When a steam locomotive goes around a curve the wheels on the outside of the curve would roll faster than the wheels on the inside of the curve, .i.e. on a curve to the right, the wheels on the left side of the locomotive would be rolling faster than the wheels on the right because the radius of the outside curve is 4 ft. 8.5 in greater than the radius of the curve on the inside. So if that's the case, the piston on the left side of the engine would also be reciprocating more rapidly than the piston on the right, and the connecting rods would also be moving back and forth more rapidly on the left side of the locomotive. Is that the case or is there some kind of differential mechanism (like cars have) to compensate for this?
The driving wheels are all at the same speed .The wheel tire has a taper to help compensate for the greater radius and , yes, there is and must be, some slip.
Ulrich When a steam locomotive goes around a curve the wheels on the outside of the curve would roll faster than the wheels on the inside of the curve, .i.e. on a curve to the right, the wheels on the left side of the locomotive would be rolling faster than the wheels on the right because the radius of the outside curve is 4 ft. 8.5 in greater than the radius of the curve on the inside. So if that's the case, the piston on the left side of the engine would also be reciprocating more rapidly than the piston on the right, and the connecting rods would also be moving back and forth more rapidly on the left side of the locomotive. Is that the case or is there some kind of differential mechanism (like cars have) to compensate for this?
Solid axles. Has to be so that you get four evenly spaced power strokes per revolution. If the axles weren't solid and the drivers got "in sync" you could get stuck at "top (or bottom) dead center" on the right and left side.
Drivers are "quartered" so that the crank pins are at a 90 degree angle to each other.
The difference in radius is made up for by the wheel taper or by some slipping if the curve is very sharp - as mentioned above.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
But on an articulated locomotive, either Mallet or simple, front and rear "engines" are not always in synch.
daveklepper But on an articulated locomotive, either Mallet or simple, front and rear "engines" are not always in synch.
That is quite true, as there is no mechanical connection between the two engines. However, each side of each engine is also in synch with the other side of the same engine.
The other thing that assists in turning is the fact that the rail head is not square, it has rounded shoulder and a slight 'dome' to the center of the railhead. Tapered wheels and rounded rail section don't bind in the manner of a straight flange and a square rail when going through a curve.
Never too old to have a happy childhood!
You;re discovering why the axles on large steam power are so stout.
A typical North American locomotive is made to stay 'in quarter' (or more precisely in a state where all the connected wheelsets share the same mutual angle) and one wheel will slip a bit going around curves.
An interesting point is that in some cases lateral motion is permitted -- how do the rods keep in proper alignment with the drivers quartered? (Timken's answer was to put a ball joint in the rod eye, with a radius equal to track gauge)
A consequence of the necessary slipping, which is amplified at the relative torque peaks during the strokes, is that the wheel tires wear more in some spots than others, and have to be re-turned to circularity comparatively often. Some people think that this condition is due to slips when starting, and yes, that does contribute -- in the same areas on the tire, and for similar reasons.
Overmod, I can see the problem with quarter-locked drivers needing to creep on the inside rail on rather tight curves approaching the minimum radius for the drivetrain wheelbase of a given steamer, but for more generous/standard curvatures, would not the tapered profile of the tire allow for minimal creep due to quartering and thereby make any slipping more likely to be due to piston thrust forces?
Crandell
selector I can see the problem with quarter-locked drivers needing to creep on the inside rail on rather tight curves approaching the minimum radius for the drivetrain wheelbase of a given steamer, but for more generous/standard curvatures, would not the tapered profile of the tire allow for minimal creep due to quartering and thereby make any slipping more likely to be due to piston thrust forces? Crandell
I can see the problem with quarter-locked drivers needing to creep on the inside rail on rather tight curves approaching the minimum radius for the drivetrain wheelbase of a given steamer, but for more generous/standard curvatures, would not the tapered profile of the tire allow for minimal creep due to quartering and thereby make any slipping more likely to be due to piston thrust forces?
The 'creep' isn't due to quartering per se, it's related to the rigid wheelbase making the wheels curve at different effective radii. In this case the coned treads don't help as they do on swivel trucks or radial-steering axles/Bissel trucks.
In any case, the slip propensity is vastly more likely to be influenced by piston thrust force than anything involving creep -- in at least two respects.
First, the creep force is relatively momentary in extent, and usually quickly arrested once a small amount of correction takes place. All the drivers that are not 'creeping' will still be exerting their proportion of adhesion (through the rods and bearings) just as they exerted force to induce the creep in the first place.
(You will note that if there is an 'insufficient' number of these correcting wheels, you will have a decreased tendency to arrest a transient slip that begins to evolve. That is part of the problem with unconjugated divided-drives -- but relatively less significant to the actual, physical problem with high-speed slipping that they may suffer...)
Second, the variability of the piston thrust with angle varies, usually wildly, and this contribues far more to slip induction than creep does, ceteris paribus. You can easily judge this tendency, for a given class of locomotive and running condition, by looking at how the ear patterns develop on the driver tires, and how frequently the driver tires have to be turned.
Compounding this, the coefficient of sliding friction is less than static friction (I know the physics and metallurgy but will spare you the building-a-watch detail). If the slip occurs at a period of peak thrust, perhaps even before the period of greatest mechanical advantage of the combination of rod angle and instantaneous cylinder pressure, there may be sufficient force (and inertia) given to the system to keep it accelerating (and hence slipping) through to the next torque peak -- which now contributes a greater force to further acceleration. Note that the comparative radius of coned vs. unconed diameter, or considerations of the relative change in contact patch, etc., are less significant than the thrust, as you surmised.
I hope this makes things clearer and not more confused.
RME
No, I think that sounds reasonable, and I thank you.
Our community is FREE to join. To participate you must either login or register for an account.