement of the scale so it comes to rest quickly.

I bought two bar magnets at Home Depot. These are ferrite magnets but because of the size, they work pretty well. If you have a couple of rare earth magnets, these would work much better, just make sure you mount them so they don't snap together, they're much stronger then the ferrite magnets.

In this case, I epoxied the magnets to aluminum angle and screwed that to a mounting plate. I use slotted holes in the angle to allow adjustment of the magnet positions. The aluminum angle that is mounted to the end of the beam is attached with a single screw. This allow some rotational adjustment to it also. The aluminum sheet on the end of the beam is 1/16" thick. The magnets are set about 3/16" apart. The dampening effect will be stronger with stronger magnets and/or putting them closer together. Of course you don't want the sheet to touch and drag on the magnets so the closer together, the more precise your construction has to be. In this case, I'm satified with how fast the scale comes to rest. The dampener is working great.

 

[Handeman]

The pans are just hooked into a screw eye. (see photo) I didn't make a knife edge for the pans, although this would also make the scale much more accurate.

The pointer was made from aluminum flashing. I cut a piece an inch wide and folded over a 1/4" on each edge to make it stiffer. The scale was done with Paint and printed out. I can move it where ever I want it to "zero" the scale. The last picture shows the slight deflection I got when I added the piece of paper to the right pan.

 

[Handeman]

My first use of the scale was a huge success. I had a "recipe" for igniter dip that I had tried once before. I didn't have a scale then and used volume instead of weight when I made it. This time I was able to use weight. Since the recipe called for percentages, I didn't have to know exactly how much each ingredient weighed, only that they were in the right proportions. What a difference! I wasn't impressed with the recipe before, but now I'm very impressed with it. It helps when you do things right!

 

[jj94]

That's nice to hear. You're balance project is one of the more different things I've seen on the forum, and I think it's really interesting. It must be cool just to have it sitting on the table.

I would have definitely made this using your plans if I didn't already have a scale. My mom works as a computer programmer at a drug testing facility where they test new medical drugs on animals before they go out on the market for consumers. The balances that they had to weigh the animals, organs, fetuses, and food were going to the trash since they got new ones about half a year ago. My mom's coworker got his hands on a few since he knew I came in the facility one time to use them for TARC, and gave them both to me in case I wanted to keep it for home. I gave the other one to my friend, another member of the TARC team.