Sheave Bushing for Subsea Application

[kander]

We are currently designing some sheaves that will be submerged in seawater for extended periods of time. We wish to make the sheaves and pins from stainless steel, however what is the best material to use for a bushing?

We think that brass or bronze against stainless in seawater will cause problems. Is this the case?

The sheaves will have a conventional grease system to lubricate between the bushing and pin.

 

[PatBethea]

Not knowing the full details, I would suggest brown delrin as your bushing material. No grease would be required. I worked as a naval defense contractor for a while - delrin works great.

Depending on your particular application, you might want to consider AL6061-T6 for the sheave material. The pins should remain stainless steel.

 

[GregLocock]

How long is an extended period of time? Will the sheaf be in constant use, or intermittent?

I agree, some sort of plastic is the obvious bearing material, I'm a bit leary of aluminium in sea water - it is better than magnesium (plink plink fizz) but if it ever comes in contact with another metal it'll dissolve.

Pick your SS carefully - 304 is not really suitable, and make sure you get it from a reputable supplier, not all 316 ss is equally suitable.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kander]

The sheaves are for a divers bell clump weight, so the clump weight is lowered to depth (max 300m) and the bell runs up and down the guide wires connecting the clump weight to the vessel.

As for service, it will be very intermittent and will be submerged almost constantly apart from diver bell runs (return to swap divers at end of 12-hrs, etc)

As it is a clump weight we need the structure to be heavy, so I think best we choose stainless 316 for the sheave material.

Would delrin be subject to swelling in seawater, as nylon is?

 

[PatBethea]

No, delrin does not swell.

Greg is correct - with aluminum you do need to be careful for any galvanic reactions. It all depends on what will be around it. Stainless is a fine choice.

 

[kander]

Thanks guys,

With delrin, you can confirm that no lubrication is required? Even for faster applications?

 

[GregLocock]

Well, if it is fast you'll have to design it as in any other bearing, but I'm sure the manufacturer will have design tables for a water lubricated bearing.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[pauljohn]

Ask these guys

http://en.lewmar.com/home/index.aspx?page=home

http://www.harken.com/blocks/blocks.php

I'm sure they could help you.