The following proportions will be found suitable for the bolts, nuts, and washers used in carpentry: -

Washers should equal half the thickness of the head, and have twice the diameter, otherwise they will indent the timber on the nut being screwed up tight.

433. Screws. - The following experiments were made by Mr. Bevan on the force necessary to draw screws of iron, commonly called wood screws, out of given depths of wood.

The screws he used were about 2 inches in length, 22/100 of an inch in diameter at the exterior of the threads, 15/100 of an inch diameter at the bottom, the depth of the worm or thread being 35/1000 of an inch, and the number of threads in one inch = 12. The screws were passed through pieces of wood exactly half an inch in thickness, and were drawn out by the weights specified in the following Table: -

* ' British Carpenter,' p. 18.

The weights were supported about two minutes before the screws were extracted.

Mr. Bevan also found that the force required to draw similar screws out of deal and the softer woods was about half the above.

From these experiments we may infer as a rule that the full force of adhesion of screws in Hard Wood is

200,000 d δt=f, and in Soft Wood is

100,000 d δt = f, d being the diameter of the screw, § the depth of the worm or thread, and t the thickness of the wood into which it is forced - all in inches; f being the force in pounds required to extract the screw.*

434. Nails. - Theoretical investigation points out an equality of resistance to the force of entrance and extraction of a nail, supposing the thickness to be invariable; but as the usual shape of nails is tapering towards the points, the resistance to entrance becomes of necessity greater than that of extraction; in some of Mr. Bevan's experiments the ratio was found to be about 6 to 5.

The following Table will show the relative adhesion of nails of various kinds when forced into dry Christiana deal, at right angles to the grain of the wood.

* 'Phil. Mag.,' 1827.

The percussive force required to drive the common sixpenny nail to the depth of an inch and a half into dry Christiana deal with a cast-iron weight of 6.275 lbs. was four blows or strokes falling freely through the space of 12 inches; and the steady pressure to produce the same effect Mr. Bevan found to be 400 lbs.

A sixpenny nail, driven into Dry Elm to the depth of 1 inch across the grain, required a pressure of 327 lbs. to extract it. And the same nail, driven endways, or longitudinally into the same wood, required a force of 257 lbs. to extract it.

The same nail driven 2 inches endways into dry Christiana deal, was drawn out by a force of 257 lbs.; to draw it when driven only 1 inch, under like circumstances required 87 lbs.

The relative adhesion, therefore, in the same wood, when driven transversely and longitudinally, is 100 to 78, or about 4 to 3 in dry elm, and 100 to 46, or about 2 to 1 in deal.

The relative adhesion, under like circumstances, to elm and deal, was found to be about 2 or 3 to 1.

The progressive depths of a sixpenny nail into dry Christiana deal by simple pressure were as follows: -

In performing the above experiments great care was taken to apply the weights steadily, and that towards the conclusion of each experiment the addition did not exceed 10 lbs. at a time, with a moderate interval between, generally about a minute, sometimes ten or twenty minutes.

In other pieces of wood the requisite force to extract the nail was different. Thus, to extract a common sixpenny nail from a depth of 1 inch

A common screw of 1/5th of an inch diameter was found to have an adhesion of about three times that of a sixpenny nail.*

It was found by Mr. Bevan that the force required to cause sliding or separation in the plane of the joint of two pieces of Christiana deal, each 7/8ths of an inch thick, when nailed together with two sixpenny nails, was 712 lbs. The time was fifteen minutes; the nails curved a little and were then drawn.

When the wood was dried Oak 1 inch thick, the force required to cause complete separation was 1009 lbs.

Dry sound Ash 1 inch thick yielded with 1420 lbs.