Re: [Amel Yacht Owners] Modified SM Prop Shaft Seal Bush


Ian Shepherd
 

Interesting.Thank you Craig. I actually put the outer seal facing inwards as per the Amel diagram. My though being that you don't want the spring exposed to sea water. I have made a note of 17-4 and will specify that next time.

You probably know Sanai in Marmaris? Loads of good machine shops there.

Regards

Ian Shepherd SM2K 414 (2003) Crusader Kastelorizo Island Greece


On 08/08/2017 15:00, sangaris@... [amelyachtowners] wrote:
 

Some time ago I asked SKF's engineering about this. Here's their reply to me:
"Craig, 
My educated guess is that the seals are not an issue in this application. The problem is with the Bronze bushing that is being used as a sealing surface. Bronze is generally quite soft and might not have the required hardness of Rockwell C 30 or higher. You have a few options to solve this issue:
• You can put SKF speedi-sleeve gold on the shaft (P/N 99830). You will probably have to install multiple sleeves as you have 3 seals next to each other.
• You can replace the bronze bushing with a hardened wear sleeve for better abrasion resistance. We can offer you a custom wear sleeve if needed.
• You can plate your bushing with a harder material (Chrome, ceramic, etc) to make it wear resistant.
Essentially, the harder the shaft is, the less grooving it will see. I also suggest you to make sure that you grease pack the area between the seals to make sure that the seals not in contact with the oil are well lubricated. A lack of lubrication can cause the seal and surface to wear quicker. Finally I would arrange the seals with 2 facing the water and one facing the oil as long as there is no pressure difference. 
Best regards, Jaydeep Laljani, Application Engineer, SKF USA

I wrote back and said:
"Many thanks for your analysis. Frankly, I have wondered why the manufacturer used Bronze in the first place and not stainless - they actually call it a "wearing-out bushing" (but then they do charge a lot for replacements :-). 
Next month I'm returning to the boat, which is in Turkey, and I can easily have a local shop turn a new bushing in 316 Stainless (the same material as the shaft and propeller, so no electrolysis issues.) I think that would solve the problem without going to sleeves or plating. 
As for the orientation of the seals, I like your suggestion of two facing the water and will do that the next time I change the seals. After all, we're really trying to keep the water out more than keeping the oil in. And, yes, indeed, I always pack the seals with a good water-proof grease and that helps, I'm sure. "

Jaydeep replied with:

"I think you might not get as much life as you want with 316 SS as it is still not up to the required hardness. If you want to go Stainless I recommend 17-4 Stainless steel which is harder. If you are going with the 316 SS I would recommend you to passivate it to improve corrosion resistance. " 

Hope that's helpful.
Cheers, Craig Briggs, SN#68 Sangaris


---In amelyachtowners@..., wrote :

Hardness of stainless steel 316 does not vary tangible. Bushing from stainless 316 will  definitely last longer, but the bushing will cost much more. 316 steel is expensive and hard to machin. But it is very good corrosion resistant material, good for salt water. If people are interested I can find mashin shop that can make them. We have to order 100 bushings or more. Otherwise setup cost is too much.

Vladimir Sonsev
SM 345
"LIFE IS GOOD"


On Aug 7, 2017 10:57 AM, "'Bill & Judy Rouse' yahoogroups@... [amelyachtowners]" <amelyachtowners@...> wrote:
 
Ian, All, and especially newbies;

....
<<<>>>
I am certainly no expert, but I believe that stainless steel comes in at least 5 types and also different hardnesses; and the Nitrile Buna-N seals are made in different hardnesses. Although I personally do not need the answer to the following questions, I believe anyone attempting to follow your recommendation should know: What type of stainless steel and hardness you used? What was the hardness of the seals? What were the engine hours between servicing?

I would guess that to obtain different results than the Amel-proven system, you have to either scientifically solve the wear issue to achieve a different balance between wear of the seal and wear of the bushing, or use a trial and error method. I assume that your method was trial and error, which resulted in changing the balance of wear between the wear bushing and the wear on the lips of the seals; which extended your haulout interval - your goal. 

We must remember that there will be wear between the lips of the seal and the wear bushing on the C-Drive. It is obvious to anyone who has serviced an Amel C-Drive that with the current Amel design the wear bushing appears to wear more than the seal lips. 

I would assume that one could scientifically change the balance to increase the interval of haulouts. However, it makes sense to me that unless a scientific approach is used, we would be playing Russian roulette with a very expensive part of our boat.

Don't take what I am saying as criticism. I am happy that you achieved your goal. My goal was to haul out every 2 years for more reasons than C-Drive maintenance.

I would urge all readers of this message to stick with proven Amel supplied parts and procedures, rather than changing something that you do not completely understand. I do not have the metallurgical and mechanical engineering expertise to develop a better system than Amel's wear bushing and seals, nor do I believe any of us have that knowledge. 

....
Until recently, I had no idea that the Nitrile Buna-N seals are available in a hardness range from 40-90, and I do not know the hardness of the Nitrile Buna-N lip seals that Amel uses. We all know that bronze, brass, and Naval brass are available in different hardnesses. I suspect that none of us know the hardness of the wear bushing that Amel uses, nor do we know the percent of copper, tin, and zinc . Without this information, I believe that changing the balance of wear between the bushing and the seal should not be attempted, unless you understand the huge risk for what, I believe, is very little reward.

Ian, it is very good to have you return to this forum. I have missed you and your posts. Probably none of us have more miles in a Super Maramu than you have.

​Best,​
Bill Rouse


 

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