How much lap?

[half whitParticipant @half-whit]

Hi All

I’m currently making a small slide valve, own design, 1/2″ bore, 13mm stroke.

The drawing here is a possible slide valve arrangement.

Is the lap too much? Any advice on calculating the lap would be welcome.

The valve rod will be 2.5mm dia and threaded 7 b.a.

Any thoughts gents?

[JasonBModerator @jasonb]

On my 12mm bore ones I tend to go for a bit less, typically 0.1mm but I do run on air and have round ports rather than slots so you don’t get quite such a large area of port uncovered at the beginning of the valves movement.

I’d make it as you have it an dif need be take a little off but it will probably be fine.

Typical one her, the width over the two holes will be 10mm and my valve is 10.2mm, same 2mm throw on the eccentric

 

[Nigel Graham 2Participant @nigelgraham2]

If someone else would care to review my calculations… It’s been a while since I examined a valve’s dimensions.

Mid-gear valve travel = 2(Lap + lead), but your design seems to have no lead, which is fine here.

I.e. the valve is only just on the point of opening to admit steam when it starts its return journey.

So Full-gear VT = 2(lap + port opening). I’m guessing this engine will work on a fixed cut-off, as typical for engines used for driving stationary plant in factories and the like. (They used governors to keep the speed constant, where necessary.)

The ports don’t normally open fully to admission, only to exhaust.

Full VT = 4mm, neglecting any slight angularity effect of the eccentric rod.

So for each end, L + P = 2mm, giving the full  1.5mm port opening and a very late cut-off with 0.5mm lap.

I’d suggest more lap, not less. 1 mm lap will give 50% port opening, probably enough for this engine unless you intend giving it some fairly heavy work (by its scale) to do.

I’d be tempted to test it with a dummy valve, perhaps made from transparent acrylic, so you can also ascertain the exhaust events by turning the engine by hand with the valve chest open, but if you make the valve a bit too long you can at least trim it back.

My club used a similar method, with a transparent valve, to prove a drawing error in a locomotive it built: there the laps so precise to drawing, were too long, making proper valve-events impossible.

 

[Charles LamontParticipant @charleslamont71117]

Nigel, the valve is shown at mid-travel and the position of the eccentric is not shown, so I don’t think you can deduce anything about the lead.

If there were 1mm of lap and the 4mm stroke were retained, the ports would be opened to 1mm, or 2/3 of their width, not 1/2.

The amount of lap can be more for running on steam, but if the engine is to run on air then it does not need much lap. Also, the higher the inlet pressure, the more lap is justified.

 

[Nigel Graham 2Participant @nigelgraham2]

Thank you for that – I did invite peer-review!

[JasonBModerator @jasonb]

here is another, Clarkson Vertical. Although converted to metric the proportions are the same as the original drawings.

3mm ports and 1mm lap so that would equate to exactly the same as HW’s 1.5mm and 0.5mm.

As for how much of the inlet is uncovered it actually has more throw at 4.5mm which would be 2.25 on HW’s engine, this takes the edge of the valve beyond the inner edge of the port.

Runs very nicely and Mr Clarkson obviously knew what he was doing.

Mid travel

Full Travel

[duncan webster 1Participant @duncanwebster1]

A better question would be what cut off should I have. If you then know the valve travel and how much lead you want (not a lot) you can work out the lap. I detailed it in my ME article a bit ago, too late now to look it up, I’ll do it tomorrow if anyone is interested

[JasonBModerator @jasonb]

I tend to ask what do you intend doing with the engine?

If it just built for fun as a display engine that may have the occasional run on a few psi air under no load, at tick over with the odd burst up to 2000rpm?

Will it run on air but be used to drive a load?

Will it get run on steam ? Again for display or to do work and if real work will it be flash steam as was used on a lot of little engines?

Are you worried about economy of steam or power produced or will you just be happy that it runs?

[JasonBModerator @jasonb]

I’m not offering but thinking about the above while in the workshop something like “designing & building for display” would make a good article.

All the stuff I have seen about valve design – lap, lead, cutoff, etc has generally been based on steam locos where getting as much out of a lump of coal while pulling as many fund raising paying passengers seems to be the main aim.

When it comes to stationary engines the majority are built to only run for display and pleasure. So what are the different requirements to get the slowest smooth running on say 5psi of air. There are certainly things that can be done at the build stage particularly for vertical engines and with no expansion (or condensing) the valve and its movement are somewhat different to the average loco’s needs.

[duncan webster 1Participant @duncanwebster1]

I agree with Jason, if you want smooth and slow use a late cutoff. Here’s the maths

 

[JasonB]

On JasonB Said:

…

All the stuff I have seen about valve design – lap, lead, cutoff, etc has generally been based on steam locos where getting as much out of a lump of coal while pulling as many fund raising paying passengers seems to be the main aim.

When it comes to stationary engines the majority are built to only run for display and pleasure. …

And those locos are BIG, whereas models are small.  Scaling down massively reduces the need for lead.  The purpose of lead is to provide a cushion so the piston doesn’t hammer the cylinder end.  Hammering is a serious problem in full-size where large heavy pistons travel at high speed.  In a model a small light piston travels slowly, so there’s no need to cushion the piston.  I suggest lead can be ignored in a model.

Steam Lap cuts the supply of steam into the cylinder off early, which by allowing the steam time to expand maximises the amount of heat energy in the steam converted into work.   Important for fuel economy in a working engine, but almost irrelevant in one that only runs for display.   Whilst input lap isn’t critical on a model engine it’s worth getting roughly right on a loco that hauls passengers on a track, otherwise don’t worry.

Pedantically, the amount of input lap needed for compressed air is different to steam, and for steam it varies with temperature and pressure.  But not so as anyone would notice in a model!  Exhaust lap, I think, is even less critical in a small model.

I think rule of thumb is good enough on a model, and calculating lap dimensions ‘properly’ won’t add value.  It’s because the scale is small.  Getting lap roughly right, by copying other people or experimenting, and tweaking to suit what the engine is intended to do should be “good enough”.   Designing a full-size working engine is another game!

Dave

 

[JasonBModerator @jasonb]

I would say they are not far off size wise, Several times I have butchered loco cylinder castings as a good cheap source of bronze for a cylinder. A G1 loco may be 1/2″ bore like the OPs and a 3.5G about the same size as a Stuart Victoria etc. And I’ve got stationary engines that would be similar bore/stroke to a 5″G loco

As for speed, the models can also run fast, That one in my first reply will tick over at 60rpm but when run at it’s intended speed will go at 2250rpm

All my models have lead so not sure why you say it is not needed. Quite likely an engine won’t run without some lead.

Economy can come into it with a model, how many times do we see posts buy makers with a small compressor where a bit of economy may make the difference between the compressor keeping up with demand or an engine that just stops. Same would apply if on a small boiler where it can only produce steam at a certain rate.

[duncan webster 1Participant @duncanwebster1]

Locos with poppet valves are arranged so that both exhaust valves are open all the time when the loco is in mid gear. Hence no cushioning. Lead is provided so that the steam inlet is open a bit so that there is full pressure available from the start of the power stroke. Inadequate lead in full size has disastrous effect on indicator diagram and power at speed. Doesn’t matter much when going slowly, and models have low piston speed.

[duncan webster 1Participant @duncanwebster1]

But having a little bit of lead makes valve setting a lor easier (with slide valves at least)

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