Free Elevator Wire Rope Stretch Calculator: Elastic Elongation, Landing Accuracy, and ASME A17.1 Safety Factor for Traction Hoist Cables

Elastic stretch is recoverable elongation that occurs whenever the rope is under load and disappears when the load is removed. It follows Hooke’s Law and is proportional to the applied force. Constructional stretch is permanent elongation that occurs only once, during the first weeks of service, as the individual wires and strands settle into their final seating positions within the rope structure. Constructional stretch does not recur after the rope is fully bedded in. For elevator hoist ropes, constructional stretch typically amounts to 0.1 percent of rope length for IWRC constructions and 0.2 percent for fiber core constructions, per Wire Rope Technical Board data.

ASME A17.1-2022 Section 2.29 requires that elevators equipped with automatic leveling devices land and maintain the car within plus or minus one-half inch (12.7 mm) of the floor landing level under any load condition from empty car to fully loaded car. This tolerance applies at every landing the elevator serves. The plus-or-minus 0.5 inch specification is the standard US commercial elevator requirement. Some modern hospital and freight elevator specifications call for tighter tolerances such as plus-or-minus 3/8 inch for smooth patient transport or forklift access, but the code minimum is 0.5 inch.

ASME A17.1 Section 2.20.4 specifies a minimum design factor (safety factor) of 7.60 to 1 for elevator hoist ropes. This means the catalog minimum breaking strength of the rope must be at least 7.60 times the maximum working tension in any single rope. The safety factor is calculated as: SF = breaking strength (lbs) divided by maximum load per rope (lbs). A safety factor of 7.60 is the absolute minimum. In practice, most well-designed elevator installations have safety factors of 10 to 1 or higher at rated load, providing a substantial margin over the minimum code requirement.

The modulus of elasticity for a wire rope is an “apparent” or “effective” modulus that accounts not just for the steel wire itself (approximately 29 to 30 Mpsi for solid steel) but for the helical geometry of the strands and the compliance of the core. An Independent Wire Rope Core (IWRC) is itself a small wire rope, which is much stiffer radially than a fiber core made of sisal or synthetic fiber. When a load is applied axially, the helical strands try to straighten and also try to compress the core radially. A fiber core compresses much more easily than a wire rope core, so more of the axial load goes into strand geometry change rather than wire tension, resulting in more elongation per pound of load and a lower effective modulus. This is why 6×19 IWRC rope stretches less per unit length than 6×19 FC rope under the same load, even though both use the same steel wire grade.

In a 2:1 roped elevator, the car has a sheave mounted on its frame and the ropes pass through this sheave, doubling the mechanical advantage of the drive machine. Each rope carries only half the load that an equivalent 1:1 system would place on the rope. However, each rope is also twice as long (two segments instead of one). The net result for elastic car drop is that the car drops only half as far as the rope segment elongates, because the 2:1 mechanical advantage also divides the car movement by two. A 2:1 roped car drops approximately half as far as a 1:1 roped car of the same weight, rope diameter, and length, for the same load. This is why underslung 2:1 roping is preferred for low-rise applications where rope length is short but landing accuracy is still important.

ASME A17.1 Section 2.20 does not mandate a single rope construction but specifies performance requirements that in practice favor certain constructions. The most widely used construction for US commercial elevator hoist ropes is 6×19 Seale IWRC (Independent Wire Rope Core), or similar Seale-class constructions within the 6×19 classification. The Seale strand pattern places the largest wires on the outside of each strand, providing good abrasion resistance against the drive sheave grooves while maintaining adequate fatigue life from repeated bending over the sheave. For high-rise, high-speed elevators where fatigue cycles are very high, 6x25B Warrington-Seale IWRC or 6×36 class rope is sometimes specified because the higher wire count provides better fatigue resistance at the cost of slightly lower abrasion resistance.

ASME A17.1 Section 2.20.3 requires a minimum of three hoist ropes for traction elevators, regardless of the capacity of the installation. The purpose of multiple ropes is redundancy, not primarily stretch reduction: if one rope breaks, the remaining ropes must still support the car safely. Adding more ropes does reduce the load per rope and therefore reduces elastic stretch in proportion. Four ropes instead of three reduces per-rope load by 25 percent and reduces elastic stretch by 25 percent. However, adding ropes also requires larger rope sheaves, more equalization hardware, and greater machine room space. In practice, rope count is selected to achieve the required safety factor at the minimum number of ropes, not primarily to minimize stretch.

Yes, indirectly. Wire rope stretch itself is not listed as a specific inspection failure criterion in ASME A17.2 (the Inspector Guide for Elevators). However, the consequences of excessive stretch are testable: floor leveling accuracy is checked during the annual inspection, and a car that consistently lands more than half an inch above or below the floor level will be cited as a code violation under ASME A17.1 Section 2.29. If leveling failure is traced to rope stretch rather than a leveling system fault, the remedy may include rope replacement, rope shortening, or adjusting the leveling zone sensors. Additionally, rope elongation due to wear or internal corrosion (as opposed to elastic elongation) can be a criterion for rope retirement per ASME A17.2 Section 8.6.

The metallic cross-sectional area of a wire rope is the total cross-sectional area of all the steel wires in the rope, excluding the voids between wires and the core material. It is not the full circular cross-section of the rope (which is pi times radius squared) because wire rope is not a solid rod. The metallic area is typically expressed as a fraction of the nominal cross-sectional area: A_m = K times d squared, where K is a construction-dependent factor that accounts for the strand and core geometry. For 6×19 IWRC rope, K is approximately 0.40, meaning about 40 percent of the nominal circular cross-section is actual steel wire. The metallic area matters directly for stretch because more metal means more stiffness (higher area times modulus product) and less elongation per pound of load.

Elastic stretch is directly proportional to rope length. Double the rope length and you double the stretch for the same load and rope diameter. A two-story low-rise elevator with 20 feet of rope might show 0.02 inches of elastic car drop at full load, essentially imperceptible. A 30-story office building elevator with 300 feet of rope and the same rope diameter and loading might show 0.30 inches of elastic drop. For very tall buildings, high-speed gearless elevators with rope lengths of 800 to 1,000 feet can experience elastic car drops of 0.5 to 1.0 inches at full load even with large diameter, high-modulus rope. This is why high-rise elevator control systems universally incorporate automatic releveling, which senses the car position and applies tiny motor corrections to maintain the car floor at landing level under varying loads.

ASME A17.2 provides the rope retirement criteria for US elevator hoist ropes. Key discard criteria include: 6 or more broken wires in one rope lay length in any strand; 3 or more broken wires in one strand in one lay length; evidence of heat damage, kinking, crushing, or birdcaging; diameter reduction of more than 3/16 inch under nominal for standard elevator rope; visible corrosion causing pitting or roughness of outer wires. Life expectancy varies enormously with speed, cycle count, sheave size, and maintenance quality. A well-maintained low-rise elevator in a small commercial building may run the same rope set for 15 to 20 years. A high-traffic office building elevator running 300 or more trips per day may require rope replacement every 3 to 5 years. Rope stretch alone is not a retirement criterion; the physical condition of the wires and strands is what drives replacement per ASME A17.2 inspection standards.

Rope lubrication does not measurably affect elastic stretch, which is determined by the steel modulus and metallic area. However, lubrication significantly affects rope life, which has an indirect relationship to long-term stretch behavior. A well-lubricated rope resists internal corrosion between the wires and strands. Internal corrosion causes pitting and reduced wire cross-section, which reduces the effective metallic area and thus increases elastic stretch over time for the same applied load. An unlubricated rope may develop enough internal corrosion within 3 to 5 years that its effective modulus and area have decreased, producing measurably more stretch than the calculator predicts for a new rope. ASME A17.1 Section 2.20.9 requires that ropes be properly lubricated at installation with the type of lubricant specified by the rope manufacturer.

The traction sheave is the grooved drive wheel in the elevator machine that grips the wire ropes through friction (traction) and moves the car and counterweight. Sheave diameter affects rope life through the D/d ratio (drive sheave diameter divided by rope diameter). ASME A17.1 Section 2.20.5 specifies minimum D/d ratios by rope construction to limit the cyclic bending stress that leads to wire fatigue. The minimum D/d ratio for most elevator constructions is 40:1 for wire rope, meaning a 1/2-inch rope requires at least a 20-inch sheave. Sheave diameter does not directly affect elastic stretch because the modulus of elasticity is a material property not influenced by bending radius. However, undersized sheaves cause accelerated wire fatigue and broken wires, which reduce the effective metallic area and therefore increase effective stretch over the rope’s service life.

There are several common field situations where calculating expected rope stretch helps an elevator mechanic diagnose problems or plan maintenance. First, when tenants report consistent floor misleveling, calculating expected elastic stretch confirms whether the observed drop is within or beyond what the rope geometry can produce, helping distinguish a rope condition problem from a leveling system fault. Second, when specifying a rope replacement for an installation being converted to a new rated capacity, the mechanic can verify that the new rope diameter and construction will maintain adequate safety factor and acceptable landing accuracy. Third, during new installation commissioning, verifying that the theoretical elastic stretch is within acceptable range confirms that the rope selection was appropriate before the system is put into service. This calculator provides the tool for each of these situations without requiring manual look-up tables or spreadsheets.

The International Union of Elevator Constructors (IUEC) administers the National Elevator Industry Educational Program (NEIEP), which is the standard apprenticeship pathway for US elevator mechanics. The apprenticeship lasts five years and combines approximately 144 hours of classroom instruction per year with on-the-job training under a licensed journeyman mechanic. The classroom curriculum covers ASME A17.1 code requirements including wire rope specifications, counterweight calculations, buffer design, hydraulic systems, and escalator mechanics. The program is jointly administered by elevator contractors and the IUEC, with apprentices receiving wages that increase progressively from approximately 50 percent of journeyman scale in year one to full journeyman scale upon completion. According to the BLS 2024 Occupational Outlook Handbook, the median annual wage for US elevator mechanics is $106,580, reflecting the specialized technical skills and code knowledge required by the trade.

In high-rise elevator installations with more than approximately 100 feet of rise, the weight of the hoist ropes themselves becomes significant relative to the car and counterweight. As the car descends, more rope hangs on the car side and less on the counterweight side, creating an imbalance even at the balance point of the counterweight. To compensate, high-rise elevators use compensating ropes or chains that hang from the bottom of the car and counterweight and loop around a compensating sheave at the pit. Compensating ropes add additional tension to the system and therefore slightly increase elastic stretch in the hoist ropes. This calculator does not account for compensating rope tension, which is a reasonable simplification for installations up to approximately 200 feet of rise. For very tall buildings above 200 feet where compensating ropes are required by ASME A17.1 Section 2.24, the hoist rope tension, and therefore elastic stretch, will be slightly higher than this calculator predicts.