Building Fasteners: Tightening a Square Head Bolt Using the Bolt Head or the Nut
It's a common, and often relevant, question. When you are tightening Building Fasteners, Line Fasteners or Marine Fasteners like square head bolts and Heavy Hex Bolts (or any bolt for that matter) is it better to use the bolt head or tighten using the nut?
Although we could get into torsion and torque, there are a few general rules to remember. The answer to this question depends a lot on the materials being joined together. If the two materials being joined are the same and the bolt is running through holes drilled in the same diameter (for example two pieces of wood being joined together to form a laminated beam or two pieces of overlapping metal being bolted together to form a sheet), it will not matter what you use to tighten the fastener. Tighten from the bolt head or tighten from the nut, either way you will not affect the materials or the fastener strength.
If however, you are bolting together two different materials (aluminum to steel, steel to wood, or even galvanized to non-galvanized steel), it's important to note that the friction coefficient will be different for each. In that case you will need to calculate whether it is a good idea to tighten from the nut face or the head face. Choose the one that demonstrates the lowest friction coefficient. Otherwise you run the risk of increasing the preload or creating such a major difference that bolt breakage is a possibility.
Another commonly seen case is when the bolt hole in each piece of material has a different radius. Generally, in these cases, it's best to tighten the bolt from whichever side has the larger radius. This is because the preload created will be less than if the tighter fitting side was used.
When a washer is used, remember to try and keep the overall diameters of each (bolt head and nut/washer combination) the same, if possible. The closer in size they are, the better the fit.
In short, there are times when it matters whether a square head bolt is tightened using the bolt head or the nut. Depending on the type of material and the diameter of the bolt, it may produce a more solid fit to tighten one way. Generally this is only an issue when you are using torque control and often the difference is a minimal one.
