Divided versus unified drive

This is one of several themes on the http://www.national-preservation.com/threads/proposed-new-builds.985610 thread, which may deserve a separate discussion.

The numbers of generally successful designs (and, come to that, less successful ones) with both arrangements indicate that neither has an overwhelming advantage. As usual in engineering, each presumably has some pros and some cons, to which different designers gave different weights in choosing which arrangement to adopt.

Differences include the patterns of stresses in the frames and the constraints on the inclination of the inside cylinder(s) and on the lengths of the connecting rods.

What are the pros and cons? Why was Gresley in particular so keen on unified drive?

 

You have to go back to Gresley's first 3 cylindered loco, and subsequently the famous meeting with Harold Holcroft. As well as Holcroft showing Gresley how a much better conjugated gear could be used, he also showed Gresley how the middle cylinder could be inclined to drive the same axle as the 2 outside cylinders.

Holcroft knew enough about valve gears to deal with the corrections required to deal with conn rods of differing lengths and the angularity that results.

Cheers,
Julian