For the last few hours I've been browsing several web sites related to railroad photography...the Reading in particular. I often look for photos of trains running on Temple Hill just north of Reading when I grew up there during the 60's and thoughout the 70's. I happened on to some very nice shots of the G3 class Pacifics and was immediately reminded of a unanswered question I've had about them.
Why did they have a boxpok driver set as a center axle and spoked on the outboard two?
I guess I had always assumed they were equipped that way since the middle axle received the direct force from the piston main rod. I've never known for certain.
Or...had the Reading discovered some other type of improved performance factor leading them to set up drivers like this. The G3's were very late entries in the world of steam...being built in 1948 and only remained in service for 9 years.
http://www.rrpicturearchives.net/showPicture.aspx?id=1726427
http://www.northeast.railfan.net/images/rdg214s.jpg
Just looking to satisfy a longtime curiosity.
Mark H
Modeling in HO...Reading and Conrail together in an alternate history.
Added strength to the main driver, I think you are right.
According to what I have read, the G-3 engines carried 260 lbs boiler pressure which was the highest of any RDG engine. This alone produced a tractive force about 15% greater than the other 4-6-2's. That factor alone might call for a stronger main driver. You are probably right about the main rod location being a factor. I think it was easier to get a good balance with this type of wheel as opposed to the spoke type.
Many roads used Boxpok, Scullin discs. or other such wheels for the main drivers when engines were built, converted, or upgraded.
Charlie
It's not just the added strength in the main driver, it's the added space for both the principal and the angled balance mass for low augment at high speed, and better seating and securement in the wheel for the main pins. (The advantage of lower deflection at the rim between the spokes may also play a role here, but if that were important, it's more likely that all the drivers would be switched to disc centers, or perhaps Web-Spoke)
Compare the details of the 1938 rebuilding of T&P 610 (for example as described in the Trains article about modern dynamometer cars behind steam), There are other examples, the CNW H class coming to mind, that got similar treatment when lightweight rods and good cast centers demonstrated their substantial augment-reducing benefits (both in revolving and reciprocating forces).
Conversely, the N&W J class, which has greatly reduced overbalance due to Voyce Glaze's methods, can make do with a spoked main even though the peak piston and rod loads are much higher than on a G3 (and the problems with the original rod design on the J show up in increased fracture of the comparatively long #4 pins, rather than fracturing main pins a la Niagara or MILW F7)