Re: [Amel Yacht Owners] Re: Toe pulley (headsail)


Mike Ondra
 

Peter’s test results are certainly discouraging although not surprising. Not sure where to go from here but would be interested in any other results from others who have tried Bernard’s sheaves. May yet try to modify Bernard’s since they are already in hand.

FYI: I did receive the following email from Bernard this morning with his response to some of our concerns.

While the original aluminum sheaves were not available from Amel for the past 6 months or so, I did receive an email from Maud December 3 indicating that she had received some.

And it may yet be worthwhile to look into a machine shop fabricating them in stainless as Annsofie & Jonas on S/Y Lady Annila, SM #232 did back in 2013.

So many options, so little time.

Mike Ondra

Aletes SM#240

 

 

Hi Mike

 

Thanks for your comments.

With my friend Jean-Luc we made tests on his SM and we evaluate the traction force of the rope (which is a 10 mm diameter kevlar with polypropylene sheathe) : we estimate it to 2 to 3 kN maximum (because above the thin stainless tube will probably bend or wear down as I found in design computations and it is consistent with the shear resistance of the screws and the aluminum cheeks).

So the design uses the elevator pulley principle : a groove with good angle which uses friction to transmit the torque.

With kevlar ropes they is a new fact : at rest the rope is soft, floppy, and when loaded becomes very hard (for 10 mm rope the transition is rather sharp around 300 to 500 N).

And on the tensile device we can see and measure an attractive fact :

when mounting the rope which is soft it goes at the bottom of the groove, and when loading the rope the free part becomes very hard with a fixed diameter of 10 mm. The rope in the groove pushes the lips of the groove but remains with a smaller diameter, so there is a new thing : to move on the pulley the rope has to be very much strained to reduce its diameter to enter into the groove. I asked a friend who is a professor in mechanical engineering and he is working to design a computation model.

The new pulleys were tested this fall in windstorm in Mediterranean Sea and Jean-Luc is pleased.

 

To better reply to your question I will try to design a torque device to the tensile testing device to measure the limit of friction force (joined picture). The main difficulty will be how to grip efficiently to the pulley (without damage it).

 

About your drawing which imitates the Amel design (face to face bumps) we had a lot of discussions with Jean-Luc and the boss of the shipyard where the careening takes place usually . We are not at all convinced of its usefulness : it reduces the area of contact and the transmitted torque, and bumps or teeth would be better alternates and not face to face. I tried a 3D printed model but it does not have enough strength (it began to breaks at 2 kN instead of no sign of weakness at 8.5 kN with the solid version, a measured enlargment less then 0.1 mm as computed).

Another comment is the weakness you introduce when drilling holes to make bumps. I have made a lot of design computations to evaluate a good compromise of the width of the pulley (joined picture) : there are 2 main dimensions, the bottom radius of the groove and the thickness at diameter 100. Above 26 mm width the maximum stress does not change, but below it grows rather quickly. So I choose the 26 mm thickness which is compatible with the space on the boat (without changing anything).

 

I hope to have answer your questions (I am currently preparing all these explanations on the internet site).

 

Happy New Year.

Bernard

 

 

 

From: amelyachtowners@... Sent: Sunday, December 30, 2018 7:28 PM
To: amelyachtowners@...
Subject: [Amel Yacht Owners] Re: Toe pulley (headsail) [1 Attachment]

 

 

Greetings all and best wishes for a Happy 2019,

 

Today I went back to the boat and did a series of tests on the Bernard toe pulley installed on the starboard side and compared it to the Amel stock pulley installed on the port side.  My conclusion is all of our concerns of slippage with  the Bernard pulley are well founded.

 

Details:  I tied the starboard car to a stanchion post with a random line of about 3 ft (1m).  Inserting a winch handle and cranking against the secured car slippage occurred with an estimated 50 lbs (23 kg) of force at the end of the winch handle.  I tightened the car line (3/8” or 10mm fairly old and weathered) and maybe improved the torque required to slip the pulley slightly.  To  test pulling the other way, the force the jib sheet exerts on the car:  I engaged the lock at the toe pulley shaft while the 3ft (1m) line I used to secure the car was quite taught.  I pulled upward at the center of that short line and could not get the toe pulley to slip.

Next I repeated all of that after emptying a bucket of harbor water on the pulley and line.  The wet system failed badly.  I could pull up on the short line and more the car fairly easily.  Torque at the winch handle to slip the pulley was half the effort it was when the system was dry.

Then I went to the port side with the original Amel pulley…..there’s no comparison… no slip as I’m sure you all have experienced.

 

So I’m in the market again for at least one toe pulley.  Do we have any details on where to purchase the stainless pulleys posted by Gary Silver?

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