10-13-2011, 09:35 PM #1
Carbon Seal & Driveshaft Questions
This is a bit long winded but stay with me.
I've just changed my carbon seal for the first time after 235hrs from new due to cavitation (also changed the boot, S/S floating ring, O rings, driveshaft bumper and C clip) Jet pump bearings, impeller and wear ring all good. I was not noticing any more water in the hull than the usual cup full but cavitation was quite bad.
After asking a few people and searching on this forum I thought it was probably due to the boot / bellows losing tension. So after removing all the old stuff I did some comparisons.
Overall thickness of old carbon ring compared to new - 25.8mm vs 26.1mm (I also measured the thickness variation on the old ring - less than 0.1mm uneven wear so alignment must be pretty well spot on)
Length of boot / bellows - Old = 75.3mm, New = 78.5mm
So, 1.) would an old vs new carbon ring & bellows difference of 3.5mm (0.138 inch) plus bellows tension loss be enough to explain my cavitation?
2.) After reading that some people use a socket in the impeller to push the driveshaft in to get the C clip off and on easier, I'm wondering what's to stop the driveshaft moving forwards or backwards in normal circumstances and releasing pressure on the carbon seal? Doesn't make sense to me.
I'm making an assumption that when every thing is installed and bolted up correctly and in running condition the rubber bumper on the PTO end of the driveshaft is pressed firm against something on the inside and the driveshaft's opposite end is pressed as far as possible into the impeller, preventing any lengthways movement. If this is true, then any variation in the inside depth of impellers or wear ring length could make a difference to the bellows tension on the carbon ring. Correct or not?
The reason I'm asking is because I have made a few changes such as replaced the plastic pump housing with an aluminium one, solas prop, new blue wear ring. These things were all done over the last 12 -18 months and I didn't notice anything individually causing any problems but maybe the whole combination has contributed.
10-13-2011, 09:38 PM #2
10-13-2011, 09:48 PM #3
I'll help contribute a little....... The biggest factor in the bellow is it looses tension which I seen you mentioned. What I've found is these things are picky, It doesn't take much to run them but to run them properly will take a little maintenance and 3.5mm will HELP cure your cavitation issue.
There is a plastic piece in the pto housing that the rubber piece on the driveshaft fits into that deals with the in and out motion of the shaft. I"m not sure on your swaping of the plastic to aluminum pumps.. i'm sure someone can fill you in on that. Hope I could help..
10-13-2011, 09:53 PM #4
10-13-2011, 09:54 PM #5
correct.. he just wanted to know if it had that big of a factor on the cavitation from what i understood.. correct me if i'm wrong.
10-13-2011, 09:56 PM #6
10-13-2011, 10:18 PM #7
10-14-2011, 08:22 AM #8
10-14-2011, 06:34 PM #9
Aaaahh, makes total sense now, seems obvious once it's explained so thanks. (feel a bit stupid now)
Still quite surprised at really how little wear it takes for cavitation to start.
10-14-2011, 11:38 PM #10
Im pretty sure my max RPMs are getting a little "softer" (less) also. Im wondering if I should be checking my SC slip also.
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