![]() I am not going to pretend to be in the realm of who will give you a marvellous formula for lap, lead, port sizes and steam chest volumes. I have designed a few simple slide and piston valves, all of which have worked, and it has struck me that, though their design appears complex, it can be boiled down to a few simple rules of thumb. Perhaps, if other readers agree, the editors will consider this suggestion?Īside from the perennial rewinding of various electrical coils (at 46 am I one of the youngest people who remembers winding their own transformers by hand?), one of the commonest requests was for the dimensions of slide valves. I think it would be a marvellous idea to recreate such a feature I have benefited from the advice of the editorial team on a number of occasions, and I would have had no qualms about this correspondence being published. These enquiries were often very practical, such as "where can I get castings for X?" or "how many turns are needed to rewind motor Y for 240 volts?". Readers had to write in enclosing a small voucher printed in each issue, to receive a response from luminaries such as ETW or even Percival Marshall himself. In the dim past, Model Engineer used to have a section for queries, separate from the main letters page. ![]() and shape it up as a proper article, Valves I designed this way have worked! Happy to receive criticism of it - it may be complete nonsense to anyone else. I wrote this six years ago (I know becauise I'm not 46 any more!), then decided others know better than me. Using your figures the total valve travel would only be 10mm. If you're not too familiar with valve gears then this might help to explain the terms used etc.:įizzy - your dimensions are half what they should be. You will also need to fit the exhaust port inbetween the steam ports. The distance between the ports doesn't affect the valve timing and is more a case of picking a suitable figure so that the valve will fit in the valve chest without hitting the ends. a fixed eccentric or a variable cut off valve gear such as Stephensons link? If you are using a fixed eccentric then you probably want the cut off to be 50 - 60% to avoid wasting steam. Will you be using a fixed valve gear e.g. If you make the port width 7mm and the lap 3mm then the maximum cut off will be 91% If you make the port width 5mm and the lap of the valve 5mm then the maximum cut off is 75% Things are complicated by the fact that the ratio of the lap to the port width determines the maximum cut off of the steam to the cylinder. So if your valve travel is 20mm then your port width + valve lap will be 10mm assuming the port will fully open. (Lap of the valve is the amount the valve land is wider than the port) Total valve travel = 2 x (port width +Lap of the valve) assuming the ports fully open. You are really designing it a**se about face as you would normally determine the port width and the lap of the valve and then determine the required valve travel. mark out from max stroke 2.5mm land, 2.5mm inlet, 2.5mm land, 5mm ehhaust, 2.5mm land and 2.5mm inlet. then divide half stroke into 4 parts, first part is exhaust, then land, then inlet then land. Ok this is off the top of my head.if stroke is 20mm then eccentric offset is 10mm, assuming you want to use full stroke. Not much to sketch, just a steam chest 44mm long and the slide having a stroke (length of travel) of 20mm ![]() I can't get my head round it and it's driving me nutsĮric, Can you provide a sketch, with some dimensions, of where you are so far? How do I work out the position of the ports and the length of the slide valve. I'm designing a steam chest/valve with a predetermined stroke of 20mm.
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