Valve Gear Basics
I've spoken to quite a few builders of locomotives etc. over the last few years and it's very apparent that some don't really understand the fundamentals of how valve gears work so perhaps a few words on this might be in order. Maybe an explanation of the various terms used may also help.
The valve gear is there to move the valve over the cylinder ports allowing steam to enter the cylinder at the correct time and then allow it to escape once it's job is done. The valve gear also controls the distance that the valve travels across the ports and the time that the ports open and close relative to the stroke of the piston. This refers to conventional cylinders with slide or piston valves and not to more sophisticated cylinders with poppet valves etc. although the principles are the same.
Usually, the port opens to admit steam to the cylinder slightly before the piston reaches the end of it's stroke and is about to return in the opposite direction. The time when the port closes and 'cuts off' the steam to the cylinder is varied by the valve gear and known as the cut off point. If the cut off is 100% then the steam is admitted for the whole of the piston stroke (100%) of the stroke). If the cut off is 50% then the port closes when the piston has reached 50% of it's stroke. The reason for this variable cut off is that after the steam supply to the cylinder is stopped, the steam continues to expand and can still do useful work, allowing the amount of steam used to drive the pistons to be reduced thus improving the efficiency of the engine. The very early steam engines did not have the facility to cut off the steam part way through the piston stroke. Steam was admitted through the whole piston stroke and the engines were very wasteful of steam and very uneconomical to run.
In our miniature versions it's usual to design the valve gear to have a cut off of at least 75% in full gear to ensure easy starting.
LAP or STEAM LAP
Lap of the valve is the amount that the valve face that controls the opening and closing of the port overlaps the steam port when the valve is central over the ports. In the example below, the lap is 0.09375". This is a common figure used for 3½" gauge cylinders with 0.125" wide ports.
Valve Lap on outside admission valve
The lap of the valve gives a period when the cylinder port is closed to steam and allows the steam already in the cylinder to expand and do further useful work. This makes the engine more efficient in the use of steam and more economical to run.
As mentioned above, the very early steam engines did not have any lap on the valves, in other words the valve face was the same width of the port. This meant the ports were either open to steam or open to exhaust. Because of this the steam could not be used expansively and made the engines very heavy on steam consumption.
No Valve Lap
In theory, you could have negative steam lap where the valve does not completely cover the port when in mid position but I don't know of any instances where that has been used?
NEGATIVE EXHAUST LAP or EXHAUST CLEARANCE
Normally, the exhaust cavity in the valve is the same width as the distance between the exhaust edges of the ports (known as 'Line for line') but negative exhaust lap is where the exhaust cavity in the valve is wider than the distance between the exhaust edges of the ports. When the valve is central over the ports both ports are open to exhaust by a small amount. LBSC was a big exponent of negative exhaust lap and many of his designs incorporate it although it doesn't seem to be used by more modern designers.
The idea is that the ports open to exhaust earlier than normal and reduce the compression of the residual steam that occurs at the end of the stroke after the port has closed to exhaust. This is supposed to result in a freer running engine. I suspect that in LBSC's case the beneficial effect was to compensate to some extent for deficiencies in the actual valve gear designs although full size locos did use it. LBSC used an exhaust lap of 1/128" (0.0078125") at each port i.e. the exhaust cavity was 1/64" (0.015625") wider than the distance between the ports and I thnk was the same for any gauge loco he designed.
Negative Exhaust Lap
It is also possible to have positive exhaust lap where the exhaust cavity is shorter than the distance between the ports. The effect of this would be to delay the opening of the ports to exhaust.
LEAD or LEAD STEAM
Most valve gears are designed to open the ports to steam slightly before the piston reaches the end of it's stroke and starts to go back the other way. It's a bit like the ignition advance on an internal combustion engine. The figure quoted for lead is the amount the port is open when the piston actually reaches top or bottom dead centre and varies according to the design of the valve gear (and the ideas of the particular designer!) It was introduced on full size engines to provide a cushioning effect when the piston reaches the end of it's stroke and has to rapidly stop and change direction which can cause undue stress to the rest of the motion. These stresses greatly increase as the speed of the engine increases. By introducing the steam before the end of the stroke the steam pressure acts on the advancing piston and helps to slow it down. Allowing the steam to enter early can also be an advantage at high speeds but allowing it to enter too early will of course have a detrimental effect as it will try and push the piston back the other way!
Lead - the amount the port is open when piston is at TDC and BDC
Valve gears such as Walschaert's are said to be constant lead valve gears. The amount of lead remains constant over the whole range of cut offs that the gear is designed to provide.
Valve gears such as Stephenson's for example are variable lead valve gears. The amount of lead changes as the cut off point changes. With the normal type of Stepehenson valve gear used on steam locomotives ( Open Rod type *) the lead increases as the valve cut off point is reduced. This can cause problems as the lead can increase to excessive levels when the cut off reaches the desired amount for economical running. To avoid this the valve gear may have to be designed to have less or even negative lead when in full gear so that it does not become excessive at normal running cut offs. Some GWR valve gears are examples that have negative lead in full gear. This can be an advantage for starting off as when the speed is low, too much lead steam may actually hinder starting.
In another design of Stephensons gear the amount of lead decreases as the cut off is reduced. This is the type that has what is known as 'Crossed Rods *' but I believe this is normally used on traction engines rather than locomotives.
* The difference between 'Open' and Crossed' rod examples of Stephenson's valve gear is how the rods appear when the crankpin is on the opposite side of the axle to the expansion link:
Note that constant lead does not mean that the valve opens the port at exactly the same point before TDC or BDC for all the range of cut offs. If you look at a diagram for Walschaert's gear showing the opening point of the port versus the piston position versus the cut off point, you can see that the port actually opens earlier and earlier as the cut off is reduced. The amount the port is open at TDC or BDC remains constant but the time the port is open before TDC or BDC or the preadmission period varies. The preadmission period increases as the cut off is reduced.
This confused me at first as I noticed that when I was setting the valves on Helen, the valves opened earlier as the cut off was reduced. I assumed that it was the lead that was changing but actually it was the preadmission that was changing.
Stephenson's gear also exhibits this characteristic of increasing preadmission as the cut off is reduced and the reason for reducing the lead in full gear is not only to prevent excessive lead as the cut off is reduced but also to prevent excessive preadmission.
Do we need lead steam in miniature locos?
The big debate is whether lead steam is really necessary in miniature locomotives at all as the mass of the reciprocating parts is insignificant as are piston speeds compared to full size so we don't need the cushioning effect at the ends of the piston strokes. However, LBSC was a big advocate of lead steam and Don Young's designs use quite large amounts of lead, often 0.030" or so. I don't think I've come across a design that doesn't incorporate lead so all the designers must consider it necessary but is this a case of everyone just following what has gone before? I tend to hedge my bets and design my valve gears for a lead of 0.010" at 50% cut off! It would be useful though to do some experiments with an engine where the lead could be varied easily just to see what difference it does actually make.
One advantage I can see for using lead is that it will compensate partly for slop and eventual wear in the valve gear. As the valve gear wears the valve will open later and later so if it's set to open early to begin with then it will probably still open at least at TDC when the valve gear is worn!
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