Could someone explain how the lead truck on a steam locomotive functioned i.e. the mechanism involved in guiding the locomotive into a curve?
Several types of mechanism were used historically. The methods used for two-wheel lead trucks are different from those used for four (or six) wheel versions. Bruce has a fairly good discussion of many of the designs used here in his book 'The Steam Locomotive in America' (search on this in the forum threads). There have also been some very good prior discussions on general design and practice, better than anything I could summarize quickly here.
Note that the job done by a lead truck is very different from that of a 'bogie' under a car, or on an axle-driven locomotive, and therefore the arrangements made to control its motion, the critical frequencies involved in high-speed operation, etc. can be very different from 'typical' modern practice. This is enhanced when the lead truck is equalized with some or all of the drivers (which is not always the case in older design practice).
The principal idea is that progressive force is applied between the truck frame and the locomotive chassis, while any tendency to develop oscillations or shock is damped or controlled. Ideally there will be very stiff compliance holding the truck 'on center' for small deviations, on the order of the perturbations produced by coned wheels running on straight track, then relatively easy compliance as the locomotive begins to enter a transition spiral or other easement of curve transition, then progressively stiffer compliance as either the absolute deflection of the truck relative to the chassis or the rate of momentary increase in deflection increases. You may have noted already that simple mechanisms cannot produce all three of these effects continuously...
In addition, there are deflections other than those strictly produced in curve negotiation which a truck must follow and compensate for. One of these is the transition to superelevation in curves; another might be low rail joints. It is not uncommon to see this compensated, in a two-wheel lead truck, by carrying the equalization via a single bar applied at the effective centerline of action; this allows the axle to cross-articulate and take up an equalized loading 'automatically' without overconstraint. This also decouples lateral guiding force from roll force ("hunting" is a combination of nosing and rolling which can produce significant problems for vehicle stability)
Further discussion on this topic should include those designs of truck that combine 'leading axle' and driver motion, like the Zara and Krauss-Helmholtz trucks -- these have been given significant attention in prior threads in this forum concerning locomotive design.
Some years back when reading about locomotives, I was a bit puzzled by the term "lifting" a locomotive into a curve. When crawling around a consolidation on display nearby, I was surprised at the complicated design of the suspension. The weight of the forward end was supported by the leading truck equalized with the lead driving axle, but the leading truck was mounted (with springs) on a platform with four links to another platform below it which bore the weight of the front of the locomotive. If you picture the locomotive starting into a curve, the leading truck would be pulled to the side, the links would hang at an angle, not only pulling the locomotive that way, but since they are at an angle the vertical distance would be shorter, thereby "lifting" the locomotive into the curve. This is just a non-technical way of explaining it the way I picture it. I couldn't find any decent illustrations in my books or on line.
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"A stranger's just a friend you ain't met yet." --- Dave Gardner
Apparently this topic was of no concern to the men in locomotive shops or to men on the road, for I could find no discussion of it in The Locomotive Up To Date. while paging through the book, I did find an interesting recipe for Tempering Chisels (pp. 778-9):
"Add 3 ounces spirits nitre, 3 ounces white vitriol, 3 ounces sal-ammoniac, 3 ounces alum, 6 ounces salt, with double handful of hoof parings, to 3 gallons of water. Heat the tool to a cherry red, and temper in the above solution. It gives very good results."
It may be hard to come by all of the ingredients today.
Johnny
John White's books on the development of the steam locomotive has some pretty good diagrams. The swing link suspension was developed around the end of the Civil War, and was used right up to the last Chinese-built steam.
John H. White Jr. (1997)
American Locomotives - An Engineering History 1830-1880
John H. White Jr. (1980)
A History of The American Locomotive Its Development 1830-1880
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