The Steam Locomotive

The main principle of the steam engine has remained the same ever since it was invented:

• Use the energy stored in coal or some other fuel to boil water in the Boiler and generate steam under pressure.
• Use the energy now in the steam to drive a piston down a cylinder
• Arrange for some valves to admit the high pressure steam and exhaust the spent coal.
• Convert linear motion of the piston to a rotary motion to drive the wheels.
• Add some clever bits of mechanics so the process is automatically repeated.

Here is a simple diagram of the steam locomotive:

Boiler and Firebox

In the very early days a boiler was simply a container with a fire under it. This is very inefficient way of transferring heat from the fire to the water.

The most common boiler design is the Tube Boiler, as seen in the diagram below:

Here the hot flue gasses from the grate are drawn through a series of boiler tubes. These tubes provide a vastly increased area for the heat to be transferred to the water.

To make the fire draw effectively the exhaust steam from the cylinders passes through a blast pipe below the stack, this arrangement is designed to produce a reduction in pressure caused by the Bernoulli effect. This reduced pressure in the smoke box then draws the flue gasses through the boiler tubes. The harder the locomotive is working the more gas is drawn from the firebox thus generating more steam.

The steam produced is collected in a steam dome on the top of the boiler where the regulator valve controls the quantity of steam delivered to the cylinders. To further improve the performance the steam is the super-heated by passing it through the top of the firebox and through the center of the boiler tubes.

Pistons and Cylinders

The high pressure superheated steam is them passed to the cylinders. Here the energy stored in the steam is converted into mechanical energy to drive the locomotive along.

Most locomotives used Double Acting Cylinders. This is a way of getting twice the power with the same number of cylinders. This is done by introducing the steam alternately on either side of the piston so the piston rod is both pushed and pulled generating power on both strokes.

The cylinder valves control the access of the steam to the cylinders. The operation can be simply described as follows by considering only one side of the double action: -

• When the piston is at the end of the cylinder the inlet calve opens and allows high pressure steam to enter the cylinder in front of the piston.
• This high pressure steam pushes the piston back.
• The inlet valve then closes and the steam in the cylinder expands and cools producing more power to the piston.
• After this expansion the steam has given almost all of its energy up to the piston and the exhaust valve opens.
• On the return stroke the exhaust steam is forced out up the blast pipe.

  The linear backwards and forwards motion of the pistons is converted into rotary motion by the connecting rods and cranks.

  Connecting Rods and Cranks

  To keep the piston aligned in the cylinder there is a crosshead that runs on a guide. This crosshead carries the small end of the connecting rod. The other end, known as the big end transmits the power to the wheels via the Crank Pin.

  Most locomotive have more than one set of driving wheels. There share the power generated by the cylinders, which are by the side rods.

  The cranks on either side of the locomotive are offset by 90 degrees. This spreads the power from two double action cylinders evenly over a complete revolution of the wheels.

  Valve Gear

  The valve gear controls the opening and closing of the valves in the cylinders, and also provide the means of reversing the direction of the wheels. There are two main valve gear systems in use on locomotives. The Walcharts Valve Gearfor use on outside cylinders, and the Stephensons Linkgear for use between frames.

  Both of these systems basically operate from a second crank or eccentric system that provides a movement of 90 degrees on the main piston rod.
  

  Copyright @1998 Matt Fields

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