Thread: 3801 to steam again!

I wonder if it was gearing rather than traction?

Originally Posted by Pedro_The_Swift

I wonder if it was gearing rather than traction?

Without putting to finer a point on it , yes .

Simply gearing. Two ways of looking at it - for high speed, you want to have the reciprocating mass (piston, con rod etc) changing direction as slowly as possible, since as the speed grows the stresses of accelerating and decelerating increase; so these have to be stronger, which means heavier, which means the stresses are higher - imposing a practical limit on the strokes per minute (you can shorten the strokes, but this reduces the available power for a given steam pressure and piston diameter). In the last quarter of the nineteenth century, many express locomotives had single driving axles with very large diameter wheels

For pulling power there are two factors at work. Smaller wheels (and more of them) allow heavier engine weight within limits of axle loading and length of rigid wheelbase, and the limit to tractive effort is a function of piston area, steam pressure, and the leverage between the crank and the wheel rim. How far out you can move the crank is again limited by needing to accelerate and decelerate the piston, plus the limits imposed by the need for track clearance. Piston diameter is limited by loading gauge, putting a limit on piston area, without increasing complexity.

There are a lot of factors that went into designing a steam locomotive, and most of them are compromises, so it comes as no surprise that a few designs were outstanding successes, a few others utter flops, and a lot were "so-so". A bit like cars or four wheel drives.

The 38 class has to be considered one of the outstanding successes.

John