chain sling design

huangyhg

chain sling design
hi,
i'm just a simple materials engineer.
can anyone please advise as to whether to include for acceleration/deceleration forces in detrmining the acceptability of chain slings using simple ratings from swl(or wll)?
thanks
a
johnmac7,
i never did, but my lifting devices were not subjected to extreme lifting conditions.  it is standard practice to not jerk loads when lifting, and major lifts should be done by experienced personnel.  but it depends on your industry and the 'culture' regarding safety in your facility.  some industries by nature have more severe requirements than others.  and some companies have a serious concern about safety, and others don't.  
as a general rule i wouldn't select a chain sling for the exact load required.  just a little bit extra would cover minor impact conditions and makes sense because the unknowns present when making a lift on the shop floor.  many times you don't know th exact weight of the lift, the sling angle, etc. a good material handling vendor might be able to assist.
high speed hoists however, can can cause lifting forces to be greater than the static load.  formulas for this can be found in the monorail specs by mma and overhead crane specs by cmaa.  if you are interested in more info about cranes and monorails try here.
in the us the following may help guide you:
asme b30.20a-2001 structural and mechanical lifting devices.
main section requires sf=3
padeye requires sf=5
cmaa crane requirements
aasho 1.6.17
steel construction manual-aisc
occupational safety and health administration (osha) 29 cfr standard 1926.753(e)(2) and 1926.251
american society of testing and materials specifications a391
naval facility command navfac-307
american national standards institute, "below the hook lifting devices"
asme b30.20-2003 20-1.2.2
"the load-bearing structure components of a lifter shall be designed to withstand the stresses imposed by its rated load plus the weight of the lifter, with a minimum design factor of three, based on yield strength of the material, and the stress ranges that do not exceed the values given in ansi/aws d14.1 for the applicable condition. ...."
this paragraph determines the maximum allowable stress for materials used as load-bearing structural components. analysis or actual max working stresses must be determined based upon working conditions including impact or other factors pertinent to the application. actual max working stress must be equal to or less than the allowable stress.
the intent of section 20-1.2.2 is that the load suspension parts of a lifter shall be designed so that the static stress calculation for the rated load shall not exceed 33% of the yield strength.
the following paper has numerous examples:
"design and construction of lifting beams" david ricker, structural engineering journal, fourth quarter/1991 of aisc
in the us i would generally use the manufacturers rated capacities for chains, slings, and attachments provided they are heat treated alloy and clearly indicated that they are intended for lifting and rated appropriately.  as i mentioned in my previous post the impact loading is usually not significant, but that's a decision you need to make knowing the application.  if it is then the specs for crane designers could give you guidance as far as calulations are concerned.
the 3 to 1 and 5 to 1 factors of safety are sometimes confusing.  when designing lifting devices i would use an allowable stress that was either 1/3 the material yield or 1/5 the ultimate strength, whichever was less.  this range helps insure ductility in overloaded devices to avoid dropping loads.  when purchasing lifting equipment you usually don't have this information so you depend on the manufacturers expertise.
-mike
from the previous post ...
"when purchasing lifting equipment you usually don't have this information so you depend on the manufacturers expertise."
that's a rather odd position, isn't it ?  i guess mike means that you can't be sure how the supplier has factored your specified load, unless you specify it in the po, and this is the experience your depending on.  surely, tho' the supplier is responsible (and accountable) for ensuring the supplier goods meet the specification.  sure, if there's a problem i'd be willing to bet that there'd be two groups of lawyers getting rich over "he said, she said".
when purchasing lifting chain and related attachments you need to get items clearly indicated for lifting use, with the appropriate factor of safety which is generally an ultimate load rating.  but don't expect the vendor to give you information on material spec, heat treatment, and yield load of the device.  years ago i had a meeting with a manufacturer of specialty heat treated devices because i suspected their lifting attachments didn't met the 3 to 1 yield requirements.  they wouldn't say, it was all proprietary information in their opinion, and the 5 to 1 factor of safety was all we needed to know.  
since many companies don't have the expertise in their maintenance or plant engineering depatments to do these calculations, they need to work with vendors that do.
-mike
dear all,
many thanks for your excellent posts.
it is stated to have a minimum safety factor of 5 on uts.
when we do the actual calculations however it is actually about 65% of yield. the acceleration/deceleration loadings are about another 20%, so quite appreciable.
this is not what you would classically describe as a shock loading application either.
my view is that potentially some allowance should be made in swl for these loads.
john