Steam Engine Cylinders

[modelsof1900]

Talltim,
yes, yes you are right. And there is more yet! The movement and behavior of the forces is influenced by many more factors than previously described.
Yes, the steam engines of a steam loco are double acting. To make the forces at changing the direction of movement of the piston and cross head as 'gently' as possible, some precautions are taken.
First, the supply of fresh steam at the movement of the piston in each direction, the so-called 'cut-off' is blocked well before the dead point of the movement, which can even set the engineer. However, the cut-off is set by the valve gear to max. 70 percent of the distance of movement of the cylinder piston. More or later (in the time scale) is not possible, but the engineer can reduce the cut-off point near to zero by using the valve gear. After the cut-off no more steam flows into the cylinder and the cylinder 'caged' steam can only produce force by which the steam pressure reduces in the cylinder - at each direction of piston movement. As a result, the force that the piston moves at the end of the movement is just before the dead point at its lowest.
On the other is controlled by the valve gear that opens the exhaust ports already nearing the end of the movement, before the dead center and thus before the change of direction of the piston. Thus one could say that the piston changes its direction of movement without force only with the inertia of its own mass about the dead center.
However, in order to reduce this kinetic energy before the end of the movement of the piston in one direction the valve gear opens the valve inlet channels of the opposite direction in order to give fresh steam to the back side of the piston. This steam in the opposite direction the piston is braked at the last part of the way and at the same time from the first point of the movement of the piston in the opposite direction, the full force of fresh steam for the piston movement in return direction is available.
All in all, this is a very complex procedure in the development of the forces in the cylinders. And so far is only the horizontal movement of pistons, piston rod and crosshead were written and how they arise from the forces due to the pressure of steam. But a second procedure must be considered. The horizontal back and forth motion and the high forces to move thousands of tons, must be converted into a rotary motion in a circle and evenly as possible, with little vibrations and swingings and often by high speed. But the balancing of linear moving masses by rotating masses and at end of steam era also by the cross balancing of both sides of driving system is an additional complex theme - but there I do not know enough about this.
I find that the steam engines, their gearing and and the entire system of the drivers are a high development of engineering. I am extremely excited them!

I hope for understanding of my post, here. I must use the Google translater for some parts of this text and I hope that it will help. In other case I would write until late night yet.