Over the past 30 years, straddle cranes have become producers' preferred equipment for moving precast/prestressed concrete components around the yard. Casting beds and stacked products are accessible to these highly maneuverable cranes, so producers can economize yard space instead of designing yard layouts for tractor crane picks and loadings onto flatbed trucks.
But producers often face the challenge of retraining newly hired straddle crane operators, who may have experience operating single-boom pivot cranes. A pre-shift inspection routine that focuses on the straddle crane's unique design not only increases safety, it reduces maintenance costs.
The crane's bolted connections between crossbeams and columns and side beams and columns are designed handle the vibration and forces created as straddle cranes carry their 50- to 60-ton payloads. But over time, these connections may weaken due to a neglected travel wheel system.
It's important to check hydraulic fluid level and for leaks in the hydraulic system on any crane, but it's especially important for straddle cranes. Wheel misalignment is one result of insufficient hydraulic fluid flow. Check the hoses and fittings at the steering cylinders for leaks.
Check the crane's parallel side members for proper alignment by checking the corresponding wheel alignment. Check tires for toe-in (fronts closer together than backs), toe-out (backs closer together than fronts), positive camber (closer together at the bottom than the top), negative camber (closer together at the top than the bottom) and caster (uneven when viewed from the side). Toe causes excessive side-frame vibration, which leads to loose bolts and weld cracks. Camber creates uneven lifting-mechanism motion or excessive vibration. Check tire-tread wear and tread patterns on the ground for signs of these conditions. Caster is a sign of an uneven frame; there's a good chance bolts are loose or welds are broken at beam-column connections.
Maintaining proper tire pressure is especially important on straddle cranes. Just as wheel misalignment puts side-to-side stress on column-beam connections, uneven tire pressure creates up-and-down stress on these connections. While proper tire inflation is important, equal inflation in all tires is even more crucial.
Uneven yard surfaces and tight turning requirements with a live load can cause wheel sprocket wear. Check the sprocket teeth for wear and for loose or missing sprocket bolts.
Even if wheels are perfectly aligned and tires are properly inflated, bolts and welds holding columns to side beams and overhead beams experience significant stress as the crane turns. Check for loose bolts (the mechanic should periodically re-torque them) or weld cracks, especially around ends and mounting pads.
After startup, check that speeds of front and rear traverse and unloaded front and rear hoists are proportional to volume control pedal movement, with no "run on" after pedal release. A visual check detects a sagging traverse cable, usually the culprit for run-on.
Also check the traverse drive drum, idler drum bearings and idler sheave for wear.
Your mechanic should check several straddle crane-specific items at recommended intervals. Use the following list, in addition to universal crane inspection and maintenance procedures, to set up your own planned maintenance program.
After every 40 service hours
Top beam trunnion
After every 80 service hours
Hoist chain and sprockets for wear; clean and lubricate
After every 320 service hours
After every 960 service hours
Wire rope sheaves for wear/pinching
Beam flange and sheave thickness for specified tolerances
Traverse gearbox oil; refill
Draw relevant data from ASME B30.2 (Overhead Cranes), B30.9 (Slings), B30.10 (Hooks) and B30.20 (Below-the-Hook Lifting Devices). The owner's manual and these standards should collectively cover all inspection and maintenance items.
As with any truck or single-boom crane, the producer should routinely check several items, common to all cranes, that could lead to a sudden stall or failure while the straddle crane is loaded. These include, but aren't limited to: Electrical wiring; hose clamps; belts; bearings; lights; cab controls; mounting bolts, hooks, worn chains; and proper lubrication of spindles, bearings, sheaves and cables. Also check the horn, strobe lights and travel alarm for proper operation. The owner's manual has specific information for your crane, including engine inspection and maintenance procedures.
While specific engineering and safety standards have not yet been developed for straddle cranes, basic crane inspection theory still applies:
If the manufacturer puts an item on the crane it must remain on the crane in working order. If not, the manufacturer or a registered engineer must advise how to keep the part installed in such a way as to prevent it from interfering with safe operation.
If the user installs auxiliary equipment on the crane, or if the user modifies the crane, an approval of the change by the manufacturer or a registered engineer must be on file.
If a part is broken or operates intermittently, it must be repaired immediately.
During inspection, move the crane away from plant operations. Turn off, lock and tag the master power switch and the starter lockout switch. Beware of sources of potential energy, including batteries, tires, radiators, trapped hydraulic pressure, suspended lifting attachments or hook blocks and spring assemblies under compression.
During inspection, lower unloaded blocks and spreaders to ground level for inspection and/or maintenance. It may be necessary to suspend them slightly to check underneath.
Assuming the operator finds no defects during the pre-shift inspection, remember that periodic inspection should take place according to hours of service, not time intervals. The last thing a producer needs is unnecessary equipment downtime.
KEYWORDS: safety, straddle crane