Crane Boom Hinge Pins/Bushings - Max Allowable Wear

[CGWorkBoater]

I am trying to specify when to renew a pin or bushing (installed in crane booms), based on wear/loss of material (slopy fit), either in the pin or bushing, relate those dimensions to the remaining factor of safety still in the pin. I didn't design the crane, so I am reverse engineering all of this. I have found simple shear formulas, but I'm looking for something more detailed, like what the guy used to design the crane (like a specific method for designing a pined connection). Have found whole-system design std specs, and "theoretical" type formulas for shear/etc., but nothing "practical" for simple component design. Any advice whould be greatly appreciated.

 

[chicopee]

What type of a crane is it? and is the boom hydraulic or lattice type?

 

[CGWorkBoater]

Chicopee, the cranes are all boom/hydraulic (all pinned connections, booms pinned to turrets and hydraulic actuators). I am primarily interested in the pinned connection between the boom and turret (most expensive to disassemble).

My org frequently removes booms to inspect pins/bushings (once evey 4 years), at great expense, I am building a case not do this as often (change long standing policy).

Right now I am looking for pinned joint shear/bending analysis. I am trying to establish factors of safety for existing pinned connections. I can get all material/dimensional properties, I just can't find historical analysis/conventions for determining the shear/bending in pinned connections, most of the stuff in books assumes pure shear, or point loads. I found a guy here who has done these type of calcs in the past and he is going to get me his old work next week, but I would still like to hear about any references/methods that can be found on the web, or elseware, that would validate his stuff. Since these types of joints have existed forever, and are everywhere, you would think it would be all over the web, but I can't seem to find it.

Thanks

 

[chicopee]

I'll get back to you in a couple of days w/ answers.
Meanwhile, I sufgest to read Shapiro's Crane and Derricks and Ontario's Safety Books on Cranes.
Pinned connections are normally designed for shear stresse eventho there is a certain amount of bending stress which is not the prevalent criteria for analysis altho it s/n be discounted depending of design of pin.

 

[CGWorkBoater]

Thanks chicopee, my co-worker who has done calcs like this before said that if the gap between the clevis and rod is significant, and if the pin fit is loose, bending stress gets significant. I'll keep digging on this end. Thanks for your help.

 

[GearmanPE]

I'm not an expert on this, but intutitivly I think that the problem changes drastically if the joint sees a load direction change. If for example, a lateral force causes one side to change load direction, now pin clearence allows a slope between the pin and ears causing high local shear and bending. (I am assumoing these are double-shear joints.)

Bottom line, take great care in reducing safety inspections, this would be a lawyers dream should something go wrong.

 

[CGWorkBoater]

GearmanPE, good advice. In my case, I can't envision a case where loading direction would flip-flop (load hanging from a rope, force always in roughly the down direction).

I have been told by equipment mfgs that my org's (Coast Guard) maintenances policies are more conservative than both Navy and commercial. We actually enter the realm of maintenance educed failures quite often (break stuff that isn't already broken by taking it apart and fixing it, for no reason other than "its time&quot. That's what I am trying to eliminate, I'm advocating more inspections/testing and fewer overhauls. But thanks, I'll proceed with caution.

I have found some analysis of clevis/knuckle joints since the first post. The problem is much more complex than you would think, because both joint loading and the loads in the pin itself are statically indeterminate, whole papers have been written on clevis/knuckle joints.

You bring up an excellent point, it seems, historically, my org cares more about the liability than they do actual safety (we don't care if you make our equipmemnt less safe by overhauling it, cause if it fails then its your liability, not ours).

 

[automatic2]

I would be concerned at the suggestion of bending forces applied to the pin joint. This does not sound like appropriate hoist engineering. Thrust bearings/washers should be employed to remove clearances that allow twisting of booms, that cause anything but shear forces to the pivoting pins. Increase of bending force indicates that pin bores clearances are excessive or that boom or nuckles have been stressed and have migrated. Boom to turret pinning as well as cylinder pinning should recieve shear only forces. Pins that rotate within thier bores without angular displacement will be OK until next inspection. Pins that sieze or demonstrate displacement should be scheduled out.

 

[chicopee]

From my source who has been in Equipment Maintenance for all of his adult life and who also had sold cranes tells me that pins either on lattice booms and on extensible booms cylinders can last for all intent and purposes the life of the cranes because these pins are not subjected to the degree of wear and tear that other components on these machines would be experiencing. When cranes are used, the booms may raised, lowered, extended or retracted for argument sake half dozen to a dozen times during the course of a day. These pins are unlike King pins which are subjected to a lot more wear and tear.
My source also indicated that these pins s/b replaced if they show about 1/16th inch of reduced diameter from wear, have mushroom heads, are loose and sloppy in their fit, bent( dont bother to straigthen them out), obviously cracked, have cracked end pins & twisted(primarily due to poor lubrication).
To tell you a little about my background, I worked for a major bridge contractor in the N.E sligthly over ten years. This firm have about two dozen cranes (50% hydraulic) ranging in age from new to 1940's and both of us had a hard time recollecting if any of these pins had to be replaced, it was so uncommon.