Hi Guys,
I thought it might be of help to add a few notes about setting up Stephensons' link equipped engines as it appears that some of you are having issues with this.
Die SlipOne of the most common mistakes made when using/setting up Stephensons' link valve gear is to move the link from one extreme of the link slot to the other when going from forwards to reverse.
Stephensons' link gear, especially on marine engines where the eccentric rods are relatively short, suffers far more from a phenomenon termed ‘DIE SLIP’ where the’ DIE BLOCK’ connecting the ‘VALVE SPINDLE’ to the ‘SLOTTED LINK’ slides sideways in the link slot rather than pushing/pulling the ‘VALVE SPINDLE’.
This occurs because of the extreme angularity imposed on the ‘LINK’ as one eccentric moves towards the link whilst the other is moving away.
This effect is much more severe on a marine type engine where the lengths of the eccentric rods is short, thus making the angular variations between the forward and reverse eccentric rods, and hence the angularity of the ‘LINK’ more severe which in turn presents the 'DIE' to the slot at a much more acute angle… resulting in a sideways movement of the 'DIE' in the 'LINK'.
On a steam locomotive this angularity problem is overcome, to some degree, by fitting the eccentrics to a more rearward axle, often the most rearward, which gives the maximum possible length for the eccentric rods, which in turn reduces the angular variation, but even here some DIE SLIP still occurs.
The way to overcome this is to not use the full length of the available ‘LINK SLOT’ but to restrict the movement to around 90% to 95% of the available slot length in either direction about the centre of the ‘LINK’
This will allow a short length of slot to accommodate the inevitable ‘DIE SLIP’
A properly designed ‘STEPHENSON’S LINK’ gear will have been designed to work at this slot position.
If the full length of slot is used then any ‘DIE SLIP’ will cause the ‘DIE’ to hit the end of the slot as the engine rotates with the result that the whole link assembly, and any connecting levers, will have no option but to move in sympathy thus leading to the perceived, and unwanted, lever movement.
DIE SLIP occurs when the angularity of the LINK relative to the DIE BLOCK is such that the DIE BLOCK will slide sideways in the link arm slot rather than move the VALVE stem in or out.
LOCKING the forward reverse mechanism at the extremes of slot travel, with no available slot length left for ‘DIE SLIP’ will only cause long term damage to the valve operating stem/bearing, and to the sliding die.
It is more usual to set the FULL FORWARDS and FULL REVERSE positions of the valve gear to leave some SLIP space available at the extremes of the LINK SLOT to allow for this DIE SLIP. I.E +/- 90% of available travel.
DIE SLIP is much more of an issue with marine engines since the eccentric rods are generally much shorter than on a locomotive and hence have greater angularity problems to overcome.
Try setting your full forwards and full reverse positions to around 90% of the available slot length in either direction and I think you will find a lot of the perceived problems become much less of an issue.
You will still need to use some form of servo buffering, since there will always be some frictional forces to deal with, but these will not be anything like as extreme as the DIE BOTTOMING out in the LINK.
John,
Lovely work, as always, on the refurbish job…. Is that the ‘MARCHER’ or the ‘BORDERER’ engine, it’s difficult to tell which from the pics, but I suspect the ‘MARCHER’.
Best regards to all.
AlexC
Author
Topic: fwd, rev & throttle control (Read 11821 times)