JIB CRANE LOAD TEST FAIL, NEED ADVISE 6

[Dat Tran]

Dear all,

Please help me on this issue.

i did load-testing for Jib Crane, some information as :
Lifting height: 9.5 meters
Crane radius : 8 meters
SWL : 3 ton

Single tapered roller bearing : dynamic load : 245 kN, static load : 365 kN
I used single tapered roller bearing (SKF) with pin and bushing (material SC45 + PWHT) for rotating (see above pic), i applied 1.25 x SWL and do load test but crane boom sags down when the load reaches the 7.5-meter, i assume the bearing selection was not correct, should i change?
Appreciate for your advise and support.

 

[rothers]

Putting the bearing design to one side for a mo, show us your design calculations for the the jib and column.

 

[Dat Tran]

Hi rothers,

See below analysis result.

 

[LittleInch]

SWL 3 tons, but force C and D isa approx 7.5 tonnes?

What's bending? The pole or the jib?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Dat Tran]

Hi Littleinch,

Force at C and D are 765 kg ~ 7650N.

 

[LittleInch]

Sorry, brain fart.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MintJulep]

Are you failing your analysis criteria, or during an actual load test?

 

[Dat Tran]

Hi MintJulep,

i failed during actual load test.

 

[geesaman.d]

When taper roller bearings need to provide stiffness you need to preload them. What preload or clearance setting did you use at assembly?

 

[Dat Tran]

Hi geesaman,

Does it mean i need to add a bearing above taper bearing to preload?

 

[geesaman.d]

Not necessarily, but two opposing taper roller bearings under preload will likely have less friction than a bushing / bearing combination.

 

[Dat Tran]

Is this what we are referring to ?

 

[MintJulep]

You don't seem to have anything that resists the (very large) prying moment.

 

[geesaman.d]

Yes, that is a pair of taper rollers, mounted as closely together as possible. Of all taper roller pairs it will provide the lowest stiffness in bending (prying moment) because the effective centers are so close together.

If maximum stiffness is the goal, use two separate single taper roller bearings, located with the effective centers as far apart as you can afford. Then make sure the metal immediately around the bearings is also very stiff.

 

[Dat Tran]

Hi geesaman,

Thank you for your great advise.

 

[Dat Tran]

Hi MintJulep,


i load tested at the closest point to center of column, start moving trolley to the farthest, and when it reached 7.5 meter, crane boom sagged down. And i start thinking about rotating mechanism had problem.

Yeah, seem like the resistance to prying moment not good enough. I might need to increase stiffness at the rotating point.

 

[LittleInch]

How did the stiffener stay vertical?

Did it bend a bit?

What's the bearing onto the pole?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SWComposites]

And you should be doing hand calculations for sizing of the assembly (what was asked for in the first reply above), not immediately diving into a FEM. You should be able to construct a free body diagram of each part with balanced loads on each.

 

[Tmoose]

What is the maximum deflection that would pass the test?
How did the "failed" deflection compare to the 70 mm deflection of the 4th analysis picture.

I'd be thinking about:
The attachment of the column to the base plate.
Bowing of the column
Bowing of the boom
The gusset under the boom deflecting the wall of the column
Stiffness of the "bearing" arrangement.

Is the jib crane still in the load test rig, so Mechanical measurements of deflections etc could be made.

Some lengths of EMT tubing and a 1" travel dial indicator can be mighty useful for deflections between components and points on an assembly.

Model verification

 

[3DDave]

The bearing at the top should not be taking any bending moment. The moment applied to the beam is resisted by the couple between the upper bearing and the lower bearing.

It looks from the images to mainly be deflection in the column. Since I don't see any column properties, I can't calculate the deflection and without knowing the limits for deflection can't calculate the required column properties.

It looks from the one section view that that column has a very thin wall. Double the thickness of the column wall and re-run the analysis;

I really don't understand how the entire jib crane is black when making a chart that is supposed to apply shades based on deflection and on stress.