I am rebuilding a tilting 'furnace body' ie an induction furnace coil unit with crucible mounted in it so it tips just over 90 degrees to pour the molten metal. Tilting is by a pair of double acting pneumatic cylinders acting in parallel, with the air controlled by a single spool valve lever up/off /down thingy. This beast has been out in the open for too long and the bearings and pivots took quite some freeing to get the rust out. Now it 'sort off works' but the control is far too crude. Raising to tilt is ok'ish but the down stroke wacks it down far too roughly. Somehow I need to set the air flow rate so things are a little more gentle and controlled. Is it just a case of throttling the air with a valve, or is there an analogue version of the spool valve rather than the digital on/off type that is fitted?
Andrew, you can set this very simply by putting a manual flow control valve directly into the 'exhaust' end of the pneumatic cylinder, then just wind in or out to adjust the speed.
Sorry Peter but these don't work as they should because air is compressible. In Andrews case as the furnace tilts it's weight alters and generally stuffs any load calculations.
A while ago i had to fault find some big doors on a CNC machine tool. that raised by air and lowered the same, much as Andrews setup I imagine, Going up was alright but coming down it smashed shut that hard the glass flew out. Throttling the air just reduced to time taken until it was nearly closed and then everything caught up and it slammed shut again.
To be honest it was embarrassing and very dangerous.
The fault lies in the fact air is compressible and if you wait for enough air to collect it increases acceleration.
What I did was to make a tank from a piece of 4" box section about 6" long and weld a top and bottom on it with a 1/4" BSP boss in both plates. The bottom of the tank went via one of these valves that Peter has mentioned to the bottom of the air cylinder, the tank was three quarters filled with hydraulic oil and the top connection went to the up side of the spool valve.
In operation when you go up air presses on the surface of the oil and pushed the cylinder up. When you come down air goes directly to the top of the cylinder and forces the oil past the restrictor valve back into the tank. No oil escapes as you have more oil in the tank than the cylinder holds and you have air space on top of the oil so it can't return to the spool valve. Because the oil is incompressible the down movement is very steady.
What you have is an air / hydraulic damper in effect.
Exactly the reason they used Hydrocheck circuits on the old pegboard machines to control feedrates.
Air just cannot be made to behave properly, if you want smooth motion.
Could you arrange another cylinder in parallel but use oil rather than air and a flow control valve to adjust resistance. The Air circuit will then work against the hydraulic circuit.
John, point taken that the load is fairly large on Andrews furnace, but I've been fitting these flow controllers on pneumatic systems for
20 years and they are very effective. If we had a particularly large download with possible shock impact at the end I would either use a cylinder with cushioned end travel or fit an Ace Linear Decelerator.
I completely agree that air over hydraulic gives a far smoother and much more linear control, but my thoughts were that this is a relatively cheap way to get control where none exists at the moment.
Now this is VERY interesting ! The two cylinders and valve say that they are pneumatic fittings, but the scrap furnace man that I got it from reckoned that the foundry had been using the cooling water for the coil (pumped round at about 45 psi). Now that gives me two problems. Firstly it leaks like a sieve - the cylinder end seals are hissing. Secondly, my furnace coolant water is a sealed system to keep air bubbles out - takes AGES to de-air it and is filled with expensive special antifreeze. This furnace body plumbing is done in 15mm stainless steel pipe and standard compression fittings between the cylinders and spool valve which I had assumed was to be proof against the odd splash of molten metal. I can see I'm going to have to strip the cylinders and see if they are repairable, I don't fancy oil (as per Johns cunning suggestion) or coolant spraying out at the wrong time. ( a few weeks ago I was doing a melt when it was pouring down outside, and a single rain drop dripped into the crucible of molten iron - made a very impressive BANG ! )
I stripped the spool valve to see if I could change any seals, and was expecting to find a cylindrical spool with 'o' rings. Instead it was a flat surface with ports cut into it, with a plastic shoe sliding over them rather in the fashon of a steam engine valve chest, with pressure of the air / water / oil etc pressing the shoe in tight contact with the face. This explains why it leaks like a sieve at lowish pressures and improves at the pressure increases.
Cheap power source, over the years I must have built 5 or 6 lifts for friends, powered by mains water supply. First is still going after more than 20 years. Since I convinced a friend to automate his glasshouse ventilation system utilizing water pressure, we have not had to buy tomatoes. :-)
My apologies for the language, and I'm not having a go at you Andrew, but you brought back some very unpleasant memories and I wouldn't want anyone to suffer like that, or worse.
Getting back on topic, pneumatics are a very poor form of motion control if you want to do anything more than slam something from one stop to another, in effect you're trying to move something by pulling on it with bits of elastic. Our production machines make extensive use of pneumatics but not for precise control, just moving from stop to stop. Yes you can restrict the flow or even get proportional valves, at a price, but at the end of the day air is springy. If you want control you need either hydraulics or motors and gearboxes/leadscrews.
You seem very skilled in assuming that you are the font of all knowledge regarding safety on this group, and that everybody else has no apptitude / skill / experience / or common sense. Many is the time I have bitten my tongue and refrained from replying to some of your irritating posts, as I have no wish for a flame war, but really at times you demonstrate a very annoying trait.
That said, as one who has lost an eye in an accident at the age of five, I sympathise with your aluminium accident.
So tell us. What are you casting in this marvelous facility and what are your rates per pound for additional projects?
Might have a Norton gearbox project in the forseable future and I'd be stuck between fabricated steel, aluminium or building a cupola and having the neighbours call the police :-)
I'm sorry Andrew but the very fact that you were operating with water dripping into the melt means you were not taking safety seriously enough. But hey, if you don't want to hear from someone who's been put in hospital from the same mistake then that's up to you, it's your life. So flame away to cover your mistake, I'm not bothered and won't waste my time answering you. But what you WON'T do is stop me from trying to help others who may not have realised how dangerous it was, you're not the only person in the world working with molten metal you know.
Andrew, if it would help you I have 2 sets of pneumatic catalogues from both Festo and SMC. Both are pretty big at 3 volumes each and you would be welcome to borrow these if it would help.
I'm going to the Midlands exhibition on Saturday and could bring them up in the car if there is anyone who could collect them from me to pass on to you.
I don't need them for a while so no rush to get them back.
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