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Torquing - Testing Torque Wrenches

Posted in How To on July 1, 2013 Comment (0)
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The torque wrench is probably the most often-used specialty tool. If you’re like us, you may still have your first torque wrench; we got our Craftsman ½-inch drive clicker-style as a Christmas present in 1972. It is the oldest tool in our toolbox. But veteran status might not necessarily mean it’s accurate. We contacted Cornerstone Metrology in Van Nuys, California, a company that does calibration work for both industrial and individual mechanics. We needed a highly accurate test procedure to measure our torque wrenches. Director of Quality Keith Chauvie showed us the digital scale that was accurate to within 0.10 of a pound, which suited our test just fine since we weren’t going to be working on the Mars rover.

We tested a variety of torque wrenches on Cornerstone’s digital bench unit and were impressed with the accuracy of the Harbor Freight tools. We tested torque accuracy mainly at 70 lb-ft, but also at 20 and 120 lb-ft.

We originally gathered a small collection of standard ½-inch drive clicker torque wrenches, including tools from Snap-on, Craftsman, and Harbor Freight. Then we discovered a digital torque adapter sold by Harbor Freight. This small unit fits in between a standard ½-inch drive breaker bar and socket. Using a digital strain gauge, it converts the torque applied through the adapter into a digital readout. At a typical ridiculous Harbor Freight price of $39.99, we decided to include it in our test. Frankly, we didn’t expect this little unit to be very accurate. But testing proved otherwise.

We ran the torque adapter through five consecutive applications of 70 lb-ft of torque to watch it hit within 0.10 of a lb-ft twice and actually hit exactly 70 lb-ft on one occasion. The average of the five tests was an amazing 70.1 lb-ft. The unit offers a digital readout and warning lights, and it even emits a progressive electronic beep when nearing the pre-selected torque.

We tested the Harbor Freight digital unit against Cornerstone’s calibrated scale and recorded accuracy within 0.20 lb-ft on five separate tests. In this case, both the Harbor Freight and Cornerstone tools read the same at 115.2 lb-ft. We were impressed.

Next, Chauvie suggested an even more interesting test. He offered to keep our Harbor Freight ½-inch clicker wrench and test it every 30 days for two months while keeping the wrench loaded at 70 lb-ft. He said the problem with clicker torque wrenches is, when the mechanic fails to return the preload to its lowest setting after each use, the tool begins to lose accuracy. Chauvie says the longer the wrench is loaded, the less accurate it will become. He has since reported on the torque wrench’s performance at 30 days and 60 days, and we’ve included a graph to show the results. This inaccuracy occurs because the internal spring loses its tension, allowing the wrench to break away (click) at a lower torque value. Chauvie says this is a common problem for all types of clicker torque wrenches. The point here is that you should always return your clicker torque wrench to the base setting before storing the wrench. This simple step will maintain the wrench’s accuracy for years instead of mere months.

View Slideshow

Parts list
Description...PN Price
Harbor Freight 1⁄2-inch clicker...239 $24.99
Harbor Freight 3⁄8-inch clicker...807 $14.99
Harbor Freight 1⁄4-inch clicker...2696 $19.99
Harbor Freight Digital Torque Adapter...68283 $39.99
Craftsman digital 1⁄2-inch...47712 $239.99
Craftsman 1⁄2-inch clicker...31423 $79.99
Snap-on click 1⁄2-inch...QD3R150 $323.00

This graph shows three tests of a Harbor Freight 1⁄2-inch clicker torque wrench that Cornerstone performed over a period of 60 days. Test 1 is the original test that shows the wrench varied between 69 and 70.3 lb-ft. Test 2 shows what happened to accuracy after leaving the wrench at the 70 ft-lb setting for 30 days. Notice how the curve retains its repeatability, but is down an average of 2.6 lb-ft. Test 3 is the same wrench after 60 days. Note how the spring in the wrench has relaxed, which has not only leveled out the testing, but reduced its accuracy down to an average loss of 4.4 lb-ft. As the chart shows, the wrench now torques to roughly 65.5 with a setting of 70 lb-ft. If the wrench were left for longer, the actual torque would continue to drop.

Sources

Sears
Hoffman Estates, IL 60179
www.sears.com
Snap-On
800-255-7011
http://www.snapon.com
Harbor Freight Tools
800-423-2567
www.HarborFreight.com
Cornerstone Metrology
818-902-9551

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