Re: NNG - Cab-Forward Cylinder Offset & Tractive Effort . . . ?
| September 26, 2021 02:26PM |
|
Kelly Anderson Wrote:
-------------------------------------------------------
> I agree with Johnson Barr, that it was done to
> compensate for the reduced thrust on the for-
> ward stroke due to the piston area taken up
> by the piston rod . . . An interesting side bar
> is that modern automobile engines sometimes
> have the cylinders offset in relation to the crank-
> shaft in order to reduce the angularity of the con-
> necting rods on the power stroke . . .
> The fly in the ointment of this theory is that SP's
> cab forwards also have their piston rods about
> the usual 2” higher than the axle centers, even
> though they ran in reverse in normal operation.
Russo Loco Wrote:
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> The cab-forwards do present an interesting
> special case, Kelly -
>
> Maybe the valve timing and/or the port sizes
> were adjusted to give equal total force to the
> push stroke and the pull stroke, or maybe the
> difference of a few hundred pounds out of a
> total thousands of pounds of {tractive effort}
> didn't matter enough to worry about.
Well, Roosso -
The decreased angle on the stronger 'push' stroke would tend to even out the downward force on the crosshead guide compared to the downward force from the 'pull' stroke, not that it would be a big difference. Per Dr. Robert Church's 1982 book on the cab-forwards, all of them from the AC-6 through the AC-12 – including even the "normal" AC-9 – had 24" diameter cylinders, a 32" stroke and operated at 250 # pressure. Assuming a 6" diameter piston rod – it's hard to tell from the drawings in the book – and no pressure losses in the pipes and valves, the maximum force on the face of the piston during the 'push' stroke would be π × 122 × 250 ≅ 113100 pounds, reduced by π × 32 × 250, ≅ 7070, or 106030 pounds during the 'pull' stroke for an average of 109565 pounds tractive effort at the crosshead, or 55,650# at the railhead when the 32" stroke and 63" wheel diameter are taken into account. Since there are two engines under the boiler, this figure would be doubled to 111,300# tractive effort, which is significantly lower than the 124,300# given in Church's book. But even if the S.P. (or Baldwin) simply ignored the loss of force on the 'pull' stroke due to the slight decrease in the effective area of the piston face, 113100# × 32 / 63 × 2 gives only 114,895# of total tractive effort. What Gives???
- Sincerely, Willie (Wm. Claude Johnson-Barr III, Esq.)
"Not All Who Have Cell-Phones Do Twitter *
"Not All Those Who Ponder Can Think . . . "
* Only TWITS Twitter!
-------------------------------------------------------
> I agree with Johnson Barr, that it was done to
> compensate for the reduced thrust on the for-
> ward stroke due to the piston area taken up
> by the piston rod . . . An interesting side bar
> is that modern automobile engines sometimes
> have the cylinders offset in relation to the crank-
> shaft in order to reduce the angularity of the con-
> necting rods on the power stroke . . .
> The fly in the ointment of this theory is that SP's
> cab forwards also have their piston rods about
> the usual 2” higher than the axle centers, even
> though they ran in reverse in normal operation.
Russo Loco Wrote:
-------------------------------------------------------
> The cab-forwards do present an interesting
> special case, Kelly -
>
> Maybe the valve timing and/or the port sizes
> were adjusted to give equal total force to the
> push stroke and the pull stroke, or maybe the
> difference of a few hundred pounds out of a
> total thousands of pounds of {tractive effort}
> didn't matter enough to worry about.
Well, Roosso -
The decreased angle on the stronger 'push' stroke would tend to even out the downward force on the crosshead guide compared to the downward force from the 'pull' stroke, not that it would be a big difference. Per Dr. Robert Church's 1982 book on the cab-forwards, all of them from the AC-6 through the AC-12 – including even the "normal" AC-9 – had 24" diameter cylinders, a 32" stroke and operated at 250 # pressure. Assuming a 6" diameter piston rod – it's hard to tell from the drawings in the book – and no pressure losses in the pipes and valves, the maximum force on the face of the piston during the 'push' stroke would be π × 122 × 250 ≅ 113100 pounds, reduced by π × 32 × 250, ≅ 7070, or 106030 pounds during the 'pull' stroke for an average of 109565 pounds tractive effort at the crosshead, or 55,650# at the railhead when the 32" stroke and 63" wheel diameter are taken into account. Since there are two engines under the boiler, this figure would be doubled to 111,300# tractive effort, which is significantly lower than the 124,300# given in Church's book. But even if the S.P. (or Baldwin) simply ignored the loss of force on the 'pull' stroke due to the slight decrease in the effective area of the piston face, 113100# × 32 / 63 × 2 gives only 114,895# of total tractive effort. What Gives???
- Sincerely, Willie (Wm. Claude Johnson-Barr III, Esq.)
"Not All Who Have Cell-Phones Do Twitter *
"Not All Those Who Ponder Can Think . . . "
* Only TWITS Twitter!
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488 stops for water at Los Pinos tank on closing day Oct. 1979. Photo courtesy Don Richter
The NGDF was started by Don Richter and is currently operated in his memory by Nathan Holmes and others. Others who have contributed to the upkeep of this board are (in no particular order): Charles McCandless, Everett Lueck, Ted Wilton, Cole Adams, James Bane, John Moellmer, Jon Bentz, Linn Moedinger, lloyd lehrer, Rod Jensen, John Craft, Steven Forney, Herb Kelsey, Bill Ramaley, Russ Sperry (El Russo Loco), Josh McNeal, John West, Jim Armstrong, Bob Bergstrom, Greg Scholl, Blake Forbes Doug van Veelen and Mark Fuller.