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Old 26-01-2013, 05:37   #1
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Yanmar SD40/50 Cone Clutch Problem solved

I had a problem with a SD50 drive (1000Hrs) where the drive would only partially engage when selecting Fwd. The propeller would turn, but would not exert the full thrust. The throttle had to be "Blipped" (revved up) a couple of times while watching the propwash until the drive engaged.
I checked both the control cables for the correct stroke length, as well as the selector up-down movement and selector (centre bolt) shimming.
When rotating the propeller by hand with the gear selector in Fwd, the propeller would rotate approx 1/2 a turn before engaging. There was also a "spongy" feel when it engaged, compared to the drive on the other engine (catamaran) which immediately engaged solidly when rotated manually.
If the drive is allowed to operate in this partially engaged situation, it would lead to rapid wear between the Cone Clutch and Cone Cup mating surfaces. This would result in a damaged/ruined "Slipping" drive and require the notorious and expensive Cone Clutch Assembly replacement, along with the associated installation and shimming problems.
The design of the helical splined Cone and Shaft assembly prevents any slippage between the Cone and Cup giving more grip with higher load exerted by the propeller. This is a fool-proof system, on condition that initial full contact is made between the Cone and Cup before load is applied by the propeller, as well as BEFORE THE CONE SELECTOR RETRACTS.
The Cone Selector has limited up-down stroke, and has a retract function at the end of the Up or Down stroke. The reason for this is to move the tapered surface of the Selector away from the tapered Groove in the Cone to prevent wear between the Selector and The Cone during operation.
The stroke of the Cone Selector is not sufficient to compensate for the wear that develops in the Thrust Washers, Collar, Split Ring and Top Nut, resulting in the Cone Selector retracting away from the Cone groove before the Cone has engaged fully with the Cone Cup.
I replaced the lower Collar, top Nut and Thrust Washers, making sure that there is limited vertical free play on the Cone Shaft.
The Trust Washers, Collar and Top Nut are prone to wear, and has to be inspected regularly for wear.
PROBLEM SOLVED- I am however in the process of developing a Thrust Bearing, revised Collar and Top Nut assembly which will eliminate wear. VOLVO has been using Thrust Bearings for years! My previous boat has a VOVO drive with Thrust Bearings, and is still going strong after 14Years!

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Old 21-04-2013, 14:43   #2
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Re: Yanmar SD40/50 Cone Clutch Problem solved

Our SD-50 started failing to engage at around the 550hr mark and by the time we finally got to a dealer in St Thomas, it would hardly engage at all on the port while we have not had a bit of problem with the starboard.
Yanmar warranty is useless and management in Georgia are not the least bit caring.
How is your fix working so far?

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Old 10-06-2013, 06:00   #3
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Re: Yanmar SD40/50 Cone Clutch Problem solved

Hi, Yes the Cluthces/saildrives function 100% Now on 1300Hrs, and still good!!
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Old 10-06-2013, 06:10   #4
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Re: Yanmar SD40/50 Cone Clutch Problem solved

I find this problem interesting . when Merc cruiser first came out with the Bravo Drive they had issues with their. Cone clutch as well . the issue has long been solved and reliably holds up to well over 500 horsepower I'm not sure why this is still an issue for other manufacturesl. for the Merc cruiser drive the synthetic drive will is mandatory and has gone a long way to eliminate any problems
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clutch, yanmar

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