Severe Load Fluctuation

[MikePhua]

The Problem Scene
A mobile crane operator notices the lift weight readout and felt response jump and fall erratically while changing boom angle or radius during a lift. The machine shows no persistent fault codes, but the load moment indicator and the hoist feel “alive” — moving unpredictably and making precise placement dangerous. This is not mere load sway; this is fluctuation of the measured/available load that can indicate anything from sensor noise to real hydraulic instability.
Why It Matters
Erratic load readings and fluctuating lift force are safety-critical. A mobile crane’s stability margin is computed from boom geometry, counterweight, and measured load; if that input is noisy or wrong, automatic limiters could cut power at the wrong time or fail to prevent an overload. Beyond safety, unexplained fluctuations cause downtime, wasted lifts, and can accelerate wear on hydraulic components and wire rope systems. Modern LMI systems and load cells are supposed to prevent these failures — when they don’t, the problem must be traced methodically.
Common Root Causes

• Load sensing / load cell faults — damaged, mis-calibrated, or electrically noisy load cells produce jittery weight readings. Calibration drift and wiring issues are frequent contributors.
  
• LMI (Load Moment Indicator) or electronics faults — sensors (boom angle, radius encoder), poor grounds, or software glitches can make the controller calculate wildly varying moments even when the physical load is steady.
  
• Hydraulic instability — pump surges, slipping control valves, relief valve chatter, or internal cylinder leakage cause actual force/pressure variation at the hook and boom actuators. Thermal effects and transient cavitation can magnify the effect.
  
• Structural dynamics and pendulum interaction — as radius or boom angle changes, dynamic coupling (pendulum motion, boom elastic deflection) can shift load inertia and instantaneously change measured load moment. This is especially pronounced on long booms or when lifting near resonance frequencies.
  
• Rigging and load geometry changes — asymmetrical slings, shifting inside the load, or snagging can change effective weight distribution mid-lift producing sudden readout changes.
  

How to Diagnose (practical ordered workflow)

• 1 — Stop and secure: Park the crane in its current safe posture and block the load if possible. Treat the lift as compromised until validated.
  
• 2 — Record conditions: Note boom length, boom angle, radius, load chart rating, operator inputs, ambient temperature, and when fluctuation occurs (during lift, when slowing, at certain radii). This creates pattern data.
  
• 3 — Check LMI and sensor health: Inspect boom angle transducer, radius encoder, and any inclinometer for mounting play, water ingress, loose connectors, corroded pins, and correct wiring shields. Replace or reseal connectors as needed. If available, view raw sensor values while moving the boom slowly to spot spikes.
  
• 4 — Verify load cell and strain sensor: Visually inspect load cell(s) and junction boxes; measure excitation and output with a meter. Swap in a known good sensor or load (if practical) to isolate. Look for inconsistent zero offset or noisy signal.
  
• 5 — Measure hydraulic pressure/flow: Fit test gauges to the hoist and boom circuits and operate the functions through the range of motion. Look for pressure oscillations, relief valve cycling, cavitation (rapid pressure drops), or pump surge under varying engine RPM. Note temperatures and compare to normal range.
  
• 6 — Observe rigging and load: Inspect slings, hooks, and the load for free movement, snagging, or internal shifting that could change load geometry mid-lift.
  
• 7 — Run a controlled test: With a safe test load and spotters, repeat the operation at very slow speed while logging LMI outputs, sensor channels, and hydraulic pressures to correlate events. Patterned data is the fastest route to root cause.
  

Representative Measurements and Thresholds to Watch

• Load cell electrical noise: RMS signal noise > a few millivolts (or manufacturer spec) indicates problem.
  
• Hydraulic pressure ripple: sustained pressure oscillations greater than 10–15% of nominal under constant demand suggest valve or pump chatter.
  
• Boom angle encoder jitter: angle jumps >0.5° during smooth movement imply sensor or mounting problems.
  
• Temperature swings: hydraulic fluid rising rapidly under the same load may point to cavitation or inefficient cooling that changes fluid compressibility and response.
  

(Exact numerical thresholds vary with crane model and manufacturer; always compare to OEM tolerances.)
Fixes and Remedies (by cause)

• Sensor/electronics: clean or replace connectors, re-seal junction boxes, re-calibrate LMI per OEM procedure, and update firmware if a known bug exists. Where possible add transient suppressors or ferrite beads to reduce EMI on sensor lines.
  
• Load cell: recalibrate or replace; ensure proper mechanical seating and clean load paths. Protect cells from overloads and moisture with appropriate housings.
  
• Hydraulics: rebuild or replace worn pumps, repair leaking spool valves, replace tired relief valves, clean/replace suction strainers, and restore correct fluid level and filtration. Install dampening accumulators in specific circuits to smooth transients if repeated pressure spikes are seen.
  
• Rigging: correct sling arrangement, use spreader bars, ensure taglines prevent rotation and internal shifting.
  
• Operational: train operators in smooth radius/boom changes, avoid sudden command inputs near rated capacity, and where available enable anti-sway or damping features and tune their gains per manufacturer guidance.
  

Preventive Practices

• Regular LMI and sensor calibration per hours cycles and after heavy impacts.
  
• Scheduled electrical inspections with connector cleaning and dielectric grease.
  
• Condition-based hydraulic maintenance: particle counts, viscosity checks, and pressure/flow monitoring to catch pump degradation early.
  
• Use logged telemetry where available: trend small drifts before they become operationally dangerous.
  

Short Case Story
A rental fleet reported intermittent severe load jumps on a 100-ton all-terrain crane. The operator logged the issue and technicians duplicated it: at a particular boom length the LMI would spike, then recover. Investigation showed that a rain-damaged radius encoder produced micro-faults when the boom rotated through that angle; the LMI compensated inconsistently and the hoist system responded to the erroneous moment. Replacing the encoder and re-calibrating the LMI eliminated the fluctuation and restored normal operations without expensive hydraulic work.
When to Escalate to Workshop / Manufacturer

• Repeated transient failures that persist after sensor and wiring checks.
  
• Hydraulic pressure oscillations that trace to pump internals or valve block wear.
  
• Any event that cannot be duplicated safely in a controlled test or that compromises rated capacity calculations. In these cases pull OEM diagnostics, loggers, and expert tech support — rapid escalation prevents catastrophic failures.
  

Takeaway
Severe load fluctuation in a mobile crane is a cross-discipline problem: electronics, hydraulics, mechanics, and rigging all interact. A structured diagnostic routine — record, isolate sensors, measure hydraulics, and run controlled tests — will reveal whether the issue is a noisy sensor or a genuine instability. Early detection, frequent calibration, and preventive maintenance minimize the risk and keep lifts predictable and safe.