Jump to content

Plummer block question


Featured Posts

There is a Plummer block on my propshaft, a little way behind the flexible coupling, in a fairly typical arrangement. It is not AFAIK by any means a thrust bearing, its sole purpose is to prevent the inner end of the shaft from waving around. 

The bearing has an inner sleeve, through which the shaft passes. There are a couple of small Allen screws in this sleeve, which bear on to the shaft. I am told that these must not be left undone because then the shaft would rotate in the sleeve, instead of the bearing rotating, which would lead to wear of the sleeve and shaft and hence cause noise and vibration. On the other hand I am told that they must not be tightened, because the shaft must be able to move back and forth as the thrust taken by the engine causes it move on its mounts (which it visibly does). Instead my boatyard engineer tells me I must tighten them gently, enough to cause rotation of the bearing but not enough to prevent fore and aft movement of the shaft; this is a careful balancing act which needs me to adjust them at frequent intervals. 

So I ask (after 28 years and two bearing replacements) what is the correct procedure?

Link to comment
Share on other sites

I have the same arrangement and share your concern. The screws were loose when we got the boat so I left them that way, but recently I heard the sleeve spinning on the shaft so for the last couple of years I have run with the grub screws fully tight, a bit tight is no good, they just work loose straight away.

 

I also can not imagine those little screws taking the full prop thrust. We have a rigid mounted engine and no sliding spline in the propshaft so they will potentially also see any thermal expansion in the propshaft.

 

..............Dave

Link to comment
Share on other sites

20 minutes ago, Keeping Up said:

There is a Plummer block on my propshaft, a little way behind the flexible coupling, in a fairly typical arrangement. It is not AFAIK by any means a thrust bearing, its sole purpose is to prevent the inner end of the shaft from waving around. 

The bearing has an inner sleeve, through which the shaft passes. There are a couple of small Allen screws in this sleeve, which bear on to the shaft. I am told that these must not be left undone because then the shaft would rotate in the sleeve, instead of the bearing rotating, which would lead to wear of the sleeve and shaft and hence cause noise and vibration. On the other hand I am told that they must not be tightened, because the shaft must be able to move back and forth as the thrust taken by the engine causes it move on its mounts (which it visibly does). Instead my boatyard engineer tells me I must tighten them gently, enough to cause rotation of the bearing but not enough to prevent fore and aft movement of the shaft; this is a careful balancing act which needs me to adjust them at frequent intervals. 

So I ask (after 28 years and two bearing replacements) what is the correct procedure?

If the engine is solidly mounted then there will be no fore and aft movement caused by prop thrust. If it is flexibly mounted then there will be so you need the specification of the bearings to see how much axial load they will take. Even if they look a bit like ordinary ball bearings they may be angular contact ones that do take significant thrust. I have seen plumber blocks using a pair of taper roller bearings that will again take axial thrust. Without knowing the exact type of bearing that is used I can not comment further.

 

Re the grub screws. The shaft is usually drilled through the grub screw hole to provide "dimples" for the grub screws to fit into. You are then relying in the sheer resistance of the screws to prevent thrust movement.

 

I think it may be the wrong type of plumber block or the bearings may be the wrong type for a flexibly mounted engine but at 14 years life I doubt it matters much and I also doubt that the shaft will turn much in the sleeve unless the bearings started to seize. If it were mine I would be investigating the bearing type s that would fit the block.       .

 

 

Link to comment
Share on other sites

Might not make much difference. You say you have a flexible coupling so if the engine moves a bit then the coupling may be quite happy to absorb that plus a bit of backwards and forwards too. I guess the plummer block is really helping to prevent wear to the stern gland and leaks. So tighten the grub screws. OR, the engine might leap around a bit and the coupling might not like fore and aft movement. So maybe leave them slack. You say you have replaced the plummer block  a couple of times, I would slacken the hold down bolts on it just to see if a couple of shims might be needed, could be worth a check. I have three or 4 plummer blocks on my shaft, its a mid engine boat so a long drive train and the first one just behind the gearbox and a flexi couping is rubber mounted as if it was all bolted down hard the movement might  wear the gearbox bearing and other stuff. Unconventional maybe but it all works.  is also quiet and smooth so I reckon its all quite relaxed and not stressed.

Link to comment
Share on other sites

My stoke boat was fitted with such an arrangement when built. As an engineer i have MY views on this set up.

If you tighten the grub screws onto shaft all forward thrust will be taken by the bearing,in this case i found more noise transmitted into the hull than i liked and some sense of vibration.With the grubscrews loose the bearing has no effect other than stopping exesive side to side motion,which would indicate missalignment between engine and shaft. I removed the bearing,removed the flexible coupling,fitted a solid coupling,(worth having for future alignment checks/adjustment) and set up engine alignment near on perfect. Refitted the flexible coupling and went cruiseing. 9 years latter its running loverly with no exsesive noise or vibration.Whilst i like the idea of the python drive type systems,a plumber block in that position is nothing like it and my opinion is that its not necessary.

I will add that ive heard mention the plumberblock set up will prevent wear on shaft where it enters stern tube,as it prevents side motion.This would not be a problem if aligment was correct.

This is purely my view,other views may be available.

This view is not relevent to mid engine set ups where plumber blocks are used for suppotr.

Edited by Klim 1
  • Greenie 1
Link to comment
Share on other sites

Surly if it was intended for the shaft to move laterally in the plummer block it would be a roller bearing and not a ball race bearing. When I use to work on electric motors they had a ball race at one end to keep the armature in the right place and a roller at the other to allow for expansion etc

Link to comment
Share on other sites

You have to allow for the fact that most boats are not built by engineers. Its all very knife and fork work at most boat builders, they don't understand the niceties of setting up a drive shaft to take end thrust.

Aquadrives and Pythons get around this by having a flange bearing mounted in rubber to allow movement fore and aft.

Link to comment
Share on other sites

1 hour ago, Keeping Up said:

The engine is flexibly mounted and it does visibly move; the bearing is, and always has been, just a plain bearing.

Plain bearing?? means just a bush of bronze or whatever.   Races can be ball, roller or tapered roller.

Link to comment
Share on other sites

Best thing to do would be to replace the plummer block with a combined plummer block and Thrust bearing.  The engine will then not move under prop thrust, because it won't feel any  and you can tighten the securing screws fully.

 

Other wise, remove the block and mounting screws.  Grease the base of the block and then insert two springwashers on each mounting bolt separated by a plain washer (  so the spring washers don't bind on each other)  and tighten up again just tight enough so the block can slide on its mountings.  Fully tighten the shaft screws and loctite them. Periodicaly lightly grease or oil the sliding area.

 

The block will then support the shaft but be free to accept the engine movement fore and aft.

N

Link to comment
Share on other sites

1 hour ago, BEngo said:

Best thing to do would be to replace the plummer block with a combined plummer block and Thrust bearing.  The engine will then not move under prop thrust, because it won't feel any  and you can tighten the securing screws fully.

 

Other wise, remove the block and mounting screws.  Grease the base of the block and then insert two springwashers on each mounting bolt separated by a plain washer (  so the spring washers don't bind on each other)  and tighten up again just tight enough so the block can slide on its mountings.  Fully tighten the shaft screws and loctite them. Periodicaly lightly grease or oil the sliding area.

 

The block will then support the shaft but be free to accept the engine movement fore and aft.

N

Trouble is there's almost certainly shims under there that would gradually shift and come out. If there isn't any shims I'd question the alignment.

Link to comment
Share on other sites

1 hour ago, BEngo said:

Best thing to do would be to replace the plummer block with a combined plummer block and Thrust bearing.  The engine will then not move under prop thrust, because it won't feel any  and you can tighten the securing screws fully.

 

Other wise, remove the block and mounting screws.  Grease the base of the block and then insert two springwashers on each mounting bolt separated by a plain washer (  so the spring washers don't bind on each other)  and tighten up again just tight enough so the block can slide on its mountings.  Fully tighten the shaft screws and loctite them. Periodicaly lightly grease or oil the sliding area.

 

The block will then support the shaft but be free to accept the engine movement fore and aft.

N

 

That assumes that the bolt holes can accommodate the degree of forward and aft movement occurring - the OP describes this as "visibly", and I suspect typical clearance holes for the mounting bolts won't be anything like oversize enough.

Link to comment
Share on other sites

42 minutes ago, Keeping Up said:

There are indeed shims.

Yes, don't dare do it.  Are there any names or numbers on the plummer block. If its only about an 1.1/2'' wide with only two bolt holes it will be a single bearing, but might still might be a thrust bearing. If its about 3'' or more wide with four bolt holes its probably a double Timken tapered roller bearing thrust block. Fenner or Dodge Fenner is a popular make.

Link to comment
Share on other sites

Only 2 bolt holes and not very wide, as this picture (taken when the engine was out) shows. Also noticeable are the shims, also the plate that had to be fitted to move it backwards after the original Borg Warner gearbox was changed to a PRM which was bigger (a smaller bearing than the original had to be used). It clearly shows the sleeve and one of the grub screws, just aft of the bearing, and as the shaft has been pushed backwards to allow the engine to be removed you can see the darker section of shaft where it is normally inside the sleeve. When this was taken the bearing (but not its housing) and shaft had both been replaced 12 months before. 

 

16675717845_f3b0421f50_b.jpg

Link to comment
Share on other sites

6 minutes ago, Keeping Up said:

Only 2 bolt holes and not very wide, as this picture (taken when the engine was out) shows. Also noticeable are the shims, also the plate that had to be fitted to move it backwards after the original Borg Warner gearbox was changed to a PRM which was bigger (a smaller bearing than the original had to be used). It clearly shows the sleeve and one of the grub screws, just aft of the bearing, and as the shaft has been pushed backwards to allow the engine to be removed you can see the darker section of shaft where it is normally inside the sleeve. When this was taken the bearing (but not its housing) and shaft had both been replaced 12 months before. 

 

16675717845_f3b0421f50_b.jpg

I very much doubt if that's a thrust block, just a standard lineshaft carrying  type of plummer block where no end thrust is involved. And the grub screws would be used in that circumstance just to prevent the inner sleeve from turning on the shaft.

Link to comment
Share on other sites

8 minutes ago, bizzard said:

I very much doubt if that's a thrust block, just a standard lineshaft carrying  type of plummer block where no end thrust is involved. And the grub screws would be used in that circumstance just to prevent the inner sleeve from turning on the shaft.

So what on Earth is the point of it on such a short length between the Centaflex (or whatever the thing on the left is) and the stern bearing?

Link to comment
Share on other sites

Just now, BruceinSanity said:

So what on Earth is the point of it on such a short length between the Centaflex (or whatever the thing on the left is) and the stern bearing?

To prevent the shaft waggling about and wearing the stern tube out. A Centaflex won't prevent that entirely.

Link to comment
Share on other sites

1 hour ago, bizzard said:

To prevent the shaft waggling about and wearing the stern tube out. A Centaflex won't prevent that entirely.

It must have been fairly effective at this, because the shaft and stern tube lasted for almost 12,000 running hours before they needed replacing. 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.