Did it all work ? - Engineering Report No 13

The answer is yes it did. ‘President’ has had a very successful working year. Two important developments were the modification to the prop shaft and stern tube bearings and fitting the flywheel.

For the benefit of new readers I will recap. An annoying knock was eventually traced to the stern tube bearings consisting of two heavy cast iron sleeves some 10” long by 4” wide bored to 2 1/2 ” dia.. These were replaced and a new shaft was made. The old Plummer block (thrust bearing) was replaced by a heavy duty marine thrust bearing; this transfers the thrust of the prop directly to the hull of the boat and not the engine bearings. An expanding insert joins the two shafts together

The first steam tests confirmed that the Knock had gone, but, the shaft was rattling so a large flywheel was sourced and fitted which transformed the running of the engine, enabling the engine to run expansively by about 50%.

It is at this point I recall my last article ended when I said that I would attempt to explain “cut off,” so here goes; If the inlet valve allows steam to flow into the cylinder until the piston has reached the end of its travel it would mean that the pressure at that point would be the same as the incoming pressure from the boiler, which in turn would mean that the pressure of the exhaust leaving the cylinder would be fractionally below boiler pressure indicating that the steam has done no work. It would also force the piston to complete its stroke harshly causing unwanted forces on the cross head and bottom ends. For many years it has been the accepted norm in steam engineering to restrict the opening of the inlet valve to 75% of the total opening which means that approx the latter half of the pistons forward movement is done by steam working expansively, thereby using less steam, also as the piston moves to the end of its stroke the exhaust valve will have been closed meaning that it arrives at the end against a cushion of steam from the previous stroke. So we now come to managed cut off, this means that the operator of the engine can further reduce the travel of the inlet valves down to approx 5% of the total opening which means that the valve admits a very short blast of steam into the cylinder where it works expansively for the rest of the stroke. This is vital to the efficient operation of rail locomotives, where the driver will fully open the regulator then by moving the reversing mechanism will admit more or less steam depending on the gradient i.e. uphill he will move the gear forward so the valve opens more admitting more steam level and down hill he will pull it back until the valve is at 5% opening. This also means that less coal is burnt and equally important the fire-mans work is reduced.

In the case of ‘President’ with the fitting of the flywheel which stores energy, cut off can be reduced to approx 50%. If you are still unsure please ask the driver next time you are on or near ‘President’ to demonstrate how the reversing gear works.

Looking ahead we have a busy work schedule for this winter improving pipe-work layouts, lubrication, fitting shelves and that most important item the steam kettle.

As we are still at the start of the New Year I wish you all a very happy new year and enjoyable work on ‘President’.

David Stott