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vrickster
09-20-2005 15:58:27
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Critical bolt connections require you to use a torque wrench, and non-critical connections work better/last longer when you use one. Selecting, using, storing, and caring for torque wrenches properly can mean the difference between a safe installation and one that fails catastrophically.
How can you accurately measure the amount of torque you're applying to a bolt while tightening it? The obvious answer is to use a torque wrench. Yes, it's a basic connection tool, but you would be surprised at how little knowledge most have about this handy device. An understanding of torque wrench basics not only can reduce your tooling costs, but may also protect you from unannounced downtime, lost revenues, and major liability. When you're working on or installing equipment susceptible to damage because of too much bolt pressure or to vibration because of too little, you need to know how much torque you're applying.
Torque-sensing tools come in both powered and non-powered varieties. Powered torque tools normally operate on compressed air and generally are not suitable for field work. They also require significant maintenance and a higher degree of care than their non-powered counterparts.
Among the manual torque-sensing tools are special wrenches and screwdrivers. The wrenches come in beam, gage, and breakaway configurations, while the screwdrivers come in gage and breakaway configurations. The same principles of selection and storage apply to all of them, and they are similar in their use.
The beam wrench is the simplest and least expensive of torque wrenches. This type of wrench reads the actual torque, but has no provision to prevent you from exceeding the desired torque. Also, you must look directly at the scale and pointer to get an accurate reading. It has limited use in electrical work.
The analog "clock face" gage wrench is another type of "actual reading" wrench. Its big advantage is high resolution, but itrequires looking at the gage and also has no provision against over-torquing.
The click or breakaway wrench is the third type of torque wrench. We use the term "breakaway" because at a predetermined torque value the handle breaks away, as though suddenly becoming disjointed (which it does). You adjust a knob, dial, or cuff until the display reads the desired torque value: That's your preset. This value tells you how much rotational force the wrench will support until it breaks away. For example, if you set the wrench for 55 ft-lbs, the handle will suddenly "fail" or "break away" when the wrench head sees 55 ft-lbs of force. The adjustment mechanism compresses a calibrated spring, which allows the handle either to stay locked or "break" at the desired torque.
The breakaway torque wrench comes in two varieties. One has a digital preset. It's a digital breakaway torque wrench (also known as a digital wrench). The other has an analog preset in the form of a vernier scale-it's a vernier breakaway torque wrench. Of the two, the digital costs a little more and requires more care to prevent breakage. To select a torque wrench, you have to know the torque requirement of the fastener, which is usually a bolt. The required torque is a function of the bolt's material and diameter. Generally, you'll work with bolts of Grade 2, 5, or 8. You can identify a Grade 8 bolt by the six hash marks on its head, each mark coming from a corner almost to the center. You can identify a Grade 5 bolt by the three hash marks on its head, each mark coming from one of three non-adjacent corners almost to the center. A Grade 2 will have no marks.
Let's say, for example, you're tightening a 1 / 4 in. bolt (Grade 5). The Table (in original article) is typical of one you'd need to consult if your product literature doesn't tell you the tightening specifications. Such tables do not account for the many variables that affect required torque, so use them only as guidelines. To tighten this bolt, you would need to apply 7 to 9 ft-lbs of torque.
Suppose your torque wrench is a "0-to-150 ft-lbs" model. Can you tighten this bolt with confidence at the correct torque value, based on this tool's range? The short answer is no. The reason? These devices depend on a calibrated spring, and thus they lose accuracy when you operate them below 20 % or above 80% of their rated range. Below 10% or above 90%, and you're asking for trouble.
The correct torque wrench would read the required torque within 50% to 75% of its capacity.
You have to know about range and span. The range of the scale is the top of the scale and the bottom of the scale. For example, a 50-to-200 ft-lb wrench has a range of 50 to 200 ft-lbs. The span is the difference between the top of the range and the bottom of the range. In our example, this wrench has a span of 150 ft-lbs (200 ft-lbs minus 50 ft-lbs). Add 50% and 75% of 150 to the bottom of the range, and you are looking at 125 to 162.5 ft-lbs.
For our 8 ft-lbs, we might choose a "0-to-15 ft-lb" wrench. If we had a "0-to-25 ft-lb" wrench, that would also be suitable. A "0-to-10 ft-lb" wrench is a little too small.
Next, inspect the wrench. Is its spring in the fully relaxed position? In other words, did the last user set the wrench to zero before putting it away? Springs work off the "metal memory" principle. When a manufacturer makes a spring, it "sets" the spring into a predetermined shape. When you compress a spring and then release it, it returns to its former shape. If you compress a spring too long, however, it experiences a "memory shift" and begins to take on the shape you've compressed it to. Eventually, springs lose their memory.
There's more to applying a torque wrench than many people think. Yes, you're turning against a spring to twist a piece of metal (the bolt) into a slightly different, but new, shape. But, there are some dynamics at work here, even though you don't see them. Here are some guidelines to follow. * Use the torque wrench on "the rotating part" whenever possible. Normally, this is the nut, not the bolt. However, when the nut is inaccessible or the bolt is entering a threaded passage instead of a nut, you can tighten the bolt. Also, locking devices work best when you insert them under "the rotating part" instead of the other end. * Always torque fasteners in steps. Related fasteners (those holding the same parts together) interact with each other, and you will not get true torque readings if you tighten them one at a time. You must torque all of them at each tightening stage.
The normal stages for the above guidelines are 50%, 75%, and 100%. Some applications require five stages instead of three. Regardless of the tightening patterns, percentages, or sequences, the principle is the same: You don't slap a wrench on a fastener and take it from slack to full tightness in one step.
For example, if you're tightening to 55 ft-lbs, you would tighten all related fasteners to 30 ft-lbs first. The torque value for this first step is not critical. Your "calibrated wrist" would recognize this as "snug." The next step would be to 45 ft-lbs, which makes the nut just a little hard to turn. The final step, of course, is 55 ft-lbs, beyond which you do not obtain useful additional tightening force.
A lot of this is common sense, too. You have to make sure your threads are clean. You have to use the right sized socket. If you're using a wrench, keep your handle perpendicular to the shaft of the fastener. If you're using a torque screwdriver, keep your handle parallel to the shaft of the fastener. You may already do this so you don't slip and hurt yourself, but this caution also increases accuracy.
The time to release the pressure on the spring is when you've made your last torque check. It's always good to "exercise" the torque wrench before putting it away. Move the torque setting through its full range of motion, then set it back to zero. As with all metal tools, store the wrench in a dry place.
Many wrenches have a small hole (with a rubber plug) for lubrication. Dirt can destroy the torque wrench, so keep this hole plugged through the life of the wrench. Be sure to inspect this plug when your wrench comes back from calibration; leaving it out is an easy mistake to make.
If you suspect your wrench has suffered damage or is not reading torque correctly, send it out for calibration. Treating your torque wrenches like the precision tools they are will help you do the kind of quality work from which good reputations spring.
Once you tighten a split-ring lock washer to its rated torque (indicated by a flattening of the lock washer), never reuse it. The reason why? On the second use, the lock washer has only 50% of its "memory," and therefore only 50% of its locking ability.
*Select a wrench that reads the desired torque at 50% to 70% of the wrench's capacity. *Ratchet extensions do not affect the torque reading, because they do not extend the lever arm. However, they do increase the tendency to "slant" the wrench from the right angle needed for an accurate measurement. Always ensure your torque wrench is parallel to the plane the fastener sits in. *"Crow's feet" and other extensions added between the wrench head and fastener cause the wrench to understate torque, because they add leverage on the end where you are measuring torque. *It does not matter, from a torque measurement standpoint, where you grip the wrench. However, for safe use, grasp it by the handle grip. *Never use a torque wrench that's been stored with the spring set above 10% of capacity. *Always "exercise" a torque wrench through its full range of settings before using it. *Always apply smooth, steady pressure to a torque wrench-do not jerk it. *When a torque wrench breaks away, stop turning with it. Continuing to turn can damage the wrench and cause personal injury. *Do not use a torque wrench for more than an hour without setting it to zero. *Always store a torque wrench with the spring set at zero. *Use a torque wrench as sparingly as possible; that is, don't use it instead of a regular wrench when you don't need to check torque. *Always follow the instructions that come with the torque wrench. *Know the location of a qualified service center to calibrate and lubricate your wrench.
If your application isn't critical, you can probably get by with this technique in a pinch. Here's what to do: Leave the wrench set to zero for one hour. Then adjust the torque setting through its full range, several times. Let the wrench sit at zero for a few minutes, then compare it to another torque wrench by testing it at 25%, 50%, and 75% of range. Using another torque wrench that wasn't stored improperly, torque a bolt to the first value (on a 100 ft-lb wrench, this would be 25 ft-lbs). Try the suspect wrench-it should break away before turning that bolt. Then, loosen the bolt and tighten it to 20% with the suspect wrench.
Check the bolt with the other wrench. If the bolt does not turn, your suspect wrench is probably good for that torque value. Repeat for the other two steps. This crude test depends on many variables, so don't use it in lieu of an annual calibration. It will tell you if you have a badly out-of-calibration wrench, but it cannot ensure you have a good wrench.
This is useful is when you cannot get to a calibration facility, perhaps because of scheduling or distance. If you are torquing fasteners on something like bus bar, do not use this method-you are better off borrowing a known good torque wrench from a neighboring facility or buying a new one than taking chances.
And remember, once you over torque a fastener or locking device, you have diminished its holding capacity forever.
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