What are the advantages of extra threads on a machine bolt?

Um, no. While long bolts with an unthreaded portion of the shank are sometimes used as cheap “axles” or shear pins, shoulder bolts (that have a shank of larger diameter than the lower threaded portion) should be used if a threaded fastening is designed in such an application. I’m not certain what you mean by “axial loading on blind screws should be avoided,” although I assume the intent is to avoid compressive loading onto the head of the screw, which is not typically a problem; otherwise, fasteners threading into blind holes should always and only be loaded in tension resulting in a clamping force, never used as a threaded standoff unless specifically designed for this (as in a jackstand or vice).

I’m also not clear what you mean about “nuts, washers, and…bolt heads…relieve the load on the internal threads.” The function of flat washers and flanged nuts or screw heads is to distribute load and protect a softer joint material, protective surface finish, and in some cases, provide for galvanic isolation between the fastener and a reactive joint material. They do not relieve load on the internal threads, as load on those threads is critical both to transmitting load to the joint and maintaining adequate friction to prevent the screw from backing out. Washers are not needed with hardened surfaces and are generally not used with socket head cap screws unless the screw tension is such that the cap screw will dig into the joint surface; split ring “lock” washers aren’t needed at all (unless you need a washer of smaller diameter than the flat washer of that size) and do nothing to prevent a fastener from backing out if not otherwise restrained by friction or positive locking mechanisms. In general, the fewer items you have in a bolted joint “train”, the more reliable and measurable the preload on the joint will be.

Stranger

[QUOTE=Stranger On A Train;12766250Um, no… [/QUOTE]

I want to get back to you on this, but I’m suddenly short on time. I think there are different practices in different application leading to some our differences, but my information comes from way back, and some of it could be wrong. For instance, I don’t remember what the ‘blind screw’ directive was about, except for the simpler ‘don’t use bolts as hangers’ adage. But I’m certain the bolts have far greater shear strength than tensile strength, but I may not understand something, and maybe you could educate me. Please send me a private message if you don’t want to continue in this thread (Groan).

My understanding from best practices in the aircraft and heavy construction industry was that the clampling action of bolts is to hold parts together so that forces are transferred between the joined parts as if they were a single piece. Those forces are supposed to be varying or transient, and constant loads should be taken in shear, perpendicular to the bolt because it is much stronger that way. I remember only vaguely the details about deformation and bolt head strength. Easy to see examples are steel frame bridges and the lug bolts that mount wheels on cars. I believe the bolts on bridges and in steel frame buildings replaced rivets at some point in time to simplify construction. In construction, gravity is the most common large force, and horizontal bolting is also preferred for the same reason as in aircraft, safety. A horizontal bolt will not fall out of a hole if the bolt head breaks. For that reason horizontal bolting is preferred in aircraft, even when the shear forces are horizontal. And I’m sure you will agree that in vertical bolting, the bolt head should always be on top so that the bolt will not fall out if the nut comes off.
Please let me know if I have misunderstood this, or if you have references, since I won’t be able to locate this information again.

Aren’t those primarily to get the alignment precise? Or is there some other means for that such as a shouldered mating profile on the housing and engine?

A bolted joint is basically a set of parallel springs, with one in tension (the bolt) and the other in compression (the joint). This combination allows the joint to be stronger than the contributions of the individual members themselves, and also makes it resistant to fatigue in cyclic loading (assuming that the correct preload has been applied). Loading the bolt in direct shear creates a combined loading scenario, and also provides an edge loading condition that may initiation or amplify a flaw in the bolt. For this reason, shear loading should be taken either by the friction between joint member flanges (imposed by the clamping load of the bolt) or by pins, dowels, rivets, or other fasteners specifically designed to function in shear. Bickford’s An Introduction to the Design and Behavior of Bolted Joints or the inferior Handbook of Bolts and Bolted Joints, also edited by Bickford. However, basic theory of bolted joints can be found in any machine design text like the standard Shigley and Mischke Mechanical Engineering Design.

Stranger

From: http://www2.mae.ufl.edu/designlab/Lab%20Assignments/EML2322L-Fasteners.pdf

One of the most common fastener mistakes is using the wrong type of thread in the wrong type of material. The basic rule for fastener selection is: fine threads are stronger when the female thread is strong relative to the male thread, and coarse threads are stronger when the female thread is weak relative to the male thread.

Therefore, if the female fastener material is weak compared to the male fastener material, the female fastener should be given the advantage and coarse threads should be chosen. If the female fastener material is strong compared to the male fastener material, the male fastener will always fail first and should consequently be given the advantage by selecting fine threads.
For this reason steel bolts and studs that thread into relatively weak aluminum or cast iron castings such as engine blocks, cylinder heads and gearboxes are always coarse threaded on the end that goes into the casting. Also invariably, the end of the stud that receives the nut is provided with a fine thread. In this way the designer ends up with the best of both worlds.
Because coarse threads are faster to assemble, they are often used in applications where strength and weight are not of utmost concern. Conversely, virtually all aerospace bolted assemblies feature fine threads. Generally, unless threading into a relatively weak material, avoid coarse threaded fasteners.