COMMON QUESTIONS AND ANSWERS
Q How critical is dimensional accuracy?
A A little use of a simulator shows the order of accuracy required in both design and erection, valve setting being particularly sensitive. The basic datum is the driving wheel centre and the weighshaft and (Walschaerts’) trunnion centre set from this. Sometimes a drawing gives dimensions for these points square to the frames. Where the gear centreline is inclined all parameters are square to the gear line and are entered into the simulator so. Cylinder positions are divorced from the gear in that adjusted piston rods and valve setting accommodate slight diversion from the drawing. Small inaccuracies will not disable an engine but the benefits of excellence are to be had by care and attention to detail.
Q My initial simulation reveals wide differences in events. How should I proceed?
A The aim should be to gain equality of 3% or less, particularly in shorter running positions. Wild inequality indicates (apart from errors of input) poor basic design. Use a design program prior to simulation: a simulator is the wrong place to start designing.
Q The drawing specifies ‘check from job’. Why is this necessary?
A In the case of a reach rod it is difficult to hold a long dimension accurately in the building. A Walschaerts’ eccentric rod is critical to the lead setting at dead centres and is the first check of valve setters. The axle should be at the working height.
Q Having set the valve for equal leads the port openings are disparate.
A Ignore port openings unless wildly disparate: the time remaining open is more important.
Q Why is the Stephenson’s trunnion offset?
A This provision causes the die to slip to the correct place in order to account for angularity errors. The actual amount is not highly critical but the absence results in poor event equality.
Q How critical is the curve of the expansion link?
A Sometimes adjusting this radius can improve events, but it is as well to find the real culprit.
Q How sensitive is Walschaerts’ backset?
A Not very. If the swings are much more than 2° disparate good equality of events may be difficult to obtain, especially in both forward and reverse gears. It has often been practice to bias forward gear if necessary, as little working is required in reverse for most engines.
Q The simulator does not appear to cover a particular gear layout in one respect or more. How should I proceed?
A There is usually a workaround sufficiently plausible to ensure good simulation. Take care with minus signs in the input. To emulate a valve crosshead guide use a long hanger. The extension bar of GW types may be constructed horizontally without compromising results.
Q Each cylinder half should ideally produce equal power outputs, but the rear half contains a piston rod.
A This can easily be accommodated if desired by aiming at an event inequality of 1 – 1.5% in favour of the rear cylinder half.
Q The engine is a poor starter.
A Either events are of poor equality or the full gear cut off is insufficient – 75% should be adequate unless the track is inclined.
Q I can’t see the piston valve in order to set it.
A Never set a valve by sight – it is nowhere near accurate enough. First determine dead centres properly – not by sight. Next, set in full forward gear and clock measure the total valve movement. Set the simulation to exactly the same travel. Read off the piston/valve relationship list where the valve is central over the ports as the piston approaches front dead centre, clock the piston to this position and lock the valve centrally. All the simulation is then preserved. If the inputs for simulation are not known, whatever the gauge or scale of the locomotive, then they must be procured by careful measurement if the best valve setting is to be gained. Assumption is not good enough!
Q “But she runs well and notches up to mid gear alright”
A Nonsense! Driving impressions may reveal bad running but are no test for efficacy. If she runs in the centre notch the quadrant reading is faulty. Apart from all too common design faults there is a large difference between skilled valve setting to precise design office directions and simply being satisfied that the wheels turn. Full size insistence on 1/64″ means keeping within 0.0013″ in 1/12th scale!
Q Can I scale the gear from the full size drawing?
A Yes, if this includes all the port and valve measurements, but is not the type of work required of the engine model different? The scale lead is often far too much for a model, where low piston speeds allow almost immediate bore filling, and this requires some redesign affecting the whole gear.
Q What is meant by American ‘all square’ Walschaerts’?
A This is somewhat of a misnomer. The return crank angle with the main crankpin can only be 90 degrees if at the dead centres the expansion link tailpin lies exactly on the horizontal gear centreline. There accrues no advantage and the pitch circle may be much larger in such a case, increasing angularity problems, unless the expansion link trunnion is lowered considerably. This can cause asymmetry between fore and back gears. It has been suggested that inclination of the eccentric rod introduces problems but the author can find no such disadvantage.
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