Here is something to waste your time thinking about. No matter how fast a steam engine is running down the tracks the side rods are constantly changing speed from dead stop to twice as fast as the engine is moving. Think about it: when the rods are all the way down they are not moving at all for the infinessimal moment then it speeds up to the top where for a lickering moment it is travelling twice as fast as the engine itself. Now I hope I haven’t ruined anybody’s sleep tonight as they lay there trying to visualize this. Odd-d
Yes, Odd-d, you are correct
Sorry, I don’t understand.
The rods move along the rails at the same speed as the loco, but also have tangential motion. They only pause and reverse their paths in appearance when veiwed from the side. It is the piston rod, in my view, that stops, and then accelerates back the way it came. But the side rods are moving in a constant circle.
The rods need to move twice as fast as the loco. For every one revoution of the wheels, the rods go up and down.
The side rods and the piston both move alternately a little faster and a little slower than the locomotive generally, but not twice and zero, because they are attached to the cranks no more than half the distance from the axle to the tire. However, there is a part of the locomotive that not only moves more than twice as fast but also briefly moves backward.
Selector----- Even though the wheels and siderods are turning at a constant rate IN RELATION TO THE LOCOMOTIVE they are constantly changing speed IN RELATION TO THE GROUND. Try it with a model locomotive. I made this observation afer watching a video taken by a camera strapped to the siderod of Nevada Northern 4-6-0 number 40. Also in the movie “the General” starring Buster Keaton he is shown sitting on the siderod of an old 4-4-0 as it moves slowly along. BTW don’t try that at home. Good luck getting to sleep tonight . Odd-d
The path that a point on a rim seems to take in planar space is called a cycloid. A point anywhere inside that circle (rim) will not be a cycloid because that point does not move at the same speed as the axle about which the rim turns…and which the loco moves. The motion relative to the ground is irrelevant because the rods aren’t attached to the ground. They are attached to the wheels, and turn in circles with the wheels. The only force that the rods experience are acceleration away from the tangent to the rotation arc, minus a smaller vector when they are arcing away from the direction of motion of the axle along the track.
Any physicists out there?
Is there a mathamatician, or physicist, in the house? (Pardon my spelling)
Ben is pretty good with that stuff, I care not to know
If anyone has a 4-4-0 at home, I got a bunch of frequent flyer miles…I’d be willing to use some to try out the Buster Keaton stunt![:p]
underworld
[:D][:D][:D][:D][:D]
Thanks to Odd-d, I now have a headache. [:(]
I hope this dosn’t turn into one of those “calculating drawbar pull” threads. [:D]
Crandell, the path taken by a point inside the rim, like the cranks and siderod, is called a “curtate cycloid”. There is also a name for points outside the rim, “prolate cycloid”. This is the path taken by a point on the flange, which is the part of the locomotive I referred to above that not only moves forward more than twice as fast as the locomotive generally but also briefly moves backward. In fact, some part of every flange of every wheel of a moving train is always moving backward!
And a Cyclops is a monster with one eye![:D]
Now that we have straight we can move on! [:)]
Here is the point that I think will illustrate my observation…if you watch a steam engine run smoothly down the track you will have to agree that at some point the siderod crankpin is behind the axle, and at other times it is in front of the axle so it stands to reason that in order to get behind the axle it must move slower than the axle on the horizontal plane of the track thus slowing down to a stop so it can accelerate again to pass behind the axle to the point above the axle where it has to decelerate again. AW NUTS!! Now I’m getting a headache. I guess I’ll take a nap and let you younger guys figure it out. Odd-d
I have a BS in physics, which I got 23 years ago.
This is problem which is going to take some analysis & I don’t have time for it now. Someone has probably done this work before & it may be on the Internet. I’d do a search.
In any event, selector is right in that the side rods just go around & around, but so what? I don’t think he’s right that the motion with respect to the ground is irrelevant – as Einstein said, everything’s relative. Said another way, one point of reference is as valid as any other.
To an observer on the ground, it’s possible that the side rod never actually goes backward, if it’s speed along the track is faster than its tangential speed around the axle. This is the same thing that happens if you throw a snow ball out the back end of a vehicle that’s going faster than you can throw the snow ball. To you, it looks like the snow ball is moving backward, but to someone on the ground, it’s just going forward at a speed slower than the car.
The forces felt by the side rod are primarly the pushing & pulling forces from the piston. There’s also the centripdal forces from going around with the wheel, but the vector associated with centripidal force points along the axis of rotation.
I really don’t want to think about this one any more, it’s giving me a headache.
Tony
Well, yeah, but your assumption that the side rod stops when it gets directly behind the axle isn’t necessarily correct. The speed of a point on the side rod depends upon subtracting the part of that point’s velocity that is parallel to the train’s direction of movement from the engine’s speed in the same directrion. If the engine is moving faster than the side rod, it will never travel backwards or stop, relative to a viewer standing on the ground.
Relative to the train itself, the side rod is just going around in circles. It all depends on where you stand when you look.
YOu really need to draw some diagrams & break out the trig to analyze this.
Tony
The side rod is always moving forward, sometimes slower, sometimes faster.
Odd-d is right they are moving forward and backward:
Odd-d,
the General is my favorite train movie and I remember that scene. Buster was really on cloud nine when he was doing that stunt.
I’d be interested to know what the valve gear is doing. I think that stops too.
Those valve gears, siderods, main rods, etc, make steam train watching so exciting. That’s why I never really cared for those streamlined steam locomotives with their sheathing.
Why hide it? Flaunt it if you’ve got it.