As mentioned in previous posts, the bearing surfaces of the rudder shaft are poured epoxy with graphite, poured into the plastic (ultra high molecular weight polyethylene) bearing. In order to pour the bearings on the shaft I need to centre the shaft in the opening of the plastic bearing and seal the bottom opening by using a plasticine rope pushed into the opening and then using gravity literally pouring the epoxy mix into the top of the opening until the cavity around the shaft is filled with the graphite epoxy. Sounds simple enough.
Famous last words. It is simple in theory, difficult in practice because of the need to align the top and bottom bearings, the need for a fixed ring around the shaft below the bottom bearing to stop the rudder pushing up any further and rubbing against the hull and because getting spacers into the gap between the rudder shaft and bearing inside surface and be removed again so that they dont in any way foul the plastic bearing.
So here are my solutions to the various problems. Firstly wedges to pack the bearing walls away from the post and the exact distance all the way around. I practised the pour on a spare bearing (that I had made years ago before I bought the rudders, to the size of the shaft that the other cat built in my shed used and I had mine made at the same time with the thought I would make my rudders the same way with the same shaft material). I tried using split ice-cream sticks to make the spacers but these are way to hard to keep standing in the space, as you pushed another in the spacer would fall over and fall out. The problem with wider spacers though is that because you are placing the spacer in between two curved surfaces, the wider the spacer the harder it is to measure an actual thickness because the wider spacer hits the sides of the curve in 2 locations instead of one, the wider the spacer the wider apart it is touching the wall and the thicker the spacer becomes, leaving a gap in the middle of the spacer in the curve at the point on the curve where you measured the spacer to fit in the first place. That all makes sense in my head but probably does not convey its meaning properly. In short, the wider the spacer is, bigger the gap it creates so instead of the carefully measured thin spacer, you have to factor in its width which effects the desired depth.
In my practice run I was not as disciplined about the exact spacing as I will need to be on the actual shafts, simply making the spacers roughly the right size (the opening on the trial run was only 3mm, the opening on the actual shafts will be 4.5mm and 5mm). I used offcuts of fibreglass plate (from the rings that will form the thrust bearing on the bottom bearing.
I made a mistake on my trial run that I wont make on the actual rudders. I made the spacers out of material that would stick to the graphite epoxy. In the actual pours I will use plastic spacers. What I did do is make sure the spacers taper away on the bottom so that I could remove the plastic bearing over them by tapping the plastic bearing down and off the poured bearing. I wont be able to do that with the rudders of course, but I will make the top spacers taper away in case the plastic bearing is tight to remove and I have to tap it up to get it off for the first time. I am sure once it is off the first time it will slide off and on easily. With the practice pour the plastic bearing was tight to remove so required a tap with a hammer but once off could be put back on and taken off again by hand. It also turned easily by hand in the plastic bearing.
There were a couple of things that did surprise me. There are tiny grooves inside the practice plastic bearings that I did not originally notice. I noticed them immediately I took the plastic bearing off because those grooves were transferred as ridges to the poured bearing. I immediately checked the actual bearings and they look much cleaner inside than the practice bearings. But because I was practising, I tried a very light sand of the poured bearing to remove the ridges. This helped with the ability to rotate inside the plastic bearing, so if there are any indentations I cannot yet see in the actual bearings, then a light sand with wet and dry paper will remove the ridges without deforming the shape or making the poured bearing loose inside the plastic bearing. I also noted that the black practice plastic bearing was not perfectly round because it binds ever so slightly at one point in the rotation meaning it is slightly out of perfect round. Hopefully my actual bearings are perfectly round, it just means that the rudders will rotate more easily and there wont be any wear.
Confident that pouring bearings is the easy part, I set about making plastic spacers using strips cut off an cheap plastic chopping board and sanding them down from the cut size of about 6mm to the exact size using a micrometer to gauge thickness. Just a tip, when you cut a plastic chopping board with a jig saw the friction created melts the board, dont try to remove the melt until it sets. Its hot. Molten hot. Bit of filing and sanding later and I had a strip of plastic exactly 4.5mm thick. Then confusion set in when it was immediately obvious that the spacers were way to tight for the top of the top bearing. A few seconds later I realised I had glassed a couple of layers onto the top of the shaft to isolate the carbon from the stainless steel tillers. Back to the micrometer to measure the new thickness of the shaft, re-calculate that I needed 3.5mm spacers at the top, back to the sander to take another millimetre off my top spacers and I was all set.
And the first set of spacers top and bottom went on really easily and in minutes I had the first pour ready. Seemed to go too smoothly. It was then I realised I had the top plastic bearing on upside down. So I took apart again and started the process over with the bearing on the right way up. Theoretically it should not matter, the bearings ought to be straight and true so that it would fit and work either way, but I figured it could not hurt to do it right way up and it took a couple of minutes to set up. Wrong. Second time around I would get 3 of the 4 spacers in and the 4th would make one fall out. Or I would get all 4 on the top of the bearing in, move to the other end and the motions of getting these spacers in would dislodge one or more of the top spacers. These spacers are only in lightly just enough to hold the shaft in the exact centre of the opening, if they were tight they might deform the spacer enough to knock it out of round. Anyway, I finally got the re-do done and the placticine on and moved onto spacing the other rudder.
The pour consists of regular epoxy mixed with graphite powder at 10:1 by volume. It is still very runny and viscous at this ratio so pouring it into the space is very easy from just the edge of a margarine container. About 110 ml is enough to pour each bearing with a little left over. I tried to keep stirring the mixture between each little pour to keep the graphite well suspended in the epoxy as it can tend to fall to the bottom of the container.
I checked that the bearing flange, which, assuming the bearings were made correctly is exactly square to the shaft bore, was square to the rudder shaft. It was. This is only part of the equation though, the bearing could be on square but all the way over to one side of the bore, but being square meant that the spacers top and bottom put the shaft plumb to the bearing and the chances of the spacers (being different thickness’s due to the extra layers of glass at the top) being offset top and bottom but still being parallel to the shaft are extremely slim given all the top spacers are the same thickness, different to the bottom and all the bottom spacers are the same thickness. So that takes care of the easier of the 2 pours. The bottom shaft bearing will be somewhat more difficult to get right and crucial to the job. The shaft can cope with being slightly off centre if both top and bottom bearings are exactly aligned, but if they are not, the rudders will bind, too far out and the shaft wont get into both bearings let alone turn. The fortunate thing about this method of bearings is that should the bottom bearing not come out perfectly aligned I can always grind the bearing off and do it all again.
The observant of you will notice the bow ladder in the right hand corner of the pic above. It is on the floor between 2 spacer blocks undergoing my “jump test”. To make sure the ladder with a gang plank above can handle the weight I jumped up and down on it, I weight about 90kgs, so by jumping up and down I make sure it can handle people walking over it. All good. Now to make the brackets to attach the ladder and gang way.
So, first bearings have set and just need to be cleaned up (edges bevelled), I have a plan that hopefully ensures that the pours for the bottom bearings are exactly aligned. We know that the inside of the tube is perfectly aligned so that when the uhmwpe bearings are in the ends of the tube they are naturally also perfectly aligned. And we now have the top half of the pair of bearings on each rudder poured and set. If I set up the top spacers and plasticine dam then slide the fibreglass tube over the bottom uhmwpe bearing carefully enough that the spacers stay in place, then insert the top uhmwpe bearing into the tube and onto the top poured bearing I can then insert the bottom spacers and pour the bottom bearing knowing that it is perfectly aligned to the other poured bearing. But that is the difficult part of the entire process. The spacers only lightly sit in place, too tight and the risk is that the pressure would slightly deform the uhmwpe bearing and the fit of the outside of the uhmwpe to the fibreglass tube is also snug (and has to be otherwise if it were loose then there is still the risk of misalignment of the poured bearings) so getting all of these snug parts into each other without disturbing the loosely fit spacers may be quite difficult. We will soon know.