Gimble Bearing

[Mark Emaus]

I don't know much about Alpha one stern drives. My question is this...Does the shaft that runs through the gimbal bearing slide back and forth as to allow for the steering and pitch control?

Thanks

Mark

[marshmat]

Mark,

Have a look at the attached diagram (from http://www.iboats.com/Mercruiser_Sterndrive_Drive_Shaft_Housing_Components_Exploded_View/dm/view_id.272968 ). This is the upper housing and U-joint assembly of an Alpha.

The driveshaft of an Alpha consists of a double Cardan joint, geometrically centred at the point where the trim and steering axes intersect. Thus, roughly constant-velocity rotation is maintained throughout the range of normal steering and trim angles. (As you can see, tilting the drive fully up to trailering position will put a fair bit of misalignment on this joint- good practice is to shut the engine off before tilting the drive to the full-up position.) The shaft protruding into the boat is splined, but should have no need to slide during normal operation. The F/N/R gearbox of an Alpha is in the lower unit, like an O/B. This general design is shared by most sterndrives, although some mount the F/N/R gear in the upper housing or inside the boat.

It is also worth noting that on the Alpha, the rubber bellows around the driveshaft is critical to watertight integrity; some other drives (ie. Volvo) have additional bearings or seals that can help protect against massive flooding if the bellows were to fail.

[Mark Emaus]

Thanks Marsh,

We are putting the alpha drive on a pontoon boat. The engine we are running is a vw air cooled. The shaft will not be in the water at all and will be close to the top of the deck. I would like to make the alpha drive move vertically up and down with no pitch. I will do this by an actuator. If there is no need for the shaft to slide in the gimbal bearing, why is the shaft splined so long? Also since we will have an air cooled engine, I assume that we can just let the water pump, just pump the water back out without worrying about heat in the alpha drive.

Mark

[Red Tide]

Drive manufacturers do recommend to keep the water pump pumping in an alpha drive even if you're not needing water from the drive to cool the engine (e.g. if you have a thru-hull pickup instead.) Pumping water through the drive also helps cool the drive.