wood hardness

[river rat]

I was talking with a friend the other day abort wood hardness He is of the opion that Lignum Vitea is the hardess wood because it is used to make bearings for ships, I diagreed that it is used for that because of the oil that is in it. Does anyone know what wood is the hardess and where that info can be found.


River Rat

 

[Bill Boehme]

There is no such thing as a measurement of hardness that is applicable to wood because there is no practical use for such a measurement. The Rockwell hardness scale is a relative hardness rating, but it is applicable to metals, minerals, and possibly some synthetic materials. The strength of wood is measured in terms of characteristics such as bending, shear, compression and torsion where the fatigue points are determined. However, none of those characteristics necessarily relate to hardness. BTW, Lignum Vitae was used for propeller bearings because of its lubrication properties when immersed in water. It is reported to be the heaviest of woods, but heavy does not equate to hard.

Bill

 

[dkulze]

The other aspects of wood that make it difficult to determine hardness by species are variances in growth patterns, soil conditions, direction of grain, age when cut, period of drying, etc.

An example would be the white oak used in building New England warships in the 1700's. The USS Constitution is built with a layer of white oak beams on the outside that earned it the nickname "Old Ironsides" because cannonballs were known to bounce off of it. This was likely an artifact of the oak being original growth and extremely dense in grain (a piece I have dating from that time has 300 growth rings in about 10 inches) but flexable and resilliant rather than brittle or "hard". A white oak that grew in the second and third growth forests of today would be much less dense and hard and probably would have earned it the nickname "Old Screen Door".

And, yup, Lignum Vitea is supposed to be the "densest" wood out there and is self lubricating and quite resilliant under pressure, making for good bearings, bushings, and fittings for ocean-going vessels. I'd also wonder, with submarines, if they weren't shooting for quiet too.

Dietrich

 

[Bill Johnson]

Isn't specific gravity indicative of wood hardness? If so, that is a term/rating that is easily referenced.

 

[river rat]

I am looking for the Janka side hardness. density is how much it wieghts per cube inch and is miss leading ie mercury is much denser than wood but it is a liquid and there for softer than wood at room temp.


River Rat

 

[Mark Mandell]

Lignum was replaced for immersed shaft bearings in the late 60's with the invention use of teflon and similar materials.

Carved a piece of LV when in college. Beautiful wood, but I wore a 1/2" off the chisels doing the piece. It was very much like cutting stone.

m

 

[woodwish]

Bought a piece of Lignum Vitae at AAW Symposium just to see what it was like. I've made some bottle stoppers and a small lidded box with very little trouble. Maybe the piece I have is unusually soft or something, but I swear the air-dried pecan I have made similar items out of is much harder.

Not sure how I would describe what "hard" really is in wood though.

 

[Bill Boehme]

Isn't specific gravity indicative of wood hardness? If so, that is a term/rating that is easily referenced.

Nope! Density has no specific relationship to hardness. I would say that crystalline structure has a more significant effect on hardness. As an example of density, look at lead, one of the heavier elements -- not very hard is it? Also, don't forget that "hardness" is an arbitrary term based on comparison. There are relative hardness scales for materials using certain criteria for making the comparisons, but there are no physical units of measure that quantify hardness.

Bill

 

[DeanGThomas]

Interesting topic. Twood that there were such a scale as Rockwell for woods, but even that might be misleading.

Lignum was one of several woods used for bearings of all types, not just because of its lubricity and its water-resistance. It's one tough wood! It was used in pulleys because it didn't need lubed, but also because it did not split or crack under strain of load. Several of the rosewoods were also used, if memory serves, for the same ability to take loads without wearing the bearing hole oval.

As implied earlier, lignum is carvable (at a cost), so it's workable, machinable wood. Many other dense woods are tough, but are very brittle and would split if used as a pulley on a metal bearing shaft. Not to mention things like tannic acid content that corrodes steel and iron, and lots of other water related issues. Lignum is not hygroscopic (did I get the right word there?). It does NOT absorb water like a maple, pine or oak would.

And doesn't pecan have a high silica content? Isn't that why it eats tools??

Others, please??

 

[jbburgoon]

Strength to weight ratio

As a sixth grader, my daughter did a science fair project where she was comparing the shear breaking strength of different spieces of wood. We found alot of scientific data at the US forestry website. Amazingle enough, her experiment concluded that based on a comparison of sample weight and breaking point, balsa was pound for pound the strongest wood tested, followed very closely by oak.

 

[Bill Boehme]

I am looking for the Janka side hardness.

Here is a link to a FPL report on Janka hardness. Sort of reminds me of the four-ball test on lubricants. The impression I get about the Janka hardness test is that it is difficult to get consistent results and that only the FPL could be interested in such things. Of course, that is not exactly true -- companies that make manufactured wood products would find this to be useful information (i.e., laminating wood under high pressure).

Bill

 

[MichaelMouse]

More important, how far the wood will crush under what load is important in structural use. Top and bottom plates, headers and truss spans come immediately to mind. Seems most folks here are speaking of resistance to abrasion as the definition of hardness. Different item. Favors the denser species, because all are using cellulose as the basic bearing material, just some don't put as much air between the fibers.

Neighbor of mine served on boats as an oiler during reciprocating days, and he said the reason for lignum was that it didn't split if the shaft was a bit out, and that water was sufficient for lubrication. No galvanic effect to worry about like using a brass bearing against a steel shaft, either. Made sense to me.

 

[john lucas]

Don't know all about that fancy test stuff but Black locust, Pecan and Osage orange are very hard but all turn nicely with sharp tools. I don't remember Lignum Vitae being that difficult to turn but that was a long time ago.
The worst tool dulling wood was oak barnwood. One pass across the face and I had to sharpen the gouge. It wasn't dull, it wouldn't even cut. About 3 passes later and the wood was more like kiln dried Oak. I suspect the outer layers had sand imbedded in them.