Having finished my Stuart Turner 10V, SWMBO complained that she what really had wanted was a beam engine. I now feel confident to make one out of the scrap box (not sure how to use the lock or the hinges :-) ), especially as my best mate has gone and bought the Brunell Models one, just to put his nose out of joint.
I've conceived of a way of producing the beam by milling from a bar and then screwing in the bushes, but what I don't know are the intricacies of the Watt's Parallel Motion.
(Apart from ensuring that the distance from the ground anchor of the swinging pivot to the parallelogram when it is rectangular is the same as the distance from the beam's pivot to the same point vertically above)
Also, I assume/guess that when the parallelogram is rectangular, that the piston should be half way along its travel through the cylinder?
-----ooooo-----
Also, in order to get a large enough flywheel, are scrap flywheels from motor cars of a suitable cast iron to be easily machined - I envisage doing the crossing out (a la clock wheels) to make the spokes?
It provides a 5th order approximation to a straight line.
Discussed on the Yahoo groups "MLProjects2" group around 5-12 January
2004, refer to their archive for more info.
Here's a spreadsheet and diagram which should help design an accurate link motion to any scale you want, and plot the error (deviation from straight line) on a chart. For a given stroke, the spreadsheet lets you find the beam length (and exact geometry) to meet any given error criterion you want.
formatting link
How it works...
All the input settings are in column B3-B11 of spreadsheet Beamer-5.xls. Points A, B etc are illustrated in the CAD sketch wattlink4.dxf. Outputs C(x,y) and CD tell you the remaining dimensions for minimul error, and Chart 1 plots the error itself.
(excerpt from above discussion)
Start by setting the beam length (radius), and desired stroke of the engine (currently 1000 and 720mm) in B3 and B4. If you are happy with the bridle and link rods (half the beam and stroke respectively), leave them, though you can enter different numbers and it SHOULD design you a consistent and accurate linkage.
Then note that the beam angle A'Stroke is output in H4. Using it, decide where you want to put the four zeroes in B8-B11. For example, choose 21 and 10.5 degrees for B8,9, and -10.5 and -21 will appear in B10,B11.
It will calculate 4 positions for D corresponding to zero error at these
4 angles, and use these (which lie on a circle) to calculate the centre and radius of the circle, which gives the position of C, and the length of CD. NOTE that if the bridle is shorter than half the beam length, CD will be longer - and vice-versa - and the error can still be very low.
(If B10,11 are not mirror images of B8,B9 then the four positions of D may not lie on the same circle, and the calculated positions of C (in columns N8-O11) will not be the same. This is a good check that the model is working...)
The resulting error is calculated in O16 to O42, involving the Cosine Law (thanks to Robert Smith for that one!) roughly as per his spreadsheet, and plotted in Chart 1. (You can modify the range to display using Chart/Source Data when the chart is displayed)
Try 21 and 13 degrees, for slightly lower error.
Or 21 and 0 degrees, it should re-create the motion according to Jeynes. (oops! try 21 and 0.001 degrees :-)
Or set the stroke to 600mm, note the beam angle is now only 17.45 degrees, and choose 17 and 10 degrees for B8 and B9. The error is under
They certainly used to be easy enough to machine. I remember machining about 2 lbs of the flywheel of my Austin Seven Special to speed up the gear changes.
My car maintainer has just confirmed this, so I've asked him to get me an old one. He says that such flywheels are very easy to drill and tap - so have we just inadvertently identified a cheap source of very usable cast iron?
How's the Simplex Boiler? I spent your £100 on a
3 1/2" part-built Hall; running chassis, although seized with a film of rust, and partly-soldered boiler (original constructor apparently died about 10 years ago) but it did come with the complete pile of ME from about
1955 in which the design was serialised.
Rather slow progress at present following a move into a new (nearly finished) house. Anyway the workshop is up and running now and I have nearly completed the frame. The boiler bits are stacked behind the lathe.
Sorry, Brian, I've just realised that your print-out has been sitting on my desk submerged for a week, and that I hadn't acknowledged your help. I don't have a means of displaying DXF files (I did try to install one of the monthly-magazine disks release of TurboCAD v4, but it trashed the computer)
I'll reply once I've had the chance to decode the calculations in the spread sheet. TA!
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.