Bowthruster shear pin keeps failing - is this right

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hoped4

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We have a Lewmar 185tt bowthruster and the shear pin has sheared today for the 2nd time in 3 months of light use, and 3rd time in total 18 months of ownership. I am wondering if this is normal and looking for reasons? I wouldn't describe myself as a heavy user of the bowthruster, having had 8yrs of yachts without bowthrusters. I did note that at least one of the ' holes' the pin goes through is oval rather than round as if through wear and wonder if this could be aggravating things. Any tips for minimizing shearing or possible causes would be most helpful.
 
Mine sheared twice in as many weeks when a piece of wood got drawn into the tube. Shear pin did its job, and I didnt query the frequency.

Do you know for certain that something didnt stop the prop, thus causing the pin to shear?
 
I have a similar bow thruster, a 12 volt 5kW 185TT and I've only broken the shear pine once, caused by a rope in the thruster.

The shear pin will only fail if, either you using the wrong type of pin, mine is solid aluminium "rod", Lewmar part: #10 - 559017 - Drive pin.
or
The torque required to drive the gearbox and prop is too high.
Are you sure the prop is free to rotate? no barnacles between the prop and the tunnel? (there's very little clearance). No monofilament around the shaft?

If the prop is free to rotate, then the gearbox is probably knackered, if it is be prepared for a shock, they're very expensive.

I needed a new motor at the beginning of last season, and it was cheaper to buy the whole a motor, gearbox, prop and leg assembly than the just the motor.
 
I am not aware of any of the occasions of any obstructions getting sucked in to jam it. On any of the occasions. It was quite a strong cross tide today but, I guess that's when bowthrusters come into their own.
 
The pin in an oval hole is not good. It will apply shock load and it will get worse. If the pin is correct size it could indicate that it has sheared often. New round hole and matched pin no slop would be a start. But there may be a cause of multiple breakage.
 
The boat is just relaunched and the prop was rotating fine, yes very little clearance. I am sure it is the right pin. Hope it's not the gearbox! Yes that 'oval' hole could be a cause couldn't it. sounds like others don't have oval holes ?
 
Does the shear pin have a tie wrap around the outside, to hold the pin in place?

The instructions say that tie wrap must the kept in place, I guess that if the tie wrap is missing, the pin could slip out, so that only a part of the pin is in contact with the "land". That just might cause the holes to go oval.

Changing drive pin 140TT or 185TT
Cut cable tie on shaft - (If Fitted)
Punch out pin parts.
Tap in new pin and secure with new plastic cable tie.
Before working on unit check battery switch is off or remove the
fuse.

I have an old motor in the garage, I can photograph the shear pin and retaining tie wrap tomorrow if it would help.

I also have an electronic copy of the "Owner’s Installation, Operation & Basic Servicing Manual", which I could put on dropbox and send you a link, if you think it would be useful.
 
Ian_Edwards said:
Does the shear pin have a tie wrap around the outside, to hold the pin in place?

The instructions say that tie wrap must the kept in place, I guess that if the tie wrap is missing, the pin could slip out, so that only a part of the pin is in contact with the "land". That just might cause the holes to go oval.

Changing drive pin 140TT or 185TT
Cut cable tie on shaft - (If Fitted)
Punch out pin parts.
Tap in new pin and secure with new plastic cable tie.
Before working on unit check battery switch is off or remove the
fuse.

I have an old motor in the garage, I can photograph the shear pin and retaining tie wrap tomorrow if it would help.

I also have an electronic copy of the "Owner’s Installation, Operation & Basic Servicing Manual", which I could put on dropbox and send you a link, if you think it would be useful.
Click to expand...


Thanks Ian. I've got a manual aboard - thank you. If I followed that last time, and I think I did, pretty sure I would have used a cable tie - I will dismantle it next time I am down on the boat and carefully analyze, the position of cable tie. I noticed the 'oval' hole last time I fitted the pin. A photo might be very useful if it's not too much trouble? Thanks, Rob.
 
hoped4 said:
We have a Lewmar 185tt bowthruster and the shear pin has sheared today for the 2nd time in 3 months of light use, and 3rd time in total 18 months of ownership. I am wondering if this is normal and looking for reasons? I wouldn't describe myself as a heavy user of the bowthruster, having had 8yrs of yachts without bowthrusters. I did note that at least one of the ' holes' the pin goes through is oval rather than round as if through wear and wonder if this could be aggravating things. Any tips for minimizing shearing or possible causes would be most helpful.
Click to expand...

That could be the cause. If the pin is at all slack in the hole then every time you reverse the thrust direction, or start or stop it, the pin is being hit against one or other end of the oval hole, which will fatigue it. A shear pin needs to be a good fit in the holes in both the solid shaft and the outer tubular part so that they turn as one with no relative movement. A proper repair might be expensive as the part with the worn holes may need replacing. On my Vetus thruster the shear pin is 3mm brass, and only sheared when a rope was sucked in to the thruster tunnel.

EDIT: There is a clue in the post above. The pin has to be punched out according to the instructions, meaning that it should be a good fit with no slack.
 
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View attachment 62569

This is a 10 year old unit, the holes don't appear to be oval, but the Aluminium shear pin looks as though it may be flattened by contact with the slot in the shaft.

It's a working unit, kept as a spare, it was replaced when it stopped working, however the problem turned out to be rusty main contactors, which were repaired by a local motor rewinder for a few pounds, but not before I'd bought a new one.
 
IMG_0837.jpgIMG_0175.jpgSo, here is a photo of the hole for the shear pin. Quite distorted by the look of things. The other is the same. Each with at least 2mm extra room for the pin to move. This looks like the probable cause given the discussion above. The pin itself was just flattened and I had to hack saw blade it in half to remove it. The cable tie was intact and then n the right position. This is now beyond my abilities and so will get someone in to fix it.
 
PaulRainbow said:
I'd say there is little question that the hole is the cause of your problem. Is it not possible to drill another hole ?
Click to expand...

Drilling the hole will need great care. Here is how I would do it.

Remove both that fitting (which I will call the collar) and the shaft that goes inside it so that you can take them to a drilling machine. This is no job for a hand held drill.
Get a new shear pin and find out its exact diameter. Use a micrometer or vernier gauge. Get a drill about half a millimeter smaller and one the correct size.
Put the piece with the oval hole into a machine vice and drill it undersize at 90 degrees to the existing hole on one side only. Take care to drill it centrally.
Put the shaft into the machine vice, carefully setting it horizontal with the hole in it lined up with the drill, but overhanging the vice so that the collar can be pushed on to it. Ideally use a close fitting metal rod in the drill chuck to put the hole in the shaft accurately in line with the axis of the drill and clamp the machine vice down to the drill table.
Now take the collar you have just drilled and put it onto the shaft and line up the undersize hole with the one in the shaft. Use the drill that is the correct size to open the undersize hole, pass through the shaft and drill out through the other side of the collar.

There other set ups you might use but this is a job needing care and accuracy.
 
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