Remember this thread? Dr. Claus Mattheck did workshops in Lake Louise and Detroit last week. He showed one slide of a bolt going through a stem at an angle indicating countersinking is undesirable as is single point bearing on wood.

He showed a set-up with two washers with one flat face and one hemispherical or domed face. The one bearing on the tree would presumably be slotted at bolt diameter to allow it to seat flat against trunk. The other would be drilled in the ordinary fashion. The two domes would contact and transmit load accomodating a wide range of tree-bolt angles.

I thought I might have seen something like thois off the shelf but can't re-locate it in a search. Claus had thought of it as a job fabricated rather than off-the shelf item.

Reply to post by Scott Cullen, on October 31, 2000 at 08:19:34:

I do remember that thread- it's left handed, if you look closely

No need to get so fancy. Only the emisphere, closest to the trunk, needs to be rounded, and only in one direction (a half-cylinder, instead of a hemisphere). The outer waher can be a regular flat washer.

The bearing surface between them is only at the tangent point. If you use two hemispheres, the bearing is at two points, on either side of the slot. If you use a half-cylinder for the slotted washer and a flat washer for the outer one, the bearing surface is large- a line along the tangent of the cylinder.

Does this make a difference? I don't know. But using a flat washer in the outer position will give the same bearing surface as using a hemisphere in that position. So there is no need to manufacture a special second piece, when only will is needed.

Drill a ball bearing all the way through, creating the slotted hole. Then cut in half, and you have two of what you need- one for each end of the cable, or for both sides of a bolt.

Reply to post by Russ Carlson, on October 31, 2000 at 08:19:34:

Good catches Russ. My description of "hemispherical" shape was inaccurate. Half-cylinder is correct... in fact how I drew them in the graphic I sent you. And, you're right the nut side washer can be flat.

But I think the bearing area, along the tangent as you describe, is pretty close in either configuration. The straight hole on the nutted side is the same either way and the slot on the half-cylinder side is the same either way. The big advantage is only one special piece is needed.

I'm not sure I agree that drilling the BB creates a slot. It creates a straight hole. You really need to mill a slot... which you can do with a drill but that's not the most efficient way. I think I'd start with half-round bar stock rather than dealing with the BBs.

The graphic I sent has an error. The slot could not taper to bolt OD on the tree side. The slot would need to be the diagonal of the bolt OD for the angle you want to deal with.

Scott

Reply to post by Scott Cullen, on October 31, 2000 at 08:19:34:

No! Sorry Scott, I ain't buying it, even if it does come from Claus!!! If the load bearing area is anything other that 90 degs. to the bolt you WILL have a shear effect!!! I desperately wanted to make it up to Lake Louise just to discuss this issue however, chartering the lear jet was not any more viable than the regular flights, There is some fundamental problem here, trying to convey the engineering concepts of loading on bolts. Please bear with me, I will try and make my case clearer.

James.

Reply to post by James Causton, on October 31, 2000 at 08:19:34:

Well, this was just one slide out of the whole day, so maybe it's just as well you didn't charter the jet just for this.... there was plenty of stuff through the day however that was quite useful, a lot of new material too.

Anyway, there is no doubt some resultant force that is not in line with the bolt, i.e. is not a pure compressive force perpendicular to grain. The real question is whether that force will exceed material strength in that vector. Claus did not address this, but it might be assumed that it is less of a concern than the uneven crushing forces on the wood and shearing forces on the bolt from unequal bearing.

Is this potentially troublesome beveled-washer resultant a shearing force? I'm not sure.

If the hole angle is axial (pointing toward the ground) is a round bolt trying to compress the wood parallel to the grain or pull the grain apart in tension? (Go back to the earlier thread... weld a bolt to the edge of your splitting maul and try to make some firewood). Using white oak as an example, Forest Product Laboratory data indicate that compressive strength parallel to grain is 3,560 psi while compressive strength perpendicular to grain is only 760 psi. So crushing from unequal bearing might indeed be more of a concern. Even if there is a tension or splitting component with the round bolt, shear strength parallel to grain is 1,250 psi. This is a typical installation... one tree-anchor point higher than another or a guying system.

Now if the hole is more or less level but is not radial, i.e. does not pass through the center of the limb (I guess that makes it tangential), that beveled-washer resultant might be trying to shear the wood parallel to the grain. Again it's 1,250 psi vs the 670 psi crushing resistance to unequal bearing. Drilling tangengtially is not a typical installation, so should seldom be an issue.

Reply to post by Scott Cullen, on October 31, 2000 at 16:48:51:

The bearing surface area will be different depending on whether you use a half cylinder or a hemisphere.

Assume using a flat washer on the nut side, you will have a plane tangent to the other object. A plane tangent to a cylinder is a straight line. A plane tangent to a (hemi)sphere is a point. Without getting into the arguement of the area of a line, there is a difference. But then, does it make any practical difference?

Reply to post by Russ Carlson, on October 31, 2000 at 17:33:22:

I think we were saying the same thing. If you use a half cylinder on the tree side either a half cylinder or a flat washer on the nut side will result in contact along a line. Convex hemisphere on either side would result in only point contact.

Now, we are left with another issue, to conform to A300 the half cylinder on the tree side would need to be on a round base. I think the intention of A300 is to promote good woundwood formation and avoid points on hardware. This would also spread the mechanical load more.

I have also discovered a detail for a "cup and washer" set with hemisperical profiles but one is convex and the other concave so there is broad surface contact between them. The limitation is that swing is no more than 10 degrees in any direction.