25-10-2012, 06:12 PM
STRESS ANALYSIS OF CRANE HOOK
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INTRODUCTION
CRANE HOOK
Crane Hooks are highly liable components that are typically used for industrial purposes. It is basically a hoisting fixture designed to engage a ring or link of a lifting chain or the pin of a shackle or cable socket and must follow the health and safety guidelines [1-4]. Thus, such an important component in an industry must be manufactured and designed in a way so as to deliver maximum performance without failure. Thus, the aim of the project is to study the stress distribution pattern of a crane hook using finite element method and verify the results using Photo elasticity.
THE PURPOSE OF OVERHEAD CRANES
The Purpose of an Overhead Bridge Crane
Overhead bridge cranes are some of the most versatile and widely employed types of cranes on the market. They can be found in warehouses and manufacturing facilities across the country.
FUNCTION:
1. The function of an overhead bridge crane is to lift a workload, removing it from one location and depositing it in another location. To perform a lift with an overhead bridge crane, a workload is rigged to the crane’s hook. A cable raises and lowers the hook. The cable is attached to a trolley, which travels side by side along the crane’s bridge. The bridge can travel along two runways to deposit a work piece to another location.
Certification
2. Only people who have been certified by OSHA (Occupational Safety and Health Administration) are qualified to operate overhead bridge cranes in America. Overhead bridge crane certification facilities are located throughout the country. For example, in Atlanta, Georgia, consumers can find OSHA training courses at Georgia Tech Institute.
Safety Measures
An overhead bridge crane must pass regular safety inspections (performed by OSHA-certified inspectors) before it can be used to lift a work piece. Overhead bridge crane operators should be careful to never lift a work piece that weighs more than the crane’s maximum load limit. Load limits are usually clearly marked on the sides of the crane, where they would be clearly visible to the crane operator at all times.
HOOK FAILURE
Failure of Hook
To minimize the failure of crane hook, the stress induced in it must be studied. Crane is subjected to continuous loading and unloading. This causes fatigue of the crane hook but the fatigue cycle is very low. If a crack is developed in the crane hook, it can cause fracture of the hook and lead to serious accident. In ductile fracture, the crack propagates continuously and is more easily detectible and hence preferred over brittle fracture. In brittle fracture, there is sudden propagation of the crack and hook fails suddenly.This type of fracture is very dangerous as it is difficult to detect.
Strain aging embrittlement due to continuous loading and unloading changes the microstructure. Bending stress and tensile stress, weakening of hook due to wear, plastic deformation due to overloading, and excessive thermal stresses are some of the other reasons for failure. Hence continuous use of crane hooks may increase the magnitude of these stresses and ultimately result in failure of the hook.
Load Testing
• Newly installed cranes and hoists shall be load tested at 125% of the rated capacity by designated personnel.
• Slings shall have appropriate test data when purchased. It is the responsibility of the purchaser to ensure that the appropriate test data are obtained and maintained.
• Re-rated cranes and hoists shall be load tested to 125% of the new capacity if the new rating is greater than the previous rated capacity.
• Fixed cranes or hoists that have had major modifications or repair shall be load tested to 125% of the rated capacity.
• Cranes and hoists that have been overloaded shall be inspected prior to being returned to service.
• Personnel platforms, baskets, and rigging suspended from a crane or hoist hook shall be load tested initially, then re-tested annually thereafter or at each new job site.
• All cranes and hoists with a capacity greater than 2722 kg (3 tons) should be load tested every four years to 125% of the rated capacity. Cranes and hoists with a lesser capacity should be load tested every eight years to 125% of the rated capacity.
Crane Overloading
Cranes or hoists shall not be loaded beyond their rated capacity for normal operations. Any crane or hoist suspected of having been overloaded shall be removed from service by locking open and tagging the main disconnect switch. Additionally, overloaded cranes shall be inspected, repaired, load tested, and approved for use before being returned to service.
[b]General Safety Rules [/b]
Operators shall comply with the following rules while operating the cranes and hoists:
• Do not engage in any practice that will divert your attention while operating the crane.
• Respond to signals only from the person who is directing the lift, or any appointed signal person. Obey a stop signal at all times, no matter who gives it.
• Do not move a load over people. People shall not be placed in jeopardy by being under a suspended load. Also, do not work under a suspended load unless the load is supported by blocks, jacks, or a solid footing that will safely support the entire weight. Have a crane or hoist operator remain at the controls or lock open and tag the main electrical disconnect switch.
• Ensure that the rated load capacity of a crane's bridge, individual hoist, or any sling or fitting is not exceeded. Know the weight of the object being lifted or use a dynamometer or load cell to determine the weight.
• Check that all controls are in the OFF position before closing the main-line disconnect switch.
• If spring-loaded reels are provided to lift pendants clear off the work area, ease the pendant up into the stop to prevent damaging the wire.
• Avoid side pulls. These can cause the hoist rope to slip out of the drum groove, damaging the rope or destabilizing the crane or hoist.
• To prevent shock loading, avoid sudden stops or starts. Shock loading can occur when a suspended load is accelerated or decelerated, and can overload the crane or hoist. When completing an upward or downward motion, ease the load slowly to a stop.