Home made centrifuge build?

[Solidus]

I was wondering about the feasibility of building a homemade centrifuge for use with microtubes, test tubes, etc.

While I have the general idea down in my head, I'm stuck as to the logistics of it (i.e. r/min considerations, g accelerations, layout of the device), so if anyone could help steer the project it would be kindly appreciated. A few questions I have to start off:

1. What RPM range does a typical centrifuge operate at (right off the motor, what is the motor RPM)?

2. How effective would using gear ratios be to modify the motor RPM input to rotor output, and what gear ratio would be a good compromise between high RPM output and overshooting on torque?

3. How easy would it be to come across and purchase a motor capable of the above? (I'm a last-year high school / first year college student, cheaper is better )

4. I have utterly no electrical wiring expertise whatsoever. Mechanics, some. Electrical, none. How easy would it be (diagrams / walkthroughs appreciated) to wire the device to a simple 3 step switch mechanism (I'll explain below) which could feed off of standard 120V wall power?

What I mean by 3 step switch mechanism is:

1 switch to power the device on
1 switch also on that main circuit to 'lock out' or safety key the device (preferably keyed, i.e. car ignition keyed)
1 switch on the lid of the device to act as a breaker or 'fail safe' device - switch is nominally down, when the lid is lifted the power to the rotor is cut for safety purposes.

Thanks, it'll be an undertaking.

 

[(*steve*)]

My wife uses these things at work, and many of the people here at the lab use them too.

They can spin at speeds up to 3,500 RPM (well the ones here do).

You have to load them carefully to ensure they're balanced and there are guards and interlocks to ensure that you don't open them while they're running, and to protect you if a tube breaks.

Those things are there for a real reason. You could probably build one, but you'd also have to be careful.

 

[daddles]

I know research centrifuges go to 100 krpm and above, but of course that's out of the realm of a hobbyist tool. I suggest you pay a visit to an appropriate lab at your local high school or college and ask to inspect some of the existing centrifuges. You'll be able to get specs and brands/model numbers of the units and look them up on the web.

In the design, you also need to think about safety. Your design has to be able to handle things like getting bumped off a table onto the floor and having a test tube shatter. Thus, for example, you'd want it to contain the test tube contents (what if the test tube contained an acid or a harmful biological material?).

 

[Solidus]

I'm fairly familiar with the systems of locks & failsafes on a lot of the tabletop ones I've used.

I've used centrifuges in the past two years of advanced level bio and chem courses (in such realms as biotech, determination of unknown solutions, etc.), so at least procedurally I'm up to spec and at least aware of safe handling.

That is why I'm also asking about the safety keys and lid switches - even though this would be for personal use and I know how to use one safely, you never know. A lot of the higher power ones fail catastrophically if a slight error is made. I plan on fabbing a shell of wood to contain the entire assembly and then waterproofing it (in case of a liquid spill),as well as having the failsafed lid switch.

The centrifuges and microfuges I've used in my chem and biotech labs so far have had lids that are primed to a switch. If it's midcycle and you lift it, it'll short the motor. Not to say we didn't strategically do that sometimes (against procedure/lab safety).

As far as construction & safety concerns, I know how to construct the enclosure. It's the electronic, logistic & mechanism concerns that I need help on.

Put it this way; I want simpler and less complex, but as long as that does not shortcut the design. From all my lab courses, building common sense, etc. I've learned that shortcutting only leads to disaster. I want a simple, safe one, not an uber-easy, questionable one.

 

[Solidus]

Would a starter motor for a car provide the necessary acceleration factor and torque?

What sort of motor would be adequate for the build?

 

[poor mystic]

Model aircraft electric motors run at 30,000rpm under load.
I had an idea for the safety enclosure - use one or more small diameter, wide track tyres such as golf buggy tyres.

 

[Solidus]

Alright, a few questions about the motor:

- Where could I get one, and for how much?
- Under how much load to they typically run (the application would probably be 2lbs max)
- How much voltage do they use?

And please elaborate on the enclosure concept - it sounds like an interesting idea.

 

[poor mystic]

As for finding out what's available in aircraft motors, and getting one, go to your local hobby supply store and talk to a salesman.
The tyre would be lying on its side, with the rotating machinery housed within the perimeter of the rubber.

 

[Solidus]

Addt'l info

That's actually an interesting idea, I'll have to see about the feasibility of it though. Worst comes to worst, I'll make an enclosure out of wood for it.

I'm also considering using a starter motor for a car. They run 3.6-4 krpm, but that's at at least 100lbs of inertial load (remember, they're designed to turn over all the moving parts of a car's engine). So they're a very high torque design. I'll look into motor and stuff as I move further with the design.

Here's my idea behind the centrifuge rotor design:

- Hot-swappable. Bore different rotors for different size tubes, unscrew & interchange.
- Particular one: 16 x 10mm wells to accommodate microtubes, 7.5cm external diameter.

 

[daddles]

I think a starter motor would be overkill in the torque area. You should do some design work and first figure out what it is you're trying to build. You need to know the maximum angular speed, the desired angular acceleration, and what kind of torque is needed to keep it running at the maximum speed. These numbers will dictate what motor you need to get. You'll also need to calculate the moment of inertial of your design. What kind of bearings should you use -- should they be ball bearings or journal bearings (or maybe something more exotic like air bearings)? Also check out a basic college freshman physics text and learn something about rotational dynamics. All of this should represent good learning experiences.

This is a good time to learn a little about mechanical drafting. You should learn to do layouts at e.g. full or double scale. Find a book on drafting and teach yourself the basics -- it's material you'll use the rest of your life no matter what field you go into.

At the very least, inspect some centrifuges that are already built and learn how they were built. It's foolish to try to design something without understanding the state of the art. What are the specs of existing centrifuges?

Oh, and start an engineering notebook if you haven't already.

 

[Solidus]

Hmmm....maybe Physics 1 this semester'll come in handy

As far as bearings, I was planning on using a tapered thrust roller bearing cassette. From a bit of research, they handle radial loads a lot better than many ball-bearing designs, which would at least at a glance make it sound a bit better for handling vibration & slight imbalances in forces (balancing a centrifuge, so I've learned in 2 years of advanced chem & bio classes, is an art, doing so PERFECTLY is hit or miss often times).

What are engineering notebooks? My strong suit is chem, not design (sorry)

 

[daddles]

An engineering notebook is nothing more than a lab notebook. If you've been taking some science classes, you should know all about those. Trust me, good lab notebook skills separate the men from the boys in later life.

I think you'll have fun designing and building the thing. You'll learn useful skills too. Since you'll probably be doing things on a shoestring, you're more likely to use found things than go out and buy new stuff. You might be shocked what e.g. new precision bearings cost (but it's not clear that you'd need them)...

You might also look for some surplus mechanical assemblies that could be modified to what you want. Reusing something is almost always easier than designing and building it yourself.

 

[Solidus]

Oh God, the good old lab notebook. Yes, now I know perfectly what you mean.

The improvisation factor. For example, I figure I'll reuse some roller thrust bearings, because I have them laying around (less to purchase), as they're better suited to radial loads. When I was younger I used to be into the higher power hobby RC cars, and I've seen what shaking can eventually do to an unprotected motor.

I figure it's a good compromise between protection via design and then simplicity - the bearings will allow the motor to withstand slight vibrations because of small imbalances - to defeat the bearings would require more force, meaning more of an imbalance, meaning I didn't have the skill to load it right and thus shouldn't be building a centrifuge

And yes, I will have fun!