U-bolt Threads and Strength

This is a topic that is not one of the most exciting that I've written about in this blog. Yet it is important since fasteners of all kinds are found in our stations. Even if you never build your own towers and antennas, a little knowledge can go a long way. That said, I'll proceed to bore most of you.

Cutting threads onto a blank bolt shaft weakens the bolt by removing material. The effective cross-sectional area has been reduced approximately in proportion to the new minimum diameter measured at the bottom of the threads. The amount of material removed depends on the bolt size and thread standard. Since a bolt without threads isn't too useful, we need to learn how to take their effect into account when choosing bolts.

A chain is only as strong as its weakest link...

...is an old saying that we use for subjects as diverse as industrial supply chains, military forces and marriages. It can also be applied to threaded bolts.

The weakening of the bolt due to the threads is not a problem in itself since you must calculate the required fastener strength (cross section and material grade) for your application. In our case the most common applications are towers and antennas. Since few of us design and construct the towers in our stations (we only assemble and erect them) that leaves us with antennas. A far larger number of hams design and build yagis, and that entails selection of many kinds of fasteners.

The style of bolt pictured above is very common. For shear applications, such as the bolts for splicing (joining) tower sections, the load is carried by the short blank shaft under the head. The threaded shaft carries no load other than that needed to keep the nut in place. Threads are more important for axial loads.

There are many styles of specialty hardware where the thread size is greater than the shaft size. For example, the guy yokes for my large towers have ½" shafts and 9/16" threads. This keeps the fastener strength that of the ½" shaft throughout since the diameter across the thread bottoms is approximately ½".

I was reminded of u-bolt specs when I was recently shopping for small stainless u-bolts for my latest antenna project: a 40 meter reversible Moxon. I ordered online from a vendor I hadn't dealt with before. The company was reputable and the specifications and price looked good. I had a close look at them when they arrived, as one should when dealing with a new supplier.

Compare the photos of the hex-head bolt and u-bolt. In the former, notice that the shaft tapers down towards the threads. For the u-bolt the opposite occurs. The reason is that on the u-bolt the threads are raised. On these u-bolts the threads are ¼"-20 (UNC) and the unthreaded shaft is less, measuring roughly 3/16".

The axial tensile strength of the u-bolt is largely determined by the diameters of the unthreaded shaft and the inner thread diameter. Although the threads are ¼", the minimum cross-sectional area is determined by the minimum diameter, which is that from thread valley to thread valley. The table (extracted from this source) contains data for most of the UNC thread sizes we are likely to encounter.

The effective cross section (Tensile Stress Area) for ¼-20 bolts is 0.032 in². We can rearrange the well-known equation A = πr² and solve for the diameter (2r). That equates to a diameter of 0.2", in agreement with the Minor Diameter in the table. That's slightly more than the 3/16" I got from my quick measurement. The shaft diameter is therefore appropriately sized. A larger shaft does not increase the axial strength of the bolt.

A second example is this large galvanized u-bolt with ⅝"-11 threads. The calipers show that the shaft is about 9/16". This is approximately the minor diameter so the shaft diameter is, again, appropriately sized. This is typical of the all the u-bolts (including muffler and saddle clamps) you are likely to encounter.

All of this is very interesting (well, I think so) but how does it help us to select hardware? There is no simple answer and there can be no simple answer. Shear strength is in some respects easier to calculate, which I did when I custom built guy hardware for one of my big towers. However, u-bolts are typically placed under axial load; that is, load along the direction of the shaft/threads axis. 

The axial load for typical antenna applications is the sum of the dead and live loads on the antenna section being supported and that from tightening of the nut against a rigid plate and tube/pipe. Use of a saddle distributes the load over more of the tube surface so that the tube is less likely to be crushed (e.g. excessive nut torque) or rotate.

Consider the following two methods for using u-bolts to support the large capacitance hats on the element tips of the reversible 40 meter Moxon that I am currently building. They may look alike until you take into account what sits on or hangs from what. In both cases the element takes the load of the capacitance hat.

In the top picture, the capacitance hat is underneath the element. The hat hangs from and is held by two u-bolts. The weight of the capacitance hat loads the u-bolts. The hat and plate both place an axial load on the u-bolts hanging from the element.

In the bottom picture, the capacitance hat is on top of the plate and the plate sits on the element. The load of the hat on the element is the same (as it must be) but the u-bolts have a lower axial load for the same nut torque. (Note that for this fitting work that I omitted lock washers or nylocs.) The u-bolts hold the tubes in place, but the only axial load is from the torque on the nuts.

Does that matter? Yes, but not by a lot. I find that hams that design and build yagis tend to stick to one method of connecting yagi elements to booms, either sitting or top or hanging below. Despite the axial load difference, I haven't heard many good arguments about which is superior. 

In a marginal case with u-bolts that are of minimum size it might make a difference. However, bolt grade, material and durability are more important in most cases. Axial and sheer strength of u-bolts become increasingly important for high load applications such as boom-to-mast clamps and rotator mast clamps. 

A failure under unanticipated stress is something you really want to avoid if you can. Size your u-bolts  by the application and fundamentals if you can, or do your best to mimic the size and grade in similar applications that have proven durable in commercial products. That is, lean on the engineering done by an engineer.