Saturday, 21 January 2017

The Vacuum Exhauster

Drawing by Kerr Stuart,  Photo by Rob Bishop

In February 1929 KS4415 was fitted with vacuum brakes to allow it to be used on the thrice weekly Dinas to Beddgelert winter service. While there are no photos of it working these trains, the vacuum pipe is very evident in the series of photos taken of the loco in Minffordd yard later in 1929. Fortunately Kerr Stuart produced a series of drawings detailing the modification. An exhauster was formed using a Blackburne 500cc engine ‘complete without timing gear and other external fittings’. The ‘other external fittings’ presumably being the carburettor and magneto.

A second drawing details the various valves and mounting brackets required to turn the ‘4HP SV Blackburne Engine’ into a vacuum exhauster. 

Isn’t eBay a wonderful thing! You can buy anything on line these days, even small single cylinder engines from manufacturers who ceased trading long ago. This a 4HP, 500cc Blackburne side valve engine:

It needs some attention, and a chain drive from the gear box. How effective it will be as a vacuum pump we will find out eventually, and whether it can really cut the mustard is in the lap of the gods. One of the challenges of exhausters driven off the engine is their (lack of) ability to create adequate vacuum on tick-over. That’s why when you see ‘Mary Ann’ on a passenger train it is revving its nuts off in the platform just to maintain 21” of vacuum. Once you get going the exhauster pulls 25” and will suck the world inside out. The Kerr Stuart arrangement is driven off the gear box. This means that when you drop the revs and dip the clutch the exhauster is not working at all. 
Having a stock of newspaper and leather bootlaces will be a must for the toolbox. For non-believers, that’s technical advice from the Reverend Awdry on how to remove leaks from train pipes .   

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