1. Rope Construction and Terminology
Understanding rope construction is essential for proper specification. Elevator ropes are described using a standardized notation system. A common designation such as "8 x 19 Seale" breaks down as:
8: Number of strands in the rope
19: Number of wires in each strand
Seale: Strand construction pattern (alternating large and small wire layers)
The strand construction pattern significantly impacts rope properties:
| Pattern | Wire Arrangement | Characteristics | Typical Use |
|---|---|---|---|
| Seale | 9 large outer + 9 small inner + 1 core | Good wear resistance, moderate flexibility | Standard passenger elevators |
| Warrington | Alternating large/small outer wires | High flexibility, good fatigue life | High-speed, high-cycle elevators |
| Filler | Small filler wires between layers | Excellent fatigue resistance | Heavy-duty freight, mine hoists |
Rope cores provide structural support and help retain lubricant. The two primary core types are:
Fiber Core (FC): Typically polypropylene or natural fiber. Offers excellent flexibility and shock absorption but lower crush resistance. Suitable for low to moderate speed elevators.
Independent Wire Rope Core (IWRC): A smaller wire rope serving as the core. Provides 7-10% higher breaking strength, better crush resistance, and higher temperature tolerance. IWRC is standard for high-speed and heavy-load applications.
2. Material and Grade Selection
Elevator ropes are manufactured from high-carbon steel wire, with tensile strength grades defined by EN 12385-5 and ISO 4344. Common grades include 1570 MPa, 1770 MPa, and 1960 MPa. Higher grade ropes offer greater strength for a given diameter but are less flexible and more susceptible to fatigue.
Selection guideline: For speeds below 2.5 m/s with moderate traffic, 1570 MPa Seale rope with IWRC is the standard choice. For high-speed (>3.0 m/s) or high-cycle (>500 trips/day) applications, 1770 MPa Warrington rope provides superior fatigue life. Grade 1960 should be reserved for special applications where diameter constraints limit rope size.
Galvanized ropes are available for environments with high humidity or chemical exposure. The zinc coating adds approximately 5% to rope diameter and reduces breaking strength by 3-5%, but extends service life in corrosive environments by 50% or more. Stainless steel ropes (typically AISI 316) are used in extreme environments such as marine applications or food processing facilities but cost 4-6 times more than standard carbon steel.
3. Rope Sizing and Safety Factor
EN 81-20 specifies that the minimum safety factor for suspension ropes must be at least 12 for traction elevators with three or more ropes, or 16 for elevators with only two ropes. The safety factor is defined as the ratio of the aggregate breaking strength of all ropes to the maximum static tension under rated load.
For a traction elevator with 4 ropes of 10 mm diameter, 8×19 Seale construction, grade 1570 MPa:
Minimum breaking strength per rope: approximately 52 kN
Aggregate breaking strength: 4 × 52 kN = 208 kN
Maximum static tension (car + 50% load + rope weight): approximately 15 kN for a typical 630 kg elevator
Safety factor: 208 / 15 = 13.9 (compliant with EN 81-20 requirement of ≥ 12)
Compensation ropes (used in high-rise elevators to offset the weight of the suspension ropes) are sized similarly but with a reduced safety factor of ≥ 8, since they do not carry passenger load directly.
4. Wear Patterns and Failure Modes
4.1 External Wear
External wear manifests as reduction in rope diameter due to abrasion against the traction sheave, deflection sheaves, and equalizing sheaves. Normal wear reduces diameter gradually at a rate of 0.01-0.03 mm per year for well-lubricated ropes in clean environments. Accelerated wear occurs when:
Traction sheave grooves are worn or incorrectly machined
Rope tension is uneven between ropes (causing unequal load sharing)
Lubricant is depleted or contaminated
Abrasive dust enters the machine room
EN 81-20 requires rope replacement when the diameter reduction of the worst rope exceeds 6% of nominal diameter for traction ropes, or 10% for governor ropes.
4.2 Internal Wear and Core Degradation
Internal wear is more dangerous than external wear because it is not visible during routine inspection. As ropes bend over sheaves, internal wires rub against each other and against the core. This produces wear debris (often called "rope dust") that accumulates inside the rope. Over time, the core degrades, reducing the rope's ability to distribute load evenly among the strands.
Internal wear is assessed by measuring rope diameter at multiple points and comparing the measurements. A "bottle-shaped" profile - where diameter is reduced in the middle of the rope's bending zones - indicates internal core degradation. Magnetic rope testing (MRT) equipment can detect internal wire breaks non-destructively and is increasingly required by insurance and regulatory bodies for high-rise installations.
4.3 Fatigue Breaks
Fatigue failure occurs when individual wires break due to cyclic bending stress. Wire breaks typically initiate at the contact points between wires within a strand or at strand crossover points. The number of visible broken wires is a primary replacement criterion:
| Location | Replacement Threshold (Broken Wires) |
|---|---|
| Within one rope lay length (approx. 6-8× rope diameter) | ≥ 6 broken wires for 8-strand rope |
| At rope termination (socket or wedge) | ≥ 3 broken wires |
| External wire wear (diameter reduction) | ≥ 6% of nominal diameter |
4.4 Corrosion
Corrosion reduces rope strength and accelerates fatigue. Surface rust is often superficial, but pitting corrosion creates stress concentrators that initiate fatigue cracks. Ropes showing deep pitting, significant rust staining, or reduced flexibility due to corrosion must be replaced regardless of broken wire count.
5. Inspection Protocol
EN 81-20 mandates visual rope inspections at intervals not exceeding 12 months. The inspection should cover:
Full length examination: Run the car through its full travel while visually inspecting all visible rope sections with adequate lighting.
Diameter measurement: Measure rope diameter at three points per rope using a vernier caliper or rope gauge. Record measurements and compare to previous readings.
Broken wire count: Examine each rope for broken wires, paying special attention to high-bend areas (traction sheave contact zone, deflection sheaves).
Lubricant condition: Assess whether ropes retain adequate lubricant. Dry ropes require re-lubrication; contaminated ropes (with dust, grit, or water) require cleaning and re-lubrication.
Termination inspection: Examine rope sockets, wedges, and thimbles for cracks, deformation, or loose fittings.
Tension equalization: Verify that rope tensions are balanced within 5% using a rope tension meter.
6. Replacement Best Practices
6.1 Complete Set Replacement
When any rope in a set reaches replacement criteria, all ropes in that set must be replaced simultaneously. New and old ropes have different elastic elongation characteristics and diameters, leading to unequal load sharing if mixed. Unequal loading accelerates wear on the new ropes and can overload the traction sheave grooves.
6.2 Rope Installation
New ropes must be installed with care to avoid kinking, twisting, or introducing internal damage. Key steps include:
Unreel ropes from the shipping reel without introducing twist (use a turntable if possible)
Thread ropes through sheaves without dragging across the floor
Apply manufacturer-recommended initial lubricant before installation
Install rope equalizing devices (if equipped) before tensioning
Tension ropes to specified preload and verify equal tension across the set
Run the elevator through 50-100 cycles without load to seat the ropes in sheave grooves
6.3 Post-Replacement Verification
After rope replacement, perform:
Traction verification (slip test) to confirm adequate friction between ropes and sheave
Overload test at 125% of rated load per EN 81-20 requirements
Re-inspection after 30 days of service to check for tension changes and early wear signs
