Engine alignment with Volvo shaft seal

[saltyrob]

Hi Folks,

I am about to replace the engine mounts on our VP2002 together with the cutlass bearing. The cutlass bearing is housed in P bracket and the shaft runs through a Volvo shaft seal to the coupling at the back of the gearbox. When the coupling is split the shaft and the part of the coupling attached to the shaft tends to sag slightly in the Volvo Shaft seal.
Therefore I would like advice on engine alignment with this arrangement so that the shaft will be properly aligned in the cutless bearing.I intend to do the initial alignment pre launch, with the final adjustments carried out afloat as the hull does flex a small amount.

Many thanks

Rob

 

[RivalRedwing]

you need to support the shaft central in the stern tube (assuming cutlass is parallel to the stern tube). Pull the volvo seal off the stern tube and measure the clearance between the shaft and the stern tube and then move the supported shaft around until it is central (the solid ends of a set of drill bits at 0.5mm spacing should be accurate enough to measure clearances). While still supporting the shaft replace the volvo and attach the engine with the shaft and gearbox flanges parallel. Relaunch, and optimise gearbox:shaft alignment in the usual way.. well thats how I would do it anyway...

 

[FullCircle]

The concentricity tolerance is quite tight at +/- 0.5mm. Although it is good practice to firmly support the shaft in its central location, you are also changing the cutlass brearing, whcih may change the included angle as the shaft will not be sagging in the new cutlass.

I was going to change my mounts this year, as I had to adjust the alignment last year after 5 seasons, and I thought the mounts were deteriorating.

I carefully put marks near my engine mounts on the engine beds and the engine side brackets and measured them carefully with height gauges and vernier. All you then need to do once you have the new mounts in place is to replicate the measurements you have made.
Then you will have a reference set of measurements as a start place for when you put the boat back in the water to make alignment adjustments. You can also then see the sag over time by referring to the notes you have made.
The other benefit is that the beds do not need to be aligned, as these are comparator to the engine position, not the hull position.

You could also do this with a face plate on the gearbox coupling, but that would not necessarily give you a fore and aft position, and the face is quite a small area to commit to a position in the hull some distance away, whereas the engine mounts to engine is pretty much a short distance fixed point.

 

[Bosun Higgs]

Hi Folks,

I am about to replace the engine mounts on our VP2002 together with the cutlass bearing. The cutlass bearing is housed in P bracket and the shaft runs through a Volvo shaft seal to the coupling at the back of the gearbox. When the coupling is split the shaft and the part of the coupling attached to the shaft tends to sag slightly in the Volvo Shaft seal.
Therefore I would like advice on engine alignment with this arrangement so that the shaft will be properly aligned in the cutless bearing.I intend to do the initial alignment pre launch, with the final adjustments carried out afloat as the hull does flex a small amount.

Many thanks

Rob

You will be able to use the old cutlass bearing at the engine end of the stern tube to ensure that the shaft is near enough central along the tube. Then line up the engine with the shaft. Disconnect the engine again and remove the old bearing, replace the Volvo seal loosely and connect up the engine again. Then ease the seal into place so that it is central round the shaft - the internal lips on a Volvo seal are easily compressed resulting in a leak if the seal isnt central round the shaft.

You will have to realign the engine again once the boat is in the water as the hull will change shape. But it should not be by a lot.

 

[Neil_Y]

Easy, use a clearance fit bearing in the P bracket. With shaft chocked you should be able to slide the new bearing along the shaft and into the carrier and still spin it by hand. This will mean the shaft and carrier are aligned at the P bracket. Use a long straight edge on the unsupported shaft to check for sag here and chocked front end or use an old bearing as suggested the shaft should be very close to its running position.

As you have already worked out a shaft just sitting in a piece of rubber will sag and it is almost impossible to confirm it is straight in that bearing or the carrier is aligned when the bearing is firmly installed. The rear end where the thrust is applied and the weight of the propeller sits should be aligned in the bearing. Running a shaft slightly skew in a bearing at start up will make it very difficult for a water film to establish and the shaft will run around the bearing increasing wear and causing vibration in the drive train.

The really interesting thing in all this aligment is that the front end is designed to move by a few mm in all directions whilst running as a result of torque, thrust and acceleration forces. The rear end should not move outside the water film supporting the shaft, which on a 30.00mm shaft is less than 4 thou or 0.10mm thick. I feel we need an academic at this point.

As a further note a bearing especially a rubber one will not hold a shaft straight as it is usually only 4 x shaft OD. Even a new hard bearing (30.00mm shaft 30.20 bearing) will allow a run out of 0.20mm per 120.00mm If you hadn't checked carrier/bearing alignment, this angle at a coupling/adapter plate equates to the same distance so a 60.00mm diameter plate will see a 0.10mm gap on one side when touching at the other in terms of angular offset. Distance offset on a 1m shaft could be 1.6mm with the shaft skew by 0.20mm in the bearing.