More on compressors

You might recall I stripped the little BTH compressors and discovered a curious Scotch Yoke type of crankshaft within. Thus, I was interested to see
inside one of the other type of aero air compressors I've collected over the years. They are the type that are fitted to the rear of a Rolls-Royce Merlin B bank cambox and this one is pictured in pics 1&2 at
One I have has been modified with the addition of a splined shaft and an outrigger bearing mounted in a 3/8" thick steel plate. This makes it suitable to be driven from a free standing power source and there must have been many thousands so modified in the post war years - I've got five of them and I used to think they were rare! It is upon this basis that I don't think they are an off topic subject ;o)) Whilst cleaning it, I found the usual host of AID numbers and the AM symbol, surmounted by a crown.
It is obvious from their construction that they were never intended to carry lubricant in their crankcase, but must have survived on splash from the cambox, there being a ring of four slots to facilitate this in the inner crankcase wall. This inner wall supports a big (2" across), thin shell bronze bush and the other mainshaft is supported by a ball race in a blind housing. The crank itself is boringly normal except that the conn rod is cast bronze.
The ali piston has an automatic valve in the crown, two rings and runs in a fine pitched, one piece, finned steel cylinder of all-machined construction. It is secured to the crankcase in an ingenious way. There is a female ring machined in the top of the crankcase halves and this accepts a male ring that is part of the cylinder, requiring assembly when the crankcase halves are fitted together. Above it, there is a fine thread to which is fitted a toothed, internally threaded brass ring that tightens against the crankcase mouth, pushing the cylinder *upwards*. As far as I can see, the only advantage this confers to the device is that the cylinder can be turned anywhere in 360 degrees, giving an infinite variety of positions in which one can fit the outlet.
The slim, cast finned ali cylinder head with its automatic inlet valve is secured to the cylinder with six Allan screws and appears to have had a thin lead gasket, now replaced with Hylomar.
It requires filling with oil before the mounting plate is secured with a ring of 1/4" bolts, but then it is easily operated with an electric drill to ascertain its abilities - and it works just as it was intended to over sixty years ago.
Interesting to think that on this auspicious weekend, this little compressor might well have been whirring away on the back of a Merlin and Doing Its Bit to wrest the Continent from the grip of the oppressor.
Kim Siddorn,
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Could the lack of sump oil be due to the need to design them to take negative G. This is something not normally experienced by the average workshop compressor.

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