The Jam Nut

The proper way to use a jam nut, and a look at engineering drawing texts


The jam nut, a locking device for nuts, is illustrated on p. 164, Fig. 382A, of the 5th edition of French, and on p. 530, Fig. 42A, of the 11th edition (see References). This illustration is, I believe, in error: the thinner nut should be on the bottom, the regular nut on top. It is remarkable that this illustration survived so many editions without notice, and probably has caused many engineers to put the jam nut on top. It looks "logical" to put the thinner nut on top, but this is misleading.

The idea of a jam nut is to cause the two nuts to press on the threads in opposite directions, so that the thread is stretched between them, and this stretch is not relieved if the nuts should turn slightly on the bolt, so that unscrewing is discouraged. If this condition does not exist, and it usually will not be if the thinner nut is on top and tightened second, then the top nut and after it the lower nut can loosen without restraint, and there will be no locking action. Two nuts in this case are not much better than one. The proper connection is made by tightening the jam nut snugly first, then tightening the upper nut so tightly that the stress on the jam nut is reversed as the bolt strains. The two nuts could be the same thickness, but it saves space if the lower one is thinner. The thinner nut must counter only part of the compressive force of the regular nut, which does double duty. It is surprisingly difficult to find illustrations of a jam nut used properly, or torque specifications. This is also a matter that is of no interest to the academic engineer, who only talks about locknuts and does not actually use them.

This matter caused me to consult the two editions of French that I had on hand. This classic text is, in my opinion, perhaps the best engineering drawing text ever written. It is interesting to compare the two editions. The fifth edition had 433 pages, while the eleventh was expanded to 787 pages. The later edition had more and larger illustrations, and added, among other things, a good chapter on manufacturing methods, and a chapter on engineering design. The mention of design was necessary at the time, when it was hoped that following some procedural framework could compensate for a lack of good sense. Pictorial drawing and sketching were actually de-emphasized, and line shading was eliminated. The distinction between third-angle and first-angle projection also was not mentioned in the later edition, so that European drawings will appear strange to the American student. It would not have been too much to say that the views were the same, only placed in different positions. In general, everything mentioned in the later edition was also present in the earlier one, so that the earlier text would be just as useful and complete as the later.

One interesting improvement is in the methods of drawing open and crossed belts between two circles. The fifth edition presented exact geometrical constructions, but the later an effective shorthand method. In this method, the direction of the tangents can easily be estimated with a triangle, which can then be used to find the exact points of tangency by using the right angle. This is quicker than the earlier way, and gives good results.

The later edition did not yet mention computer drafting, which has become very important in industry. Contrary to popular belief, computer drafting is actually more difficult than drawing with instruments, and places greater reliance on geometrical methods and descriptive geometry. The great difference is that the result is machine-readable and machine-plottable, easily modified, and benefits from all the advantages of computer handling (such as transferring details between different drawings, use of standard parts, or making 3-dimensional representations). On the other hand, computer drafting does not involve the manual activity that is such a valuable accompaniment to drawing with instruments and sketching. Indeed, the relation to sketching, a valuable tool for the designer, is totally absent. The novice draftsman on a computer is far more likely to create drawings that are largely useless in the shop.

Computer drafting and machine plotting does away with the inking of drawings, which was necessary for making good copies. Inking was always the hardest part of mechanical drawing for the novice, though with practice it was not hard to do, and the result was pleasing. Modern drafting pens made it somewhat easier, but the older ruling pen was not hard to use. The elimination of inking requires less skill in draftsmen, so they can be paid less, always a prime consideration.

There is a field of drawing that is intermediate between T-square or computer drafting and sketching, and that is not well presented in French (or any other text). This method uses "engineering paper" ruled in 1/10-inch or millimetre squares on one side; the other side is used for drawing, where the ruling shows faintly and is a guide to drawing horizontal and vertical lines. Triangles are used in pairs to draw parallel and perpendicular lines, just as with a drafting machine. Circles are drawn with compasses. Leads should be F or H, somewhat softer than for drawing on a drawing board. Good results can be obtained from sketches with freehand straight lines to accurate drawings, and very little equipment is required.

References

T. E. French, A Manual of Engineering Drawing for Students and Draftsmen, 5th ed. (New York: McGraw-Hill, 1935).

T. E. French and C. J. Vierck, Engineering Drawing and Graphic Technology, 11th ed. (New York: McGraw-Hill, 1972).


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Composed by J. B. Calvert
Created 31 December 2003
Last revised