Cylinder CL vs Axle CL

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Fender
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Re: Cylinder CL vs Axle CL

Post by Fender »

This diagram illustrates inclined cylinders, with the centerline of the cylinder aligned with the main driver.
Dan Watson
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daves1459
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Re: Cylinder CL vs Axle CL

Post by daves1459 »

I have a completely different perspective. I believe the purpose of the dimension is to ensure running height of the cylinder relative the axle center line is maintained so that the valve gear will function as designed. That is to maintain equal valve motion for equal steam distribution in both ends of the cylinder. Early locomotives with Stephenson valve gear slanted the cylinder center line directly at the main axle center so that they didn't have to deal with the angularity of the main rod and resulting effect on the position of the piston. With the angled cylinders the point of suspension of the expansion link was in the center of the link radius center line. Draftsmen later found that by moving the suspension point rearward of the center line the main rod angularity could be compensated for and give even valve events. By the time Walsharts(sp?) valve gear can alone main rod angularity was well understood. If I'm not mistaken the eccentric crank position. eccentric rod length, and back set of the expansion link to eccentric rod connection point all have to do with compensation for main rod angularity due to the vertical distance of the main driver and cylinder center lines.
BClemens
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Re: Cylinder CL vs Axle CL

Post by BClemens »

The valve gear and its suspension and attitude toward the main crank pin and axle center line are only related to the piston by its tdc and bdc and therefore the crank pin position. The drawing showing the 2" increase in height from the center line does not depict anything about the valve or its motion. I'll buy the suspension theory with its 'break-in' but the basic design should compensate for that with a line for line position or 'as designed' not what is going to happen in the future with that locomotive - near or far. I don't believe this has been explained with any plausibility although the suspension explanation is the most realistic theory. I can't imagine what a locomotive would feel like with this uneven main motion at 100 miles per hour.

BC
Last edited by BClemens on Fri Jan 20, 2017 4:06 pm, edited 1 time in total.
Steve Bratina
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Re: Cylinder CL vs Axle CL

Post by Steve Bratina »

"This explanation doesn't make sense"
Sorry. .........Bye
JKreider
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Re: Cylinder CL vs Axle CL

Post by JKreider »

I do not think that the offset in question has any significant affect on either valve events or the forces on crankpins or driver vertical loads, or at least I have never seen any documentation that supports that thinking. Of course that does not mean that it does not exist somewhere. But consider the following;
On Walschaert and Baker valve gears the primary valve events are controlled by the combination lever at fdc and bdc. At this point the valve gear is just along for the ride. When the crosshead moves from these points the valve gear comes in to play to provides the required valve travel, or cutoff, as required by the engineer.
The crosshead, combination lever, and cylinders, don't move, of course, from their vertical relation to each other.
The Walschaert gear, being an "all square" gear, is designed such that the link foot swings tangent to the piston rod centerline and the link backset is imposed to allow equal swing of the link for and aft to give somewhat equal valve travels for and aft.
With the Baker gear, being an "inclined gear", a forward set to the upper combination lever pin is generally imposed due to the unequal swing of the bell crank in the gear, and likewise in an effort to give somewhat equal valve travel for and aft.
In either case however valve travels are rarely precisely equal for and aft and the difference becomes greater as the cutoff increases.In either case valve port opening is sufficient and close-enough for good performance.
Can the offset in question have any significant affect on the above parameters? It is not obvious to me how it would?!?
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Re: Cylinder CL vs Axle CL

Post by Soot n' Cinders »

JKreider wrote:I do not think that the offset in question has any significant affect on either valve events or the forces on crankpins or driver vertical loads, or at least I have never seen any documentation that supports that thinking. Of course that does not mean that it does not exist somewhere. But consider the following;
On Walschaert and Baker valve gears the primary valve events are controlled by the combination lever at fdc and bdc. At this point the valve gear is just along for the ride. When the crosshead moves from these points the valve gear comes in to play to provides the required valve travel, or cutoff, as required by the engineer.
The crosshead, combination lever, and cylinders, don't move, of course, from their vertical relation to each other.
The Walschaert gear, being an "all square" gear, is designed such that the link foot swings tangent to the piston rod centerline and the link backset is imposed to allow equal swing of the link for and aft to give somewhat equal valve travels for and aft.
With the Baker gear, being an "inclined gear", a forward set to the upper combination lever pin is generally imposed due to the unequal swing of the bell crank in the gear, and likewise in an effort to give somewhat equal valve travel for and aft.
In either case however valve travels are rarely precisely equal for and aft and the difference becomes greater as the cutoff increases.In either case valve port opening is sufficient and close-enough for good performance.
Can the offset in question have any significant affect on the above parameters? It is not obvious to me how it would?!?
I think youre right, it wouldnt have any noticeable effect on Walschaerts, Baker, or even Stevensons. Would only produce an effect on gears like Southern or Hackworth/Marshall.

I think the difference in the drawn centerlines is for suspension settling after construction and when the boiler is filled with water. If a boiler holds 2000 gallons for normal operations, thats 16000 pounds more weight than when the locomotive is empty. That'll definitely make the locomotive ride lower.
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Fender
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Re: Cylinder CL vs Axle CL

Post by Fender »

I've read several hypotheses for the reason for difference in centerlines, but only one with any supporting evidence. Which is, that (as described by RWilliams from actual experience) the difference disappeared after new springs were broken in and the boiler was filled with water.
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Re: Cylinder CL vs Axle CL

Post by JKreider »

Filling the boiler with water certainly contributed some but on modern locomotives where axle loadings were in the range of 60 to 70000 lbs, it could not have been a major item for the difference.
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Re: Cylinder CL vs Axle CL

Post by Dick_Morris »

The USATC 2-8-0s designed by ALCO also used 2". I just reviewed some engineering notes that I have on these locomotives. The horizontal location of suspension points had a resolution of 1/8" but there is no mention of the vertical dimension. There was a lot of engineering that went into determining total weight and weights on axles. For this design, it appears that a weight was determined for each part down to individual hardware items, a preliminary locomotive weight was determined, and then the completed locomotive was weighed. In one weight sheet that I have, the locomotive was apparently incomplete at it's first trip across the scales. It was weighed with 2 gauges or water, 200 pounds of steam pressure, 2 men in the cab, and with specified weights of fire brick and sand on the running boards. On another sheet the center of gravity was calculated. Some other notes suggest that the steam and water weights were calculated.

Added - The locomotive weighed 141,000 pounds, water in boiler 13,800 pounds, fire 1,100 pounds, and sand 1,200 pounds. Another sheet allowed 350 pounds for "men."
BryceGTX
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Re: Cylinder CL vs Axle CL

Post by BryceGTX »

Here is a statement from "Modern Locomotive Construction" by Meyer Dated 1892 page 150 center of page:

"In the first place, the centers of the cylinders are usually placed from 1 to 2 inches above the centers of the driving wheels when the engine is in the ordinary good running condition."

This statement clearly indicates common practice is the cylinders are placed higher by design. And not because of suspension travel.
Bryce
Last edited by BryceGTX on Thu Jan 19, 2017 8:35 pm, edited 1 time in total.
BryceGTX
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Re: Cylinder CL vs Axle CL

Post by BryceGTX »

We also know that the engine in forward motion is always exerting force on the upper cross head guide rather than the lower except at the dead centers. So both forward and backward piston motion wears the upper guide gibs while in forward.

Just another piece of the puzzle..
Bryce
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Re: Cylinder CL vs Axle CL

Post by BryceGTX »

When the engine is in forward motion..

During the forward stroke of the piston, the vertical inertial force of the main rod drives the cross head up into the upper cross head guide.

During the backward stroke of the piston, the vertical inertial force of the main rod drives the cross head down into the lower cross head guide.

These inertial forces increase with increasing engine speed and are nearly zero at low speed.
Just another piece of the puzzle..
Bryce
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