Pins for boom truck crane - fit tolerance
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  1. #1
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    Default Pins for boom truck crane - fit tolerance

    Hi all, I'm working on an IMT 16000 boom crane on a truck for the local lumber yard. needed a bunch of work including repinning and bushing the crane. The pins are hard chromed cylinder rod, that ride in Garmax synthetic bushings, so no grease. The haave a collar welded on one end that keeps them from rotating in the boom.

    So to illistrate, if they fit together like this -=, the "inner" boom has the garmax bearings and the "outer" boom has anti-rotation knobs that keep the pin from rotating in it (as it's steel on steel). What we are finding is that the steel of the "outer" parts is worn, and new pins rattle around in them. I'm afraid if we put it back together, it will just pound the pins in short order. I'm thinking of line-boring and then rebushing down to size, but curious what sort of fit I should really be shooting for?

    I've attached a picture of one of the original pins installed. As you can see, it got loose, and the anti-rotation collar broke. 20190827_091452.jpg

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    A press fit would be nice.

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    If you bush it use at least a 1/4" wall to keep from hammering it out. I usually weld and rebore.

    Ed.

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    Quote Originally Posted by deerefanatic View Post
    I'm thinking of line-boring and then rebushing down to size, but curious what sort of fit I should really be shooting for?
    The pin sheared off the welded end cap when the worn bearing forced the pin to rotate, If the pin had been fixed into position with a press fit in the collar so that it could not rotate, the pin would have sheared in torsion. A sheared pin failure is dangerous. It may cause the crane boom to drop if the sheared pin halves fall out. It may cause significant damage to the welded frame if the pin halves stay in place and pivot downward with the applied load.

    The collar repair should retain the feature of allowing the pin to rotate when the plastic journal bearing seizes.

    The pin fit in the collar does not need to be tight. For example, the pinned boom frame sections on cable cranes that are assembled at the work site have pins that are sliding fits. It does not seem to be a problem. A tighter tolerance would be needed if there were reversing loads on the pin as would occur in a backhoe frame.

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    Quote Originally Posted by Robert R View Post
    The pin sheared off the welded end cap when the worn bearing forced the pin to rotate, If the pin had been fixed into position with a press fit in the collar so that it could not rotate, the pin would have sheared in torsion. A sheared pin failure is dangerous. It may cause the crane boom to drop if the sheared pin halves fall out. It may cause significant damage to the welded frame if the pin halves stay in place and pivot downward with the applied load.

    The collar repair should retain the feature of allowing the pin to rotate when the plastic journal bearing seizes.

    The pin fit in the collar does not need to be tight. For example, the pinned boom frame sections on cable cranes that are assembled at the work site have pins that are sliding fits. It does not seem to be a problem. A tighter tolerance would be needed if there were reversing loads on the pin as would occur in a backhoe frame.
    ...And yet.... shops that doo this for a living weld up the bore, and remachine....

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    Quote Originally Posted by digger doug View Post
    ...And yet.... shops that doo this for a living weld up the bore, and remachine....
    Yep- but not necessarily to a press fit

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    Quote Originally Posted by jmead View Post
    Yep- but not necessarily to a press fit
    No press fit all, they weld up the bore, and negate any need for a bushing....

    The OP asked about bushing the bore of the non rotating section.

    On these connections, you want the non rotating parts to...wait for it
    "not rotate" and for the parts designed TO rotate...wait for it yet again "Rotate".

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    So my take is this should be a "tap in with a deadblow" fit.

    I'm fairly certain my collars would keep the new pins from rotating, but I'm afraid that the chucking action of the slop will wear it out. They complained that the thing was "sloppy and loose" compared to the other truck with identical crane.

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    I'd shoot for about .010" diametral clearance. Yeah, you could go for .005 but then you'd need a guarantee from the customer that they would go out and grease those every day, even if the machine is not being used. It's a bitch when 'nice fits' rust together.

    While a press fit may sound ideal, in the real world, people just want stuff to go together readily. For some, even .010" clearance isn't 'readily' because they can't stand back 5 feet and throw the pin into the hole.

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    Ive done a lot of truck cranes,pins should be a firm slide in fit,the self lube bearing can be tight-ish,as they cant seize.....And smear the whole pin with pure moly,so it doesnt fret in the steel bushes.....Never a hard drive in fit,because the big sledge hammers will be applied,and bend the light steel these cranes are made from...With knuckle boom type cranes ,pins and bushes flog out because the drivers are too lazy to fold the crane.......IMHO,making pins from hardchrome ramshaft,dont use some of the very soft cheapo shafting,it needs to be HT 4140,preferably induction hardened skin.

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  14. #11
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    Quote Originally Posted by HuFlungDung View Post
    I'd shoot for about .010" diametral clearance. Yeah, you could go for .005 but then you'd need a guarantee from the customer that they would go out and grease those every day, even if the machine is not being used. It's a bitch when 'nice fits' rust together.

    While a press fit may sound ideal, in the real world, people just want stuff to go together readily. For some, even .010" clearance isn't 'readily' because they can't stand back 5 feet and throw the pin into the hole.
    There are qty (2) fit's being discussed here.....

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    I'm not sure what the big question is here. Look up the bearing on the GGB website and bore the housing to the correct size. A 2" heavy wall Garmax bearing specifies a 2.500/2.501 housing for example. They give you the design clearance, if your pin is larger or smaller than nominal you can adjust the housing fit to maintain the clearance.

    Part Finder >> GGB Part Finder


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