I bought some 1 1/4" 10-24 bolts and nuts at the hardware store. They also have 10-32.
I understand #10 is the size. 24 or 32 threads per inch are the choices. It’s important that the nut matches the threads on the bolt (i.e. use a 10-24 nut on a 10-24 bolt).
I’ve wondered what the advantages are for extra threads per inch?
Especially with a nut that is less than 1/8" thick? How many extra threads could possibly be inside a 10-32 nut vs 10-24? Perhaps 1 or 2 threads? What can that extra thread do?
Well, for one, if the nut starts to rotate backwards on the bolt so that things become loose, the nut has to rotate MORE to achieve the same level of looseness.
Oh, and a finer threaded nut/bolt would perhaps be less likely to come loose in the first place because the thread “ramp” so to speak is not as steep on a fine thread than a coarse thread.
And, when tightening something like that up, it would allow for a finer control on getting the correct amount of tightness.
As you mention, strength could be an issue as well, with possibly more threads being engaged by the nut.
:eek: That’s right. I’d forgotten 32 threads per inch are finer (more shallow). They don’t cut as deeply into the rod.
24 tpi are coarse and deep. Making the threads stronger. But vibration is more of a problem. The bolt is slightly weaker because of the deeper threads.
How do you get at that ballpark? I purchase lots and lots of fasteners, and there’s no discernible difference in my price for the same material category and size regardless of the thread pitch (or threadless). I’d have to assume that that 20% is passed along to somebody.
simply put, better leverage.
A bolt is just a lever to hold a pin in place. But the lever is round to convert torque into tension. The more threads per inch the longet the lever.
On small bolts course threads are easier to strip and fine threaded scrrews are easier to twist the head off.
Ok, there’s some confusion between Threads per Inch, and fine/coarse threading. Traditionally, fine threads are cut to more precise dimensions, with less play between the mating threads. These threads are smoother, and turn more readily, with consistent travel as the screw is turned. These are the factors desired in threaded motion control systems and adjusting screws. These screws can be more subject to rotation from vibration because of the smooth surfaces, but the lesser incline of the thread allows them to be tightened better. Coarse threads were cut more roughly. They required more force to turn because of the increased friction between the mating surfaces, and were less affected by vibration, but may have exhibited play and inconsistent travel.
Threads per Inch factor into the use of the screw. As a fastening device, screws should are best placed in a shear relative to force, i.e., perpendicular to the shaft. Here the threading is not so important. When the force is parallel to the shaft, coarse threads are thicker, and deeper, and provide more strength.
In modern screws, all threads are cut so precisely that the coarse/fine distinction is usually only important in terms of TPI. The exeption are high strength screws, subject to vibration, such as lug screws on that attach wheels to cars. The main reason finer threads are used in small applications is the greater ability to tighten the screw. The shallower thread depth also makes them more suitable for threading into plastics and composites, where the material compresses around the thread. Most modern designs are based on finer threads when smaller sizes like a #10 screw are in use, and the cost factor is insignificant. For larger bolts and screws, coarse threading may cost less. But with two standards for threading (and I think there are other specs, like extra-fine), the different thread types have proliferate, and will continue to be manufactured and used. One factor that defines thread type in non-critical applications, is the type of screw available when a design prototype was made. So sometimes the original choice is just arbitrary.
Screws used for adjustment provide finer adjustment with finer threads, and I’ve seen 1/4-100 screws for this purpose. Also there are screws with tapped center holes, for example 1/4-100 on the outside and a smaller hole with 80 threads per inch on the inside, so you can use the difference for adjustment (the screw turns around a center bolt that moves relative to the outermost nut).
Threaded things that must be hollow can have a larger inside diameter if the threads are shallow. The externally threaded ring in the front of a nice camera lens, holding the element in, is a nice example. I have a tap for 0.535-40 threads that taps a hole bigger than a half inch, but with twice the thread count of a 1/4-20. This thread is also called a SM05 thread. The bigger diameters are even more impressive.
I went to the McMaster-Carr catalogue and checked out a few examples of fasteners that were the same except for thread pitch. For those of you unfamiliar, McMaster-Carr is a hardware supply house that has, no shit, EVERYTHING under its roofs.
Screws should never be placed in shear, if it can at all be avoided.
The threads of a screw create a stress riser which can lead to the screw shearing off at that point.
Look at how a bell housing attaches to an engine, there are usually two shouldered bolts, two guide pins, or a couple of hollow dowls to take the shear forces.
Bolts hold in tension.
Screws can accept a moderate amount of shear, and with thin wall connections (like one can inside another) it is typically the parent joint material that will tear out or buckle before shear failure of the fastener, but yes, in general, you either want to avoid shear loading of fasteners by applying enough clamping preload such that bending and direct shear forces don’t overcome the friction in the joint, or use shoulder bolts or dowel pins to accept shear load directly. The link cited by Gary T nicely summarizes how coarse thread versus fine thread (UNC versus UNF for the Unified Thread Standard fasteners, also a UNEF that I’ve never seen in practice) are selected for a particular application. The one nit I have to pick with it is that while it is true that UNC threads are less prone to galling than UNF, the thread class (which is a measure of the prescribed tolerances of the thread) is even more critical for galling considerations.
You are correct in your interpretation. Machine bolts with an unthreaded shank are designed for shear loads, where the load is place on the unthreaded portion. I should have been more specific about that point. Axial loading on blind screws should be avoided as well. Nuts, washers, and the screw or bolt head are needed to spread the load over a greater area, and relieve the load on the internal threads.
Good point. Usually the bolt is a little longer than needed. I can see how more threads would resist the nut backing off.