Building Logs

Non aligned

Posted by Paul

Well, pouring rudder bearings is dead easy. So much so, I will be doing mine twice. What’s not so easy is using spacers to centre an uhmwpe bearing on a post in order for the poured bearings to be aligned. That’s not quite so easy. So much so, I will be doing mine twice.


So far I have poured both top bearings and although the top and bottom openings of the top bearings looked evenly space all around, as you see above, they were not square to the post, despite me using a square and checking the flange against the post. One of them was probably acceptable, the other was out enough that there is no way that pouring the bottom bearing was going to work or the rudder not bind up when mounted in the rudder tube.

2013-12-17-10.59.38-e1387256362862 2013-12-17-10.59.58-e1387256650205

After I removed the plastic bearings to admire my work I cut the fibreglass rudder tubes to length so I could insert the top and bottom plastic bearings into them and mount the top plastic bearing back onto the top poured bearing to see where it left the bottom bearing in relation to the shaft, that is, where the post exited the bottom bearing and the size of the gap all around it. Non centred unfortunately. So poorly aligned on the poorer of them that one side of the shaft at the bottom was touching the plastic bearing side leaving no room for any poured bearing surface at all. And rotating the shaft in the top bearing did not move it away. If I had forced the shaft into the centre with spacers and poured there, it would bind and not rotate and probably require some force to remove once set, and quite possibly would not have gone back in again due to its misalignment. (The other rudder that was not quite so bad was also not centred but not by much and I felt that perhaps I might have got away with it but if I am going to do one again I may as well do both and see if I cant get them closer to perfect.)

I was a little baffled. I carefully measured and placed equally thick spacers along the top of each bearing and the same at the bottom, equal thickness spacers and what looked a uniform gap all around. But I guess if one spacer falls into a low spot on the shaft and the bearing is not on the shaft exactly parallel to the fraction of a mm straight and true, then over the half meter length of the shaft to the other bearing it becomes 3 or 4 millimetres out at the other bearing which is enough for it to be all the way over one way and the shaft touching that bearing side.


So I decided the solution lay in two heads being better than one, and when one of them is an engineer all the better. Terry (building an Easy 37ft cat) has an engineers eye and mind so he came up with the solution and checked all openings to ensure as close to centred and true as we could get. I ground off the top poured bearings to start again. We checked the inside of the fibreglass tubes with a straight edge and they are straight. We also assume the plastic bearings to have been made with the centre tube, that the rudder shaft and poured bearing goes in, would be set in the exact middle of the bearing. That is, that the walls of the uhmwpe bearing are a consistent thickness and therefore the outside walls are parallel to the inside walls. We know they are all consistently the same size outside in 2 ways, one we measured, and 2 they fit into the fibreglass tubes and are interchangeable, that is they can all be fit into any end of the 2 tubes so that makes them consistently the same outside diameter. 2 have 69mm inside diameter and the micrometer shows that, these are the top bearing designed to be slightly smaller than the bottom bearing opening deliberately so the rudder post passes through the bottom opening easily, and the bottom bearing is 70mm and measures so.

This all points to straight bearings and rudder tubes, and we checked the rudder shafts and they are not perfectly smooth surfaced nor are they anywhere near round, but they are for all intents and purposes straight. We found that at the bottom the shaft is more elliptical than round wider fore to aft than it is left to right by about 2mm. But there is no reason we should not be able to make 2 rudder bearings line up in the tube and pour perfectly round (again assuming the uhmwpe to be round) and in line, taking all of the imperfections of the rudder post out of any contention and spaced so as to be as close to centre of the shaft as we can get it. (Notice also in the pic above that the timber dowel inside the carbon shaft is also not centred, again this should not matter other than the wall thickness is slightly less on one side. I checked the other rudder and its dowel is centred.)

On this basis we concluded that if we get the bearings top and bottom on the shaft as close to centre as we can but more importantly that a straight edge on the outside surfaces of the plastic bearings all around shows them to be aligned then that would mean that the pours will also be parallel and that we should be able to fit the plastic bearings inside the glass tube and remain aligned and the shaft with poured bearings will still go into each bearing and rotate freely.

So we started with 3 straight edges for each rudder (I used 4 wedges, whereas Terry told me 3 would still ensure a centred shaft), and cut them off perfectly square to same the length as the rudder tube. What we are really doing is simulating the rudder tube but with open sides to allow for the setting of dams and pouring from above (not possible with the tube on) and for both top and bottom bearings to be poured at the same time rather than pour one like I did and then try to align the bottom bearing to it.


Then with the top and bottom uhmwpe bearings now tied to each other in a way that ensured they were aligned we centred them on the shaft as best we could, keeping in mind we know the shaft is not round and wedged them in place so they could not move, sealed the bottoms with plasticine ready to be poured. As a further test we rolled one of them on the kitchen table, this was an accident, we stopped for lunch and I took the joined bearings inside and rolled it along the table to show Terry what I had done with the second set and it rolled on the flange edges perfectly so it proved to be another helpful hint that everything was now in line, and it rolled without any wobble or bumps indicating 2 round flanges in perfect alignment to each other. Out of alignment would have resulted in a slight wobble. So from the image above we added a bit more of a more robust tape, the masking tape being just to get it all in place, nice and tightly to ensure it would not move and then wedged it on the mast post and rolled the plasticine dam into the bottom opening of each bearing. Then had lunch!

After lunch I mixed up another batch of epoxy graphite and poured them. I had a few issues with leaks, my plasticine dams didnt hold up and I ended up with a nice run down one rudder blade which I had to clean up with metho and a rag but otherwise as I said at the start of the post, pouring bearing is easy.

The moment of truth will be when they are set and we try to reapply the uhmwpe bearings from inside the rudder tube. If all goes to plan they should still go on. So check back in a couple of days, if I dont post straight away I have issues!

Its no biggie if we are wrong. We just grind them off again and start again. All we are wasting is time and about $2 worth of graphite each pour and about 50ml of epoxy. But if we dont get it right this time I will be at a bit of a loss as to how to get these bearings poured in perfect alignment. The final solution may be to cut away a part of the fibreglass tube in order to pour the bearing from inside the tube (it would be easier to just pour the bearing from inside and have the plasticine dam on the outside extremities than to try to lay a plasticine dam through a small opening all the way around the shaft and pour from the extremities.) But I would prefer not to do that, as the tubes need to remain water tight. Of course it is possible to cut a hole in the tube, do what I have to do through that hole then replace the cut-out and re-glass it so as to make the tube water tight again, but lets cross that bridge…..

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