Thrust Bearings - Do I Need them?

[big_s]

Hi all,

I have a 10kw motor powering a propeller via a 2.8:1 pulley gear and lay shaft. The lay shaft is mounted in a rigid frame using two simple flange bearings (ball bearings) and retaining collars. The lay shaft connects to the propeller shaft via an R&D flexible coupling. The 2 blade propeller has a diameter of 14" and a pitch of 10".

While this setup takes care of radial loads it doesnt consider axial loads. Do I need to introduce thrust bearings into the system and if so how?

I have only found one thread that considers the problem but the solution was described as non-ideal by the op; he used a load bearing and a thrust bearing for forward and reverse.

Others have suggested use of tapered roller bearings but I cant find any that are housed.

My initial thought is to glass in a fixed plywood cross member in the bilge that the thrust bearings are attached to. This would then isolate all thrust from the electric motor frame itself, with thrust forces being passed directly to the hull.

Any advice or insight much appreciated. Links to actual products would also be a great help.

Many thanks.

 

[vyv_cox]

What are you calling 'simple flange bearings'? Deep groove and angular contact bearings have thrust capability. You would need to calculate whether their capacity is sufficient for the duty.

 

[billskip]

Others have suggested use of tapered roller bearings but I cant find any that are housed.

We used to use F.A.G. housed tapered roller bearings. It may be the size you want is restriction.

 

[rogerthebodger]

I have a thrust bearing which is a simple deep groove flanged pillar block bearing on a 40 mm dia shaft

This is the type of bearing I have but I added taper lock bushes to take the trust in both directions

 

[big_s]

Hi all, and thanks for such quick replies. Working through them:

We used to use F.A.G. housed tapered roller bearings. It may be the size you want is restriction.

Will check their catalogue out now. Out of interest is it possible to swap the bearing parts of housed bearings to whatever you want?

 

[big_s]

I have a thrust bearing which is a simple deep groove flanged pillar block bearing on a 40 mm dia shaft

This is the type of bearing I have but I added taper lock bushes to take the trust in both directions

Did you use two of these, one for forward thrust and another for reverse? How did you know this would take the thrust forces for your boat?

 

[big_s]

What are you calling 'simple flange bearings'? Deep groove and angular contact bearings have thrust capability. You would need to calculate whether their capacity is sufficient for the duty.

I suppose this just shows my ignorance wrt bearings. The housed bearing units I typically find are like the pic shown in rogerthebodger's post. I know nothing about how to calculate the thrust of the electric motor/propeller in order to select a suitable bearing - can you help here?

 

[Hepne]

I built an electric propulsion unit with toothed belt drive on a 30 foot Albin Ballad 25 years ago and used a 25 mm flanged ball bearing unit as shown on rogerthebodger's post. The flanged ball bearing unit (and the ball bearing within it) can take both lateral and axial loads, where axial load capacity typically is 25% of the lateral capacity. The SKF catalogue gave the figures.

SKF

SKF

PBO and other magasines have measured bollard pull when testing propellers.The bollard pull will be the thrust acting on the bearing. A typical test I found, show thrust from a 21 hp engine to be peaking at 270kg=2,7kN

 

[rogerthebodger]

Did you use two of these, one for forward thrust and another for reverse? How did you know this would take the thrust forces for your boat?

I only used one as they will take thrust in both directions.

I did use a shaft fitting to transfer the thrust from the shaft to the bearing. The size of the bearing was determined by the diameter of the prop shaft wis is 40 mm and more than capable of taking the thrust applied from the propeller.

The maximum prop trust can be obtained from the prop thrust graph for the prop you have, and the spec of the bearing should give the maximum thrust that the bearing is designed to take

 

[Supertramp]

A proper thrust bearing takes all the forward and reverse loads off the engine gearbox and ensures proper alignment of the shaft through stern gland and cutless. Mine has a CV joint between bearing and gearbox so that any movement at the engine is not transmitted to the thrust bearing. Mine is an Aquadrive which is not cheap but is designed for the purpose. The CV joint can be custom made to a length.

My understanding is that the larger you specify or can fit in, the longer it will last although for leisure use the standard size will probably outlast the engine.

 

[vyv_cox]

I suppose this just shows my ignorance wrt bearings. The housed bearing units I typically find are like the pic shown in rogerthebodger's post. I know nothing about how to calculate the thrust of the electric motor/propeller in order to select a suitable bearing - can you help here?

Roger says the bearings he shows are deep groove types. These accept thrust from either direction. Angular contact bearings have a higher thrust capacity but only from one direction. It is customary to use them back-to-back, or reversed on either end of a shaft.

The calculation is very simple, as Roger has said. Your propeller manufacturer will advise the maximum thrust deliverable at the maximum revs that your gearbox will supply. All bearings are stamped with a code. The maximum radial and axial load capacity is provided by the bearing manufacturer.

It is well worth looking at or downloading the SKF bearing catalogue, which contains a vast amount of information on the subject. https://www.scribd.com/doc/97653382/SKF-Bearing-Catalogue

 

[rogerthebodger]

Bearing life calculations cn be quite complicated which I used to do many years ago.

The size of the prop shaft should give a good indication of the size of the bearing.

Do not just rely on the grub screw in the bearing to take the thrust a proper clamp would be the best.

Like this

I made mine from 316 stainless steel as I do have a lathe

The same type of fitting was also used to back up the stainless Roter on mt PSS stern seal.