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!!!

 

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2 thoughts on “A Suitable end to a winter’s engineering!

  1. cornbeam

    Fantastic work. Good to see and hear it turning over, must be great feeling to have got a working engine. BTW we found a shed member who can put the steam engine head on his milling machine.

    Reply

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