Steam loco models valve gear.

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The middle number please? (6): 6

Steam loco models valve gear.

Post by ben_issacs » 28 Feb 2018, 03:07

An interesting feature of the setting of the Walschaert valve gear on almost every N, HO, OO and O gauge steam loco model is that it is set in what is called the 'mid gear position'.
To take this a bit further, a look at the Walschaert valve gear is in order.
Starting at the driving wheel, off the driving wheel crank is a short, offset crank, the return crank.
This drives the return crank rod which goes forward and connects at the lower end of the expansion link.
The expansion link is the curved, slotted link, sitting vertically, and pivoted at its centre so that it is swung back and forth by the return crank rod.
In the slot of the expansion link is the die block. which is part of the forward going radius rod.
By means of linkages from the reversing lever or wheel in the cab. the aft end of the radius rod and the die block can be moved up and down in the expansion link slot.
The forward end of the radius rod is connected to the top of a nearly vertical rod, the combination lever, the bottom end of which is connected to the piston crosshead.
A short way down from the top of the combination lever is the connection to the valve spindle.
From all this, it can be seen that the movement of the valve is affected by the return crank and to a smaller, but fixed degree by the combination lever from the piston crosshead.
If the die block is in the lowest position in the expansion link slot, this is 'full forward gear', and the valve moves its maximum distance, opening the cylinder ports fully to give forward motion.
If the die block is in the top position in the expansion link, this is 'full reverse gear', and the valve moves in the opposite sequence to that of 'full forward gear'.
'Full forward gear' would be set for starting off and for climbing grades or accelerating.
On the level, the driver would wind back from 'full forward gear' to somewhere closer towards the centre of the expansion link slot, thus reducing the valve travel and the amount of steam required to maintain the train speed.
With our model steam locos, they are always in 'mid gear'. this could happen in real life, the train could be coasting with just a whiff of steam going into the cylinders to keep them hot.
A loco could be standing in 'mid gear' with the loco brakes on. so even opening the regulator shouldn't move the loco.
But, from the modelling view point, does this fixed 'mid gear' position really matter?
It's very unlikely that in the smaller scales the die block could be made to move up and down in the expansion link to give the correct forward or reverse positions.
So, Reckon that we all live with our QJ's, JS's etc always standing or coasting!

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