Plastic plain bearing/bushing on shaft

[Fritz Koschmann]

I am working on refurbishing a 26' powerboat. I have never had it in the water. I took the 5'-6' shaft out today and it goes through a fiberglass tube. It has a plain plastic bushing on the inside end and a regular cutlass bearing on the aft end. The plastic bearing was torn up. Here it is before removal. I might add that it is 30 years old.



here it is out:




According to the previous owner the boat had one of these plastic bearings at each end originally. He replaced the aft one with a cutlass bearing. It originally had a conventional stuffing box but I will be installing a PSS shaft seal. Perhaps I don't need the forward bearing. If I replace it I don't know what it is made of or where to get one. McMaster-Carr has a variety of non-metallic bearing that might work but I have no idea which one and none of them have the thick and wide diameter flange. I do wonder about having a close tolerance bearing with the PSS and water flow. By the way the shaft is 1.25"

[wizbang 13]

Is it a fast turning shaft?
Is it a smooth runner?(gas or diesel)
The conventional SB went over the blue plastic so it stayed wet right?
Any ide of how many hours in 30 years?
I am having issues with my 30 yo fg tube mostly due to my poor/lazy alignment.
I am going to go back to a conventional SB and ditch the pss.

[Fritz Koschmann]

Diesel Yanmar 3200 rpm, 2.83:1 reduction
Yes the conventional SB went over the blue bushing and I assume enough water got through to the bushing.
Hours are not exact but maybe around 1200

[wizbang 13]

Thanks Fritz.
Your rig is very similar to mine, except I never had an inside bushing.
My damage is from plain old lazy engine alignment. After 2000 hours I noticed leaking from the tube and when I pulled the SB hose I saw the tube was paper thin on one side. Shaft has been rubbing the tube.I had run 2 other engines over the decades without incident, thousands of hours.
I think if the engine is just aligned well , it is fine without the blue bushing. My shaft is only one inch so it might ocilate more.
The blue bushing would make alignment easier, as I have to pull the SB hose to be sure the shaft staysin the middle of the tube. Pita in the water.
BIt of a catch 22.

[nedL]

Only a 5 or 6. Foot shaft (or 5'6",... makes no difference), I think a second 'bearing' is only complicating things and asking for trouble. No need for more than a cutlass bearing aft, the stuffing box (which in no way acts as a support element), then the coupling to the engine. (Effectively, support only at the coupling and the cutlass,.. this would be pretty normal for a shaft of that length.)

[artif]

A few questions
1 is the engine on flexible mounts or hard mounted
2 Is the shaft fitted to the gearbox with a rigid or flexible coupling
3 does it have an Aqua drive or similar coupling
3 Is the cutless bearing a rubber lined one.

[John B]

Vesconite is the shaft bearing a lot of people are using now. Like a cutless bearing it is an interference fit bushing. You put it in the freezer overnight and fit it cold.
Face seals like PSS don't generally require a bearing at the inboard end of the log because of the way they work and the fact they are more tolerant of mis alignment. Lip seals generally do/ must have .

[Fritz Koschmann]

"A few questions
1 is the engine on flexible mounts or hard mounted
2 Is the shaft fitted to the gearbox with a rigid or flexible coupling
3 does it have an Aqua drive or similar coupling
3 Is the cutless bearing a rubber lined one"

I just installed new flexible mounts (Bushings inc.) but haven't checked alignment yet.
No flexible coupling but I was thinking of adding one - R&D
no Aqua-Drive
Yes rubber lined cutlass

In going through the previous owners misc parts I just found the bushing that he removed when he installed the new cutlass bearing. It is the same as the destroyed forward one. The fit seems like it would allow enough water through.

[artif]

"A few questions
1 is the engine on flexible mounts or hard mounted
2 Is the shaft fitted to the gearbox with a rigid or flexible coupling
3 does it have an Aqua drive or similar coupling
3 Is the cutless bearing a rubber lined one"

I just installed new flexible mounts (Bushings inc.) but haven't checked alignment yet.
No flexible coupling but I was thinking of adding one - R&D
no Aqua-Drive
Yes rubber lined cutlass

In going through the previous owners misc parts I just found the bushing that he removed when he installed the new cutlass bearing. It is the same as the destroyed forward one. The fit seems like it would allow enough water through.

If you have flexi mounts, then ideally you should have a flexible coupling.
As already mentioned the inboard bearing isn't required for that length of shaft in this application however I would be inclined to retain the original stuffing box.
I'm sure you already know but I do a rough alignment on land and do the final alignment in the water, I've got lucky a few times but normally it gets worse.

[MN Dave]

Speaking just about the type of plastic, with no useful advice on the design, there are three plastics that seem most likely to be the material used here:

It looks like UHMWPE in the pictures. It appears to be somewhat translucent, soft and the way it frays are reminiscent of UHMW polyethylene. That would indicate a lightly loaded bearing that provided alignment and possibly a water seal, but wold not take much heat or pressure in operation. UHMW would be very easy to get and machine to make a custom bearing. (the thick flange is unusual, so custom seems likely, but that is speculative.

A blue high temperature plastic bearig material that might look similar is Techtron HPV PPS. PPS is harder to get and more expensive, but would make a good bearing.

Turcite A is the only other blue plastic bearing material that seem at all likely, but I don't remember it being around 30 years ago.

Nylons rare less likely because they swell in water and would tend to bind the shaft unless the bearing was loose when installed. Of course there are many other plastics, but blue is not a common color.

UHMW will melt, drip and burn readily if you hold a match under the scraps. No burning plastic smells good, but polyethylene is basically paraffin wax on steroids, so the smell is pretty tame by plastic standards. The bearing grade PPS would eventually char and since it is a sulfide, it will smell awful. The SO2 in the PPS smoke is toxic.