Spreader problem

[Sir Rondo Normal]

Ya it does have flat sides.

[osirissail]

Originally Posted by chala

This is not a true compression tube. The tube or special bolt is introduced from one side of the mast by a 7/8 hole witch mean that the opposite side can still collapse under load. A true compression tube is placed internally and in such a drawing both holes would be 5/8.

Wrong! You forgot that there are tang plates at each end of the bolt and/or tube/bolt. The "whole assembly" prevents the mast tube from being compressed. The mast tube "rides" in between the tank plates without compressing the mast tube sides.

[Blue Stocking]

Originally Posted by osirissail

Wrong! You forgot that there are tang plates at each end of the bolt and/or tube/bolt. The "whole assembly" prevents the mast tube from being compressed. The mast tube "rides" in between the tank plates without compressing the mast tube sides.

+1 This is what I described in my post #20.

My wooden mast has 2 which pass thru the mast at the spreader position.
My mast tangs are 1/8" stainless, V-shaped projecting above the spreader 18+", and below, to carry the double lowers. Been that way since 1965.

[chala]

Originally Posted by Capt Wraun

Now I don't know what the lateral load placed on those screws is

I can see 5 screws but do not know what grades they are. Just to compare with other way of fastening a tang I will assume that they are ¼ imperial, 600 MPa tensile strength.
1 MPa = 1N/mm2 = 0.10197 kgf/mm2
Min tensile strength… Fu…………………...600 MPa
Stress at 0.2% offset... Fy…………………...360 MPa……..(= 600 X 0.6)*
Max permissible tensile stress Ft……...........…216 MPa……..(= 0.6 X 360)
Max permissible shear stress Fv…..........…….118.8 MPa…...(= 0.33 X 360)
* The second digit in grade designation stands for 0.6(grade 4.6, 8.8)

Bolt size…………..........……….¼……………M12………..…..M16
Shank area mm2
((3.18*3.18)*3.1416) = ..31.77………….113………..…...201

“Stress” area mm2….....……..?……………..84.3…………...157

Core area mm2…….......…..“22.56”……..….76…………..…144
“Stress area” that part of the thread of a bolt that when cut in half include the core plus part of the thread.

Safe loads (vary from countries): kN (kg)
Axial tension at 216 MPa….?………..18.21(1856.9)….33.91(3457.8) on “stress” area

Shear at 118.8MPa
Threads excluded ……3.75(382.4)…13.33(1359.3)....23.72(2418.7) on “shank” area
Threads included ……2.66(271.2)…..8.97(914.7)…….16.99(1732.5) on “core” area

Using pure shear and “threads included” for 5 screws 271.2kg * 5 = 1356kg. An M16(1732.5kg) will be required. 1732.5/271.2 = 6.39 or 7 screws will better M16.
Lower shrouds I assume 8mm 1x19 316 (1/2” pin) breaking load 5400kg. Turnbuckle 8mm breaking load 5430kg (only data available from manufacturer).
Having considered the above nominal breaking load and having checked my own rig, a M20 at 600MPa with a Core Area of 225mm2 with a SWL of 2039.4kg would be needed. This represents 2039.4 / 271.2 = 7.52 or 8 screws.
A A4 70 “threads excluded” bolt would be preferable to an all thread rod with an UTS of 550 MPa.
To minimise crushing of the wall of the mast I use a maximum of 80 MPa, 8.157kg/mm2. 1 screw 271.2 kg/8.157 = 33.2475mm2, width of core 5.35mm.
Thickness of mast wall 33.2475/5.35 = 6.214mm.
Thread length required 5.2mm OK.
M20 2039.4kg/8.157 = 250.0184mm2 width of core 16.92mm.
Thickness of mast wall required 250/16.92 = 14.78mm

What I like about the fitting is:
Wall thickness requirement
Strength
Spread of fasteners.
Load distributions of spreader, may not require a compression post.
The possibility to make it stronger than a M20

What I find as a problem:
Dissimilar material, the possibility of corrosion.
Weight.
Small spreaders lateral resistance.
Cost, may be a reason why no longer in favour.

[chala]

Originally Posted by osirissail

Wrong! You forgot that there are tang plates at each end of the bolt and/or tube/bolt. The "whole assembly" prevents the mast tube from being compressed. The mast tube "rides" in between the tank plates without compressing the mast tube sides.

When there is a pull on the tang on the 7/8 hole side there is nothing to prevent the compression tube from coming out of the mast if it is the tang on the other side of the mast. A good pull on the tang on the 7/8 hole side of the mast and the opposite side will collapse.

Originally Posted by osirissail

For instance, if you put a standard bolt through a mast section and tighten down the nuts on each end you would start to squeeze and deform the shape of the mast tube.

This should not happen. The tangs should be free to line up with the shrouds.

[Sir Rondo Normal]

Thanks for that Chala.
An update, we went up the mast yesterday had a closer look. There is a thru-bolt between the spreader sockets so that should put to rest the debate about compression tube.
The movement of the spreaders was deemed to be acceptable, normal and equal with the other side.
We went sailing in about 15 knots of wind in the afternoon and none of the spreaders appeared to move. Although, I was unable to monitor them closely while tacking so we'll keep our eyes on them.
All in all, it appears that we're in pretty good shape for sailing local waters for the time being.
And again I thank all who have responded and shared their knowledge.