The Test Rig
Easy: Just weld a bolt on and pull on the wrench, right? It’s a little more complicated than that — but not too much.
First we needed a way to measure the amount of weight we’re applying. We called around to some friends who’re into the whole outdoors thing and managed to score a kick-butt 125-lb-capable spring scale used to weigh (large) fish. It’s made of machined aluminum and not only has a great padded handle but “jaws” that can be opened or closed to grab the fish.
To achieve the AutoWrench’s rated torque of 225 ft-lb by pushing on the 8″ wrench handle, we’d need to apply a 337.5 lb force — the equivalent of standing on the wrench. This seemed impractical, and besides, our (very heavy duty) spring scale only reads to 125 lb.
What we needed was a longer lever arm. If we were to increase the lever arm’s length from the stock 8″ to, say, 36″ we’d only need to apply 75 lbs of force to achieve the desired result. We’d also have the ability to apply as much as 375 ft-lbs of torque should the AutoWrench prove stronger than its claims. A longer lever arm also lowers the risk of injury from things flying around when the wrench eventually lets go.
We noticed that the AutoWrench almost perfectly fit into a bit of 1″ x 2″ square tube we grabbed from the bin — just the rounded battery cover prevented it from sliding right into the tube. So, we laid the wrench on the tube and marked the offending section in silver pencil.
We then used the Hobart AirForce 250A plasma cutter (from our long-term test toolbox) to cut out the marked slot. We could have used the band saw, but the AirForce let us cut just one side, leaving us with a stronger overall piece. A little flap-disc cleanup with the angle grinder, and we’ve got a nice solid fit.
To create a lever arm of known (and easy-to-calculate) length, we slid the wrench in as far as it’d go then measured and marked a point exactly 36″ (3′) from the center of the AutoWrench’s jaws. Since the spring scale was really designed to hold fish by their gills, we cut a triangular notch into the lever arm to grab the scale’s catch and assure a solid, no-slip fit on the scale end.
To give the AutoWrench something beefy to grab on to, we inserted a piece of 12-gauge 1″ square tube through the hardy hole in our 125 lb anvil — which is massive enough to handle withstand a decent amount of twisting torque.
On page 3 we perform the test.