That’s a question I get often with regards to the ones I’ve made, and I think people expect that I can put a number on it. Assigning an exact value would be difficult, since I don’t have the time or equipment to test these to the breaking point. Another reason is the variability of wood – ultimate strength relies on a number of important factors, like grain direction, density and defects. One clamp could test high and another could fail fairly quickly because it has a knot or other defect in the bar at the right place.

Like the old saying about a chain, clamps of any type are only as strong as their weakest link, or component. If that’s the connection between the fixed jaw and the bar, then expect that to fail first. If it’s grip the moving jaw has on the bar, then that will be the first to go.
But pushing a clamp to it’s limits doesn’t happen often in woodworking. Instead we use the clamp with moderate pressure – enough to get the job done – and nothing fails outright.

Over the many wooden clamps I’ve made I’ve learned a lot about how strong these parts, or links, should be, and now it more or less comes down to the strength of the bar itself. In particular, how well it resists bending and breaking while bending.

The best way to stiffen the bar is to make it bigger – thicker and deeper. However, that adds bulk and makes the clamp less usable. Another way is to use a stronger material, like hardwood as opposed to softwood. And as it happens, I have bar clamps made from both hard and softwood to directly compare the two. I set up a simple test to measure how far the bar bends (deflects) for a given load, and included a steel pipe clamp as well for reference:

There’s not much to the test – I just clamped down each clamp to the edge of my workbench so that they were all exactly 1″ above the opposite edge. I added pieces of wood below the bars to make the cantilevered distance the same for each, and used the same load at the same point.

Surprisingly, all three deflected by about the same amount, with the ash bar being the strongest. I thought that the steel pipe would deflect the least, and the pine the most, but both of these were nearly identical, with the ash bar just marginally better.

I should point out that this was less than scientific, but I think it’s value is to show the relative strength of these three materials doing the same job. While steel is much stronger than any type of wood, it needs to be the right size and shape to fully take advantage of that lead. Likewise, although hardwood is clearly stronger than softwood, the relative difference is fairly small.

What prompted this was a recent email asking if plywood or softwood can be used to make my wooden bar clamps. While it’s not a good idea to use plywood for the bar, as shown here, softwood or even a softer hardwood (like poplar) will perform adequately. More important than the specific species is the straightness of the grain and selecting stock that have a few defects as possible.