Question about UK steam engines

Garrett Spear writes

The (vast?) majority were right-hand, mostly 90 degrees (Lord Nelson

4-6-0s at 120 degrees were an exception that springs to mind). Some other exceptions included ex-SECR types with left-hand lead. Don't know why some engineers (designers) had a preference for left over right.

Rob writes

It's to do with the position of the coupling rods in their rotation cycle - they can't be in the same position on both sides otherwise the loco would shake itself to pieces and/or damage the permanent way. Normally the right hand rod is a quarter-turn further round than the left, hence quartering with right-handed 90 degree lead. The balancing weights on the wheels are another component of the complex business of keeping a steam loco running smoothly and safely.

HTH

Thanks, I thought you meant the train driver. duh

Pennsylvania

Roderic,

It's a bit more basic than that, and more to do with the transmission of torque between two or more axles fitted with cranks.

In a basic two axle setup with one crank on each axle, there are points on the rotational cycle when no torque can be transmitted from one axle to the other - i.e. when the coupling rod lies in a horizontal line through the axle centres. To ensure transmission of torque between the axles, you need to supply a second set of cranks and a coupling rod, and the cranks should be at a different angular displacement to the existing ones so that one set can transmit sdome torque when the other set cannot. If the two sets of cranks are at

90 degrees displacement to each other (i.e. quartered) then you get the best set up for transmission of torque. Note that if you don't quarter the cranks and they are at the same displacement or at 180 degrees, then the same situation of zero torque transmission will occur when they are horizontal. You also get the problem of the driven axle possibly rotating in the opposite direction when starting from the horizontal setting, which cannot happen when the cranks are quartered.

When you have more that two axles, then the same situation can occur if the driving axle is one of the outer ones. If the driving axle is an inner one - say the middle axle in a three axle machine, then torque can be transmitted to the outer cranks if there was only one set of cranks, or two sets in zero or 180 degree phase, and the coupling rod was solid. But this would put a great bending stress on the coupling rod around the centre crank pin, for which coupling rods are not normally designed. They are normally designed to withstand tension and compression along their length. And locomotive couplings rods are normally jointed to allow for individual movement of axles, so the above situation could not apply.

Jim.

Lord Nelsons were 4-cylindered locos with cranks at 135 degrees. Most

4-cyl. locos had cranks at 90 degrees.

3-cyl. locos would have cranks at 120 degrees.

Roderic Camer>

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