Prm 150 and propeller shaft weight

[Tresfjord 26]

Hello

 

I have a beta 43hp 2800rpm engine and a prm 150 1:2.09 gearbox on my tresfjord 26 boat

 

the propeller shaft is 35mm and 1.8 meters long. Weights aprox 13,59kg

propeller is a 17x14 3-blade weight 7.7kg

 

Im now fitting a vetus flexible coupling Bullflex type 8, it weights 11.9kg

A smaller Bullflex type 4 that weight 6.9kg would otherwise be good for my aplication but there is no model for a 35mm shaft

 

My consern is that is there too much combined weight on propeller shaft for prm 150 gearbox to handle, will it wear out faster?

Is bullflex coupling going to take care of exrta weight By flexing when engaging a gear?

 

shaft, coupling and propeller weight combined is 32.2kg

 

Best regards

Henri

 

 

Edited January 27, 2018 by Tresfjord 26

[Guest]

11 minutes ago, Tresfjord 26 said:

Hello

 

I have a beta 43hp 2800rpm engine and a prm 150 1:2.09 gearbox on my tresfjord 26 boat

 

the propeller shaft is 35mm and 1.8 meters long. Weights aprox 13,59kg

propeller is a 17x14 3-blade weight 7.7kg

 

Im now fitting a vetus flexible coupling Bullflex type 8, it weights 11.9kg

A smaller Bullflex type 4 that weight 6.9kg would otherwise be good for my aplication but there is no model for a 35mm shaft

 

My consern is that is there too much combined weight on propeller shaft for prm 150 gearbox to handle, will it wear out faster?

Is bullflex coupling going to take care of exrta weight By flexing when engaging a gear?

 

shaft, coupling and propeller weight combined is 32.2kg

 

Best regards

Henri

 

 

I have usually found PRM to be very helpful and I would think a quick email or phone call would be a good starting point. Also, the length of the shaft from the stern bearing to the Bullflex will have an effect on the gearbox bearing.

[Tresfjord 26]

1 minute ago, catweasel said:

I have usually found PRM to be very helpful and I would think a quick email or phone call would be a good starting point. Also, the length of the shaft from the stern bearing to the Bullflex will have an effect on the gearbox bearing.

I have just asked prm for their opinion aswell..

I would just like to hear some opinions and experienses when using a prm150 gearbox and bullflex 8 

 

[MoominPapa]

I guess the downward gravity force on the bearings will be fairly insignificant. What matters are the forces to accelerate the shaft and prop as it goes into gear or, worse, goes from forward to reverse and vice-versa. To get some idea of that you'll need to know the  moment of inertia of the shaft and prop, which is related to, but not the same as the weight. A kilo at the end of a propeller blade counts for more than a kilo in a shaft which is close to the axis.

 

MP.

 

[Tresfjord 26]

6 minutes ago, MoominPapa said:

I guess the downward gravity force on the bearings will be fairly insignificant. What matters are the forces to accelerate the shaft and prop as it goes into gear or, worse, goes from forward to reverse and vice-versa. To get some idea of that you'll need to know the  moment of inertia of the shaft and prop, which is related to, but not the same as the weight. A kilo at the end of a propeller blade counts for more than a kilo in a shaft which is close to the axis.

 

MP.

 

 

[BEngo]

Crudely, the weight  of the back half of the shaft and the coupling plus bullflex  etc. at the gearbox end is taken by the gearbox back bearing.  The weight of the prop and the outboard half of the shaft is taken by the stern-tube bearing, not the gearbox.  We can ignore the possible  effects of misbalance and whirling as the shaft and coupling should not do that!

Find the size and type (deep groove ball, or roller, taper roller or angular contact ball ) of the back gearbox bearing. You want  outer diameter, inner diameter and thickness of the races.  Look that bearing  up on say the SKF website and it will tell you the radial and axial  loads the bearing is capable of sustaining with a reasonable life expectancy.  Compare the RADIAL load  with the sum of your weights multiplied by 10, to turn them into Newtons, and see if it is overloaded or not.  My money is on it being nowhere near a concern.

 

N

 

 

[Tresfjord 26]

3 minutes ago, BEngo said:

Crudely, the weight  of the back half of the shaft and the coupling plus bullflex  etc. at the gearbox end is taken by the gearbox back bearing.  The weight of the prop and the outboard half of the shaft is taken by the stern-tube bearing, not the gearbox.  We can ignore the possible  effects of misbalance and whirling as the shaft and coupling should not do that!

Find the size and type (deep groove ball, or roller, taper roller or angular contact ball ) of the back gearbox bearing. You want  outer diameter, inner diameter and thickness of the races.  Look that bearing  up on say the SKF website and it will tell you the radial and axial  loads the bearing is capable of sustaining with a reasonable life expectancy.  Compare the RADIAL load  with the sum of your weights multiplied by 10, to turn them into Newtons, and see if it is overloaded or not.  My money is on it being nowhere near a concern.

 

N

 

 

I allso belive that its nowhere near to a point that i should be concerded.

 

The thing that troubles me is the shaft weight that the gearbox has to rapidly accelerate when engaging a gear. Will this bullflex absorbe the impact of rapid acceleration eaven tough it is quite massive and add extra weight to my shaft configuration

or eaven if i have to do a emergency forward to backwards gear change

Im worried about gearbox clutch plates wearing out faster than normal beacose of this.

 

[David Mack]

Isn't that exactly what the coupling and gearbox clutch plates are designed to do?

[Tresfjord 26]

13 minutes ago, David Mack said:

Isn't that exactly what the coupling and gearbox clutch plates are designed to do?

It is, but im guessing there is limit to everything. Including the weight of shaft.

 

Maby bullflex is flexing and accelerating faster compared to propeller shaft when engagin gear and gearbox clutch plates will engage fully before the whole shaft starts spinning, then added extra weight does not matter

[BEngo]

Sorry, I thought you were concerned about the weight rather than acceleration forces.  These are driven by the inertia of the various components in the driveline.

The inertia of the shaft is negligible, because it has a relatively small radius.  Your prop is also quite small at 17 in diameter, again as props go.  You could probably treat the prop as a bronze disc with the same mass and diameter then work out its  mass moment of inertia.  Add to this the figure for the bull flex and use that to work out the torque needed to accelerate the whole lot to shaft  tickover speed in about 0.3 seconds.  Multiply by the reduction ratio  to get the torque on the box.  Compare that with the PRM's torque limit.

I still  would not expect a problem.  The PRM will deal with a much larger diameter prop with correspondingly (radius squared)  higher inertia.  The bullflex will also withstand some serious torque, according to the data.   PRM boxes have a reputation of being tough, and they are very popular on the  canals because they will survive mistreatment like logs or other debris  in and round the propeller as well as withstanding much more frequent and numerous reversals than is common in open water work.

 

N

 

 

[David Mack]

18 hours ago, Tresfjord 26 said:

It is, but im guessing there is limit to everything. Including the weight of shaft.

Indeed, but there's nothing untypical about the shaft and prop you are using.

 

[Tresfjord 26]

21 hours ago, BEngo said:

Sorry, I thought you were concerned about the weight rather than acceleration forces.  These are driven by the inertia of the various components in the driveline.

The inertia of the shaft is negligible, because it has a relatively small radius.  Your prop is also quite small at 17 in diameter, again as props go.  You could probably treat the prop as a bronze disc with the same mass and diameter then work out its  mass moment of inertia.  Add to this the figure for the bull flex and use that to work out the torque needed to accelerate the whole lot to shaft  tickover speed in about 0.3 seconds.  Multiply by the reduction ratio  to get the torque on the box.  Compare that with the PRM's torque limit.

I still  would not expect a problem.  The PRM will deal with a much larger diameter prop with correspondingly (radius squared)  higher inertia.  The bullflex will also withstand some serious torque, according to the data.   PRM boxes have a reputation of being tough, and they are very popular on the  canals because they will survive mistreatment like logs or other debris  in and round the propeller as well as withstanding much more frequent and numerous reversals than is common in open water work.

 

N

 

 

You are probably right, it might be that im over thinking...

I have allso e-mailed to Nevage prm and im waiting for their opinion asswell

[Bee]

I also had concerns about this very issue. The PRM goes from neutral to engaged with quite a clunk and with a heavy prop, long shaft and lots of inertia with couplings etc it worried me, there must be wear on keyways etc.  the bigger PRM's have a 'soft start' option but not the 150, if it had then I would invest in it. However the engine / box and the rest have now done many hours and had some abuse and all seems to be well. I now do what my wife does when the car makes funny noises - turn the radio up.

[Tresfjord 26]

31 minutes ago, Bee said:

I also had concerns about this very issue. The PRM goes from neutral to engaged with quite a clunk and with a heavy prop, long shaft and lots of inertia with couplings etc it worried me, there must be wear on keyways etc.  the bigger PRM's have a 'soft start' option but not the 150, if it had then I would invest in it. However the engine / box and the rest have now done many hours and had some abuse and all seems to be well. I now do what my wife does when the car makes funny noises - turn the radio up.

What kind of setup you have on your boat? 

[Bee]

Same engine and gearbox as yours, then a Fenner tyre coupling (not too heavy) close to the gearbox, this takes out any movement from the engine rubber mounts, then about 3 metres of 1.5 " (a bit less than 40mm) shaft then a Python drive flexible coupling where the shaft has an angle in it and another 1 metre shaft with an 18 inch propeller. It sounds complicated but its not really.

[Tresfjord 26]

On 28.1.2018 at 19:33, Bee said:

Same engine and gearbox as yours, then a Fenner tyre coupling (not too heavy) close to the gearbox, this takes out any movement from the engine rubber mounts, then about 3 metres of 1.5 " (a bit less than 40mm) shaft then a Python drive flexible coupling where the shaft has an angle in it and another 1 metre shaft with an 18 inch propeller. It sounds complicated but its not really.

How many hours have you used your engine? What is your propeller pitch and max engine rpm you can get?

Your combined shaftline weight is much more than with my setup.

[Tony Brooks]

I am just going to observe that the PRM 150 uses multiplate clutches operated by hydraulic pressure so there will be a short time where the clutch slips giving a softer take up than a cone clutch mechanical box. The higher the inertial the slightly longer the box will slip for but as we are talking wet clutches I am sure any extra wear will be minimal.

I think the OP is over thinking this.

[Bee]

1 hour ago, Tresfjord 26 said:

How many hours have you used your engine? What is your propeller pitch and max engine rpm you can get?

Your combined shaftline weight is much more than with my setup.

Probably 3000 hours , think its 18 x 12, max rpm? seldom had it over 2000 rpm, It'll certainly do 2,500 rpm though.