Steam Loco Wheel Size

[farecoal]

Why is it that the primary driving wheels on more recent steam locomotives were so large? And why do diesel and straight electric locomotives usually lack large wheel sizes? Any answers would be highly appreciated.

 

[captainkman]

Why is it that the primary driving wheels on more recent steam locomotives were so large? And why do diesel and straight electric locomotives usually lack large wheel sizes? Any answers would be highly appreciated.

I can answer the first part.

A larger wheel radius means that you go further per wheel turn when compared to smaller wheels.

I think the reason many diesels and electrics have smaller wheels has something to do with the fact that with a smaller wheel, a larger percentage of the wheel is available for traction, therefore minimising wheelslip.

 

[bendorsey]

Overall passenger locos have larger diameter drivers (for speed) then feright locos (for power).

A very general rule of thumb with steam locos is the (theoretical) max speed is roughly 1.2 times the driver diameter in inches so a loco with 50 inch drivers should max out at about 60 MPH. One with 80 inch drivers around 96 MPH. Obviously there are exceptions and other factors that apply. One important one is the amount of weight in reciprocating motion (side rods, connecting rods, cylinders, etc). The more drivers the more weight so a 4-4-2 isn't as hard on the track then a 4-8-4 at the same speed. The overall weight of the loco gets involved as well. Too much weight in reciprocating motion can be a design problem. The VGN 2-10-10-2 had so much (including 5 ft diameter front cylinders) it was limited to about 15 MPH. Any faster and it would not only tear the track up it would tear itself up as well. Diesels do not have any weight in reciprocating motion associated with the wheels (the actual diesel engine inside does of course, lol).

Diesels have the advantage of a gear box (tho its done electrically rather then mechanically). Steam locos don't although changing the cutoff does something somewhat similar. A diesel might start out with the motor windings configured (wired) as a 72 pole motor for max power. As it speeds up it can be changed to a 60 pole motor, then 48, then 32, and so on. Power decreases but speed increases.

Smaller diameter wheels on diesels allow more wheels in a given length (ridgid wheelbase). More wheels in contact with the rails for more traction (as mentioned above). This also spreads the weight of the loco over more distance decreasing axel loading so lighter rail can be used.

Lots more to this I'm sure.

Ben

 

[whitepass]

Most older Diesels do have mechanical gear boxes.

 

[SD45T-2]

Also due to the nature of Diesel's and Electrics, the wheel size between locos make be only inches. A SD60I has 33 inch Wheels, while an SD80MAC has 36 Inch wheels. That allows it to go faster. If the SD60I had 36 inch wheels it would go faster to.

They made the TGV Duplex go faster By adding 40 something inch Wheels. And thus made a world record.

 

[bendorsey]

Also keep in mind the faster the wheel turns the more expensive the bearings are. I worked on jet engines and some of those bearings cost $50,000 each! Granted loco wheels don't turn anywhere near as fast as those in a jet engine but on the other hand they have a lot more weight on them.

Ben