The Steam Locomotive

A steam locomotive was developed in the early 1800s to improve speed of the carrying of goods on tramways. These had been in use for some years using horses.

The principle of the steam engine has remained the same ever since

• 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.
• Convert the 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.
• Voila One Steam Engine!!

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 the heat from the fire to the water

The most common design of boiler is the Tube Boiler. 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 Bernouli 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 then Super-Heated by passing it through the top of the Firebox and through the centre of the Boiler Tubes

Pistons and Cylinders

The high pressure superheated steam is then passed to the cylinders. Her the energy stored in the steam is converted into mechanical energy to drive the Locomotive along.

Most locomotives use Double Acting Cylinders. This is a way of getting twice the power with the same number of cylinders 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 cylinder:-

• When the piston is at the end of the cylinder the inlet valve 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 the 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 locomotives have more than one set of driving wheels. These share the power generated by the cylinders by the Side Rods.

The Cranks on either side of the locomotive are offset by 90°. 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 rotation of the wheels. There are two main valve gear systems in use on locomotives. The Walscharts Valve Gear Walscharts gear(an annimation) for used on outside cylinders and the Stephensons Link gear for used between the frames.

Both of these systems basically operate from a second Crank or Excentric system that provides a movement 90° advanced on the main piston rod. As we saw above the efficiency of a steam engine can be improved by allowing the steam to Expand in the cylinder before it is exhausted. Unfortunately this adjustment depends on the speed. When starting the pistons only move slowly and a brute force 'Blow' system is the most effective. However at speed a dramatic improvement in efficiency can be obtained by increasing the amount of expansion allowed.

This adjustment of the amount of expansion is know as adjusting the Cutoff; ie. setting the point in the cylinder cycle that the inlet valve closes allowing the steam to expand before the exhaust valve is opened.

The Walscharts Gear is characterized by being fitted completely on the Outside of the driving wheels. The 90° offset is obtained from a Sub Crank mounted on the out side of the main driving wheel Crank Pin. The length and angle of this pin is arranged so that its journal is exactly 90° displaced from the main pin. This drives a Slotted Sector pivoted in the middle, the connecting rod to the valve gear is driven by a peg running in this sector. By moving the pin from the top to the bottom of the sector (this is done by the Reversing Gear controlled from the cab) the valve sequence can be reversed and hence the direction of rotation of the wheels reversed.

The Cutoff Adjustment is done by adding some of the motion of the piston to the motion of the valve gear. this is done by the Drop Link which joins the Main Crosshead to the valve assembly. By moving the pin closer to the centre the relative proportion of the piston movement can be increased, thus increasing the degree of Cutoff.