Tag Archives: air pump

Edwards and Feed Pump VIdeoz

Here’s a better video (with iffy commentary) showing the feed and air pumps (and alternator) drive in action.

The Edwards pump is working, but I’m not sure that the flap valves are holding pressure (as it seems to have to start from atmospheric on each stroke…) but clearly the ball valves on the feed pumps are very sound (they actually hold 50psi for hours)…..


A Suitable end to a winter’s engineering!

The plan is to work with the seasons and transition from boat building when it’s warm to mechanical engineering when it’s winter – and we are clearly heading into spring and temperatures in the boat house are becoming tolerable, and suitable for working with Epoxy; however, I was loath to make the switch back to boat building until I had concluded the design and construction of the pump and alternator drive assembly.

This has proven a very slow process, and as my good friend  David Mattingley pointed out there is a world of difference between building something from drawings and designing and building from scratch.

The design process – solid modelers

pump/alternator drive

Isometric of the planned pump and alternator assembly

I used PunchCAD’s ViaCAD 3D to draw the basic arrangement of this assembly, and this solid modeler was a new experience for me having previously only used 2D CAD systems (like Autocad). With this new solid modeling approach you manipulate 3d objects (cylinders, cubes etc.) to construct a computer model of the thing you are planning to build. You drill holes in things (by subtracting a “hole-sized” cylinders from the object) and merge primatives to create more complex shapes (like the alternator and gear box in the attached pic). This approach allows you to see clashes of components and get a good feel for clearances and shape of the final assembly. It requires a very different mindset. When the model is complete it is possible to create dimensioned 2D drawings of each part (although I confess I found it hard to get this feature to provide exactly what I wanted.)

This got me to phase 1, with the worm box, chain-wheels, v-belts and alternator all in position, and running smoothly (although it did not predict that the chain would need a tensioner as the centre distance I had chosen for the chainwheels forced us to fit a chain that was almost exactly one link too long) 😦

The lengthier part of the build/design process was to then mate this assembly to the air and feed pump assembly we completed a month or so back.

Feed Pump Drive

I had decided to use the worm-box output shaft to drive two cranks, each being used to form a scotch crank assembly (see this nice video if you are not familiar with how a scotch crank (AKA Scotch Yoke) works). This meant I could remove the uneven stress on the worm box if we only drive the pumps from one side. It became clear we should mount the pumps over the worm box, to avoid adding too much height to the engine assembly, and so an arrangement with 4 “side levers” running down from the pump crosshead to the scotch cranks seemed right. Mounting the pump assembly required a stiffened plate which I fabricated from some 3mm Brass plate I had in the “stock pile”.

The cranks are fitted to a keyed shaft, and each crank pin is 10mm silver steel. I found some thin section ball races (10mm ID, 15mm OD and 4mm wide) from Bearing Boys, and used two on each crankpin. However, I was unsure of the load capacity of the bearing, and the spreadsheet I had constructed for the pumps suggested that the four bearings needed to support a dynamic load of almost 100lbs (when pumping @ 200psi), and while this theoretically would be shared across 4 bearings, this would require “perfect” conditions. During an initial test I accidentally jammed a plastic mug! into the assembly stalling the engine – this resulted in two of the bearings failing with split outer races. I suspect the jam caused the bearings to be axially overloaded, but none the less this failure was worrying.

I contemplated fitting needle roller races to raise the load capacity, but as an initial “fix” I fitted 3mm thick outer “retaining rings” in silver steel to the bearings to strengthen the outer races… with this fix in place and some hose pipe to prime the pumps I ran a test with a pressure gauge on the feed line, and a 200psi safety valve to regulate the pressure (as you can see we reached over 250psi in practice).

As the video below shows we appear to have a working system (with hints of Tardis – don’t you think!!!) – the variability in pressure is to some extent caused by the safety valve, but also I think the pump valves may be slow to seat…

I feel I can now get on with the “wood work”, with a job well done!!!


Gaskets and Unions

Well, the last week or so has been dedicated to finishing the air pump and feed pump assembly. The original owner of the castings had partially machined and assembled the main air pump castings, but they needed “fettling” to fit. This ranged from minor adjustment of PCD bolt holes, replacement of pump rod, and other minor work thru to remachining / truing of all of the mounting faces, fitting o-ring to undersized piston, reboring the feed-pump rams (to provide increased capacity needed for geared-down operation), the making of all the feed-pump valves and manufacture of all (bar one) of the pipe unions.

This involved a variety of odd-ball threads… I think this assembly now includes all the following:

  • 1/4 x 25 TPI British Standard Fine
  • 3/8 & 1/2 British Standard Pipe (parallel)
  • 9/16 x 26 TPI (actually a British Standard Cycle Thread)!
  • 2BA (British Association)

The larger ones of these were either entirely screw cut on the lathe or screw cut and then “chased” with a standard split die. With the new insert/index threading tools this all went really well.

Interestingly (and as a demonstration of the value of “keeping everything”) a great many of the bolts on the engine are 1/4 BSF, and luckily at a club Auction at Guildford Model Engineering Society (I used to be a member), I managed to purchase a bucket of 1/4 BSF Allen Bolts for about a fiver …. I just had to look after them from 1985 ’till now to find a use for them!

The assembly also needed a number of gaskets (about 7) and these were all cut using a ball-peen hammer and centre punches to tap them out from the components they were sealing. This is such a simple and effective process (but one that many people seem not to know) that I included some pictures of the process in the slideshow below….

(also see link about drawing errors on this part here)

This slideshow requires JavaScript.

A fix for the Air Pump

So, following some communication with the steam boating forum we are all agreed that the Edwards Air Pump as drawn is missing  anything to hold the valve-plate in place, or seal its contact with the pump body. This means that:

  1. The whole valve tends to move up and down with the piston rod, (and I think this needs to be a “good” fit to prevent leaking round the pump-rod, so this is unavoidable).
  2. There is nothing to seal the lower valve plate (the one with the holes) where it sits on the ledge of the pump body…

So, my solution is to:

  1. fit a gasket under the valve plate
  2. make and fit a nylon block to hold the plate down – made 10 thou too long, so that the top cover provides some “squishing pressure”

As other members suggested this may help pump performance by removing “deadspace” (but it is beyond the valves so I am not sure if this is correct), and secondly it needed to be made in a way that ensured the outlet port is not cut-off if the block rotates; so, as can be seen in the pics below, I have made it with an exhaust annulus and internal ports to the valve chamber cut in the bottom…

I think this approach will also prevent the valve opening too far….. a test in the kitchen sink proves that it all works!

Here are some pics of the block and completed pump assembly.

air pump stuffing block in situ

air pump stuffing block in situ

air pump valve chest stuffing block

air pump valve chest stuffing block

air pump valve chest stuffing block

air pump valve chest stuffing block

Don’t think the air pump will work

I think there is a problem with the air pump design/drawings…

This looks like it’s not a problem with the drawings, so much as an oversight in the design…

The outlet flap valve assembly is not fixed/located by anything vertically within the outlet chamber… neither is the lower valve plate – part 3 on P20 of the book – (the one with the holes)  sealed against the lip it rests on above the cylinder….so (as I noticed when I completed a trial assembly) the whole valve (both plates and the rubber washer) move up and down with the piston rod, and even if the vacuum held it down, I feel sure any hard won pressure difference would be lost as the air leaked back into the pump round and under the valve plate.

My current plan is to make a nylon “block” which is a light push fit in the outlet chamber, has “ports” to let the air-water out to the  delivery port, and is slightly longer than the available space, so that when the top cover is fitted it will press the lower valve plate down onto a gasket I am fitting under the valve plate….

does anyone have any better ideas/opinions? am I missing something?

Retirement Beckons!

A change of pace and circumstance

Well it seems I have not posted since November and the arrival of Befur’s trailer. Since then a lot has happened (so Happy Xmas, and Happy New Year)… I have had the fortune to be made redundant, and have (with Louise’s kind support) agreed to turn that into retirement – so from the end of February there will be no more working interruptions, and as I am only “on call” now, progress should be faster. So with the shingles finally subsiding, and hopefully the last of the winter colds and the left shoulder starting to free up,  there can be no more excuses – so 2015 looks bright indeed 🙂 Continue reading