Introduction: Poor Man's Centrifuge and Lazy Suzan

Introduction + Math and design


Centrifuges are used to separate materials by density. The greater the difference in density between materials, the easier they are to separate. So in emulsions such as milk, a centrifuge can separate out some of the fattier cream to the top whereas the skim milk goes to the bottom. Sigma aldrich has a good article discussing the physics of centrifuge separation as well as what is possible to separate at different centrifugation speeds.

The centrifuge I made should be within the low speed category that they specify (2,000 - 6,000 RPM). At these speeds, if you had a suspension of Eukaryote cells in solution, you could possibly pellet them (as in, separate them from the solution). Suspensions of things like flour or dirt particles could also be separated at these speeds.

Lazy Suzan

A motorized lazy suzan is a slowly rotating platform usually used for videography. You can place something like a statue or food item on it, and get a smooth video of all sides of the object. Revolution speed is ideally < 1 revolution per second.

Motor Speed, Force, and RPM

In a product description of an electric motor, you will usually see two things: the voltage and the RPM. You can probably safely use voltages at + or - 50% of the specified voltage. In theory, higher voltages mean more force behind the motor, and higher RPM. But let's say you have a motor that has standard RPM of 1000 revolutions per minute. How can you make something spin faster or slower than 1000 RPM with this motor? With gears.

Changing gear in a bike works in a similar way to what you have to do here. If you have a gear with 20 teeth attached to the 1000 RPM motor. Then a gear with 5 teeth in contact with the 20 teeth gear, will spin at 4000 RPM. The relationship being: Target RPM = (Motor RPM)*(Teeth on Motor Gear)/(Teeth on revolving gear). FYI, the number of teeth on the gear should be directly proportional to the radius of the gears, so radius can be used instead. So in this example, the 5 teeth gear is the revolving gear and would have the centrifuge platform and tubes attached to it.

But that's in theory, in reality, the revolving gear will spin at a lower speed than expected. This is because there will be a higher amount of friction, especially if you add weight to the 5 teeth gear.

Instead of gears, you can use rubber bands, which is what I use for my lazy suzan. And the math works out similarly, in terms of circumference or radius instead of teeth.

Step 1: Materials

General List:

  1. Empty containers like the ovaltine and oatmeal containers I used. Soup cans could also work.
  2. Motors. Rated 12V or higher for the centrifuge. Around 3-6V for the lazy suzan.
  3. Hot glue gun
  4. Duct tape - optional
  5. Super Glue - optional - wear gloves if using
  6. Wires, gears, rubber band, rotating platform, batteries, etc - I will discuss in more detail in following steps

The motors I used are regular, 2-lead, electric motors, not stepper motors. I got one (rated 12V and 1200 RPM) from a Canon printer that stopped working, and another (possibly 3V, < 300 RPM probably) from a dvd player that stopped working. If you don't have any motors on hand, there are tons on eBay and other sites for <10 dollars that should work.

The gears and axles that I have are from an "RC Snap Rover kit" which I took apart. In reality, all you need is a wheel that can rotate freely on an axle and a way to make it spin faster or slower than the motor, depending on what you wish your final product to be.

Step 2: Centrifuge

Ovaltine containers have a metal bottom. To form a hole for the top of the motor to go through, I recommend using a hammer and nail and going around in a circle, slowly punching out material. Using a drill or a dremel would work as well. Make sure the motor head will fit and be perpendicular to the base of the can, and then apply hot glue to the area around the motor head and set it in place. I used tape to support the motor while the glue hardened. I recommend attaching long wires to the motor before gluing it to the container. I simply wrapped the stripped wire ends around the motor leads.

Once the glue is hardened, you can poke two holes out the side of the container and pull the wires through. I have the battery outside of the container so I can manually touch the leads to the battery to turn the centrifuge on. Ideally a switch would be used but I don't have one.

I then super-glued a large gear to the motor head. I used sandpaper to roughen the gear surface before applying glue, which in theory should increase bond strength. I marked an area on the ovaltine base where the next gear should go (as in, a point where the teeth for the two gears will interlock) and poked a hole in that location with a nail. Then I placed an axle slightly through this hole and hot glued it in place. This is most visible in image 2 for this step.

The revolving gear has a thick paper disk super-glued onto it. I cut the disk out of the back of a notebook. The disk, being paper, should bond better than plastic, but is strong enough to hold up to the speed of the motor. A hole should be placed in the center of the paper disk, in line with the hole in the gear. This way, when the revolving gear is placed on the axle, the axle will end at a point above the paper platform. I then placed a bead of hot glue, visible in the header picture on the first page, on the top of the axle. Which will harden above the platform, and ensure that it won't fly up off the axle.

The mp4 file is a video of my centrifuge spinning. You may have to download it to view, the file isn't that large. The tubes I use are plastic and they have milk in them. I capped them with plastic wrap. I wouldn't recommend using glass tubes given there may be some risk of them flying off the disk while spinning. The tubes are simply taped onto the disk. I use a 9V battery in the video, which is less voltage than the motor is rated for. To increase voltage levels, you can connect batteries in series.

Step 3: Lazy Suzan

For the Lazy Suzan, the gear in the image isn't actually acting as a gear, it simply is able to pivot around the axle. The wheel which is attached to the gear provides a base for me to place a plate on top of it, and then an item on top of the plate.

Image 2 shows the axle which has been sunk through the base of the oatmeal container and then hot-glued in place. The motor was positioned and hot-glued in a location that ensures the rubber-band is taut when pulled between the gear and the motor (image 1). If you simply have a wheel that spins on an axle, the rubber band can be stretched directly around the wheel, just note that if the wheel is large, it will spin at a much lower RPM than the motor.

I have to run this at 6 volts actually. Which is 4 1.5V batteries in series. Image 3 for this step shows how I recommend connecting batteries in series (if you only have 1.5V batteries). Basically a strip of aluminum foil folded onto itself several times and placed onto duct tape. The tape then secures the aluminum in contact to the + and - battery ends, and secures the batteries together. So when things are set up like in image four, you fold the loose end of tape up onto the batteries and seal it together. Make sure the aluminum foil is folded up at least 10 times or it will heat up a concerning amount.

I could have also run the motor with a 9V plus a resistor but I wanted it to be simple. Just know that there are several ways of reaching optimal voltages or currents.

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