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Post by John Baguley on Sept 20, 2020 15:22:56 GMT
A better design of pump has the bore for the ram lengthened so that it extends part or all the way into the valve box drilling. That way there is nowhere for the air to get trapped as it is pushed straight out of the pump outlet. No need for any pins on the end of the ram.
John
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Post by Roger on Sept 20, 2020 15:34:24 GMT
If the pin is only "a few thou" smaller, when the pin enters the hole it could cause a partial hydraulic lock. I wonder if this is the reason some locos with axle pumps can be jerky? Good point. I don't really understand why the nose of the plunger can't be close enough to the valves to make the volume small anyway. This is the way I've done mine. You can see that the plunger goes between the valve seats. The volume could be made even smaller if the valves were moved in even closer. I'm not expecting this to be an issue since the pump is gravity fed from the Pannier Tanks anyway. Sectioned pump assembly by Roger Froud, on Flickr
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Gary L
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Post by Gary L on Sept 20, 2020 15:49:56 GMT
The purpose of the 'pin' is to displace as much air as possible out of the pump during the priming stage. With a volume of compressible air inside the pump and boiler pressure working against it, you can have the effect of repeatedly compressing the air bubble without actually displacing anything. Ie the pump won't prime. I guess you said that in the post above and suggest using the bypass route without backpressure to complete the priming. Still, with a stuck suction and volume of air that is spongey it's a problem. My traction engine suffers from this. Of course, the pin must have space around it to pass water at the inward extreme. It is just a solid piece to take up air space. No problem with reducing the clearances between the crown of the ram and the barrel to a minimum in order to expel any residual air, but could an air bubble really lodge in the outgoing waterway?? Whenever I see air bubbles in water they display a marked tendency to rise, especially when assisted by a flow of water. I can't imagine for a second that a bubble could lodge in that position so that the 'pin' would obligingly drive it out into the delivery pipe. I agree that if a 'pin' is formed in the crown of the ram, then there must be a clearance around it to allow the water from the final portion of the delivery stroke to escape around it, but I submit it must be more than a 'few thou'. Let's say the 'pin' is ⅛" high and ⅛" dia, the pump bore is ½", and assume that the crown clearance is 1/32". That means that the volume of water that has to pass around the 'pin' is Pi x r 2 x 3/32" less a trifle for the volume of the pin itself. It's not a massive amount of incompressible water, but it is still a lot to force past an ⅛" dia pin with "a few thou" clearance in the space of a few milliseconds. It's worth saying that I've never seen any clearance shown on any drawing, much less quantified. Is this where complaints of 'noisy pumps' comes from I wonder? The transient rise in hydraulic pressure must be quite significant, not to mention stress on the drive components. Gary Edit: just seen John Baguley's and Roger's drawings, which are a far superior design IMHO
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Post by steamer5 on Sept 20, 2020 16:15:07 GMT
Hi Guys, The few thou is / was a relative term.
Cheers Kerrin
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mbrown
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Post by mbrown on Sept 20, 2020 16:28:45 GMT
I could never work out why LBSC specified an "anti-airlock pin" on axle pumps, on the grounds that the clearance at the end of the ram should be as small as possible, but left a tremendous amount of clearance around the ram on his crosshead pumps where the ball valves were usually half way along the body, necessitating clearance for about half the stroke of the ram.
I can only say that the crosshead pump on my Bagnall has clearance around the ram for half the stroke, and works a treat....
Malcolm
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Post by suctionhose on Sept 20, 2020 22:54:01 GMT
I contributed what I believe the thought behind the pin was. Quite likely the problem it solved was academic in the first place - much about model engineering is I suggest - we do like to theorise on our design choices without much testing to substantiate the benefit.
Agreed, Roger's etc is a better arrangement. I hope Roger has something to stop the delivery ball blocking the outlet hole?
In general reciprocating pump terms, space inside is not ideal. Liquid can vaporise under vacuum (on the suction stroke), air can compress on each delivery stroke (as it does in a flow shock absorber bottle thing). Consider a boiler clack that hasn't seated properly...steam blowing back, pump hot, feed water hot...
Whether these symptoms occur in mini pumps is conjecture but they certainly occur in larger pumps in many applications.
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Post by Roger on Sept 21, 2020 6:44:33 GMT
Agreed, Roger's etc is a better arrangement. I hope Roger has something to stop the delivery ball blocking the outlet hole? Hi Ross, Yes, this is the outlet union and the same arrangements of pockets has been machined in the body for the inlet valve. Outlet valve union by Roger Froud, on Flickr
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Post by suctionhose on Sept 21, 2020 6:53:39 GMT
Hi Roger! I expected no less...
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uuu
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Post by uuu on Sept 21, 2020 10:12:05 GMT
Have you considered the effect of the ball pounding away on the edges of the pockets? Will it work its way up until it blocks the hole - I have seen pumps do this.
At least it's easily replaced.
Wilf
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Post by Roger on Sept 21, 2020 12:52:50 GMT
Have you considered the effect of the ball pounding away on the edges of the pockets? Will it work its way up until it blocks the hole - I have seen pumps do this. At least it's easily replaced. Wilf To be honest I hadn't, but I'd be staggered if it does that because there's a lot of room around the ball. If there's insufficient room, I could see how the ball could hit the edge hard, but I calculated the area to be the same as the delivery pipe. Maybe this is the explanation for the damage you saw?
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uuu
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Post by uuu on Sept 21, 2020 13:13:12 GMT
I imagine you're on the right lines making the passages as large as possible - and limiting the lift of the ball, so it can't get up any speed.
Wilf
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Post by Roger on Sept 21, 2020 18:39:02 GMT
I imagine you're on the right lines making the passages as large as possible - and limiting the lift of the ball, so it can't get up any speed. Wilf Agreed, I worked out how much lift I needed to be the same as the pipe cross section. It's not much.
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dfh
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Post by dfh on Sept 21, 2020 20:38:08 GMT
I imagine you're on the right lines making the passages as large as possible - and limiting the lift of the ball, so it can't get up any speed. Wilf Agreed, I worked out how much lift I needed to be the same as the pipe cross section. It's not much. I came across this site which gives some useful information about ball lift, seat dimensions etc which maybe useful. ibls.org/mediawiki/index.php?title=Design,_Care_and_Feeding_of_Check_Valves David
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Post by andyhigham on Nov 15, 2020 18:51:30 GMT
The prototype Hunslet Jack has an axle pump. It is controlled by a valve on the suction side and an air bleed valve. When the boiler is not being fed the suction side valve is closed and the air bleed opened. To feed the boiler the suction line valve is opened and when water sprays from the air bleed the bleed valve is closed. The pipe leading to the bleed valve can be seen on the RHS of the spectacle plate, the exit of the valve is just a hole in the side of the cab Jack 006 by Sigma Projects, on Flickr
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dscott
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Post by dscott on Nov 16, 2020 1:23:55 GMT
While spending endless hours on this Workshop Extension, I had a growing idea for a REAL CHRISTMAS TREE? As many will know of my lots of projects the idea is to hang as many dome bell things, Bright shiny metal bits and bright red bits upon it. Hopefully make it to our Front Cover for Christmas.
Shall I hang various Axle Pump castings to the lower branches?
I am now fitting them in pairs to various projects including the Dock Tank of course as a pair of castings for turned up. New Design. New Scale but using the existing castings, some from Butch. A neat idea of having them facing each other and using a single ram. A Yoke drives them from an eccentric.
David, and Lily still newish to Christmas.
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