Vulcan Beam Engine 4 – The Beam Assembly
What I am going to look at today is the Beam, Trunnion and Trunnion Bearings, and how I made and set these up. Also I will discuss some modifications I have made from the drawings and why.
Unfortunately I don’t have images of the Beam Casting or Trunnion Bearings before they were machined, but I’ll give a brief description of how I arrived at where we are now. The Beam Casting needed quite a bit of fettling with the Dremel to get rid of the rough surfaces, and the outside edge is a job for the belt sander or disc grinder, finishing off with files and wet and dry. It’s not difficult, just time consuming! The set up for milling the thickness was just a case of setting it up in the vice using a couple of lengths of mild steel bar as parallels and removing about 1.5mm from either side, being careful to make sure the bosses were centralised. To be honest, I was in danger of ‘over engineering’ this operation, and I initially struggled with figuring out how to clamp the beam. They say the simplest is usually the best, and it don’t come much simpler than clamping it in a vice
The Trunnion Bearings are designed to be in two parts, the bearing and cap. The top and bottom were cleaned up and the holes drilled 8BA tapping. The Bearing was then split using a hacksaw (how often have I seen people setting up a slitting saw in the milling machine for this sort of job?) The faces were then cleaned up and the lower holes tapped and the upper holes opened up to the outside diameter of the bolts, not clearance at this stage. Assemble the two halves and drill the 6mm hole making sure it is perpendicular, no need to set the four jaw up on the lathe for this just be careful setting up in the drill vice. Now it’s time to move onto the lathe to clean up the faces. I have a 6mm mandrel which is slightly tapered so parts can be a push fit, believe me when I say friction will do the rest, light cuts and your there.
Drilling the beam is straightforward, or so I thought. The central hole for the trunnion is simple, you drill it centrally, but when you mark the outer holes for the con-rod and suspension link, the drawing measurement of 96mm comes in at a couple of millimetres from the end of the beam and nowhere near the centre of the bosses. I don’t think the measurement is too critical so it’s a case of messing around with the dividers until you reach a compromise which centralise the holes on their respective bosses and keep them equidistant from the centre. Fortunately for me the hole for the inner suspension link works out OK with the drawing dimension of 48mm from the outer hole, because the holes in the bridle links need to be exactly the same distance, and I have already made these. All the holes I have drilled 8mm, apart from the pump rod coupling hole which is 4.8mm.
Setting up the Trunnion Bearings is pretty simple, I used a length of 6mm silver steel bar to align them on top of the side frames and drill and tap. Be careful here, theres not a lot of spare material to play with on the base of the bearings.
To the Trunnion itself, I’m not too keen on the design which calls for the centre section being a push fit through a 12.7mm (1/2”) hole in the beam, to me theres a danger of the beam pivoting on this rather than the whole assembly pivoting in the bearings, so it’s off to the drawing board (or AutoCad in my case).
I decided that the Trunnion would be better if made in two parts which screwed together, that way there would be a solid face to mate with the beam, and less chance of wear once fixed in place. The dimensions were taken straight from the model, measure the gap between the bosses, the thickness of the beam at the centre boss, deduct the width from the gap and divide by two and you have your starting point. I took my time over turning the two parts to ensure they were as accurate as possible to minimise any float or movement of the beam. I have to say this is an instance where my new quick change tool post really earned its keep. So that’s about it for this blog post, a bit long and drawn out but I think I’ve more or less caught up with the model as it stands. Next job, I think, will be the Connecting Rod.