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Re: Six wheel trucks on narrow gauge lines?
Author: SP5103
Date: 08-28-2011 - 08:08
Regardless of gauge, the reason for more axles is to spread the weight on the rail. A Baldwin catalog suggests that a limit of 300 pounds per wheel per pound of rail size.
40# rail (common on early railroads and narrow gauge) works out to about 12 tons per axle. (50 ton 4-axle locos and cars)
60# rail (common on branchlines and some narrow gauge) works out to about 18 tons per axle (70 ton 4-axle locos and cars)
In the case of passenger cars, the western US often used 6-axle trucks compared to the east, in part due to light rail (some 4 axle passenger cars were converted to 6 axles when air conditioning was added) and to improve the ride. Most narrow gauge cars were just toolight to justify the expense and wear of 6-axles.
Another issue with locos are to get enough traction motors and weight on drivers to use the available power. Compare a GP40 to an SD40 - both are 3000 hp. At 10 mph, a GP40 will only produce 2000 hp (derate by the electrical system) because it does not have the adhesion to use 3000 hp at low speed. Above 15 mph, an SD40 and GP40 will both produce the same tractive effort, but the SD has an advatage at not slipping at low speeds but hauling around extra unusable weight above 15 mph. Even an SD45 derated from 3600 to 3000 hp at 10 mph.
Modern adhesion systems allow greater use of available tractive effort at low speeds, and the use of 6 axles is more due to loco design weight and the ability to carry 5000+ gallons of fuel.
For narrow gauge (especially 36" and less), the traction motors are neccesarily physically much smaller, and have a restricted electrical capacity compared to standard gauge. In higher horsepower locomotives, the six powered axles are needed both to spread for weight and electrical output.
The A-1-A design doesn't make sense in most applications. There is only one purpose for the unpowered axles, to carry more loco weight. On passenger locos, this isn't that big of an issue unless the loco is expected to operate at low speed on long grades. This is why the EMD E-units fell out of favor on western US railroads in favor of the F unit. Three E units had around 6000 hp and 12 powered axles. To get 6000 hp, you needed 4 F7s, but got 16 powered axles and potentially more steam generator capacity. The PA did better compared to an E unit because the GE752 traction motors had more capacity than the contemporary EMD traction motors.
The other use of the A-1-A arrangement is on freight locos to reduce weight. The issue here is what the hp per powered axle is, and if it can use full power at its lowest operating speed (drag rating). GE published an article years ago arguing against unpowered axles, and actually built few diesel-electric designs that had them.
Time will twll if the BNSF/GE C4s are actually that valuable. Even with AC drive and weight transfer system, they still don't belong on heavy trains at low speed. THey need to be kept on high speed trains, but most modern US railroads prefer any of their mainline locos to work the same pool without restriction.
40# rail (common on early railroads and narrow gauge) works out to about 12 tons per axle. (50 ton 4-axle locos and cars)
60# rail (common on branchlines and some narrow gauge) works out to about 18 tons per axle (70 ton 4-axle locos and cars)
In the case of passenger cars, the western US often used 6-axle trucks compared to the east, in part due to light rail (some 4 axle passenger cars were converted to 6 axles when air conditioning was added) and to improve the ride. Most narrow gauge cars were just toolight to justify the expense and wear of 6-axles.
Another issue with locos are to get enough traction motors and weight on drivers to use the available power. Compare a GP40 to an SD40 - both are 3000 hp. At 10 mph, a GP40 will only produce 2000 hp (derate by the electrical system) because it does not have the adhesion to use 3000 hp at low speed. Above 15 mph, an SD40 and GP40 will both produce the same tractive effort, but the SD has an advatage at not slipping at low speeds but hauling around extra unusable weight above 15 mph. Even an SD45 derated from 3600 to 3000 hp at 10 mph.
Modern adhesion systems allow greater use of available tractive effort at low speeds, and the use of 6 axles is more due to loco design weight and the ability to carry 5000+ gallons of fuel.
For narrow gauge (especially 36" and less), the traction motors are neccesarily physically much smaller, and have a restricted electrical capacity compared to standard gauge. In higher horsepower locomotives, the six powered axles are needed both to spread for weight and electrical output.
The A-1-A design doesn't make sense in most applications. There is only one purpose for the unpowered axles, to carry more loco weight. On passenger locos, this isn't that big of an issue unless the loco is expected to operate at low speed on long grades. This is why the EMD E-units fell out of favor on western US railroads in favor of the F unit. Three E units had around 6000 hp and 12 powered axles. To get 6000 hp, you needed 4 F7s, but got 16 powered axles and potentially more steam generator capacity. The PA did better compared to an E unit because the GE752 traction motors had more capacity than the contemporary EMD traction motors.
The other use of the A-1-A arrangement is on freight locos to reduce weight. The issue here is what the hp per powered axle is, and if it can use full power at its lowest operating speed (drag rating). GE published an article years ago arguing against unpowered axles, and actually built few diesel-electric designs that had them.
Time will twll if the BNSF/GE C4s are actually that valuable. Even with AC drive and weight transfer system, they still don't belong on heavy trains at low speed. THey need to be kept on high speed trains, but most modern US railroads prefer any of their mainline locos to work the same pool without restriction.
