Sizing Acetal Ruffrr Bushes

[Fred@Makani]

Hi, I m in the process of replacing my rudder bushes with Acetal (Delrin) as it was originally built (Jeanneau Sun Charm 39, 1990). I m in a little town in Mexico where the machinist are doing anything but boat parts. I would like to provide the exact dimensions (tolerances) where fit, swelling and relative thermal expansion are already accounted for. I struggle a bit for the press fit and more specifically for the water swelling. Indeed material specs mentions a %age of weight changes due. to immersion , how does that translate into dimensions? my shaft is 50mm, and OD of bushing is 80mm. Temperature of machine shop is 25C. Shall I Ieave the bushing in water a week prior machining to compensate for some water swelling? Thank you for your help.

 

[coopec]

I didn't think Acetal swelled much under water? (I know some of the other plastics do) When I was making up bearings I did a fair bit of research but that was a while ago)

"Acetal:Components made from acetal rod are dimensionally stable even under extremely wet or humid conditions and will not swell like nylon in these conditions. Additionally, it resists most organic solvents. Natural white acetal is an USDA/FDA approved material for food processing applications. Acetal is relatively easy to machine and does not burr easily. Acetal is a generic descriptive name for two polymers: Celcon® – a copolymer made by Celanese – and Delrin® – a homopolymer made by E. I. DuPont Nemours. Both types are tough enough and strong enough to replace metal for many applications.

https://www.qbcbearings.com/TechInfo/PDF/PlasticBearings.pdf

"Acetal: Has been used for inexpensive bearings in a wide variety of automotive, appliance, and industrial applications. It is particularly useful in wet environments because of its stability and resistance to wet abrasion "

General Characteristics of Plastic Bearings and Non-Metallic Bearings from SDP/SI

 

[vyv_cox]

When I had the acetal bearings made described here http://coxeng.co.uk/stern-gear/sadler-34-rudder-bearings/ I only specified a small clearance, just a snug fit over the rudder stock. They have been installed for a couple of seasons now, no noticeable swelling.

 

[RichardS]

Current thread on same issue: Acetal rudder bearing question

Richard

 

[vyv_cox]

Current thread on same issue: Acetal rudder bearing question

Richard

The OP of this thread revived the other one.

 

[RichardS]

The OP of this thread revived the other one.

Confusion is running amok.

Richard

 

[rogerthebodger]

Hi, I m in the process of replacing my rudder bushes with Acetal (Delrin) as it was originally built (Jeanneau Sun Charm 39, 1990). I m in a little town in Mexico where the machinist are doing anything but boat parts. I would like to provide the exact dimensions (tolerances) where fit, swelling and relative thermal expansion are already accounted for. I struggle a bit for the press fit and more specifically for the water swelling. Indeed material specs mentions a %age of weight changes due. to immersion , how does that translate into dimensions? my shaft is 50mm, and OD of bushing is 80mm. Temperature of machine shop is 25C. Shall I Ieave the bushing in water a week prior machining to compensate for some water swelling? Thank you for your help.

This rudder bush design program may help you

Marine Rudder Calculator – Metric - Vesconite

 

[Fred@Makani]

This rudder bush design program may help you

Marine Rudder Calculator – Metric - Vesconite

Thanks a lot, that will help for sure. It seems that vesconite as even less water a
absorption than acetal. Anyhow I will go with this... Thanks