CABLE TERMINATIONS by Dan Culhane and Delbert I. Hall. Published in TD&T, Vol. 37 No. 4 (Fall 2001)

Transcription

1 CABLE TERMINATIONS by Dan Culhane and Delbert I. Hall Published in TD&T, Vol. 37 No. 4 (Fall 2001) Theatre Design & Technology, the journal for design and production professionals in the performing arts and entertainment industry, is published four times a year by United States Institute for Theatre Technology. For information about joining USITT or to purchase back issues of TD&T, please contact the USITT office: USITT 6443 Ridings Rd. Ste 134 Syracuse, NY tel: USITT ( ) tel: fax: [email protected] web: Copyright 2001 United States Institute for Theatre Technology, Inc.

2 CABLE terminations By Dan Culhane and Delbert L. Hall Scenery and lighting instruments are routinely suspended by cables above the heads of performers, technicians and audience members. In order for cables to be fastened to loads, some type of end termination must be applied to the end of the cable. Because these terminations support the entire load placed on the cable, they are of critical importance. This article will discuss the proper method for making some of the most common end terminations used in stage rigging, as well as discuss the strength and dependability of each. CABLE Before beginning a discussion of end terminations, it may be helpful to review the cable itself. What is commonly referred to as cable is technically a type of wire rope (figure 1). All wire rope has the same three basic components: wires that form the strand, multi-wire strands laid helically around a core, and the core. The core is the foundation of the wire rope. Most wire rope used in theatrical applications has an independent wire rope core (IWRC). Wire rope is identified not only by its component parts, but also by its construction the way the wires are laid to form strands and the way the strands have been laid around the core. What is called aircraft cable is more precisely 7x7 or 7x19 IWRC wire rope. The notation 7x19 means that the wire rope has a total of seven strands (six strands wrapped around a core strand) and each strand is made up of nineteen wires. Therefore, a 7x19 cable has a total of 133 wires, whereas a 7x7 cable has only forty-nine wires. The more wires in a cable, the more flexible it will be. The trade-off for flexibility is strength. Everything else being equal, the fewer the number of wires in a wire rope, the stronger it will be. Galvanized aircraft cable, the most common type of wire rope used for theatrical rigging, is manufactured to meet specifications MIL-W (Type I, composition A) that provide both high strength and flexibility. There are other factors in wire rope specifications: direction of lay, grade of rope, lubrication, and coating. Most of these specifications are not factors for most people when selecting wire rope. Although nylon or vinyl-coated cable is not commonly used in theatres, it is commonly sold in hardware Figure 1. Wire Rope stores and, therefore, does sometimes find its way onto the stage. The extruded, flexible coating can extend the service life of a cable by reducing wear. It is important to note that the nylon or vinyl coating must be removed at the ends before the cable termination is applied. STRENGTH In most instances, strength of the cable and efficiency of the end terminations are the most important factors in choosing a cable. There are two ways that strength of a cable can be indicated: breaking strength (BS) or working load limit (WLL). Breaking strength is the minimum load at which that item is rated to fail. Working load limit is the maximum load under which the manufacturer recommends use of that item. The 10 F A L L TD &T Copyright 2001 United States Institute for Theatre Technology, Inc.

3 Figure 2. Thimbles WLL is established by applying a design factor (DF), sometimes called a safety factor (SF), to the breaking strength. Since the termination is usually weaker than the cable itself, and the whole cable assembly can only be as strong as its weakest part, the efficiency of the end termination needs to be accounted for in determining any working load limit. The end termination efficiency (T) is multiplied by the breaking strength and that result is divided by the design factor. If a cable has two different types of end terminations with different efficiencies the lower efficiency is used to determine the WLL for the whole assembly. Manufacturers of termination devices swages, cable clips, etc. rate the efficiency of their devices as a percentage of the breaking strength of the cable. For example, oval swage fittings are commonly rated with an efficiency of between 85% and 100%. The breaking strength (BS) for aircraft cable is commonly listed on spools. Table A shows the breaking strengths for many common sizes of galvanized aircraft cable. The formula for determining the working load limit of a cable system is (BS T) DF=WLL. The problem with this formula is that, in many instances, the user does not know one of its variables: the design factor (DF) used by the manufacturer. Fortunately, the theatre industry has adopted common design factors based on how the cable will be used. For most uses, a DF of five is adequate. However, in some situations where the cable is used to move an object swiftly, in a severe environment where corrosion or abrasion will be a problem, where shock loads are a concern, or where there is greater than normal risk of injury in the event of a failure increasing the DF to eight, ten, or even twelve is appropriate. Thin Pattern Thimble Heavy Duty Pattern Thimble Endless Thimble CABLE THIMBLES No matter how a cable is terminated, it is important that the end be protected from damage and wear. End protection is built into some termination methods, such a Positive-Grip end fitting. In methods where it is not inherent, cable thimbles (figure 2) are used to protect the cable. The most common type of cable thimble is called a pattern thimble, of which there are three varieties: thin pattern aircraft thimbles for light duty application (meets Mil. Spec. AN-100), standard pattern thimbles for light duty applications (galvanized thimbles must meet Fed. Spec. FF-T-276b Type II), and heavy duty pattern thimbles for medium to heavy duty applications (galvanized thimbles must meet Fed. Spec. FF-T-276b Type III). Open thimbles are similar in size and construction to standard pattern thimbles, but are U-shaped, having not been bent closed into the familiar teardrop shape. Pattern thimbles are available in galvanized steel or in Type 304 stainless steel. Stainless steel hardware is commonly used in saltwater or in corrosive environments. Galvanized thimbles are best for most theatrical applications. An endless thimble, available only in Type 316 stainless steel, has no end to catch on things. Table A. Stop Sleeve Strength Comparisons Cable Size CableBreaking Sleeve No. Nicopress Stop Sleeve Percent of (inch) Strength (pounds) Tool No. Tested Strength Breaking (pounds) Strength 3/ B4 51-B % 1/ C 51-C % 3/32 1, J 51-MJ % (Yellow) 1/8 2, J 51-MJ % (Red) 5/32 2, M 51-MJ 1, % 3/16 4, M 51-MJ 1, % (Black) 7/32 5, M 51-MJ 2, % 1/4 7, F6 3-F , % 5/16 9, F6 3-F , % Note: All stop sleeves are plain copper. Certain sizes are colored for identification. Source: FAA-AC No B, Section 8. Inspection and Repair of Control Cables and Turnbuckles. Table 7-7. Copper Stop Sleeve Data. Copyright 2001 United States Institute for Theatre Technology, Inc. TD &T F A L L

4 Figure 3. Crimping Order Figure 4. Oval Sleeve in Jaws of Swaging Tool TERMINATION DEVICES Many types of cable terminations are used in the theatre; two of the most common are swage-type fittings (Nicopress) and cable clips. Other termination devices are Fist Grips, Strandvises, and Gripples. These other devices are not as common in the theatre but should not be discounted as they all have their uses, advantages, and disadvantages. With any type of termination, it is very important to read, understand, and follow the manufacturer s instructions. Safety is the highest concern in an industry where people work underneath suspended objects. SWAGE FITTINGS Swage fittings, or sleeves as they are commonly called, are available in two different patterns: oval and stop. Oval sleeves are used for making an eye termination, while stop sleeves put an end on a cable, preventing the cable from being pulled through a hole in a piece of steel. In the theatre, oval sleeves are more commonly used. The two largest manufacturers of swage fittings in the United States are National Telephone Supply Company of Cleveland, Ohio, which manufactures the widely used Nicopress brand, and Loos & Company of Naples, Florida. When applied correctly, a single oval copper sleeve will achieve a 100% cable strength termination when the cable is looped around a thimble. This is outlined in both manufacturers testing data and catalogs and is also outlined in FAA documents and the Military Standard MS It should be noted that the Wire Rope Users Manual uses a conservative 95% efficiency rating for cables one-inch in diameter and smaller. Oval sleeves made of plain copper, tin-plated copper, galvanized copper, stainless steel, and aluminum are available. The tin and galvanized sleeves are intended for use on either galvanized or bright steel aircraft cable. A plated sleeve would be used in very humid areas, such as Florida, where there is a concern of electrolysis between the two dissimilar metals. Stainless steel sleeves are for use only with stainless cable. Aluminum sleeves, intended for use on either galvanized or bright steel aircraft cable, are a cause of some debate within the theatre community. National Telephone says that oval aluminum sleeves are rated for 85% of the rated cable strength while Loos & Company rates them for 100% of the cable strength. Loos & Company says that the difference between aluminum and copper sleeves is that the copper is military in origin and has its own specification while the aluminum is industrial and therefore does not have the same extensive testing and evaluation as the copper sleeve. Aluminum sleeves are cheaper than copper sleeves about half the price. The military only endorses the use of copper sleeves. The FAA says that sleeves other than copper are permissible only when pull-tested for the application in which they are being used. Among theatre consultants and theatre rigging manufacturers the use of aluminum sleeves is not acceptable for any type of permanent overhead suspension. Swaging sleeves are crimped onto a cable using a special tool. The number of crimps needed to secure the sleeve depends on the sleeve and the tool used to crimp it. Typically oval sleeves for 1/16 cable require only one crimp, whereas oval sleeves for 1/8 cable require three crimps. After the sleeve is crimped, it should be checked with a Go No-Go gauge to ensure that sufficient pressure was used to hold the termination. If a crimp is not sufficient to allow the crimp to slide into the appropriate slot on the Go No-Go gauge, the tool must be adjusted if the tool is adjustable; some are not and re-crimped. According to Barry Grothause, vice-president of sales with Loos & Company, crimps made with insufficient pressure to pass the Go No-Go gauge test do not need to be replaced. He says the sleeve can simply be re-crimped after the tool is properly adjusted. It is also possible to over-crimp a sleeve. Over-crimping is practically impossible with a hand swaging tool, but is quite possible with a hydraulic swaging tool that isn t set properly. Grothause estimates that overcrimping would reduce the breaking strength of the termination by ten to twenty percent. Sleeves, tools, and Go No-Go gauges come as systems from the manufacturer, and they are not interchangeable with each other. Do not use a Nicopress swage tool with a Loos & Company sleeve or the other way around. If a Nicopress tool is used, use Nicopress sleeves and check with a Nicopress Go No-Go gauge. When making a thimble eye splice with an oval sleeve it is important to first determine the proper swaging tool and sleeve, along with the correct number of crimps needed. This information can be obtained from the manufacturer s instructions supplied with the tool or the FAA document, Advisory Circular B, Section 8. Inspection and Repair of Control Cables and Turnbuckles. The swaging tool must be in good working order and in proper adjustment, or the crimps may not be properly made. 12 F A L L TD &T Copyright 2001 United States Institute for Theatre Technology, Inc.

5 The cable end should extend past the compression sleeve slightly, as the sleeve will elongate during the compression process. When the compressions have been completed, the cable end should still extend past the sleeve slightly. If the cable end is inside the compression sleeve when finished, the termination may not hold the full strength of the cable. A good rule of thumb is to have a length of cable sticking out of the finished sleeve an amount equal to the diameter of the cable that is used. The thimble needs to be firmly held in place within the loop of cable. When a sleeve requires three compressions, the FAA recommends making the center compression first, followed by the compression next to the thimble, and the third compression farthest from the thimble (figure 3). When doing the compressions in this order, it is easy to judge how much cable will be sticking past the sleeve when all of the compressions are done. To make a compression in a sleeve, have the sleeve wellcentered in the tool groove (figure 4). Have the major axis of the sleeve set into the tool jaws so that when the jaws are closed they are compressing the major axis of the sleeve. If the sleeve does not appear to be aligned with the tool after the start of the press, stop, realign the sleeve with the tool and then continue the press. Stop sleeves are available in plain copper only. A typical use for stop sleeves is to terminate a cable onto a winch drum. (There must be three full cable wraps on a winch drum at all times before this termination is considered a full strength termination.) Stop sleeve terminations, as opposed to the oval sleeves, vary greatly in their holding capacity when compared to the breaking strength of cable, the highest being 109% for 1/16 cable and the lowest being 38% for 3/16 cable. Add a safety factor of 8 to that 38% and the working load is only 4.75% of the cable s ultimate breaking strength. For this reason it s very important to know exactly what loads and loading situations exist when using stop sleeves, as all sizes are not rated the same. See table A for specific cable comparisons. CABLE CLIPS Cable clips (figure 5) consist of two parts: a U-bolt with nuts and a saddle. The saddle is somewhat controversial in the theatre because there are two types of saddles commonly used and not everyone understands why one is better than the other for certain applications. In one type of cable clip, the saddle is made of malleable cast iron. The casting process can leave hidden imperfections in the saddle, including pockets of material other than iron and voids or air pockets. These imperfections can cause the saddle to break under a heavy load or a shock load. For this reason, malleable cable clips are not to be used for critical applications, such as overhead lifting; this is written into many different codes and standards by OSHA, ANSI, ASME, and ASTM. Malleable cable Figure 5. Cable Clip Table B. Cable Clip Information. Clip Size Rope Size Minimum Amount of Torque in (inch) (inch) No. of Clips Rope Turn-Back Ft. Lbs. * in Inches 1/8 1/ / /16 3/ / /4 1/ /4 15 5/16 5/ /4 30 3/8 3/ /2 45 7/16 7/ /2 1/ /2 65 9/16 9/ /8 5/ Note: If a greater number of clips are used than shown in the table, the amount of turn back should be increased proportionately. * The tightening torque values shown are based upon the threads being clean, dry and free of lubrication. Source: The Crosby Group Inc., Crosby Clips, Warnings and Application Instructions. clips can be found in most hardware stores and are very cheap to buy. The other type of cable clip saddle is made of forged steel. In the forging process, the steel saddle is repeatedly heated red hot and stamped into forming dies. The manufacturing process actually determines what metallurgical properties the saddle will have. Besides having a stronger base material, the saddle in a forged cable clip is larger than a cast iron one, and has more surface area in contact with the wire rope it is holding. An eye termination (loop) using a thimble and forged cable clips will hold eighty percent of the breaking strength of the cable when properly applied and torqued. The number of cable clips required for a loop is determined by the diameter of the cable: for 7/16 cable and smaller use two clips; for 1/2 through 5/8 cable use three clips. Above 5/8, consult the manufacturer s usage guide. When creating a loop in the end of a piece of cable, it is important to have the U-bolt in contact with the dead end of the cable. This is the piece of the cable after the loop that does not go anywhere; this usually is the end of the cable closest to the loop. The saddle needs to be in contact with the live side of the cable. This is the piece of the cable that actually is doing the work. The U-bolt, when tightened, pinches or crushes the cable, which is why it is important that the U-bolt be over the dead end of the cable. If the U-bolt were applied to the live side, the efficiency of the connection would be lower, as there would be a pinch point in the cable where the U-bolt was applied. A catch-phrase helps technicians remember the proper way to orient cable clips, Never saddle a dead horse. To make an eye termination in a length of a cable, first Copyright 2001 United States Institute for Theatre Technology, Inc. TD &T F A L L

6 determine the amount of turn-back beyond the thimble and apply the first clip one saddle width away from the end of the cable (see table B). Remember to have the U-bolt over the dead end of the cable. Alternate tightening the nuts evenly until reaching the proper torque on both nuts. The second clip is applied as close to the thimble as possible. Put the nuts on the U-bolt so that the same amount of thread is sticking out of the nut and that the nuts are firm in tightness. If a third clip is necessary, put this one in the middle of the first two. Now tighten all of the nuts on the U-bolt(s) evenly, alternating between nuts until reaching the proper torque for all the nuts. The next step is very important. Apply a load equal to or greater than the expected load, then re-torque all of the nuts to the proper torque. The cable stretches when this initial load is applied, ever so slightly, and the diameter of the cable gets smaller. This creates more room between the cable, U-bolt, and saddle, and the nuts will not be as tight as they were before the initial loading. Cable clips need to be inspected periodically and the nuts re-torqued. Due to the stress imposed on cable clips during installation, they must be carefully inspected and meet the follow requirements before they are re-used: 1. The U-bolt must fit into the base without requiring a forceful change in the U-bolt spread. 2. The clip assembly must be properly installed and capable of being torqued to its full recommended value without stripping the threads or twisting off the U-bolt leg. 3. The saddle in the clip base must be undamaged. that they only be used for static loads, such as guy cables. Strandvises are also made for static loads and are rated by the manufacturer, Maclean Power Systems, to hold 90% of the rated strength of the guy cable. When asked about their use with aircraft cable, Holly Flannery of Maclean Power Systems said, Technically they cannot approve their use with aircraft cable, and that any use would be at your own risk. If these devices are used in theatrical applications, it is common practice to use a cable clip as a cable end stop to prevent a catastrophic accident in the event of a failure. A relatively new termination device is the VERLOCK (figure 9), a patented suspension device made especially for the entertainment industry and sold by VER Sales. Using the VERLOCK is simple just loosen the knurled nut, insert the cable to the desired position and retighten the nut. No tools are needed. The VERLOCK Sr. is designed for use with 1/8 aircraft cable and is rated with a safe working load of 250 lbs. Although we have not tested the VERLOCK, it is reported to hold the breaking strength of the cable. Indeed, if a design factor of eight is applied to the breaking strength of 1/8 GAC (2,000 lbs.), you get the rated safe working load of this device. VER Sales says, Correctly installed, the VERLOCK provides an ample percent of the nominal breaking strength of the cable to which it is applied. VER Sale also warns that VERLOKS are not recommended for prolonged outdoor use and recommends that they be tested before each use. Tapered body contains jaws which hold cable. Figure 6. Fist Grip Figure 7. Strandvise OTHER TERMINATIONS A Fist Grip (figure 6) is a type of forged cable clip with two saddles. Like the cable clip it also has an efficiency rating of 80%. Because the Fist Grip has two saddles it eliminates the problem of installing a cable clip backwards; a situation which reduces the efficiency of the termination. While the amount of turn back and torque values are different, the physical installation is basically the same as the cable clip. The cost of a Fist Grip is approximately three times that of a single cable clip. Some cable termination devices, such as Strandvises (figure 7) and Gripples (figure 8), are used almost exclusively in the power transmission and agriculture industries. Gripples are rated by the manufacturer to hold 85% of the breaking strength of the cable, but the manufacturer also recommends DESTRUCTIVE TESTS As noted earlier, the working load limit (WLL) of a cable in an application must take into account the method by which the cable is terminated. We performed some destructive tests at the material lab at East Tennessee State University to evaluate the efficiency of a particular termination. Professor Bill Hemphill of the Department of Technology supervised these tests. The two types of terminations we tested were oval swaging sleeves and forged cable clips, the two most common means used in the theatre industry for terminating cable. In all tests, 1/8 galvanized aircraft cable with a rated breaking strength of 2,000 lbs. was used. In our first series of tests, we used solid copper oval sleeves to terminate the cable in an eye, utilizing a cable 14 F A L L TD &T Copyright 2001 United States Institute for Theatre Technology, Inc.

7 thimble. A single oval swaging sleeve was used to terminate each end of our ten test samples. The cables were then pulled until destruction using a calibrated tensile testing machine. Three crimps were made on each sleeve, as required by the manufacturer, and the crimps passed the Go No-Go gauge check. In all cases, the cable broke at the point where it entered the sleeve. The results of the tests are shown in table C. In the ten tests, the highest load before breaking was 2205 lbs. (110% of the cable s rated breaking strength) and the lowest load was 2065 lbs. (103% of rated breaking strength). The mean load in these ten tests was 2144 lbs. (107% of rated breaking strength). We also did tests with oval sleeves in ways other than those recommended by the manufacturer. In terminations made using two oval swaging sleeves on the ends of the cable, we found no statistical difference in the breaking strength of the termination than in terminations made using a single oval swaging sleeve. These tests included instances where slack was intentionally placed in one of the cables between the two sleeves. Despite the fact that these terminations appeared flawed, their breaking strength was in line with all other tests using oval swaging sleeves. In no test using terminations made with oval swaging sleeves did a cable pull through the sleeve nor did a specimen fail at less than 100% of the rated breaking strength of the cable. In practice, every crimp made on swaging sleeves is not Figure 8. Gripple zinc alloy housing plastic end cap sintered steel locking wedge stainless steel spring Table C. Destructive Tests of 1/8 Galvanized Aircraft Cable with Solid Copper Oval Sleeve. Test Number Load at Failure lbs lbs lbs lbs lbs lbs lbs lbs lbs lbs. checked with the Go No-Go gauge. The generally accepted rule is that the first crimp of each work session is checked and afterward checks are made after every fifty crimps. We also tested terminations made using forged cable clips. We used two cable clips, the minimum recommended by the manufacturer for 1/8 cable, to make our terminations. The nuts on the cable clips were not torqued to a specific foot/ lbs. as required by the manufactured, but tightened by hand with a standard nut driver until it was not possible to tighten them further (consistent with how most technicians would make these terminations in the field). After the terminations were made, each specimen was placed on the testing machine and loaded with approximately 50 lbs. of tension. Once tension was on the cable, the nuts of the cable clips were re-tightened by hand using the nut driver. In all cases, it was possible to further tighten the nuts on the cable clips once the cable was under a load. The results of this series of tests are shown in table D. In the ten tests, the highest load before breaking was 2045 lbs. (102% of the cable s rated breaking strength) and the lowest load was 1670 lbs. (83% of rated breaking strength). The mean load in these ten tests was 1869 lbs. (93% of rated breaking strength). Table D. Destructive Tests of 1/8 Galvanized Aircraft Cable with Forged Cable Clips. Figure 9. VERLOCK Test Number Load at Failure lbs lbs lbs lbs lbs lbs lbs lbs lbs lbs. Copyright 2001 United States Institute for Theatre Technology, Inc. TD &T F A L L

8 We also conducted a test where the cable clips were applied incorrectly, where the U-bolt rested against the load-bearing end of the cable. In this test the cable broke at 1360 lbs. or at 63% of its rated breaking strength. The results of our tests were consistent with recommendations made by the manufacturers. Terminations made with swaging sleeves are rated by the manufacturer at 100% of the breaking load of the cable (our low number was 103%), and terminations made with cable clips are rated by the manufacturer at 80% of the breaking load of the cable (our low number was 83%). CONCLUSION The type of termination used on a cable must be based on many factors: type of installation (permanent or temporary); load rating and design factors applied; ability to check terminations after installation; size of cable; and result of an accident if the termination fails. Because of their holding ability, swaging sleeves are the best and cheapest means for terminating small cables (1/4 diameter or less). No matter what means of termination is used, it is important to follow the manufacturer s installation instructions. Doing so will not guarantee that the manufacturer will stand behind you in the event of an accident, but an accident is far less likely if you follow the manufacturer s instructions than if you do not. The best way to know that your terminations are good is to test them. Setting up a simple destructive test that will test to see that a termination will not slip is fairly easy. Many universities have destructive testing facilities and can help you determine the exact breaking strength of your terminations. Finally, if you are not confident of your ability to make a good termination using the means you have selected, find someone who is. Knowing the breaking strength of different termination methods and knowing that you can properly make these terminations is paramount to safe rigging. REFERENCES The Crosby Group Engineering Journal. Tulsa, OK: The Crosby Group, Inc Technical Data Sheet (Document # 450-1). Tulsa, OK: The Crosby Group, Inc. Federal Aviation Administration Advisory Circular B, Acceptable Methods, Techniques, and Practices Aircraft Inspection and Repair, Section 8. Inspection and Repair of Control Cables and Turnbuckles. (Published on the Internet at 300/pdf/3a-ch7_8.pdf.) Gripple Rope Clip Strandvise United States Institute for Theatre Technology Standard for Rigging and Stage Machinery: Wire Rope Terminations Swage Fittings. Syracuse, NY: USITT, Inc. United States Institute for Theatre Technology Standard for Rigging and Stage Machinery: Wire Rope Terminations Wire Rope Clips. Syracuse, NY: USITT, Inc. VERLOCK Wire Rope Technical Board Wire Rope Users Manual, 3rd edition. Washington, DC: Wire Rope Technical Board. Dan Culhane has worked as technical director for Great Lakes Theatre Festival, StageWest, The Children s Theatre Company, and The Guthrie Theater. Currently, he is the engineering manager for SECOA, which manufactures and installs theatre rigging and related equipment. Delbert L. Hall is professor of theatre at East Tennessee State University, and president of Hall Associates, Inc. Flying Effects. 16 F A L L TD &T Copyright 2001 United States Institute for Theatre Technology, Inc.