steerable trucks on Australian BHP locos

[miyakoji]

I've always been interested in these trucks, they look cool.  I infer from the text with this picture that non-steerable trucks provide less tractive effort than steerable types; I also guess that the steerables require maintenance.

 

Are the more simple looking trucks on EMD SD locomotives, for example, also steerable using a different method?  If not, does anyone know how much this reduces total tractive effort?  Does this only make a difference when starting load on a curve?

 

http://www.railpictures.net/viewphoto.php?id=126306

 

Example of SD trucks (note that maintenance crew is on a different track :))

 

http://www.railpictures.net/viewphoto.php?id=555890

 

Also, the class(?) lights are cool, kinda like Conrail marker lights.

[kvp]

There are two basic types of steerable bogies, one is the articulated frame which has various rods and other parts for semi active or active steering. These are relatively maintenance intensive. The more classic variant if the passive steering or swiveling axle bogies, which were used since the early days of streetcars. These are more simple systems with more flexible primary springs and no rigid side guides (only some rods and shock absorbers), so the axles can rotate in curves on their own. The only tricky point with the latter is the motor/power shaft to axle connection.

 

Bogies with horizonal freedom are only good to reduce loads on the rails in curves, but do not actually increase the tractive force, just decrease power loss caused by wheel and rim friction. To help maintain tractive force, you need vertical freedom and most primary suspension systems provide this. Some locomotives went as far to make the bogie frames flexible, but this is not reallly needed in most cases.

 

The third type of freedom required is side tilt freedom for really bad tracks, where the two railtops are not level and for this, you can use per axle independent suspension or fully independent axles, like the ones on the 3 axle bogie of an DE10. Those bogies have 3 steerable/independent axles. Good bogies on bad tracks are especially important as these rail plane errors can and usually do lead to traction force loss, due to wheels lifing off the tracks. (in the worst case up to 3 wheels per bogie for a 3 axle one)