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Traction for Field Personnel, Part One

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Traction for Field Personnel, Part One Continuing Education Traction for Field Personnel, Part One Principles of traction and how they apply to the safety of electric elevators by John W. Koshak Unintended loss of traction, resulting traction from occurring due to unnoticed from component deterioration, for example, wear and inadequate maintenance, is very hazardous and can allow an elevator inspection and testing. How do the to move uncontrolled. Four factors suspension members not slip on a traction determine and control traction: sheave? What traction force prevents the 1) Te traction ratio of car and ropes from slipping on the sheave from the counterweight empty car load to a fully loaded car? 2) Te area of contact of hoist ropes on the Maintaining traction falls under elevator drive sheave maintenance. 3) Te coefcient of friction/friction factor Tis course is divided into two parts: Part Value: 1 between the hoist ropes and drive sheave One will be the principles of traction at a contact 4) Acceleration/deceleration level a mechanic should understand; Part Tis article provides information on Two will be the components used in elevator hour traction requirements to prevent loss of systems that provide safe traction. Te (0.1 CEU) principles of traction will be applied, and indicators of traction problems, how to test This article has been Learning Objectives traction and the code requirements to assure peer reviewed by traction degradation does not occur will be Martin Rhiner and This Continuing Education article will explained. George W. Gibson. It provide education needed for elevator All major components in elevators, is approved for personnel who work on electric elevators escalators, moving walks and dumbwaiters Continuing to understand the importance of have design requirements in the code. Te Education by component condition, code code in the U.S. and Canada is ASME A17.1/ NAEC for CET® and requirements, measurement and testing CSA B44. Field technicians and mechanics CAT®. of traction to ensure the highest level of have no design experience and are not safety. In this article, the reader will learn: EW Continuing responsible for design, but knowing the ♦ Maintenance on suspension members Education is design basics can help one understand what is required by code. currently approved is happening. ♦ Maintenance must include in the following measurement of rope and sheaves at Design Requirements states: AL, AR, FL, adequate frequencies to ensure A17.1/B44 code requirements call for GA, IL, IN, KY, MD, compliance. traction to be provided, maintained, tested, MO, MS, MT, OK, PA, ♦ Te types of friction and how they and repaired if components show signs of VA, VT, WV and WI. apply to elevators noncompliance. To understand traction Please check for ♦ Which sheave grooves are used in the applications in an elevator system, it is specifc course veri- elevator industry helpful to relate it frst to something we fcation of approval ♦ How a capstan winch can help with experience, and identify the principles in at www.elevator- visualizing traction force play. books.com. Continued August 2017 • ELEVATOR WORLD 77 In the simplest case, when you park your vehicle on a hill and ♦ Skin friction is a component of drag, the force resisting the set the emergency brake, there must be some force preventing it motion of a fuid across the surface of a body. from sliding down the hill. Te emergency brake is not the force ♦ Internal friction is the force resisting motion between molecular providing the road/rubber frictional interface, though friction elements making up a solid material while it undergoes prevents rotation of the wheels with a brake pad/brake surface deformation. interface. Te friction of the rubber tire mounted to the wheels is Elevators utilize the principles of dry friction when considering also preventing the tire from spinning on the wheel, but the the frictional relationship between the suspension members (ropes rubber/road interface is where traction is required to prevent the or belts) against a traction sheave turned by an electric motor. vehicle from slipping down the hill on the road surface. Unbalanced masses suspended on each end of the suspension Force inherent in the rubber tire and the road surface holds the members (the car and counterweight) create a variable normal vehicle from slipping down the hill. If you drop an ink pen on the force (the support force exerted upon an object in contact with same steep hill you park your vehicle, there is inadequate traction another stable object) measured in Nm2 (Newtons per square to stop and hold the pen, yet the vehicle stays parked like a rock. meter), kPa (kilopascals) or psi (lb./in.2) over the traction sheave. 1 Why does one stop and not the other? Friction. psi = 6,895 Pa or 6.9 kPa. Friction is a component of the science of tribology. Friction is Like where the rubber meets the road in the example of a the force resisting the relative motion between two solid surfaces, parked vehicle, this article will be about where the suspension fuid layers and material elements sliding against each other. Tere members contact the sheave and the resultant force that prevents are several types of friction: the heavier counterweight from dragging the empty car up into the ♦ Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction (“stiction”) between nonmoving surfaces and kinetic friction between moving surfaces. ♦ Fluid friction describes the friction between layers of a viscous fuid moving relative to each other. ♦ Lubricated friction describes fuid friction where a lubricant fuid separates two solid surfaces. Identify a Cylinder Head with Figure 1: All surfaces have a roughness; this depicts the surfaces interlocking. the Interactive Head ID Tool! Texacone On-Site Attributes Manufacturers Set Number Go Online or CYLINDER TYPE Roped Download the App Single Stage Telescopic BOLTS/FASTENING Ball Bearing Bolts 1 Row Select Attributes Bolts 2 Rows SEARCH NOW Find Cylinder Head www.texacone.com/IECHI Figure 2: Normal force is the force perpendicular to the Texacone Interactive Elevator Cylinder Head ID - Patent Pending surface of contact, irrespective of gravity. 78 www.elevatorworld.com • August 2017 overhead or fully loaded car from dragging the lighter As asperities are invisible to the naked eye, scientists continued counterweight into the overhead (slipping traction). to test diferent materials and their qualities, and his further Friction is a dimensionless value created by physical properties experimentation led Amonton to his Second Law of Friction: the when two surfaces touch and forces are applied to the two surfaces force of friction is independent of the apparent area of contact. We in diferent directions. It is a variable force that changes based on know today that the coefcient of friction and the available friction the materials, loads, lubrication, etc. Because there are multiple sometimes change. However, centuries ago, empirical testing items, together, they have variable properties and a coefcient of showed friction could vary with varying areas of contact, friction.[1] depending on the materials tested and variable forces being Dry friction arises from a combination of materials, inter- applied. surface adhesion, surface roughness, surface deformation and Two types of dry friction are static and sliding (kinetic). Tis surface contamination. Te complexities of these interactions also confounded early scientists in understanding the friction make the calculation of friction impractical and necessitated the relationship. Given the same materials and applied forces, static use of empirical (real-world) testing for analysis and the friction was always higher than sliding friction. For example, if a development of theory. Leonardo da Vinci Continued wrote the earliest recorded rules of sliding friction afer such empirical testing. Further testing was documented by Guillaume Amontons and resulted in Amontons’ First Law: the force of friction is directly Schumacher Elevator Company engineers proportional to the applied load. and manufactures standard and custom No matter how smooth the surface feels elevators for customers worldwide. or appears, there will always be some quantity of jaggedness — high and low spots at a molecular level — called asperities. As shown in Figure 1, both the surfaces have high and low areas. A classic example at a macroscopic scale is sandpaper: the higher the grit value, the smaller and closer the asperities. For sandpaper, the characteristics of asperities Hydraulic & Traction are a function of the materials used to In–Ground, Holeless, MRL, fashion it. Geared, Gearless Te presence of contaminants and lubricants afects how the friction Specialty Elevators relationship of the surfaces will behave. Personnel/Manlift, LULA, Tink of a rubber tire on the road again: on Residential a dry road surface, there is higher friction, but if oil is present, there is lower friction, because the voids in the asperities fll up, and the oils can create rolling balls of liquid, eliminating the locked nature of the asperities. Continuing with Figure 1, if only the high points of the two materials are touching, what may look like the entire surface touching is, in fact, not substantially touching. When the materials are pressed closer together with a force, there is more Group 1 Packages locking interaction of the materials because Partial Packages, Bucher Valve, of the high points of material deformation Components, Power Units, Controllers or breaking of, creating more interlocking Elevate of asperities. Bolted connections utilize this your experience. property; the higher the bolt torque, the 319–984–5676 · Denver, IA higher the dry friction preventing slipping www.SchumacherElevator.com of the bolted connection. [email protected] August 2017 • ELEVATOR WORLD 79 car is at rest, then accelerating, the car will move when the drivetrain turns the tires, but, if you gun the motor and spin the Ff ≤ 1 * 98N (Equation 2) tires, the car does not move forward as quickly.
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