Nuts & Bolts

Building Basics BUILDERS

USE

TORQUE

Using Your Wrench '*•'•• A dual-beam wrench gives a continuous readout of the torque as you apply force to it. And it works in reverse. You can pull to loosen an already t i g h t nut and know quickly what torque it was at (mi86

JANUARY 2001

nus the issue that tightening at 40 foot/pounds probably releases above 40 f t / I b s ) . To get the Measuring a fastener's "tightness" most accurate reading you should look BRIAN MORK straight down on the scale to avoid a parallax error, but this isn't always possible when your hands are twisted inside an engine compartment and your neck is reaching up and sideways from a crawl cart. This is the biggest drawback to dual-beam wrenches. My heavy-duty wrench is a dual beam that doesn't have a readout scale, but rather a sliding disk about the size of a nickel compressed between the two bars. The disk is connected to the beam not under stress, and it nudges sideways until it falls off a 1/16-inch ridge on the (stress) beam that's connected to the handle. When the disk falls, it makes a click or snap noise, and you have to reset it to make another measurement. You can't see exactly how much torque is being applied prior to the target setting, but you can get a rough guess of sorts (e.g., half way there) by watching how far the disk has until it falls off the ridge. You There are two basic types of torque can remove a bolt with this type of wrenches, the internal spring (left) wrench, but there's no way to read the torque needed to remove it. and two versions of the dual-beam An internal-spring wrench doesn't wrench. : have a continuous readout. You tighten a bolt in the blind until the wrench clicks at the target pressure. To test the release torque, you have to set a low torque, pull to click, and then repeat the process until the nut moves before the click, and this gets boring quickly as you try to release a bolt at 10 ft/Ibs, then 15 ft/Ibs, then 20 ft/Ibs, then 25 ft/Ibs, then...(you get the idea). The precision of this measurement drops to whatever in-

Torque Wrenches

wrenches to ensure the proper pre-loading or "tightness" of various fasteners, such as prop bolts, and other threaded items like spark plugs. There are two basic types of torque wrenches, dual beam and internal spring. There are two types of dual-beam wrenches. The first one has a fixed panel with numbers on it and a pointer that appears to move back and forth as you push the wrench one way or the other. It earned its name because the pointer beam is not under stress, so it remains straight. The bottom beam is connected to the handle, and it bends as you apply torque, moving the scale underneath the pointer. The second type of dual-beam torque wrench has a different indicating mechanism—the pointer, a sliding disk, moves parallel to the wrench instead of perpendicular to it—but it works on the same principle. A micro-adjusting torque wrench has a twisting handle that sets the target torque. This wrench has an internal spring mechanism that pops when it reaches the proper torque. Wrenches that have a wider torque range are physically longer, and this falls back to the wider tension ranges that can be put onto a longer spring. (I have one of these wrenches, but I haven't been brave enough to pull it apart to see in detail what's on the inside.)

How accurate a torque wrench is also depends on how much it has been used and how well it's been treated.

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One version of the dual-beam wrench has a pointer that indicates the torque load as you apply force to the wrench.

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The other version of the dual-beam wrench uses a sliding disk "click plate," which gives an audible signal when you reach the desired value.

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crement you step up the torque, as

you try repeatedly to loosen the nut or bolt. t . s, v; • Internal-spring wrenches are instruments, not tools, and mine came with instructions, certification of calibration, and a plastic storage case. You should treat any torque wrench with care, and the FAA's airframe and powerplant handbooks recommend that mechanics have them recalibrated after 5,000 cycles, or after you drop them. Readability & Mechanical Accuracy The scale on my s m a l l 1/4-inch socket, dual-beam wrench goes from

0 to 60 inch/pounds (to determine

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Building Basics

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ft/Ibs, divide the inch/pounds by 12), with divisions every 5 in/Ibs. I think I can read repeatedly to the nearest 2 in/Ibs, with some accuracy lost when in awkward physical positions. Internal-spring wrenches essentially have zero readability errors. When the wrench clicks, you're at the desired value. Any operator error such as quickly yanking the wrench and overshooting the target value is not included. My internal-spring wrench came with a certification of calibration, and it claims accuracy within 4 percent overall when tightening

bolts and 6 percent when loosening. When below 20 percent of full scale, the accuracy worsens to two scale increments (typically 1 ft/lb on 3/8-inch drive wrenches). In comparing the accuracy of my dual-beam and internal-spring wrenches, they were surprisingly close. The dual beam has a gently increasing error rate at the lower torque values because the dominant error (reading the scale) stays fixed and becomes a larger percentage of the reading. In contrast, the internal-spring wrench has a steady percentage of error until it reaches the lowest values, and then it quickly jumps up to excessive amounts at the low end of the range.

Warranty & Pricing There are a dozen or more brands of torque wrenches available, and they range in price from around $30 for a dual-beam wrench to several hundred dollars for professional-grade instruments with a dial readout. You can find them at

Y = Indicated torque (dial) reading

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The internalspring torque wrench often has a wider range of available torque values, and the spring gives an audible "pop" when you reach the desired value.

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JANUARY 2001

How accurate a torque wrench is also depends on how much it

has been used and how well it's

been treated. Springs wear out with use, and dropping a dualbeam wrench can realign the

pointer or fixed scale.

stores such as Sears (Craftsman) and Home Depot (Husky) and on the weh from tool sellers such as SnapOn (www.snapon.com), Avery Tools (www.avery tools.com), Cleaveland Aircraft Tool (www.cleaveland tool.com), and others. When looking for a torque wrench, important features are a lifetime guarantee and recalibration services. CM)

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The FAA's Airframe and Powerplant Mechanics General Handbook (AC 65-9A) and Acceptable Methods, Techniques, and Practices—Aircraft Inspection And Repair (AC 43.13-1B) offer more how-to torque wrench information and give torque tables. These publications recommend that whenever specific torque values are not called for by the designer or manufacturer, use a standard torque table as a guide in tightening nuts, studs, bolts, and screws. Do not lubricate nuts or bolts except for corrosion-resistant steel parts or where specifically instructed to do so. For corrosion-resisting steel nuts, use torque values given for shear-type nuts. Always tighten by rotating the nut first, if possible. When space constraints make it necessary to tighten by rotating the bolt head, approach the high side of the indicated torque range. Do not exceed the maximum allowable torque value. Only use the maximum torque ranges when materials and surfaces being joined are of sufficient thickness, area, and strength to resist breaking, warping, or other damage. The use of any type of drive-end extension on a torque wrench changes the dial reading required to obtain the actual values indicated in the standard torque range tables. When using a drive-end extension, the torque wrench reading must be computed by use of the proper formula, which should be included in the handbook accompanying the torque wrench. When tightening castellated nuts on bolts, the cotter pinholes may not line up with the slots in the nuts for the range of recommended values. Except in cases of highly stressed engine parts, the nut may be over tightened to permit lining up the next slot with the cotter pinhole. You can use the torque loads specified for all unlubricated cadmium-plated steel nuts of the fine or coarse threads that have approximately equal number of threads and equal face-bearing areas. These values do not apply where special torque requirements are specified in the maintenance manual. If the head end, rather than the nut, must be turned in the tightening operation, maximum torque values may be increased by an amount equal to shank friction, provided the latter is first measured by a torque wrench.

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