Simple Syringe Pumps

Written by: admin@makezilla

Many projects, like 3-D printing, require some means of pumping fluids. Although many types of pumps are available they may be too expensive for your project budget or they may not pump thick fluids and/or the small volumes and low flow rates you need for your application. If your budget is tight, your project requires small volumes and/or uses thick fluids then one of these two syringe pump designs may be just what you need.

Please note that this instructable is only intended to explain the basic operation of a syringe pump. Selection, assembly, testing and adjustment of the individual components will be completely up to you depending on your particular application. You need to plan on experimenting a bit to get everything just right. An internet search will show you some of the the hundreds of syringe pump designs and applications.


Step 1: Choose a Gearmotor

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There are thousands of gear motors available so getting the right one may be a challenge.

The fist thing to decide is the voltage you want to use to operate the motor. The project you have in mind will be the determining factor here.  

Next you need to choose which gear assembly to use. This involves determining the needed input to output ratio of the gear motor unit. I suggest using a ratio of at least 400:1 (10 to 15 RPMs) for thin fluids like water. Thicker fluids like thin oils may require a 1200:1 ratio. 6000:1 ratios (1 to 4 RPMs) or even higher may be required for very thick oils, resins or icing. 

You will also need a pulley to attach to the gear motor shaft such as the one shown here. 

Step 2: Choose a Syringe

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Some hardware or kitchen supply stores sell syringes. In most applications these syringes actually work better than the type sold in drug stores due to the larger output tube that is easier to attach tubing to. These syringes usually have a larger capacity too which works out well in most applications. Again, your project is the deciding factor.

You can also purchase syringes at most drug stores. However, the medical type may be a bit hard to attach tubing to because of the luer lock connectors they use. It is legal to purchase syringes nearly everywhere. However, some drug stores just won't sell them to you without a prescription for some injectable medication. It has nothing to do with it being legal. They just have a company policy not to. I caught a lot of grief at one major drug chain when I tried, unsuccessfully, to purchase syringes even without needles.  A drug store right across the street sold them to me with syringes without a single question.

Thanks to the criminal element in our society some things are just more difficult to acquire these days, Edison would be hard pressed to get anything done in today's world of suspicions,  regulations and restrictions.  

Glass and metal syringes are also available, but disposable plastic syringes will work fine for most applications and are much less expensive.  

Step 3: Choose a Check Valve

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You will need two (2) check valves in order to pump fluid using a syringe. There are many types available, but the simplest and least expensive I have found are the type used in aquarium air lines to prevent back flow into the air pump. Nearly every pet store sells these and there are many types available 

The spring operated type of check valve pictured here (the clear one) should work for most applications. 

The other type of check valve shown (with the yellow-orange center) uses a rubber or latex valve that opens to allow fluid to pass and collapses to prevent back flow. 

Step 4: Putting it All Together

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Now all you have to do is mount the gear motor and syringe to some sort of base, find a way to connect the syringe plunger to the pulley on the gear motor with some sort of connecting link and hook up the check valves to start pumping.  The syringe can either be fastened securely to the base or mounted in such a way that it pivots so the connecting link/plunger assembly stays straight during operation. Again, your application will be the determining factor here. 

If your gear motor is too fast or your liquid is too thick the neoprene seal on the end of the plunger (inside the syringe) will simply pull off and the pump will quit working even though the gear motor and plunger continue to work.  If this happens you will need to slow your pump down by slowing down the motor or by replacing the gear motor with one of a higher ratio (slower RPMs).

Step 5: More precise Control Using A Screw Drive Stepper Motor (Linear Actuator)

Picture of More precise Control Using A Screw Drive Stepper Motor (Linear Actuator)

If your project requires a higher degree of flow precision than the gear motor can provide then you may want to consider using a screw drive stepper motor.This is a type of linear actuator. With this setup you will need a stepping motor controller and devise some way to fasten the drive's screw to the syringe's plunger. Screw drives are avaliable from some sources for as low as $30.00 each and a controller can be purchased for about the same price.


Step 6: Taking it Further

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The possibilities here are only limited by your your budget and imagination.

- If your project requires a closed system where the pump output goes back into the pump inlet then you will need to add an expansion cylinder to the system. This can be something as simple as a length of tubing with one end crimped and the other end hooked onto the outlet side of the pump. Make sure the tubing stays vertical and is filled with air. 

- You can add a speed control to the gear motor to better control the flow.
- The fluid flow of a syringe pump is not smooth and constant. It pulses as the syringe cycles through the filling and emptying stages. You can smooth these pulses out and make the flow more constant by adding one or more additional syringes. Think of a two or four cylinder gasoline engine and you will have the idea. Keep in mind that additional syringes may require a stronger motor.

- Additional syringes will also increase the flow rate. 

- This pump would be easily adapted to 3-D printing of icing, liquid resins and other thick fluids.

- Replacing the connecting link between the pulley and the syringe plunger with a long screw would allow for more precise control of the flow in 3-D printing applications. 

I hope this instructable will spark some ideas and help you in whatever project you are building.