Deck cranes do not usually fail at the wire or the hook. They fail at the connections — a slewing bearing whose bolts have been quietly shedding preload for three years, a jib heel with a crack running out of a weld toe under intact paint, a pedestal seat wasted where water has sat against it since the last docking. These are slow failures with an abrupt ending, and the thing that makes them dangerous is that a crane in this condition lifts perfectly well right up until it does not. Nothing in a routine load test finds a bolt at half preload. What finds it is a measurement taken against a baseline recorded when the crane was new, repeated at the same points, by someone who knows that a rising number matters more than an absolute one. This guide covers the crane’s structural hot spots, how slewing bearing wear is actually measured on a ship — including why the rocking test most crews were taught is unreliable here — bolted connection practice, NDT of the jib and pedestal, reading cracks through paint, hydraulics as a structural indicator, and the conditions that stop the crane on the spot. Start a free trial of Marine Inspection to hold bearing measurements, bolt records and NDT reports per crane across the fleet.
Structural hot spots — where cranes actually crack
Six places carry most of the load, and most of the risk
Stress concentrates at connections and section changes. These are the locations worth NDT, worth photographing every survey, and worth stopping the crane over.
3Jib heel and foot pin
5Slewing ring bolted connection
6Pedestal to deck connection
1Jib head and sheave mountings
4Luffing cylinder mountings
2Jib mid-span welds
How deck cranes actually fail
Bolted connections losing preload
A high-strength bolted joint works by clamping, not by the bolts carrying shear. As preload is lost to settling, corrosion or one bolt failing, the remaining bolts see load reversal instead of steady clamp, and they fail progressively. By the time movement is visible at the joint, the connection has already been sharing load unevenly for a long time.
Fatigue cracks at weld toes
Every lift is a load cycle. Cracks initiate at weld toes and section changes where stress concentrates, grow slowly under normal service, and only become visible late. They are found by inspecting the right locations deliberately, not by looking at the crane in general.
Corrosion at the seats and pockets
The pedestal-to-deck connection, the slew ring seating and any pocket where water sits are the places section loss happens unseen. Wastage there reduces the very area carrying the overturning moment.
Bearing raceway deterioration
Raceway wear increases clearance and changes how load is distributed through the ring and into the bolts. It develops over years and is only visible as a number if somebody is measuring it against a baseline.
Overload and shock loading
Snatch loads from a rolling ship, lifting a load that is stuck, and slewing against a swinging load all put peaks through the structure that no routine test records. What they leave behind is fatigue damage that shows up later somewhere else.
Nobody holding the history
The common thread in structural failures is not an absent inspection but an absent baseline. A measurement with nothing to compare it to cannot tell you anything, and crane records are exactly what gets lost at management changes.
The slewing bearing, and why the rocking test misleads on ships
Most engineers were taught the rocking test: apply a tipping moment, measure how far the ring opens. It works on mobile cranes because they carry a counterweight that lets the structure rock. Ship cranes generally do not have one, so the crane cannot rock in the way the test assumes, and the resulting figure does not describe the bearing’s condition. What does work is measuring axial clearance through rotation, at marked positions, against a baseline taken when the crane was new.
Slewing ring in plan — measurement positions and bolt sampling
Bolt count and sample proportion are illustrative — the maker’s manual sets both. What matters is that the sample is spread around the ring, that the same measurement positions are used every time, and that results are compared with the baseline rather than with the previous reading alone.
01
The baseline is taken when the crane is new
Measurement points are marked around the circumference at commissioning and a zero reading recorded with a dial gauge reading to 0.01 mm. Everything afterwards is a comparison with that figure. A crane with no commissioning baseline has to have one established and then be trended from there — which costs years of usable history.
02
Measure close to the rolling contact
The reading is taken between the companion structure and the bearing ring bolted to the upper structure, as close to the rolling elements as access allows. Measure further out and you capture structural deflection as well as bearing clearance, which makes the number larger and meaningless.
03
Rotate and repeat at every marked point
The upper structure is slewed round and the measurement repeated at each marked position, because wear is not uniform — it concentrates where the crane works most. A single reading at one position can miss the worn sector entirely.
04
The limit comes from the maker, by bearing type
Permissible wear depends on bearing type and rolling element size, and is given in the manufacturer’s tables. There is no universal millimetre figure, and when the permissible value is exceeded the guidance is to take the equipment out of service rather than to monitor it more closely.
05
Rising values mean measuring more often
Wear is not linear. Once the numbers start increasing, the interval shortens — an annual check that was adequate for a decade stops being adequate the moment the trend turns upward.
06
Grease tells you what the gauge cannot
Ferrous content in a grease sample shows raceway deterioration before clearance changes enough to measure. Sample it, purge until clean grease emerges at the seals, and keep the results with the clearance record rather than in a separate file.
A measurement without a baseline is just a number
Bearing clearance at the marked positions, bolt checks, grease results and NDT reports held per crane and trended across surveys — so the history survives crew changes, superintendent changes and management changes.
Bolted connections: the rules that are not negotiable
High-strength bolts are not re-used once loosened. Slackening a preloaded bolt to check it changes it. Bolts that have been loosened are replaced, not retightened.
Renew as a set, not as individuals. A single new bolt among worn ones carries more than its share. When renewal is needed, the connection is done as a set to the maker’s procedure.
Tighten to the maker’s figure and sequence. Preload, not feel. The sequence matters as much as the value, because tightening out of order distorts the ring and leaves uneven clamp.
Sample around the ring, never in one sector. A cluster of checks in the accessible quadrant tells you about the accessible quadrant. Spread the sample so a worn sector cannot hide.
A ping test finds the obvious, not the marginal. Sounding bolts will find a badly loose or broken one and will not find a bolt at half preload. It supplements torque checking; it does not replace it.
Re-check after the first period in service. New and renewed connections settle. The maker specifies an early re-check for exactly this reason, and it is the one most often skipped.
Look for what movement leaves behind. Rust staining weeping from under a bolt head, fretting, cracked paint at the joint line or a witness mark out of alignment all mean the joint has moved.
Protect them from water. Corrosion under the head and in the thread both reduces section and makes any future torque reading meaningless. Sealing and coating the connection is structural maintenance.
NDT: where to look, and with what
NDT is only as good as the locations chosen. These are the areas that repay the effort on a ship’s deck crane, with the method that suits each. See how NDT findings are held against a location map per crane so the same spots are examined each time.
Location
Method
What you are looking for
Jib heel and foot pin area
Close visual plus magnetic particle at weld toes; ultrasonic where thickness is in question
Fatigue cracks at weld toes, elongation or wear of the pin bore, distortion of the lugs
Jib head, sheave and pin mountings
Magnetic particle or dye penetrant; visual on sheave grooves and bearings
Cracks at lug welds, worn pin bores, sheave groove wear and bearing play
Jib mid-span welds and splices
Close visual over the full length; magnetic particle at any suspect indication
Cracking at transverse welds, section changes and any previous repair
Luffing cylinder trunnions and lugs
Magnetic particle at the lug welds; visual on pins and retaining arrangements
Cracks at the weld toes, pin wear, movement of retaining plates
Slewing ring seating and upper structure
Visual with the ring accessible; ultrasonic thickness on the seating where wastage is suspected
Distortion, wastage, cracking around bolt holes, evidence of joint movement
Pedestal shell and stiffeners
Close visual; ultrasonic thickness at suspect areas and any pocket that holds water
Section loss, buckling, cracks at stiffener terminations
Pedestal to deck connection
Magnetic particle at the connecting weld; ultrasonic thickness on the deck plating around it
Cracking in the connection weld, wastage of the deck under the crane seat
Deck structure beneath the crane
Internal inspection of the supporting structure in the space below
Cracking and deformation in the underdeck girders and brackets carrying the crane load
Machinery house frame and mountings
Visual; magnetic particle at the mounting welds of heavy items
Cracks at winch and gearbox seat welds, loose holding-down arrangements
Previous repairs, anywhere
Magnetic particle over the repair and the heat-affected zone
Re-initiation at the repair boundary — a repaired crack is a location to watch permanently
Paint is a crack detector
A straight crack line in sound coating
Coating is brittle relative to steel. A crack in the steel opens the paint above it in a thin, straight, often rust-stained line that does not follow the pattern of general breakdown. Treat it as a crack until NDT says otherwise.
Rust weeping from a joint
A stain running out from under a bolt head, a lug or a weld means water is getting into a moving interface. Movement and water together are how a connection deteriorates fastest.
Fresh paint in an odd place
A newly painted patch on an otherwise weathered structure is worth a question. Coating applied over an indication hides it for exactly one inspection cycle.
Photograph the same views each survey
Fixed viewpoints at each hot spot make deterioration comparable instead of remembered. It is the cheapest structural monitoring available and almost nobody does it consistently.
Hydraulics as a structural indicator
Cylinder drift under a held load points at internal leakage or a load-holding valve, but persistent drift also loads the structure in ways the design did not assume. Measure it rather than describing it.
Working pressure creeping upward for the same lift suggests increasing friction somewhere — a stiffening bearing, a seizing pin, a slew drive fighting the ring. Structure and hydraulics are reporting the same problem.
Slew hunting or jerking can be contamination in the control valves, but it can equally be bearing or pinion damage. Check the cleanliness code before stripping the drive.
Relief valve lifting during normal lifts means the crane is working harder than it should for the load. Find out why before adjusting the setting, which only hides it.
Leaks at trunnion-mounted cylinders put oil onto the very welds you want to inspect and mask indications. Fix the leak so the structure can be seen.
Slew gear backlash measured and trended shows pinion and rim wear, and rising backlash changes how shock loads enter the ring.
Stop the crane and do not use it until assessed
Any crack found in a primary structural member or connection weld
Bearing clearance at or beyond the maker’s permissible value
A broken, missing or visibly loose slewing ring bolt
Visible movement, fretting or rust weeping at the ring joint under load
Unexplained noise, knocking or roughness through the slew
Metal particles or heavy ferrous content in the bearing grease
Load drifting down when held, or a luffing cylinder that will not hold
Deformation anywhere in the jib, pedestal or deck seating
Inspection intervals
Before use
Operator walk-round: leaks, obvious damage, wire and hook, controls and stops. Noise and behaviour noted during the first slew and luff rather than ignored as normal.
Monthly
Grease the bearing and pinion, purge until clean grease appears at the seals, check for water in the grease, inspect seals and the condition of the ring joint.
Quarterly
Visual survey of all six hot spots with photographs from fixed viewpoints; check bolted connections for movement, staining and cracked paint at the joint line.
Annually
Bearing clearance measured at the marked positions and trended; bolt sample checked to the maker’s figure; slew gear backlash recorded; grease sampled for ferrous content.
Per the maker and class
NDT of the hot-spot locations to the crane’s own scheme, with the findings recorded against a location map so the same points are examined each cycle.
After any event
Overload, snatch load, contact with structure or cargo, or a lift that stalled — inspect the hot spots and re-measure the bearing before the crane is used again.
Frequently asked questions
Why is the rocking test unreliable on a ship’s crane?
Because the test depends on being able to apply a tipping moment that opens the bearing measurably, which on a mobile crane comes from its counterweight. Ship cranes generally have no counterweight and so do not rock in the way the test assumes, and the figure produced does not describe the bearing’s condition. Axial clearance measured through rotation at marked positions is the workable method.
What is the wear limit for a slewing bearing?
There is no universal figure. Permissible wear depends on the bearing type and rolling element size and is given in the manufacturer’s tables. When the permissible value is reached the guidance is to take the equipment out of service, not to shorten the interval and carry on.
Where should the dial gauge be placed?
Between the companion structure and the bearing ring bolted to the upper structure, as close to the rolling contact as access allows, using a gauge reading to 0.01 mm at points marked around the circumference. Measuring further from the rolling elements captures structural deflection and inflates the reading.
Can slewing ring bolts be re-used after loosening?
No. Loosening a preloaded high-strength bolt changes it, and it is replaced rather than retightened. Renewal is carried out as a complete set to the maker’s procedure, torque figure and tightening sequence.
Is a ping test enough for bolt checking?
It will find a badly loose or broken bolt, which is worth knowing, but it will not find a bolt sitting at half preload. Treat it as a supplement to checking a sample against the maker’s torque figure, not as a replacement for it.
How do you find cracks before they are obvious?
Inspect the known hot spots deliberately rather than looking at the crane in general, read the coating — a straight, rust-stained line in sound paint usually sits over a crack — and use magnetic particle inspection at weld toes. Photograph fixed viewpoints each survey so change is comparable.
What does ferrous content in the grease mean?
Raceway deterioration, usually before clearance has changed enough to measure. Grease sampling is the early indicator; clearance measurement confirms it later. Keep both results together so the two trends can be read against each other.
What stops a crane immediately?
A crack in a primary member or connection weld, bearing clearance at or beyond the permissible value, a broken or loose ring bolt, visible movement at the ring joint, heavy ferrous content in the grease, a load that drifts when held, or any deformation of jib, pedestal or deck seating.
Structural condition is a trend, and trends need somewhere to live
Bearing clearance at the marked positions, bolt records, grease results, NDT findings against a location map and fixed-viewpoint photographs — captured at the crane and held across the fleet, with due dates that arrive before the survey does.
