Free Lifting Device Resources
  • Analyzing Hooks and Lifting Lugs  

Lifting Lugs are often mis-analyzed for strength.

For hooks, combined normal stresses typically govern the design at the
section of maximum moment.  Normal stresses from bending and direct
factors (usually in the range of .75 to 1.3) are applied to account for the
shift in the neutral axis of the section due to hook curvature.  Calculations
should consider both the tensile stress along the inside radius and the
compressive stress along the outer radial surface.  Machinery Handbook
and Marks' Handbook offer good calculation approaches and
approximation tables.

What about lifting lugs?  Next time you see a lifter in operation on a crane
hook take a real good look at the eye area.  Is there enough "meat" above
the eye to carry the load?  How much is there?  How much do you think
you could shave off and still be OK?  This is a great area on lifters to add a
little material and get a whole bunch of extra safety.  Too often, I see lifting
lugs considered for only shear and tension through the eye section, when in
fact bending through the eye in a manner analogous to that of a hypothetical
hook, attached on both sides, will typically govern the behavior and leads
to a safer more conservative design. A good portion of the lifting devices
under evaluation will have their weakest point at the eye of the lug, a place
where it is very inexpensive to have initially designed in some added
strength.

Many forged hooks are designed as non-constant cross section for
efficiency.  Simple Computer Aided calculations to account for variable
area and area moment can be used for analysis and stress calculation.

Also, the engineer must consider and include any other contributing stress
factors like stress concentration effects, thermal effects, welded joint
stresses, impact or dynamic loads.

A good reference text is Omer Blodgett, Design of Weldments.  
  • Proof Testing of Lifting Devices

Some lifting device suppliers suggest proof testing of lifters at 2 times their
rated capacity.  Stress Analysis Solutions does not recommend this
practice.  Overloading of a lifting device should not be a planned event.  
and HAZ in weldments. Combined with the dynamic crane affects, failure
can occur due to intentional overloading during the proof testing process.
If devices are accidentally overloaded, inspect the devices for cracks prior
to the placing back in service.  The best action you can take is to have a
experienced professional lifting engineer review the design and place the
lifter in service.
Useful Links:
American National
Standards Institute (ANSI)


Mechanical American
Society of
Engineers
(ASME)
"Helping you design safely"
  • Determining Allowable Stresses for Lifters

Current industry specifications are lacking in clear definition of the structural factors
involved.  For example-- the 5:1 should apply to only low carbon steel Static
Stress and thus the 5:1 safety is actually very misleading.  Cranes and lifters are
inherently dynamic systems and many can be  subject to long term fatigue effects.  
Thus the actual, in service, Safety Factors are much less then 5.   ASME has
recently generated a new set of specifications called ASME BTH, covering
requirements for  below the hook lifting devices in more detail.

As far as materials, ASTM A36 or AISI 1020 Hot Rolled Steel are the best
material choices for steel lifting devices due to their moderate strength properties,
good weldability and high ductility.  An allowable static stress of 10 ksi Normal
Stress and 5.8 ksi Shear Stress are reported material strength data using several
sources.  Make sure of the condition of the steel.  Be prudent in the use of heat
treated components and increase safety factors to account for decreased ductility.  
As always, have all Lifters calculated and reviewed by a licensed engineer and lift
industry professional.  Consult our office for additional assistance.
A Series of Short Articles by Jim Grek, PE, MSE