After spending the winter tearing out wet foam and rebuilding my rudder, it is finally time to start thinking about the reinstall when the snow melts in a couple of weeks. I have contacted a local machine shop who are going to be machining a new rudder bushing. How much larger than the rudder shaft should the inside of the bushing be for a ~6.5" shaft. The machinist is going to be measuring the shaft to get an accurate measurement on the OD of the sleeve, but I need to give him the acceptable tolerance for the ID of the bushing.
Rudder bearing tolerance
- Thread starter waXrules
- Start date
Dupont says the absorption of Delrin is a little less than 1%, which won't change a thin wall bushing much. A greater variable may be what the bearing guys call "close down" which is how much the bushing swages down when pressed into the housing - the plastic generally being much less stiff than the housing. Kind of a guess, but I'd go for around 0.005 diametric clearance, after assuming 1% growth in wall thickness and 100% shrinkage from the OD interference fit. You may also have misalignment to accommodate if the two bearings are not exactly aligned, housing may be tapered, shaft may be out of round, etc. 0.005 will feel nice and tight without dragging (piece of paper is about 0.004). 0.020 will probably clunk noticeably (typical credit card is about 0.030). Measuring all that and getting it right isn't a trivial exercise.
SASSAFRASS
Super Anarchist
Definitely get advice from a yard or shop that does this we saw a boat recently in Panama that almost had to haul again due to too tight of a tolorance in the new bearing. There are standards based on shaft diam and as DDW said it would be a good idea to fit and re-check the bore. Not a huge deal to remove and skim cut if needed. One of the NA's around like Zonker can probably give you a good number or ratio.
SloopJonB
Super Anarchist
One thing I learned the hard way is to ensure the top of the rudder shaft has a small chamfer or roundover.
If it just has the square cut edge you will have trouble sliding it in - it will tend to "bite" into the bushings with the slightest misalignment
If it just has the square cut edge you will have trouble sliding it in - it will tend to "bite" into the bushings with the slightest misalignment
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G
Guest
Guest
also the difference between tight and loose is not much at all, a friend had uhmwpe bearings that were too tight, he reamed them with a drum sander on a drill ever so lightly- now its clunk clunk
- Thread starter
- #9
Working with the Design a Bearing tool on the Vesconite website and plugging in the numbers for my boat (6.5"shaft, 7.25"tube, 3"bearing length), their recommendation is an internal diameter of 6.521". Note that I am not press fitting the bearing into the boat; will be epoxied in place. Also interesting is that the internal diameter increases to 6.539" if I select a press-fit application.
They may be accounting for some swelling in the water. In my opinion 0.021 is a fairly loose fit. Is that Vesconite guidance for a prop shaft log or a rudder stock? Shaft log would have additional clearance.
Epoxied in place you have a better chance, with the additional problem: How will you line it up? Could use the rudder, but that requires some care.
Epoxied in place you have a better chance, with the additional problem: How will you line it up? Could use the rudder, but that requires some care.
- Thread starter
- #11
Plugged the numbers into both calculators and the numbers are the same. Only difference is the recommendation to add grooves into the shaft log for cooling. Just for fun, I plugged the dimensions into their general application calculator (https://www.vesconite.com/design-and-technical/) and the recommendation came out at 6.509"
I agree that .021 seems fairly loose. I've seen a couple other recommendations suggesting just slight drag on the bearing.
I will most likely be using the rudder to reinstall the bearing. The top bearing is bronze and still in place. It was only the bottom bearing that was worn; I could move the bottom of the rudder over an inch and the movement of the shaft within the bottom bearing was easily visible. No appreciable movement at the top bearing.
I agree that .021 seems fairly loose. I've seen a couple other recommendations suggesting just slight drag on the bearing.
I will most likely be using the rudder to reinstall the bearing. The top bearing is bronze and still in place. It was only the bottom bearing that was worn; I could move the bottom of the rudder over an inch and the movement of the shaft within the bottom bearing was easily visible. No appreciable movement at the top bearing.
IStream
Super Anarchist
- 11,009
- 3,180
The top bearing is probably worn too, just in a different way, so it's making line contact rather than surface contact with the shaft. Once you put the new lower bearing in and constrain the lateral play, it'll start wearing the top one faster. If I were going to the trouble to pull the rudder and replace a bearing, I'd do them both.
Rain Man
Super Anarchist
Please believe me when I tell you that Delrin is the wrong material for this application. Use UHWMP instead.
I can pretty much guarantee that Delrin bearings will wear out within two years.
Rain Man
Super Anarchist
This type of adjustment should only be done on a lathe. Also it is very important to be sure that the shaft is at water temperature, not room temperature, especially if the shaft is hollow. On our recent bearing build, I couldn't get the shaft into the bearing at room temperature. Take it outside (5-7C) and it slid in nicely.
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G
Guest
Guest
Agree, not a big fan of plain bearings, vesconite is excellent but the most expensive. If I didnt have needle rollers that’s what I’d probably use.
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Vesconite is pretty cheap in the high end engineering plastics biz. Price out some PEEK or Torlon or Ultem. But at the lower end I'd probably use UHMWPE rather than Delrin. The cost is about the same between those two.
Does Vesconite give figures for water swelling? 0.009 on a 6.5 D sounds better to me, so I'm still thinking that they expect some growth.
Does Vesconite give figures for water swelling? 0.009 on a 6.5 D sounds better to me, so I'm still thinking that they expect some growth.
SASSAFRASS
Super Anarchist
Aren't there specific composites for this, that have lubricant quality in the material. UHMW doesn't seem to hold up very well as a bearing. Lots of thermal movement too.
- Thread starter
- #20
The boat is a J/28. Rudder is about 52"deep by 29"wide. Probably weights in around 80lbs. Space between center of upper and lower bearings is 31".
Someone just asked me why don't I make a new bearing out of fiberglass using the rudder shaft as a mold with graphite powder added to the bearing layer. Sounds like an interesting idea and I have everything needed except the graphite. Can even vacuum bag the layup around the shaft to get better consolidation in the layup.
Someone just asked me why don't I make a new bearing out of fiberglass using the rudder shaft as a mold with graphite powder added to the bearing layer. Sounds like an interesting idea and I have everything needed except the graphite. Can even vacuum bag the layup around the shaft to get better consolidation in the layup.
