Winson 5" Gauge Britannia
I've now cleaned up both combination levers and also the crosshead drop arms, the union links and the valve crossheads.
Next item on the agenda were the guide blocks for the valve crossheads. Originally these were simple blocks of brass on each end of the pin that goes through the valve crosshead fork and the combination lever. These blocks run on top of the valve guides and are retained by a metal strap on the top.
Needless to say, these were a poor fit and had been Loctited onto the ends of the pin to stop them falling out. Not a very good solution but there was nothing else to hold them in. Unfortunately, I don't have a drawing of these guide blocks but I would guess that the Spink drawings show that these blocks have a flange on the inside to hold them in place. I decided to go that route with the new ones.
I started off with some 0.5" steel bar in the lathe, drilled and reamed a 5.5mm hole in it and then parted off four slices, one for each of the four guide blocks. These were then milled to a square shape but leaving a 0.030" flange on the top and the bottom.
I was a bit worried about getting the fit right for the blocks but it went a lot easier than I thought.
The first thing was to make sure that the valve rod was located centrally in the valve liner. The valve rods were actually a really sloppy fit in the end cover of the valve chest and the gland so I made a couple of PTFE bushes that were a good fit on the valve rod and in the packing recess in the end cover. I may retain these and use them as the valve rod packing.
I also needed to centralise the other end of the rod as it could still wave about in the breeze so fitted the piston valve with the rings temporarily to act as a steady. The valve rod was also a sloppy fit in the valve bobbin but that was correct as the bobbin needs a bit of clearance on the rod to centralise itself in the valve liner. To get round this I made a thin brass bush to take up the clearance.
With the valve crosshead fitted to the valve rod and a length of silver steel through the holes of the fork I could measure the distance of the pin above the valve guide face and hence the thickness that the underside of the valve guide block needed to be.
I found that the easiest way to measure the gap was to put equal shims under each side until they were a nice sliding fit. The blocks were then milled slightly oversize and the thickness reduced a few thousandths at a time until they too were a nice sliding fit on the valve guide.
The tops of the blocks were then milled down until there was a small amount of clearance between the top of the guide block and a bit of bar placed across the valve guide top.
The last job was to mill down the front and back of the blocks to give a bit of clearance at each end of the valve travel.
Incidentally, one of the valve rods was bent on the thread that screws into the valve crosshead but I managed to straighten it out ok. I really didn't want to have to make a new one. Also, the inside arm of the lefthand valve guide was bent and had to be straightened out. It must have had quite a thump to bend it as it took a lot of effort to straighten it again.
You've probably noticed that I finished up taking the cylinders off the chassis for this job. I had tried to avoid doing this but it made the job a lot easier being able to take the cylinders into the workshop for the fitting process.
I did notice that all the cylinder fixing bolts are a very sloppy fit in the frames so I'm going to have to line the cylinders and slidebars up again when I come to refit the cylinders properly. I might even be able to get rid of that shim that I had to fit in the RH slidebar assembly.
In the last photo, I had fitted the combination lever just for show and I noticed that it won't swing back very far before the front of the oil box on the top front of the lever hits the inside of the valve crosshead slot. I can see that being a problem and could explain the bent combination levers. I may have to try and get some more back travel by a bit of judicious filing. I won't know though until all the valve gear is back together.
I've also read on the Britannia Builders website that on John's Britannia the valve guide blocks were not central in the valve guides when the valve was at mid travel. I think they were too far forward? John solved the problem by moving the expansion link brackets rearwards to shift the whole valve gear further away from the cylinders, thus moving the valve crossheads further back.
Steve Sharman also found this problem with his own build to the Spinks drawings so I'm wondering if I may have a similar problem. There will be very little clearance between the valve guide blocks and the ends of the valve guides when the valve gear is in full travel so I haven't got a lot to play with. Only time will tell!
I decided to check the swing of the combination levers today. I made some pins for the combination lever and the union link joints so that I could assembly the combination lever etc. temporarily. I rolled the chassis back and forth on the stand and marked the extremes of travel of the crosshead on the slidebars. I disconnected the connecting rod from the crosshead so that I could just slide it up and down without having to roll the chassis. I didn't want to risk bending the combination levers again if they did foul the slots in the valve crossheads. I then refitted the drop arms to the crossheads and fitted the union links. I positioned the valve crosshead as far forward as it would go and slid the crosshead backwards as far as it would go. It would only travel about half as far back as it needed to before the oil box on the front of the combination lever hit the end of the slot in the valve crosshead. It would seem that this is indeed why the combination levers were bent backwards. When someone had tried to push the chassis along or run it on air the combination levers would have hit the end of the slots in the valve crossheads and bent them back. It would also have forced the valve crossheads upwards and caused the bent retaining bars on the valve crosshead guides and the bent valve rod.
I had to remove quite a bit from the slots in the valve crossheads before the combination levers would swing back enough to allow the connecting rod crossheads to move all the way. I did this by milling away the top end of the slots at an angle to give the necessary clearance.
There was already enough clearance in the slots for the combination levers to swing forwards. It's the oil box on the front that causes the problem with the backwards swing.
I temporarily assembled the expansion link and the radius rod on the right hand side to see what that looked like.
Much to my surprise everything seemed to line up ok and moved freely. With the radius rod in mid gear and the combination lever vertical, the valve crosshead guide blocks seemed to be pretty much central on the valve guide which is as it should be but we'll see how it is when the eccentric rod is fitted and the return crank set. One problem that I can forsee is that the top back edge of the expansion link hits the expansion link bracket before what will be the end of the valve travel in reverse. Some material has already been removed to give clearance but I think it will need more to give enough swing for the expqansion link. This doesn't seem to be a problem in forward gear though as there is plenty of clearance. Hmmm.
One thing I noticed when I was rolling the chassis along the stand was that the front coupling rod oil box hits the end of the bottom slidebar.
I hadn't noticed that before and that's with the axleboxes right at the bottom of their travel. The problem will be worse when the axleboxes are higher in the horns. The other side doesn't quite hit but it's very close. It will help if I can get rid of that shim that I fitted by jiggling the cylinder position but I may have to reduce the width of the oil boxes on the rods. Something else to sort out!
Not a lot done over the last week or so due to other things.
I decided to get the reverser and the lifting arms sorted out. The lifting arms are built up from two side plates and a central boss which is pinned to the weighshaft with a roll pin. Both of the bosses were a very sloppy fit on the weighshaft so I made some new ones. The originals were brass but I made the new ones out of steel as the arms were steel. I also decided that I would replace the roll pins with proper taper pins when the time came to refit the arms to the weighshaft.
The Spink drawings show the arms silver soldered to the boss but these are just held with a couple of bolts.
All the arms were a bit bent and battered but I managed to get them flat again and looking a bit more respectable. The sides of the slots in the lefthand arm that take the nut of the reverser were especially bent so I straightened them out, bolted both sides together and then carefully milled the slots square again taking as little off as posible. The sides are a bit flimsy anyway so I didn't want to weaken them anymore.
The Spink plans show the reverser nut with a pin either side of it with two steel dies that run in the slot. The Winson affair was just a groove turned in each end of the nut which was machined from brass bar. I decided to use the proper die blocks as shown on the drawings as that is a much better arrangement. The pins on the original nut were a sloppy fit in the lifting arm slots to begin with and I had made the slots slightly wider now when milling them true. To modify the nut I just held it in a collet in the lathe and machined the ends down to 0.1875" diameter. The dies were made from 0.3125" square steel bar and drilled and reamed 0.1875" diameter to fit the pins on the reverser nut and then two sides milled down to fit the slots in the lifting arms. The die was then sawn off leaving a flange to hold them in place. The rough sawn flange was then milled down to 0.020" so that the dies and the nut would fit between the arms of the lifting arm.
The result was much nicer than the original.
I would have liked to have made a new nut altogether as it's not a good fit on the screw but it's a lefthand thread and I don't have any lefthand taps. I didn't fancy trying to screwcut a new one.
I thought I had better try the lifting arm and reverser screw in place before I went any further and that's when I found out that the reverser screw doesn't line up with the nut!
The screw lines up vertically but it's too far from the frames by about 0.125". Several rude words later and I noticed that the screw is not parallel to the bolting face of the main bracket, in fact it's miles out!
I set the bracket and screw up in the mill vice and compared the axis of the screw with some gauge blocks and you can see how far it is out of line. I checked it with my Wixey Digital Angle Gauge and it's 1.9° out of true.
The hole for the rear bearing bush has obviously been drilled in the wrong place and needs to be moved by about 0.125". I'm not sure if there is enough meat on that part of the bracket to do that easily. I may have to move the front bush as well to get the screw to line up properly. More problems!
I decided to just move the rear bearing to begin with and see how everything lined up after that. I knocked the rear bush out and Loctited in a plug turned from aluminium bar. I also knocked out the front bush so that I could drill through the hole.
The bracket was then set up in the mill vice and the front bush hole centred using a DTI as per normal practice.
The idea was to then redrill the hole for the bottom bush in the correct position and refit the bush. All went well until I was using a 7mm drill and then the Loctited in plug broke loose. To be honest, I half expected this to happen and I should have known better! The 7mm hole only left a crescent shaped piece of the plug remaining and there wasn't enough contact area left to hold it in place. The plug really needs to be of a large enough diameter so that the new hole doesn't break through the edge of the plug and weaken it.
So, I started again using a larger 10mm diameter plug. The hole in the bracket was now oval shaped so I had to enlarge it with an endmill rather than try to drill it out to 10mm which would have been a bit tricky to do.
The new 10mm plug was then Loctited in place.
I then repeated the process of drilling the new hole and finished it with an 8mm drill to take the bush. It all went ok this time!
I temporarily assembled the reverser and bracket and fitted it back on the chassis to see how everything lined up.
The reverser screw could still have done with going over a bit further but that would have meant moving both of the bushes rather than just the one. It does just line up with the nut but the outside lifting arm is touching the inside of the slot in the bracket. I think it will be ok though.
I'm not sure yet how well the lifting arm lines up with the radius rod. I've only got the lifting link assembled with some of the old pins which are a very loose fit in the opened out holes and I think that the lifting arm could do with moving inwards by maybe 0.063". I'll check it when I make and fit the new pins for the link. If it does need to go over a bit then I think the easiest way of doing that is to machine some off the bolting face of the bracket and move the whole assembly nearer the frames. Hopefully, it may not come to that.
You may notice that I've drilled and tapped the end of the reverser screw to take a bolt and washer. This is to control the end float of the screw in the bracket. Originally the end of the screw was cross drilled to take a split pin and there was a lot of end float in the screw which caused the lifting arms to move up and down quite a lot. There seems very little play between the reverser nut and the screw so hopefully that will be ok.