TR Footnotes FN.1010 : Page 1

TOWING&RECOVERY October 2010 Keeping industry pros on their tows TOW SAFETY Drivers In Danger Towing can be hazardous duty Page 12 TRUCK ART Look At That!! Good design gets attention Page 14 TIRE TECH Why Nitrogen? It can work better than air Page 15 TOW DOCTOR Hurtin’At Work When towing leads to pains & strains Page 19 Don’t let your wire rope become a deadly striking snake By Wayne Olson You do your mandatory walk-around truck inspection before you start your day. You take it seriously, looking for anything that could cause a problem: loose lugs, cracked rims, flat tires. Brakes that will and lights that won’t. Then you lean toward the winch, giving the spooled cable a quick check, and wonder if there are any surprises waiting for you in that coil of steel. How old is that thing? Has it been misused and abused on my days off? When was it lubricated last, by whom and with what? Did they use a good quality cable lube, or just old motor oil? What lurks hidden within the outer strands of that cable? Broken strands? Corroded and worn wires that are half the diameter of the outer ones? Was that birdcage I saw in the line the other day bad enough to worry about? Will the winch drum, a “revolver” that can spit steel if the line breaks, nail me today? You swallow hard. You cringe a little but dismiss this troubling thought and jump in your truck. Has the winch become a roulette wheel that you now gamble with? Cable accidents happen on a regular basis. Sometimes you dodge a bullet. Sometimes you do not: “Tow Truck Driver Killed In San Volume 21, Number 6 ❘ $3.95 © 2010 Dominion Enterprises. All Rights Reserved. Towing&Recovery Footnotes® 10 Bokum Rd. Essex, CT 06426 Bernadino. September 1, 2009. A 52-year-old tow truck driver died when a cable snapped, causing a vehicle to come loose and crush him to death, PRST STD U.S.POSTAGE PAID PEORIA, IL PERMIT NO. 315 authorities said. Randy Cole of San Bernardino was unloading a vehicle from a flatbed tow truck when the acci-dent occurred about 11:30 p.m. in the 100 block of East Central Avenue, according to San Bernardino police. Cole died at the scene, according to the San Bernardino County Cor-oner. The San Bernardino County Police Department is investigat-ing the tow truck death, along with the state’s division of Occu-pational Safety and Health.” Killer Cable This brief review of winch and boom design and some wire rope maintenance solutions may help reduce the concern that your wire rope may someday become a striking cobra. Tow truck wire rope, or cable, is not usually subject to the thorough inspections received by large crane-winch cables used in construction, marine, and mining equipment. It’s not cost-effective to do magnetic flux leakage inspection of the cable, or have the cable lubricated with a pres-surized collar to force the lubricant through the cable for complete pene-tration because the length is too short. It’s just cheaper for towers to replace the whole thing. While regular maintenance can cer-tainly extend the life of towing cable, it would provide more peace of mind to have a printed readout showing every inch of the cable to identify exactly where the breaks, corrosion, and wear are occurring and how bad they are, but that’s not an option for the average towing company owner. He must use his experience and settle for an occa-sional manual lube and replace the cable before its service life is over — very likely long before it’s over — but he sleeps better. But owners who just use old motor oil to lubricate their cable, even though it has broken down and lost most of its anti-friction properties, may be in for a surprise someday. So let’s use that unpleasant possibility to launch a look into the properties of wire rope and the safety and mainte-nance issues: D/d Ratio This is the ratio between the winch drum diameter and the diame-FOOTNOTES ® www.trfootnotes.com ter of the cable that we refer to as the “D/d ratio.” In short, the larger the drum diameter is in relation to the cable diameter, the happier the cable will be. The American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME) recommend a minimum 18:1 ratio for lifting applications (B30.7-2001: Base Mounted Drum Hoists). To find the ratio of your winch, add the diameter of drum barrel (A) plus the diameter of cable (E). Then divide it by the diameter of the cable (E). Tow truck winches typically have low D/d ratios, which create more bending stresses in the cable. I surveyed the models of two major U.S. winch manufacturers and found ratios of between 9:1 and 11.5:1, well See STOP THE COBRA! , page 3 www.trfootnotes.com FREE! GET US

Stop The Cobra!

Wayne Olson

Don’t let your wire rope become a deadly striking snake You do your mandatory walkaround truck inspection before you start your day. You take it seriously, looking for anything that could cause a problem: loose lugs, cracked rims, flat tires. Brakes that will and lights that won’t. Then you lean toward the winch, giving the spooled cable a quick check, and wonder if there are any surprises waiting for you in that coil of steel. How old is that thing? Has it been misused and abused on my days off? When was it lubricated last, by whom and with what? Did they use a good quality cable lube, or just old motor oil? What lurks hidden within the outer strands of that cable? Broken strands? Corroded and worn wires that are half the diameter of the outer ones? Was that birdcage I saw in the line the other day bad enough to worry about? Will the winch drum, a “revolver” that can spit steel if the line breaks, nail me today? You swallow hard. You cringe a little but dismiss this troubling thought and jump in your truck. Has the winch become a roulette wheel that you now gamble with? Cable accidents happen on a regular basis. Sometimes you dodge a bullet. Sometimes you do not: “Tow Truck Driver Killed In San Bernadino. September 1, 2009. A 52- year-old tow truck driver died when a cable snapped, causing a vehicle to come loose and crush him to death,Authorities said. Randy Cole of San Bernardino was unloading a vehicle from a flatbed tow truck when the accident occurred about 11:30 p.m. in the 100 block of East Central Avenue, according to San Bernardino police.Cole died at the scene, according to the San Bernardino County Cor-oner. The San Bernardino County Police Department is investigating the tow truck death, along with the state’s division of Occupational Safety and Health.” Killer Cable This brief review of winch and boom design and some wire rope maintenance solutions may help reduce the concern that your wire rope may someday become a striking cobra. Tow truck wire rope, or cable, is not usually subject to the thorough inspections received by large cranewinch cables used in construction, marine, and mining equipment. It’s not cost-effective to do magnetic flux leakage inspection of the cable, or have the cable lubricated with a pressurized collar to force the lubricant through the cable for complete penetration because the length is too short. It’s just cheaper For towers to replace the whole thing. While regular maintenance can certainly extend the life of towing cable, it would provide more peace of mind to have a printed readout showing every inch of the cable to identify exactly where the breaks, corrosion, and wear are occurring and how bad they are, but that’s not an option for the average towing company owner. He must use his experience and settle for an occasional manual lube and replace the cable before its service life is over — very likely long before it’s over — but he sleeps better. But owners who just use old motor oil to lubricate their cable, even though it has broken down and lost most of its anti-friction properties, may be in for a surprise someday. So let’s use that unpleasant possibility to launch a look into the properties of wire rope and the safety and maintenance issues: D/d Ratio This is the ratio between the winch drum diameter and the diameter of the cable that we refer to as the “D/d ratio.” In short, the larger the drum diameter is in relation to the cable diameter, the happier the cable will be. The American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME) recommend a minimum 18:1 ratio for lifting applications (B30.7- 2001: Base Mounted Drum Hoists). To find the ratio of your winch, add the diameter of drum barrel (A) plus the diameter of cable (E). Then divide it by the diameter of the cable (E). Tow truck winches typically have low D/d ratios, which create more bending stresses in the cable. I surveyed the models of two major U. S. winch manufacturers and found ratios of between 9:1 and 11.5:1, wellBelow the recommended ratio of 18:1.Tow truck winches can partially compensate for this acute bending of the wire rope by using 6x37-type wire rope, which is more flexible than the 6x19 type. It is difficult to understand why winch manufacturers do not build winches to satisfy this minimum recommendation when we are talking about winch drum diameters that would only be a few inches larger. For example, the drum diameter required to achieve an 18:1 ratio for a 5/8-inch wire rope would be an extra 4.625- inches (6.0” vs. 10.625”) The main point here is that towers should be aware of the higher bending stresses inflicted on wire rope from these low D/d-ratio winches. At the other end of the winch spectrum are marine winches that commonly have high D/d ratios of 40:1 or greater, providing long life to the wire cable due to low bending stresses. I attempted to contact two of the major winch manufacturers for a response to their decision to use low D/d-ratio winch designs (in case I was missing something that I had not considered) but did not receive a reply from either company. We are left to guess what their reasoning is. I am sure they would tell us that other lifting or pulling applications where their winches are utilized have much more room to play with than some tow trucks have. Fleet Angle This is the angle between the center line through the lead sheave and the center line of the rope leading to the drum. The angle should not be more Than 1.5 degrees from center line. If the angle is more than this, the cable will rub on the sheave flange and against its neighboring wrap on the winch drum contributing to wear. The width of the winch drum affects the placement of the sheave. The wider The drum, the further the sheave must usually be from the drum to stay within the recommended fleet angle. Forward/rear winch setups make it easier to stay within this angle too.Booms with side-by-side winches must have sheaves at a greater distance from the drums than forward/rear winch arrangements, due to the inherently larger fleet angle imposed by this setup. If winches are mounted side by side, the degree angle from center line to the sheave is much bigger since the cable is put so far out to the side of the center line with this type of winch arrangement.The closer to center line the cable can be kept the better. Lay of the wire rope: The mounting position of the cable’s bitter end depends on the lay of the rope, which is the way the cable strands fall as you look down the length of wire rope. If it falls to the left, it is said to be a left lay.If it falls off to the right, it is said to be a right lay. Most tow truck winches use the anchor point on the left side of the drum so right lay wire rope should always be used. “Candy wrapper” rope: Breaks in the inner wires can be detected by taking a section of the cable in your hands and bending it. Run the bend back and forth through the length and listen for noises coming from the rope. Do this at several points along the rope. If you hear a crackling or popping sound, this indicates that you have broken wires inside the rope and it should be replaced. Service and maintenance: Reverse the wire rope end for end. The half of the cable that usually remains on the winch drum is often in very good condition and may extend the life of the cable. You will have to have the hook swaged on the new end but it’s cheaper than replacing the whole cable.Think of it as flipping your mattress over after sleeping on the same side for a few years. Unnecessary Bends The enemy of running wire on cranes is bending fatigue. The fewer sheave wheels that the cable is required to pass over during its life, the better. Also, one large sheave is better than two small sheaves. Each turn that it must bend past shortens the service life of the cable, whether it’s a boom sheave or a snatch block. Most tow trucks use 6x19 or 6x37- type wire rope, but by using a wire with higher fatigue resistance such as eight strand Wire, it will have less friction than a six-strand because the friction force is shared between eight strands rather than six. No Reverse Bends A reverse bend will cause spooling problems and damage the wire rope. When installing a cable over a winch drum, sheaves, or snatch blocks, make sure that the drum is spooled from the top and that the reel feeds from the top. The cable has a memory since it was wound on the spool at the factory so work with it. This will help prevent loosening of the strands. Lubricate Regularly Inner wires wear much faster than outer ones for a number of reasons: friction from neighboring wires, loss of lubricant, and retention of water causing rampant corrosion. Lubrication is reduced as you go deeper into theCable because of the problem of proper penetration. The outside of the cable may find lubricant on the sheaves as it passes over, but inside the cable it is often dry as a bone. Lubricant that may have once been there can also be squeezed out by cable tension in its day-to-day operation.This internal friction works away unseen and from every angle. Internal gaps in the wire that once held lubricant now have empty spaces that can hold water, accelerating the hidden corrosion within. It is not unusual to find the outer strands in perfect condition, only to find many of the inner wires broken, corroded, and worn badly from friction. Be aware that some cable manufacturers do not lubricate their cable from the factory, so your cable may have never received a thorough initial lubrication.Use a high-viscosity (oil), highqua lity cable lube which penetrates the cable better than grease, and let it dry before use. Low-viscosity lube (grease) has better lubricating qualities, but needs to be applied under pressure to penetrate the cable completely. And the sticky quality of grease does not endear it to the person who has to grab the cable daily. Some tow truck manufacturers lubricate their wire rope prior to delivery, but some leave it up to the new owner to do it. Some manufacturers still use an asphalt-based lubricant on their wire rope, but it is obsolete and is inferior to modern lubricants. More Rope Dope Optimize rope diameter: This refers to the maximum service life of wire rope based on the number of bending cycles achieved depending on its diameter. Of course sheave size has to be taken into account and respected if there is to be any benefit to changing the rope diameter, but it is a way to maximize this component of the system. There is a well-established formula used by wire rope manufacturers in the industry for predicting the wire rope “discard” and “break” points. Contact the wire manufacturer to obtain the projected bending cycles of their wire rope. It will provide a curve for the bending cycles “until discard,” and a separate curve for bending cycles “until failure.” They can give you a good estimate of the service life that you can expect from your cable. They can factor in the D/d ratio too if you provide them with the winch drum diameter. The winch specs are readily available on most winch manufacturer websites. Optimize sheave diameter: The larger the sheave the less bending stress the wire rope will see during its Service life. Fewer bending cycles means longer service life. This may not be practical to do on tow trucks but for illustration: If you increase the sheave diameter by 25 percent, you will increase the number of cycles until the discard point by 125 percent. Loose strands and birdcages: These are usually caused by twisting the wire rope around its own axis, its own length. Spooling wire rope on the drum causes a slight rotating tendency of the rope due to the spiral lay of the strands. This tendency can be used to an advantage in keeping the winding tight by working with the lay of the rope, which is usually a right lay, as mentioned above. If the fleet angle that we talked about is more than 1.5 degrees, the wire will rub on the sheave flange then roll down to the bottom of the groove. Lubrication or replacing the sheave with a wider groove will help.Increasing the groove angle from 35 to 60 degrees, for example, will reduce rope twist by 50 percent. Tight sheaves can also cause loose strands and birdcages because the rope is being compressed when rolling over the sheave. Ideally, the groove radius should be 0.53 to 0.54 times the rope diameter. For example: a 3/4-inch (0. 75”) diameter wire rope x 0.53 equals a groove radius of 0.3975. This provides a groove diameter 3.6 percent Larger than the rope diameter. The sheave size is a factor that is often forgotten or ignored when a cable is up sized, resulting in premature wear and reduced service life. Looking Ahead Winches are designed for specific jobs and each design has inherent strengths and weaknesses. The same is true with cable-strand types. By being aware of the design limitations, you can deal with them in a safe and knowledgeable way. Living within their limitations is not a problem when you approach the issues with a plan that can compensate for some of their shortcomings. Perhaps in the future, tow truck manufacturers will enlarge their sheave wheel diameters and winch manufacturers will find a way to incorporate larger winch drum diameters, extending the service life of wire rope. Until then, low D/d ratios and the resultant high bending stresses on wire rope are factors that tow truck operators must anticipate and account for in the design and maintenance of their trucks. Good lube and good wire rope are not cheap but they will give you longer life and peace of mind. Especially if it is kept lubricated right from the beginning and faithfully re-lubricated.And you will probably sleep better too.

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