I used ‘search’ but wasn’t satisfied so here goes…
What happens to the slack (on straight and level ground) when using mid-train helpers, pushers, or DPU’s?
Do they distribute the power leaving the most HP on the head end?
Is any slack -action change noticeable to the engineer?
Does this stress the couplers more than usual?
TIA
Anyone?
Hello?
Do I have to post signage to get a response? (I can…) [:)]
If I recall aright, the mid train units are cut in a little ahead of the halfway point (or thirds, quarters or however many places there are extra units) so that the the train is still “strectched” rather than “bunched”; my guess is to avoid a mix of forces acting on the same train. I’d guess having to deal with simultaneous slack in and out (like on hilly lines) would be more than most companies want to deal with. Stress on the couplers would be less, I’d think, because the head end cars aren’t pulling the whole load.
How the handling characteristics (slack action) changes, I’m not sure. Any throttle jockeys out there to help us armchair railroaders out?
Slack is actually needed in a train.
If all the cars were attached with a ridged draw bar with no slack, the locomotive(s) would be trying to start all the combined weight of all the cars at once…never happen.
But with slack, or play in the connections, when you begin moving, what is really happening is you start the first car moving, when the slack runs out, you begin to pull the second, with the combined weight of the locomotives and the first car…then the locomotives and the two cars begin to pull the third…all the way down the line. The mass and weight doing the pulling increases as the “load” gets smaller.
On really heavy trains, there are times the engineer will bunch the cars together, so he can maximize the action when he moves forward and “break 'em lose”…inertia is a hard thing to overcome…but once you use it to your advantage, it helps.
With DPUs, the slack is still there, but less of it is felt on the head end as the mid train units will “absorb” their share, and the rear unit, if there is one, will bunch it up too.
Keep in mind that all locomotives do not load up or react to control surface changes at the same time…even in a simple MU, one or the other unit will move first in response to a throttle setting, even if the MU is made up of the exact same locomotives.
One of them will also begin to stop first, when you set the brakes, there is no way to “adjust” them to work perfectly in sync…you can get close, and engineers will have their preference for certain pairs of motors, ones they have found that “work good together”…and develop a phobia about others that don’t work well together or load up at a noticeable different rate.
An engineer will know within a few minutes if all the units in his consist, whether a simple MU or DPUs “work” well together…if they do, he will give you a really smooth ride…if not, or if he isn’t used to using DPU, you can get the stuffing beat out of you.
As for wear