My friend Brian and I have been on about building a cider press for ages.
There are many simple designs we could build using screw threads or bottle jacks, but Brian is an engineer - so that would be far too simple for him!
He talked me into trying to make a press that uses mains water press for the power to press the apples. This idea is nothing new as you can buy them on-line but they are rather expensive, our idea was to make one using cheap materials and then have the plans available for anyone that wants to make them.
We decided to build press out of 3/4" ply (18mm) and use a wine bladder from a box of wine as the pressing part. We built the ply box to the size of the wine bladder, we knew we were taking a risk with this method as the bladders aren't designed to work under lots of pressure or if they are we have no idea what that number is.
Brian adapted the screw cap on the front of the bladder to take some 1/2" copper pipe, this would make filling the box easier. Not shown on the pictures is the corner fillets we fitted in all the corners so the bags wouldn't be going into a sharp corner.
Our mashing of fruit was pretty basic - a mallet and a plastic bag!
The finished prototype! We added second side pieces so that dowels could be inserted to hold the top on and Brian fitted a pressure gauge so we could see the pressure we were working to.
And now the Science bit, straight from the brain of Brian :-
When we were pressing on the "traditional press" the "cheese's had an area of about 600 x 600 and the max force we could apply was 20T. We probably applied less as we bent the frame before maxing the jack out. Say we applied 10T of force.
So max force was 10,000kg or in engineering speak 100,000 N
The area was 0.6 x 0.6m so 0.36m2
Therefore the max pressure we could achieve would be Force/Area or 100,000N/0.36m2
which we can round to about 300,000 N/m2 or 300,000 Pascal (a Pascal is 1N per m2, i.e. bugger all)
There are 100,000 Pascal in a bar of pressure. A bar is also known as 1 atmosphere. So the max pressure we were applying to press in a traditional way was 3 bar. THe important thing to take away is the pressure in in the order of a couple of bar and not hundreds of bar which is used for olive oil presses.
Mains water pressure varies between 1 and 5 bar in the UK and I happen to know that we get around 2 bar at our place.
This is because you can also calculate pressure in "head" terms, which is essentially the pressure generated by a vertical pipe of water. The important thing is that the static pressure (the pressure when there is little or no flow) is only dependent on the height difference between the water supply and the delivery point. For every 10m of height difference the pressure will be 1 bar. I happen to know there is at least 20m of height difference between the back shed and the water tanks, so there is at least 2 bar of pressure available to squeeze apples (assuming the bladder doesn't pop)!"
|Goggles, we decided, were essential!|
We had the science sorted, we had the box built and we had some pears to squeeze.
Early signs were good, and juice started to flow. The pressure was making all the ply bend and there was some serious creaking going on.
We added straps around the outside as extra support, and then increase the pressure, then there was a small hiss and we decided to stop the experiment.
|Unfortunately the bag failed. A small hole formed where the bag creases in the corner.|
Unfortunately the bag had failed. It was only the smallest of holes, but we knew we were working with a bag that wasn't designed to be taken to this sort of pressure.
It was good fun to design and build something like this even though it didn't work properly.
I'm ever the sceptic and I couldn't quite believe the pressures that I was told we'd get off the tap (Theirs is from spring water with a header tank), but when we were pressing with it and the ply was trying to form the shape of a ball I understood the forces we were working with!
So although it's back to the drawing board I learnt a lot from this project and the next one will be even better! Although I don't think we'll bother until next year now though!
Anyone else spent time designing and building things only for them to fail?