I had an interesting chat with Mike of Base Camp this afternoon when placing an order for a Petromax lamp nozzle and a pre-WWII filler cap/air screw. The lamp I’m working on is THIS Petromax pattern EFAR 608 (India) and I happened to mention to Mike that although I’d oiled a serviceable pump cup washer I was experiencing occasional missed strokes. “There’s the reason” said Mike, “people mistakenly leave pump leathers to soak in oil and it makes matters worse.” Mike’s view, which makes perfect sense, is that there needs to be a certain amount of friction between the leather and the pump tube and oil disrupts the sealing effect that the friction generates. He advocates a thin smear of oil, the excess wiped off with a piece of kitchen towel. I went back to the lamp, removed the excess of oil from the pump cup leather so that it was flexible but not wet, and there was an immediate improvement of the pumping action, with no ‘missed’ strokes. I fear I’ve been labouring under a misconception for a long time! Your views? John
Thats intresting john I know what you mean the missed pump strokes. I have always just spread the leather to try.improve function Will try this next time great stuff Cheers pete
A factor here I think is that of a ‘fixed’ pump cup leather as in a Petromax lamp or Geniol stove, where the leather cup has to pull away from the pump tube inner surface to allow air to pass it on the ‘pull’ stroke, then make close contact on the ‘push’ stroke to trap the air and compress it. With a Primus type ‘floating’ pump cup assembly the leather remains in close contact with the pump tube inner wall on push and pull strokes and the air passes through, or is trapped, by the one-way valve action of the pump cup assembly sliding to-and-fro on the pump rod making a seal between the chamfer on the rod and the mating surface of the brass component in the pump cup assembly. See THIS post by Kerophile on CCS. I wonder if the two different working principles put more emphasis on one type or the other in terms of how oily the pump cup leather is? No matter how oily the Primus type leather is it always pumps reliably. My experience now with the Petromax type on the EFAR lamp suggests it’s more critical of the degree of oiliness of the leather.
An interesting observation. That could possibly hold plenty of volume. On the 'fixed' pump type without the 'movable' piston piece, the air purely enters the tube through a tiny 'gap' around the peripheral contact point with the tube's inner wall. The increment in volume at the pump tube's lower section during the 'pulling' stroke creates a reduced pressure in that volume of space.(something like a vacuuming effect). The potential difference in pressure between the atmosphere and that lower-end volume of the pump tube behind the leather cup during the pull stroke creates a current of air by forcing an opening or gap between the leather cup periphery and the pump tube inner wall. The direction of air current is from high pressure point(atmosphere) to low pressure point(lower-end volume in the pump tube). The 'forced opening' is only possible due the the leather cup's flexible nature. Like a diaphragm. With an excessive oily or 'dripping wet' leather cup and an often, equally oily pump tube inner wall, there is a very high likelihood of this occurring during the pulling stroke:- -- The 'vacuuming' effect draws in considerable amounts of oil at the expense of air into the pump tube's lower-end volume. This prevents to some extents, the attainment of an equi-potential in air pressure between the atmosphere and the pump tube's lower-end volume at the end of the pulling stroke. That's to imply that lower-end pump tube volume hasn't been completely filled with air as supposed, but remains as a "partial vacuum". The pumping sequence is then succeeded or followed by the 'push' stroke. As logic would have it, the resistance offered by the partial vacuum is always way less than that offered by a full volume of air.(it feels like an empty stroke, which, is in fact pretty empty). This can also be felt as seemingly "friction-less". In more extreme cases where the draw of air is neglibible, the lower-end pump tube volume remains in a state of vacuum. (At this point, it would be evident that the pump stem and leather cup assembly would be subsequently 'sucked-in', if you should ever release your hold on the pump knob at the end of the pull-stroke).
I have noticed this effect on old pump leathers that I have tried to "rejuvinate" in neatsfoot oil, but not new ones and assumed that it was because the old one was worn out, even though it looked fine. Interesting
