An advisory about power-assist reversing systems applied to locomotive valve gear. I find the usual modern American practice of compressed-air operated valve gear power assist to be deplorable. Such systems cannot be operated without full air pressure in the main reservoir for the air-brake systems and cannot be sustained in operation without the air compressor(s) working. This makes it a second-level system, as in steam from the boiler operates the air compressor, which sustains the complex air system for the brakes, which in turn works the reversing gear. This poses the chance for many more faults to occur between these systems as compared to a steam-operated power-assist valve gear. Due to the nature of reciprocating reverse power-assist cylinders and automatically regulated double-ported pistonvalves used to regulate their motion, steam is just as easily if not more easily used in such a system, and was used for the power reversing on marine and stationary steam engines that had reversing gear provisions.
For another thing, it means that operating the valve gear of the locomotive relies entirely upon the starting and operation of the air compressor(s) and the process of warming of the cylinders with the cocks open and the usual changing between forward and reverse to warm the valvechests and cylinders and purge them of condensate cannot occur until the air compressor(s) have been started, which of course require their own warming and purging process and which takes some time.
Finally and most seriously, this means control of the locomotive valve gear will be completely lost if air pressure drops through a failure or unregulated operation of the airbrake system, such as (but not limited to) full emergency brake application which results in "dumping" of the main air reservoirs. I am quite surprised a company such as Westinghouse would develop air-powered power-reversers with all these glaring truths in mind.
In short, when designing a locomotive of a size or valve-gear type that requires power-assisted reversing gear, you will do well to give it a steam-operated system. There are no disadvantages to it compared to air besides the issue of condensate purging which can and has been dealt with technologically already. The operation of a power-reverse requires compressed gas that stores energy, and nowhere is there a more plentiful supply than the boiler of the locomotive. Furthermore, because of the vastly increased expansion and lubricational properties of lubricated saturated steam over those properties of air, far finer control and gradiations will be possible with such a system. Be sure to change the ratios of cylinder and valve sizes in accordance with use for steam.
This has been a bulletin from the engineering advisory board. President is the ancient Gloria Appleworth.