LEAD is a term with which most steam engineers are familiar but largely as a dimension within the valve and ports, not as a dynamic effect upon the valve events in the steam distribution cycle. It is cited as the amount by which the port is open to live steam when the piston is at dead centres and can be physically dimensioned and measured when the crank is at 0o and 180o. Since it is such a small fraction of other dimensions within the cylinder and valve gear area it follows that care and accuracy will be required in order to obtain the correct figure in practice. It cannot be ‘machined’ in the manner that lap is achieved, for instance.
Expanding the term, lead constitutes an amount of steam admitted before dead centre in order to cushion the mass of piston and associated parts. In most applications this will be required equally at each end of the stroke but large vertical engines obviously create a condition whereby more cushioning is necessary at the bottom end of the cylinder(s) than at the end of the upstroke (normally achieved in conjunction with unequal laps). Moreover, on starting or at slow speed lead has virtually no function.
As piston speeds rise cushioning may become desirable according to the compression characteristics and an earlier start to the process of getting steam into the cylinder may be advantageous. Having now introduced the time factor it makes sense to consider how long the port remains open before dead centre, rather than by how much: lead as a dimension during the design of the valve gear and ports becomes a quantity of preadmission, an advance of the whole timing. It is the acceptance of the behaviour of preadmission that determines a suitable lead dimension, not the other way round.
Since the masses that require cushioning are virtually absent in models (even large ones) and the piston speeds are ridiculously slow by comparison to full size, is lead desirable at all? The quick answer depends on what the engine is expected to do, but the technical answer is that one cannot intrinsically ignore lead. Its effects are present because notching up the gear alters the preadmission as a function of timing. Even the constancy of lead attributed to Walschaerts’ gear is not scientifically absolute. Preadmission always increases with shortened cut off.
Stephenson’s gear exhibits an increase in lead as mid gear is approached (except in the case of cross rods), whilst most radial valve gears are said to have constant lead. Many GWR engines had a negative lead setting in full gear so that towards mid gear the resultant increase to positive lead would not be excessive. This perhaps gives the wrong impression. Lead determines the preadmission characteristics affecting compression towards the end of the stroke; in other words lead gives rise to a timing device, and one which advances all valve events as cut off shortens, irrespective of the valve gear employed.