Intro: From 12 V Car Compressor to Vacuum Pump
This project arose from the need to have a small vacuum pump for some of my new purposes, starting from a simple material like a tyre mini compressor, buyed for 8 € at Mercatopoli.
Then the idea came on watching some tutorials on the net, but because of mechanical differences inside the machines I decided to try to do it myself...
Step 1: Some Mechanical Notions...
The compressor I used is like that of kind of the "reciprocating air compressors", that is the more common you may meet up.
This type of compressor uses piston/cylinder arrangement to compress the air. Whenever a piston moves back and forth it's considered as moving in reciprocating motion.
There are two sets of valves (SEE PICTURES ABOVE) taking charge of air intake and exhaust. The compressor takes inside successive amount of volume of air from intake valve and confined it in closed surface and at that time piston moves downward with the closure of this valve. Then there is compression of air by reducing its volume. Now the piston moves upward and compress the air and then displace the compressed air through outlet valve.
And then again intake take place and so on, in a cycle.
Looking for the process from a reversed perspective, it is possibile to use the compressor as a vacuum pump. Since a vacuum pump is "merely" an air compressor running backwards, with the inlet attached to a machine you want to apply vacuum to and the outlet open to the air. The important is to find the valves in our compressor and to connect to the intake valve with a proper hose.
I will describe how I did it....
Step 2: Disassembling the Compressor
The compressor is cheap and "made in china", but I think it can be good for this purpose, provided that one doesn't demand a strong vacuum.
First of all, I removed the screws in the back side, then I unplugged/cutted the the electrical wires (coming from the cigarette lighter plug of the car) and I set apart them, since I don't use them at the moment.
Carefully, I extracted the electric motor together with the pump device that is bound to. I unscrewed the T-shaped metallic joint (that is linked to the air oulet and the pressure gauge) and then I removed the three screws of the cylinder head so that I can lay the piston bare.
Finally, I had to find the two valves....
Step 3: Finding the Valves and First Drilling Operations
In this model of compressor, the first thing I noticed is that the valve of air intake is not on the "head" as commonly the compressors have, but on the piston itself (In the first picture you can see the reed on the piston).
So I had first to remove the reed (and keep it for the next step) and fill the hole.
Besides, on the cylinder head I had to make a new hole...that it is simply the valve "transposed" for convenience, because it would be impossible to use it in a mobile piston!
The other valve instead, that relating to the air outlet, was kept as such with no modifications. This valve does not have the reed but a small spring with end seal (SEE PICTURE IN THE PREVIOUS STEP).
After having drilled a new breach (the hole diameter is 2 mm), both sides of the head with the two valves are showed (PIC. 2 AND 3).
Finally, in the inner side of the cylinder head, another small hole was made of 1 mm diameter in order to fasten the reed I saved before. It was screwed on the metallic body, near the valve itself (LAST PICTURE).
Step 4: Filling the Holes and Verifications of Mechanics
The first thing to do was to fill the hole left open on the piston, after removing the reed.
I used for this a epoxy resin (bicomponent), specific for metals objects. I put the two part on a plastic tray and mixed them in the ratio 1:1 with a toothpick. Before using the paste I cleaned well where I meant to use it, with some alchol and a cotton fioc.
In the last picture is shown the valve of the piston filled by the epoxy and a small spheric piece of metal.
After that, I filled also the hole accomodating the pressure gauge, on the T-shape joint, inserting a small screw too. At the cylinder head, where is the new exhaust valve I plugged a small piece of aluminium tube, slightly crushed in the center otherwise it didn't fit.
For both operations I used another small amount of the epoxy (SECOND PICTURE).
Step 5: Assembling the Pump and Final Observations
The appearance of the pump at the end is the one shown in the pictures.
At the small aluminum tube connected to the inlet valve I added a sufficient length of rubber hose and I put it out of the box through a hole in the plastic.
Now it's ready to use with normal 12 V car battery.
I must make one comment about the electrical power supply. Since the original compressor has a high current demand (up to 11 A idle, up to 15 A on working) a proper power supply is needed. For this purpose, using it at home I employed a modified PSU from an old PC. Nevertheless, in the standard 12 V the device doesn't work so I lowered to 5 V and in this way it worked.
Maybe, I may have lost a little bit in terms of motor speed (and then in the suction capacity) but the result seems satisfactory.