I've studied a number of photos of Gresley Pacific cylinders and cannot see such a fitting on any others - however there does appear to be something similar where the longer stud is on Bittern's gland cover.
At a guess I would say that this fitting was there to drain excess moisture away from the piston glands. I would imagine that now asbestos is outlawed an alternative has to be used and it may be that it is more susceptible to moisture. I await further comment?
Saint Johnstoun wrote:I've studied a number of photos of Gresley Pacific cylinders and cannot see such a fitting on any others - however there does appear to be something similar where the longer stud is on Bittern's gland cover.
At a guess I would say that this fitting was there to drain excess moisture away from the piston glands. I would imagine that now asbestos is outlawed an alternative has to be used and it may be that it is more susceptible to moisture. I await further comment?
Thank you for your reply to my question. I had come to the conclusion that the fitting was post-preservation and likley to be a drain.... just that there is nothing similar on any of the other running A4 engines.
If the packing is no longer graphited asbestos rope then what is the acceptable packing material today?
Why not ask the chaps at Ropley (MHR) nicely, and see what they have to say on the matter? I'm sure they won't mind answering as they were the ones who in all likelihood fitted the device.
First thought -piston relief valve? then realised it was the connection into the cylinder but some of my follow up mail has gone absent so start again-might it be to fit a pyrometer connection? when coupled to a dynomometer car?
Jim Brodie.
Last edited by James Brodie on Thu Oct 07, 2010 4:33 pm, edited 1 time in total.
James Brodie wrote:Piston relief valve..................jim Brodie
I just love these highly informative replies, especially from those who are so eager to share their obvious wealth of knowledge that they dont even bother reading the question properly.
I'm fascinated by this addition to Bittern. If it is a drain as you suggest, then it would need to be capable of closing automatically when the locomotive is under steam, otherwise there would be a steam blow on each forward stroke of the piston. I would expect a ball valve with a light spring under it which would close under pressure hence the complicated the housing.
My guess is that as the preserved locomotive does have long periods between turns, any water lying in the gland housing eventually causes some corrosion which leads to steam leakage when the engine is next used and someone has come up with the idea of the drain. The fitting may also be a means of introducing additional oil to the gland to help speed the process of lubrication following a lay up.
Modern LNER locomotives never had asbestos yarn piston gland packing, the packings being pairs of split cast iron rings sandwiched between bronze rings which had spherical outer faces, all in a casing and pressed against the outer gland housing by a strong coil spring. The piston rod is floating, suspended between the piston head and the cross head and the gland design allows for any misalignment generated by wear in the sliding components. The long top and bottom studs on the gland housing (noted by Saint Johnstoun) are there to aid assembly of the gland. It requires substantial force to close the gland spring sufficiently to start the nuts on the normal length studs so the nuts on the long studs are used to compress the spring in the first instance.
Asbestos has been replaced by new materials including ptfe, which perform in much the same way and are used in valve spindle packings, smokebox door seals, piston glands, boiler doors and pipe flanges etc.
