Jump to content

Rudder bearing tolerance


Recommended Posts

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.

Link to post
Share on other sites

Depends somewhat on the bearing material. Plastics swell to varying extents in water. Something like Vesconite is ideal, there is technical info on their website giving recommended clearances. Delrin isn't too bad for swelling, nylon is horrible. 

Link to post
Share on other sites

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. 

Link to post
Share on other sites

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.

Link to post
Share on other sites

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

Link to post
Share on other sites

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

Link to post
Share on other sites

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.

Link to post
Share on other sites

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.

Link to post
Share on other sites

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.  

Link to post
Share on other sites

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.

Link to post
Share on other sites
22 hours ago, waXrules said:

Sorry, forgot to mention that the new bushing is being made of Delrin.  

Please believe me when I tell you that Delrin is the wrong material for this application.  Use UHWMP instead.

Ultra High Molecular Weight Polyethylene (UHMW-PE)

I can pretty much guarantee that Delrin bearings will wear out within two years.

Link to post
Share on other sites
9 hours ago, Caecilian said:

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

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.  

Link to post
Share on other sites

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. 

Link to post
Share on other sites

But with a 6.5" dia and maybe 3" height, that is a lot of area - don't know how big the rudder and boat are. Also available is ptfe filled Delrin, which will last a bit longer as a bearing. 

Link to post
Share on other sites

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.

Link to post
Share on other sites

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.

Link to post
Share on other sites
5 minutes ago, SASSAFRASS said:

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.

There are specific composites for this, Vesculite is one and TriStar Ultracomp is another. For that small a boat with those bearing sizes, I think almost any plastic will work. The thermal expansion of any plastic is high, acetal (Delrin) and PE not that different. Carbon filled plastics are more stable, but not what you want here. 

Link to post
Share on other sites
2 minutes ago, waXrules said:

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.

I would guess it's highly unlikely the stock is going to be perfectly round due to wear. You also would have to maintain enough tolorance in the release agent to be able to remove and machine a bore with appropriate ID.

Link to post
Share on other sites
On 3/20/2021 at 4:07 AM, Rain Man said:

Please believe me when I tell you that Delrin is the wrong material for this application.  Use UHWMP instead.

Ultra High Molecular Weight Polyethylene (UHMW-PE)

I can pretty much guarantee that Delrin bearings will wear out within two years.

Bullshit. The Delrin bearings in our 32’ IOR have lasted 25 years. 
 

If you know how to make shit it lasts. If you don’t...

The thing that wears out the bearings is the shaft was never machined correctly or go damaged. The bearings were too small in the first place(very common) The bearings were not installed correctly. The bearings were too loose to start with and then wore prematurely. 
 

I would use UHWMP as well at this point in time. Delrin is an excellent material that has now been replaced by better materials. It still has its place for many applications. 

Link to post
Share on other sites
38 minutes ago, CaptainAhab said:

Bullshit. The Delrin bearings in our 32’ IOR have lasted 25 years. 
 

If you know how to make shit it lasts. If you don’t...

The thing that wears out the bearings is the shaft was never machined correctly or go damaged. The bearings were too small in the first place(very common) The bearings were not installed correctly. The bearings were too loose to start with and then wore prematurely. 
 

I would use UHWMP as well at this point in time. Delrin is an excellent material that has now been replaced by better materials. It still has its place for many applications. 

Personal experience:  I bought my Dash 34 when it was one year old in 1982.  Two years of active racing later the original Delrin bearings were worn out.  I replaced them with new Delrin bearings.  Two years of racing later they were worn out again.  On the advice of a friend I replaced them with UHMWP.  When I sold the boat in 2019 those bearings from the 80's were still in the boat with zero play.  

I have a friend who is big into Martin 242's.  He made the new UHMWP bearings for my SC 27.  He says that they have been replacing the bearings on the 242's with Delrin and are very unhappy with the longevity - they get a couple of years at most on actively sailed boats.  He is now making his new 242 bearings out of UHMWP.

It could be that correct machining makes the difference.  Somehow I doubt it.  YMMV.  I can't explain why your Delrin bearings lasted a long time and mine didn't.  Is the boat raced actively?

Link to post
Share on other sites
12 hours ago, andykane said:

Does a 28' boat really have a 6.5" rudder shaft? I would expect more like half that.

Very likely to accommodate a rectangular rudder post. The way these are commonly built is a SS ring is potted in place around the shaft because of the square pegs in round holes problem. 

Link to post
Share on other sites

Were you under the impression that Jefa bearings were cheap? Let me disabuse you of that notion.....

Also, being aluminum housed, Jefa bearings are not forever in salt water. 

Link to post
Share on other sites

Jefa makes non-metalic lower bearings, inlcuding both fixed and self aligning roller types. As noted they are not a budget item, but if the boat is apart already and you plan to epoxy the lower into the hull it may be worth a look. The self aligning version will allow some flexibility in installation as well as reduce binding if there is any issue with the top bearing or flex between the cockpit sole and hull. I do not know if the J28 has a solid rudder tube or not, if not binding is less likely. 

This was an issue on my boat as it had a the lower JP3 bushing in an aluminum housing. It is in salt water so the housing failed, traping the bushing so it could not align, similar to  your lower bushing fixed to the hull. My boat does not have a continuous rudder tube, which allows for an under deck autopilot. Under load such as beating upwind the deck and hull would flex in different planes enough to bind the rudder shaft with a large increase in steering effort. Changing to the jefa lower system solved the issue, and stearing feel is better than new.

All this may be overkill for your situation, and does not answer your original question, but it might be good info if you are worried about final alignment or noticed a stiff helm under sail before the slop started. In an odd way a worn lower would reduce binding but if a new solid bushing is too tight it might become an issue. 

Link to post
Share on other sites

If the boat had 'sleeve' bearings there will not be enuf room to switch to any other roller bearing unit. The entire rudder tube would have to be cut out & replaced. Not a budget item.

Link to post
Share on other sites

Yeah I too am surprised with a 6.5" rudder shaft - even with a rectangular carbon shaft and s.s. round sleeve that's big for a 28' boat. Perhaps it was early days of carbon rudder shafts and they were being conservative.

Link to post
Share on other sites

Looks like a fiberglass post, maybe that's why so big. I think my Jefa bearing is 6" on a much bigger boat - but carbon post. I'd definitely keep the aluminum out of it. I've actually had more problems with the upper bearing, which gets washed with saltwater but sits above the waterline. The lower one is immersed and has faired better - but when I replace them there will be no aluminum involved. 

Link to post
Share on other sites
6 hours ago, longy said:

If the boat had 'sleeve' bearings there will not be enuf room to switch to any other roller bearing unit. The entire rudder tube would have to be cut out & replaced. Not a budget item.

Got it, I missed your earlier post indicating you only have 3/4" to work with. Since the boat is 30+ years old it must have done just fine with a cylindrical fixed bushing with no need for added boat surgery to change to something more complex. 

Link to post
Share on other sites
On 3/20/2021 at 3:06 PM, Rain Man said:

Personal experience:  I bought my Dash 34 when it was one year old in 1982.  Two years of active racing later the original Delrin bearings were worn out.  I replaced them with new Delrin bearings.  Two years of racing later they were worn out again.  On the advice of a friend I replaced them with UHMWP.  When I sold the boat in 2019 those bearings from the 80's were still in the boat with zero play.  

I have a friend who is big into Martin 242's.  He made the new UHMWP bearings for my SC 27.  He says that they have been replacing the bearings on the 242's with Delrin and are very unhappy with the longevity - they get a couple of years at most on actively sailed boats.  He is now making his new 242 bearings out of UHMWP.

It could be that correct machining makes the difference.  Somehow I doubt it.  YMMV.  I can't explain why your Delrin bearings lasted a long time and mine didn't.  Is the boat raced actively?

Some plastics don't like sliding against themselves.  If the pressure gets high enough, or speed fast enough the two different parts sort of weld together and then break the bonds.  Obviously not good for longevity.  I spent a couple minutes going through a Delrin design manual, and there is pretty significantly higher friction between Delrin - Delrin parts versus say Delrin and anything else (like Nylon or steel).

I haven't worked a lot with UHMWPE but it wouldn't surprise me that they behave differently than other thermoplastics.

Link to post
Share on other sites

UHMWPE will definitely do that - it is similar to galling in SS. In a self aligning bearing I'd make the housing out of something harder, either a harder plastic or SS. Ti would be ideal but it is pricey both to buy and to machine. Ultracomp seems very good at this - I have some Ultracomp spherical bearings that are quite small and running at pretty high loads without complaint. 

Link to post
Share on other sites

Am I missing something here?  The boat in question has SS cylinders glassed to the FRP post.  So the bearing surface in question will be SS and UHMWP.  No galling as suggested in the last two posts will occur. 

Here is a good comparison summary of Delrin vs. UHMWP.  What stands out for me is the significantly lower coefficient of friction and lack of water absorption for UHMWP.  Delrin expands slightly when placed in water and this creates problems when it is used as a lower bearing in a sailboat.

https://readingplastic.com/uhmw-vs-acetal/

Link to post
Share on other sites

^^^^ from above link comparing the 2 materials.

 UHMWP -  “it is does not absorb water readily”...and...”It is also very resistant to moisture”

Acetal - ”and with low moisture absorption, molded acetal parts can perform reliably in environments that experience humidity changes.”

So is there really measurable difference? They don’t quantify any absorption rates or dimensional changes. 

Link to post
Share on other sites
7 hours ago, wick said:

^^^^ from above link comparing the 2 materials.

 UHMWP -  “it is does not absorb water readily”...and...”It is also very resistant to moisture”

Acetal - ”and with low moisture absorption, molded acetal parts can perform reliably in environments that experience humidity changes.”

So is there really measurable difference? They don’t quantify any absorption rates or dimensional changes. 

With a bit of googling, Delrin absorbs water at 0.3% by weight, while UHMWPE is 0.01%.

I recall being told that Delrin will expand by 2% when immersed, and that expansion had to be allowed for when building rudder bearings. UHMWPE does not expand, so no correction is needed.

  • Like 1
Link to post
Share on other sites
44 minutes ago, Rain Man said:

With a bit of googling, Delrin absorbs water at 0.3% by weight, while UHMWPE is 0.01%.

I recall being told that Delrin will expand by 2% when immersed, and that expansion had to be allowed for when building rudder bearings. UHMWPE does not expand, so no correction is needed.

I am replacing my lower rudder bearing as well, which is a simple flanged sleeve bearing on a 1.5 inch stainless steel rudder post.  Looks like the old one may have been acetal.  The replacement is going to be Rulon LR.  Similar to UHMWPE in terms of water absorption.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...