Lifting chain hooks 5

[tigermoth]

Hi

Can anyone offer advice on the machining of lifting hooks from solid steel plate (instead of forging). Obviously, grain direction is not optimal but is it actually a terrible idea? (provided detailed fatigue analysis is carried out, the steel properties are well characterised, grain direction and surface finish specified and the analysis validated by appropriate Break Force, Proof Force and fatigue type tests?) Must be better than non-forged castings which are historically the only other way of attaining the required finished shape.The only standards around seem to be those covering forged hooks.

Thanks

 

[MintJulep]

To your list I would add analysis of the hook after it has been in service for a few years - that is, is no longer a perfect specimen, but has acquired its share of dings, chips, scratches, hammer blows, torch assaults, and whatever other abuse it will be subjected to.

I'd also do everything based on worst-case for grain direction.

 

[EngineerTex]

I'm assuming that you have looked at hook manufacturers and determined that their products will not work.

So with that in mind:

1) For this critical service, I would recommend against using plate. Use a bar/round stock which should be available in diameters up to 36" without having to search to the ends of the earth for it. A coworker went down a similar road a few years ago and without getting into gory details, found that even the best-quality plate from the most reputable manufacturers (here in the US and western Europe) have so many (almost every piece we examined) laminations and inclusions that this experience has directed us to using round stock on all applications where stress flows in multiple directions in critical service.

2) Perform 100% ultrasonic testing on all raw material prior to machining.

3) Perform 100% magnetic particle testing on all parts after machining.

Can you really not get your hands on exactly the hook you're looking for? What kind of SWL are you talking about here?

Engineering is not the science behind building. It is the science behind not building.

 

[racookpe1978]

Buy a hook.

You really, really, really, really do not want to put a load on a "plate" that will go over people's equipment, buildings, pipes, and wires.

 

[tigermoth]

MintJulep - I take your point about allowing for the most adverse grain direction. However, I would have thought that deterioration of the hook in-service would be covered by the 4:1 safety factor on SWL which is specified in the various international and national standards for chain sling components. Most national Codes of Practice also seem to require periodic inspection and rejection if the defects exceed certain level.
EngineerTex - the 'hook' is more of a special-purpose lifting lug which is difficult to forge but relatively easy to CNC machine. SWL is 10 tonne or so. It is actually machined from round bar and has performed satisfactorily in service for >20 years. (Now requires certification however). Without prying too much into the 'gory details' - did the plate material defects actually show up as real problems under eg fatigue testing?
racookepe1978 - yes that is my instinctive reaction too.
Is it mainly on grain direction/fracture toughness/fatigue strength thing? Some standards (eg EN1677) do mention hooks consisting of profiled plates groove welded together - I think for large, one-off applications

 

[EngineerTex]

The plate problems showed up when the well exploded with our equipment on top of it. Imagine two parallel plates separated by a distance of 48 inches and four legs welded between them at the corners. Load this assembly in tension at the centers of the plates and you have an approximation of what we had.

The design was "homemade" and we found that there were others in service by our field personnel. After loads of investigation, we determined that rolled shapes, especially plates, should never be loaded in tension in the through-thickness direction. There are rolled plates that you can purchase that have improved through-thickness properties, but I wouldn't trust these based on what we found in our research. However, this all relates to rolled plate. You can purchase plate which comes forged from the mill, but that's not the same thing as rolled stock. I've never heard of a problem with forged plate, but I also have only used it once (so far, the part has worked fine).

However, that being said -- if you're talking about a lug, i.e. a pad eye, then I'm mostly unconcerned about your application. In fact, I would even skip the ultrasonic testing for your plate stock and I wouldn't even make a recommendation to use the round/bar stock, assuming that all of the loading is in the plane of the plate. For this type of application, I use common rolled stock all of the time.

Additionally, on pad eyes for skids, spreader bars, guying anchor points and other parts, I have never made a consideration for grain direction. Someone may differ with me on this, but I have also never read any requirement to take it into consideration in any specification that I have come across.

Now another point -- if you are talking about using plate and cutting it with a CNC, even if you are using water jet cutter or something equally fine, remember that you can cut the shape with the CNC, but ALWAYS ALWAYS ALWAYS drill the eye. It doesn't have to be reamed or bored, (although you can) but NEVER cut the hole with a CNC, no matter how fine the process, unless it's a CNC mill. The reason is because with a plasma cutter, flame cutter, water jet cutter and even EDM wire, they will leave cutting marks in the wrong direction, i.e. parallel to the axis of the hole, which WILL make a difference when it comes to fatigue.

So still assuming that you're using a pad eye and a shackle:

1) It is best to have a tight-fitting pin in the eye, so mill/drill the hole diameter no larger than 6% greater than the nominal diameter of the shackle pin. Try to stay as close to 6% as you can in order to prevent contaminants like paint and rust from allowing for free rotation of the shackle in the eye. This number comes from DNV 2.7-1 Rules for Offshore Containers. If you use this limit, then you will be safe as far as contact stresses go. According to the spec, allowable contact stress is twice yield. (yes that's a 0.5:1 SF for contact. But only for contact)

2) Make sure that the thickness of the plate is at least 75% of the width of the jaw of the shackle. (also DNV 2.7-1) I try to make sure that it is about 90% of the width of the jaw so that nobody can "accidentally" use the wrong size shackle in the pad eye. The next shackle smaller won't fit over the plate and the next shackle larger can't fit the pin into the hole.

3) Only use bolt-type shackles with secondary cotter pin retention (see Crosby model G-2130) and don't use screw-type shackles as these have an annoying tendency of backing themselves out in service. My organization has completely banished the screw-type shackles and personally, I don't think that the screw-type shackles should even be on the market.

4) With a 4:1 safety factor, I wouldn't waste much time worrying about fatigue. Just make sure you have a reasonable inspection regime.

Are you talking about using shackles and pad eyes? If so, then I don't think it's a terrible idea at all.

Engineering is not the science behind building. It is the science behind not building.

 

[metman]

EngineerTex,

Would give you a star but something called privoxy prevents me from giving stars. This deserves special attention anyway. Years ago I did design work with Industrial Lifters Div. of ACCO (American Chain and Cable cOmpany) but never saw anything quite this elegant.

Especially the following:

I try to make sure that it is about 90% of the width of the jaw so that nobody can "accidentally" use the wrong size shackle in the pad eye. The next shackle smaller won't fit over the plate and the next shackle larger can't fit the pin into the hole.

 

[boo1]

In the US the following may help guide you:
ASME BTH-1-2008 Design of Below the Hook Lifting Devices, Chapter 3
ASME B30.20a-2003 Structural and Mechanical Lifting devices. Section 20-1.2.2 Construction
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"

Analysis or actual max working stresses must be determined based upon working conditions including impact, fatigue or other factors pertinent to the application. Testing and certification is required for hooks used in overhead lifts. PE stamp on design documentation is required to be on file for life of product. Product marking must include weight limit.

 

[tigermoth]

EngineerTex - thanks for sharing your experience - very interesting!. My 'lug ' terminology is perhaps a bit vague - it is more of a chunky L-shaped hook with various protrusions and incorporates a clevis at the top for permanent pinning to the lifting chain (To which your points 1, 2 & 3 are relevant). There is potential for the grain to be at any orientation relative to the clevis 'ears' and to the corner of the L-shape so I guess cutting from round bar stock (as we do) is the way to go - with the addition of the NDT you suggested. The shape is such that the nominal stresses are relatively low - this may be why it has worked well historically.
Boo1 - Do you know if the ASME codes require forged hooks?

 

[boo1]

The ASME B30 Standards contains provisions that
apply to the construction, installation, operation, inspection, testing, maintenance, and use of cranes and other lifting and material-handling related equipment.

ASME 30.10 Hooks does not specify material or process. It does specify ductility.

10-1.1.2 Construction
(a) "The hook material shall have sufficient ductility
to permanently deform before losing the ability to support
the load at the temperatures at which the specific
hook will be used."

Recommend ASME BTH-1-2008 Design of Below the Hook Lifting Devices, and ASME B30.20a-2003.

 

[chicopee]

Here are some drawings of plate hooks we used to make in our plant over 10 years ago. Plates are 1-1/2 inch thick. and pretty big. We never had any problem with them

 

[tigermoth]

Thanks boo1 - yes I can appreciate it is important to have the component give sufficient sign of failure before rupture. Also relates I suppose to strain energy/work done & fracture toughness - we have done Charpy impact tests on the bar stock in each direction although not at this stage on every piece.

chicopee - this would be in keeping with EngineerTex's comment regarding all the loading being in the plane of the plate. Most plate rolling defects would be of a lamellar nature and affect the thru-thickness strength, not the in-plane. - the jpeg appears to be blank?

 

[chicopee]

tigermoth, the jpeg is there.

 

[boo1]

If you back off a screw shackle 1/4 turn before loading, binding will be minimized.

Crosby APPLICATION INFORMATION page 72 of catalog

http://www.thecrosbygroup.com/html/default.htm