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Halyard sheave, bronze bushing: only 5 years lifetime, really ? ?


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My main main halyard sheave is 5 years old (came with new mast). This season, it seized (would not rotate anymore  when hoisting). I just removed it and the inner bronze sheave is destroyed - crumbled into pieces.. The sheave itself is - I think - some zinc-plated steel (it seems heavier than aluminum). It rotates on the  bronze  bushing, itself  rotating on an StainlessSteel pin, which is inserted in the mast jaws (mast is aluminium).
And then, this: my boom attaches to the gooseneck with a SS bolt, which traverses  2 holes in the boom’s  aluminium jaws  - those holes were getting worn out by the constant friction against the SS bolt (15 years, understandable), so last year I inserted bronze bushings between the aluminium holes and the bolt. Those bushings were destroyed within one  season. Like for the halyard, the bushing broke into segments, was crumbly. 

I can’t think that it is the mechanical forces that destroy the bronze ? (my '71 Ducati 350 mono still runs on some of the original bronze bushings FFsake!!)
I any case, what should I use instead ? SS bushings ?

MainSheave.jpg

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Looks like an aluminum sheave to me...bcuz the scoring and lack of rust clues. Could be zinc, but that also reacts with aluminum and makes a very poor bearing.

No part of an aluminum mast should be bronze (or any copper alloy). That is foolish. Bearing sleeves can be stainless steel. S/s also reacts with aluminum but it is not so violent and can be slowed with various goops. No bearing surfaces should be aluminum either if it can be avoided. Steel on steel is far preferable to that.

Replace the bronze bushings with s/s. Might require reboring because of the damage. That sheave looks to lack cheek washers. Looks like aluminum on aluminum wear....gummy. S/s washers there would be a huge plus.

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1 hour ago, El Borracho said:

Looks like an aluminum sheave to me...bcuz the scoring and lack of rust clues. Could be zinc, but that also reacts with aluminum and makes a very poor bearing.

No part of an aluminum mast should be bronze (or any copper alloy). That is foolish. Bearing sleeves can be stainless steel. S/s also reacts with aluminum but it is not so violent and can be slowed with various goops. No bearing surfaces should be aluminum either if it can be avoided. Steel on steel is far preferable to that.

Replace the bronze bushings with s/s. Might require reboring because of the damage. That sheave looks to lack cheek washers. Looks like aluminum on aluminum wear....gummy. S/s washers there would be a huge plus.

thks, good point for the washers.

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Delrin is good too, yes, but might not be tough enough for a masthead sheave bearing application...depending on sheave size and boat size. 

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"Oilite" bronze bushings in aluminum sheaves were the industry standard for decades. New bushings available all day long from your local bearing dealer or Mcmaster.com. From the pic, this seems to be what you had. Oilite bushing would appear granular. Your original bushings look quite thin for the application.Should have lasted longer, tho. I have never seen a stock mast assembled with any lubricants/corrosion prevention. From the marks, your bushing froze to the axle pin, then alum sheave was rotating on the outside of the bushing.. If the sheave can have the hole correctly re-drilled (on center) a large assortment of bushing thicknesses are available to suit the new center. They should be pressed in. LUBRICATE the axle!! Tef gel or wheel bearing grease or similar. Long life is wanted, not slipperiness per say. RPM's are very slow. Or replace the entire sheave with a high strength plastic with an epoxy (?) bushing. Delrin is a bit soft for this narrow sheave. Axle pin will also need to be polished to remove scratches where bushing rides.

    Gooseneck wear: the number one culprit here is using a bolt length that has thread bearing against the jaws. The thread will cut thru aluminum quickly. Re-bush with ss inserts. Use a ream to ensure holes stay lined up.

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How big is the sail and how big is the axle? Those will determine the load on the bushing. The best implementation would be a larger fixed SS sleeve which is pinched by the fixing bolt such that it cannot rotate, and a PTFE/Nomex bushing. That's about $10 - 15 worth of materials. If sticking with plastic, PEEK is a better choice than Delrin for anything but light loads. 

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22 hours ago, DDW said:

How big is the sail and how big is the axle? Those will determine the load on the bushing. The best implementation would be a larger fixed SS sleeve which is pinched by the fixing bolt such that it cannot rotate, and a PTFE/Nomex bushing. That's about $10 - 15 worth of materials. If sticking with plastic, PEEK is a better choice than Delrin for anything but light loads. 

Efforts are not enormous. The sail is 450 sqft.  The axle diameter is about 3/4 inch. 

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If the axle is 3/4 diameter and 3/4 wide, you've got 9/16 in area (that may seem odd but that's how the bearing people do it). 450 sq ft sail has maybe 800 lbs halyard tension, x2 on the bushing so 1600 lbs / 9/16 is 2800 psi. The limit for Delrin (acetal) is usually given at around 2000 lbs. I was going to send you to McMaster for a PTFE nomex bearing but they seem to have stopped selling them. A Tristar CJ bearing trstar.com is the sort of thing you are looking for. Bore your existing sheave and press in the bearing. It is what Harken uses for high load sheaves. Should be about $4. They are self lubricating and require no maintenance, take a dynamic load of about 30,000 psi. Not sure who has inventory anymore, might have to give them a call. I may have some kicking around the shop as I buy them 100 or so at a time, depends on exact size.

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1 hour ago, DDW said:

If the axle is 3/4 diameter and 3/4 wide, you've got 9/16 in area (that may seem odd but that's how the bearing people do it). 

The reason the bearing folks do it that way is that they look at the area normal to the load. So it just looks like a cross sectional area. Super useful to know when you're talking to a mechanical engineer and they say something that sounds insane like that. :-) 

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Like I said, that's how the bearing guys talk. In fact any sleeve bearing has clearance, and that means the pressure is going to vary from zero to a peak on the line normal to the load. But that may also be why they allowable stress is usually quoted lower than the compressive limit of the materials. Something like Delrin is squishy enough it may behave almost like a fluid. On a pressure lubed bearing, the journal floats on the hydrodynamic pressure, so the pressure metaphor is valid. 

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Definitely not an expert here, but a quick glance at a Galvanic chart show Al and Bronze to be closer together than Al and 304 or 316 series SS. I replaced aluminum sheaves with Bronze bushings on my 1/4 tonner last year because they were to thin after switching to stripped halyards. Other than being too thin they were fine, and probably 30 + years old. See my super technical drawing below.

image.png

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3 hours ago, JoeBleaux said:

Definitely not an expert here, but a quick glance at a Galvanic chart show Al and Bronze to be closer together than Al and 304 or 316 series SS. I replaced aluminum sheaves with Bronze bushings on my 1/4 tonner last year because they were to thin after switching to stripped halyards. Other than being too thin they were fine, and probably 30 + years old. See my super technical drawing below.

image.png

Ttks for the data, and it ties with the fact that Al/ bronze assemblies are a standard setup.

So I wonder why I see them failing that fast.   

Anyway. I dont feel like trying SS instead of Bronze. It is pretty clear that it will be at high risk of seizing with corrosion.

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SS bushings in an Al sheave doesn't seem like a good idea. I might try calling the mast manufacturer, or Zephyr werks and talk to them about what's going on.

14 hours ago, Ganzi said:

Ttks for the data, and it ties with the fact that Al/ bronze assemblies are a standard setup.

So I wonder why I see them failing that fast.   

Anyway. I dont feel like trying SS instead of Bronze. It is pretty clear that it will be at high risk of seizing with corrosion.

 

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16 hours ago, Ganzi said:

Ttks for the data, and it ties with the fact that Al/ bronze assemblies are a standard setup.

So I wonder why I see them failing that fast.   

Anyway. I dont feel like trying SS instead of Bronze. It is pretty clear that it will be at high risk of seizing with corrosion.

Looks like cast iron is the way to go!

 

Wait....

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“UHMW coefficient of friction is significantly better than nylon and acetaland is virtually the same as Teflon® but has superior abrasion resistance. UHMWin sliding applications will out wear steel 10 to 1 and stainless steel 6 to 1." BTW, acetal is the generic name for Delrin.”

Pressed UHMW brushing would stop any corrosion. Apparently the weak link would be the plastic wearing thru the stainless axle pin!

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If you are using plastic, UHMW is a better choice for the bushing than acetal, but better still is PEEK. The CJ bearing I referenced above would be much stronger and longer lived than any plastic. 

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14 hours ago, jackolantern said:

Is anyone out there making lightweight or titanium sheaves? I know Harken makes ball bearing ones but none of their dimensions work.

I have an awfully heavy tulip at the top of the rig. 

Aluminum is lighter weight than titanium, and almost everyone makes them. Halyard sheaves are usually plain bearings, ball bearings will not handle the load (except for a very small boat). In practice, a good sleeve bearing will be freer running than a roller bearing.

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Referring to the original photo of the aluminum sheave with the worn and broken bronze sleeve bearing, I see these quite often, as it's my business.

There are a few things going on here.  That sleeve bearing was apparently very thin to start with, which means it has very little oil storage.  Heavy wall oilite bearings will last far longer than thin wall bearings. The original clearance between the shaft and the bearing is unknown.  The closer to correct tolerance the axle and bearing to start with, the longer it will last.  Stainless and oilite bronze are a very good bearing pair.  Stainless and stainless are not optimum.  

It is quite possible the fit between the aluminum and the bronze bearing was not optimum to begin with.  As soon as the bearing becomes loose in the sheave, things go downhill fast.

For most applications, I have found an acetal sheave with a heavy wall oilite sleeve bearing machined to fit with a 316 stainless axle to work quite well.  For higher load applications, I use anodized aluminum sheaves also with oilite bearings.  The oil in the bearing seems to mitigate any tendency to corrosion between the two.

Titanium, though totally cool, is not a particularly light material for sheaves, though it does do away with corrosion issues, is very strong so can be lightened by machining, can be anodized to may colors, and has a great wow factor, especially when the price is quoted.

493572882_Isomatcompleteset.JPG

aluminum sheave red anodized.jpg

Ttianium sheave - Copy (800x600).jpg

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