Maintenance Practice
Unequal Rope Tensions: The Cheapest Fix in Lifting
In brief Multi-rope suspension only delivers its designed safety factor and rope life if every rope carries a near-equal share of the load. Tensions drift with groove wear, stretch differences and spring settlement; the overloaded ropes then wear at multiples of the design rate while the slack ones surf their grooves. Checking and equalising tensions — at fitting, after bed-in, and at survey — is among the cheapest interventions in lift maintenance and one of the most life-extending.
Ask what keeps a lift safe and everyone points at the big items: the ropes, the brake, the safety gear. Almost nobody points at the distribution — how evenly the load is shared across the rope set. Yet unequal tensions quietly decide how long ropes last, how the lift rides, and how fast the sheave wears. Equalising them is an afternoon’s work with hand tools and a meter. Pound for pound, nothing else in roping maintenance pays back like it.
The design assumption nobody maintains
A suspension system’s arithmetic assumes sharing. Hang the load on multiple ropes and each carries its fraction — that is what the safety-factor calculation, the sheave pressure figures and the fatigue life predictions are all built on.
But the sharing is not automatic. Each rope is its own spring, hanging on its own termination, seated in its own groove. Equality is an initial condition, set at installation — and service spends it:
- Constructional stretch differs rope to rope. No two ropes bed in identically, so a set fitted equal drifts apart in early service (why new ropes need shortening).
- Groove depths diverge. A rope in a deeper-worn groove travels a different effective circumference and picks up creep against its neighbours — unequal grooves force unequal tensions no matter how often you adjust (grooves and rope life).
- Suspension springs settle. The compression springs at the hitch are the load-sharing elasticity; tired or mismatched springs turn the hitch plate into a rigid argument the stiffest rope wins.
- Terminations creep. Wedge sockets seat; threaded adjustments are disturbed by other work and not re-checked.
What inequality costs
Suppose one rope in a set runs meaningfully over its share — a completely ordinary state of affairs on an unequalised installation:
Rope life. Fatigue is brutally non-linear in load: the overloaded rope’s crown wires see higher bending-plus-tension stress every sheave passage and fail early. The set is then condemned by its most-loaded member — you bought five ropes’ capacity and consumed one rope hard. The premature-wear survey that follows usually finds the classic signature: broken wires and rouging concentrated on one or two members while the rest look almost new.
Groove life. The overloaded rope presses hardest into its groove and wears it deepest — manufacturing exactly the uneven-groove condition that makes tension inequality permanent. Unequal tensions and uneven grooves are not two problems; they are one self-reinforcing loop, and equalisation is how you interrupt it while it is still cheap.
Ride quality. Ropes at different tensions have different natural frequencies. Under acceleration and braking they respond out of phase — felt in the car as vibration, drumming or that indefinable “roughness” that generates complaint tickets no control adjustment ever fixes.
Traction. The sheave’s grip calculation assumes distributed pressure. Concentrated load changes local contact conditions and accelerates the undercut wear that erodes traction margins.
The fix: measure, adjust, verify
Equalisation is honest, unglamorous work at the hitch plate:
- Inspect the suspension first. Spring heights compared by eye, springs and plates checked for collapse and corrosion — tired springs are replaced, or the adjustment is building on sand.
- Measure each rope. Tension meter per rope (or frequency comparison), recorded — the “before” column.
- Adjust at the tie rods. Nuts worked in sequence across the set, converging every rope toward the mean — within a few percent is the working standard.
- Run and re-check. Tensions redistribute over a few trips; measure again, trim, record the “after” column. That document goes in the same file as your examination reports.
On a Re-Ropes installation this happens at fitting and at the post-stretch follow-up; on any installation it is part of every survey, because the tension spread is itself diagnostic — a set that cannot hold equality is reporting groove or spring problems, and says so in numbers.
The takeaway for owners
If your lift has ride complaints that maintenance visits never quite cure, or an examination report noting uneven rope wear, the tension spread is the first cheap question to ask. It is measurable in an hour, fixable in an afternoon, and the alternative — ignoring it — is buying rope sets on a subscription you never signed up for.
Book a survey and we’ll measure the set, equalise it, and tell you in writing whether the inequality was the disease or the symptom.
Frequently asked questions
How do unequal tensions show themselves?
Through the symptoms they cause: one or two ropes wearing visibly faster than the rest (broken wires and rouging concentrated on the busiest members), ride vibration or drumming as differently tensioned ropes resonate differently, uneven groove wear developing on the sheave, and suspension springs sitting at obviously different heights at the hitch plate — the quick visual check every engineer starts with.
How are rope tensions measured?
From simple to instrumented: comparing spring compression heights at the hitch, plucking ropes and comparing the note (a surprisingly serviceable field check — tension changes a rope’s natural frequency), and dedicated rope tension meters that measure each rope individually for a documented before-and-after. Adjustment is then made at the threaded tie rods at the terminations.
How equal do tensions need to be?
Working practice in the industry is to bring each rope within a few percent of the mean — commonly quoted as ±5%. Beyond tightening the numbers, what matters is addressing why they diverged: tensions that will not hold equal usually point at uneven groove depths or tired suspension springs, which is why equalisation belongs inside a survey rather than as a standalone ritual.
When should tensions be checked?
At fitting (always), again after the constructional-stretch bed-in period on a new set, whenever ride quality or wear patterns raise questions, and as part of every rope survey. Ropes stretching at slightly different rates in early service is normal — which is precisely why the follow-up equalisation visit is part of a properly run re-roping programme.