shaft bearings ideas please

MJ

"...Do you us it as a tunnel liner?.."

The first ones were like a liner, if i recall, for class requirements, as they were very conservative (this was well over 10 years ago now, so my memory has faded a bit). But where possible a complete tube made from GRP.

Otherwise stern tube suffers poorly too...see attached.

 

Aslo why we always specified for the tube to be epoxy coated but with oil or grease filling there is no problem.

That illustrated corrosion is also very similar to the corrosion in alloy watertanks when uncoated.

 

MJ

you mean like this?

This was as a result of two aspects which are often over looked.
1)The alkaline/acidity of the fresh water
2) alloy grades

The internal stiffening was a different grade of ally to the plate. It was this, that caused simple bimetallic corrosion. Never encountered this before between two different gardes of ally, so close on the scale too. Solution, all stiffening on the outside of the tank now.

 

all noted: and thats why 2205 is so successful, it is very corrosion resistant in salt particulary warm brackish stuff You can also reduce in size too, as I did argue successfully with NZ MARINE DEPT at time using 80mm as against 100 mm rudder stock
I AM at odds with your theory ideas on soft mts Mike, you do give the gearbox bearings a big chance by clocking shafts in
i remember years before I was into building I was a field service engineer, Cummins Eng co I went to new installation on a small trawler, down In Newcastle Well the noise from the twin disc was unbelievable, the thing was owner built and they were ready literally to beat the crap outta me, they were all drunk, I split the coupling and that shaft never came withing 2mm of kissing the gear flange Misalignment is just poor workmanship
I have mussel harverster with 20000 hrs on scaniaS with centreflex couplings still oiriginal It is just something I have always done
however on cv types like aquadrive , misaligment is advised or apparently they will wear in one spot
with greased stern tubes one must be careful that the grease is kept away from ceramic seals
in the past I have bored the tubes from solid 5083 square sections, leaving the section square too, except where it exit the hull, Reason being 5083 is not to my knowledge available in solid round or tube
ttp://www.azom.com/Details.asp?ArticleID=958#_Mechanical_Properties
The corrosion behavior of 2205 duplex stainless steel was compared with that of AISI type 316L stainless steel. The 2205 stainless steel is a potential orthodontic bracket material with low nickel content (4 to 6 wt%), whereas the 316L stainless steel (nickel content: 10 to 14 wt%) is a currently used bracket material. Both stainless steels were subjected to electrochemical and immersion (crevice) corrosion tests in 37° C, 0.9 wt% sodium chloride solution. Electrochemical testing indicates that 2205 has a longer passivation range than 316L . The corrosion rate of 2205 was 0.416 MPY (milli-inch per year), whereas 316L exhibited 0.647 MPY. When 2205 was coupled to 316L with equal surface area ratio, the corrosion rate of 2205 reduced to 0.260 MPY, indicating that 316L stainless steel behaved like a sacrificial anode. When 316L is coupled with NiTi, TMA, or stainless steel arch wire and was subjected to the immersion corrosion test, it was found that 316L suffered from crevice corrosion. On the other hand, 2205 stainless steel did not show any localized crevice corrosion, although the surface of 2205 was covered with corrosion products, formed when coupled to NiTi and stainless steel wires. This study indicates that considering corrosion resistance, 2205 duplex stainless steel is an improved alternative to 316L for orthodontic bracket fabrication when used in conjunction with titanium, its alloys, or stainless steel arch wires. (Am J Orthod Dentofac Ortop 1997;112:69-79.)

 

2mm is still gross misalignment I just smile when I see someone chasing the last few thou on a soft mount.

I'll tell a tale as well

I just looked at a used boat recently purchased by a local , the gearbox output shaft bearing had failed and after a new bearing and seal, the engine was in perfect alignment BUT on low power it all sounded fine on high power it sounded harsh and horrible and he knew something was wrong. The mech he had used was obviously useless and he had replaced broken bolts on the coupling and checked the alignment again dialed the shaft to check for a bend and found nothing amiss.

I saw immediately the engine was soft mounted and there was no flexible coupling. After nearly $3k for a coupling its all sweet and the stuffing box has stopped leaking too.

Engines move on flexi mounts especially under torque and that does interesting things to alignment particulallry on the models where the mounts are off the shaft plane or a drop box changes the line.

 

A few notes to add to the shaft alignment discussion:
1. most of the boats we align, older boats of traditional design, don't have the clearance around the couplings or access to allow use of a dial-indicator. Feeler gauges have the advantage of being shock- and water-proof, and cheap to replace.
2. I've found that the final few thousandths are quick to achieve, by that time the system for moving the engine is perfected. Assuming there is deflection from thrust, I've always felt the total mis-alignment, and therefore stress, would be less if the starting-point is zero. I just don't feel right bolting up a coupling with a runout of more than a few thousandths.

 

when all said and done, some of us build for a living
It is up to us to follow good and sound practice, to remove doubt when things go wronrg etc and to stop people suing when and if they go wrong
About the corrosion in the tube, shaft log, call it what you will, although tunnel is not, big ships have tunnels where the shaft spins and greasers, oiliers engineers walk, to place knowledged hands on bearing cases
If you pressed in , light interference fit a liner if that green eng plastic, line bored it, then that would stop any corrosion issues
Old fashioned stuffing glands had white metal bearings, the inboard end of the shaft was supported These days with dripless seals, you dont get a (free) inboard bearing
who uses what? vesconite? In NZ Chatfield eNG WILL supply the bearing machined to allow for swelling
Who can remember the recommended distance between bearing? for given shaft Dia?

 

"...Who can remember the recommended distance between bearing? for given shaft Dia?.."

You don't. It's called whirling calculations, and you have to calculate it for your own individual layout to satisfy yourself and/or class.

 

Stu
OK stern tube on a smaller vessel .

Interesting on the new ships that knowledgable hand is replaced by sensors and that engineer sits in front of a computer on land and monitors the fleet with remote telemetry.

The basic procedure is to calc the critical whirling speed and change the shaft dia and bearing spacing and or shaft material until whirling will not occur.

Society rules give specific calculations that incorporate this transparent to the user.

 

That's the catch, unless you are really into rotating machinery you're interest ends in doing a highly accurate static matchup and then you are out of there feeling happy.
How many go back and actually look at what the whole setup does under full power which is where you should analyse it. Just look at a soft mounted engine as its cycled through its power range and you will visually see it move with most soft mounts. Then there should really be a pre-load to account for that so some initial mis-alignment may actually be beneficial, but thats why the flexi-coupling is there.

 

"...Reason being 5083 is not to my knowledge available in solid round or tube.."

5083 is available in any form, you just have to ask for it.

I've used solid bar and tube 5083 endlessly. It is also a class requirement for inlets too...so you just need to find the mills that produce it regularly.

 

you can NOT buy this stuff in NZ or Au, in fact I just rang the al co James, to get heavy walled tube, you need a mill run, min 500kg
nothing and I mean nothing is stocked in 6082 0r 5083 Point me to any site that will supply 2 m of this and .6m of that
even solid bar or squares are not available from which one could machine from.
there are rules thumb rgds bearing, I know in Skeins, but I can not find my Skeins
ok I found my skeins which is an old Bible trusted, argue with that, no doubt you will, the recomendation for 1 1/2 is five feet and five inches

 

I just assumed someone who has been building/working with aluminium since the 70s would know where all the mills are and how to get it by now!

Onesteel, Soon lian, Pechiney etc etc...all make/stock it...but like everything, small quantities come at a price. So, just try local mills/yards that use it regularly if no luck, be prepared to pay a premium for it.. or use a size that is not ideally suited, as would often be the case, since these are almost always in stock somewhere.

 

To compare our requirements for survey under USL is calced:
0.142 X cube root (d squared) ...........in meters
d is dia in mm


Gives 1.6m or 5' 3 " with a recommended 12d minimum from the last bearing to the coupling.

 

thanks Mike, I will up to 40mm I should just squeeze in
on the Bullflex website , the coupling speed max is halved if the misalignment is 2 degrees
40 40^(1 / 3) = 3.41995189 x.142
does not tally?