I was up at Quorn during the early years and NB30 was giving trouble because of the limit switch which detected when the forward/reverse dogs on the DB gearbox were fully engaged. I was asked to take a look when NB30 refused to move; I did get it going by some judicious fiddling with the limit switch settings. I recall the look I got from Charlie Rosewarne when I pulled a little brass bristle brush (as used for suede shoes) out of my tool kit to polish the contacts.Locomotive NB 30 had the same engine and transmission, presumably a spare that became available as the NDHs went out of service. Has anyone looked at NB30, and is the engine an upright version?NB 30 has an upright 6-71 rated at 140hp and had an Alison 500 class (630) torque converter attached. This was joined by a small tail shaft to a David Brown forward/reverse reduction gearbox which in turn drives the jackshaft which drives the conrods of the original steam engine. The Alison torque converter has been replaced with a Twin Disc unit from the old SAR recovery crane. The advantage of the Twin Disc is obvious to everyone who ever had to drive the thing and try and change between forward and reverse on the David Brown. By leaving the changing mechanism on the DB alone, it is very easy to change direction with the Twin Disc unit. Fitting the Twin Disc had its problems with the tail shaft needing to be shorter and the resulting angles reduced, but it works.
The Allison/DB combination relied on a transmission brake so the dogs could engage but the same brake could stop the dogs engaging so there was a pneumatic arrangement to rock the shaft until the dogs slid into place, hence the limit switch arrangement.