what would be the easiest way to measure overhead crane deflection during load test 5

[Fowzan]

any experts here who can advise on how to measure the deflection for the overhead crane under load test.
Measuring the distance under no load condition is easy with a laser distance measurer. but the issue arises when the distance needs to be measured in the same place, when the crane is lifting the test weights, as the line of sight is blocked by the test weight. what would be an easy technique to measure the deflection?

 

[JStructsteel]

Can you go from the top of the crane to the roof? Then load it and do the same measurement?

String across the crane? load up and measure?

Are you checking the overall deflection or just the crane beam?

 

[ryaneng]

Could you measure the deflection 8' away from your load, so that the load isn't obstructing your laser sights?

Alternatively, could you measure the difference in hoist line position relative to the top of load at a specified height above the ground? This would incorporate the tower rotation, beam deflection and rope stretch, so it may not be what you're looking for.

 

[rb1957]

a string pot (potentiometer) ?

I imagine you can predict the direction of deflection, so a string on a spool (so it can extend in the direction expected) with a witness mark moving over a scale ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Compositepro]

Attach a precision ruled scale to the place you want to measure, and use a theodolite to read the scale. You can read down to 0.005" this way.

 

[SWComposites]

Use 3 laser measurers set up like a tripod aimed at a single point on the crane or beam. Then back out the vertical displacement.

 

[dold]

The easy way: strap a 2x4 to the crane beam at the point of interest and extend it out beyond your load. Then just use the end of 2x4 as a target for your laser measurer aiming straight up. The end of the 2x4 will move the same amount as the point of attachment.

 

[IRstuff]

So, let's assume a deflection measurement resolution requirement of 1 mm.

As mentioned above, measurement from the ceiling would avoid the obscuration issue. However, a vertical line of sight isn't necessarily required, since an offset can be calculated with some trigonometry.

Alternately, one could even use a theodolite to do a before/after measurement from a similar distance, or even from across a room; even assuming a relatively poor 5 arcsec measurement resolution, a 1-mm change in position can be detected from 100 ft away.



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[Fowzan]

some good suggestions here. measurement from the ceiling works in cases where there is a ceiling and there is access to the crane bridge. but in many cases it is not so or not feasible. as of now i use Pythagorean theorem to calculate. heres how i do it
1. i bring the crane to the testing point and mark a point right at the bottom of it. i pick a spot on the crane bridge where i would point the laser when taking my measurements. this becomes difficult if you cannot find one, which happens alot.
2. i then mark a reference point on the floor in the same plane which would not be blocked when the test weight is lifted. i measure the distance between the 2 points on the floor. so now i have 1 of the dimension required for the calculation.
3. then i lift the load and measure the distance from the reference point. now i have the 2nd dimension for the calculation. i then use the Pythagorean theorem to calculate the height.
4. then just substract the height (crane loaded with test weight) from the height when crane had no load.
for this procedure, all i need is a laser distance measurer and i do not need access to the top of the crane. but for some reason i do not feel this is the right way.
maybe someone has a better and easier technique.

 

[Tomfh]

Dolts way for sure. Just strap a survey marker to the beam and then measure that.


You could also hang your tape measure from the beam, and shoot a laser level at it, and watch the deflection.

 

[dvd]

Not sure what type of crane is being referred to, but the hoist rope could be marked when load is zero, but at the cusp of being transferred to the hook and measurement of rope hauled to lift the load would reflect the combination of deflection and rope stretch. Rope stretch can be calculated based on the rope's particular elastic modulus and length through all parts of reeving.

 

[Fowzan]

Dolts way is definitely a good suggestion. hanging a tape measure or thread to the 2x4 can make it even easier, since pointing the laser at a tiny cross section becomes a challenge when the distance is big. but the problem is that this requires access to the crane, and that is not feasible in some cases.

 

[dauwerda]

Fowzan, you are currently doing this, correct:

I don't see an issue with this as long as the accuracy of your laser measurer is much greater than the deflections you are measuring (of course that would be true for the direct measurements as well).

 

[271828]

What is the predicted deflection? If it's small then you'll need something like a wirepot (

https://www.micro-epsilon.com/displacement-position-sensors/draw-wire-sensor/)

that was mentioned earlier, an LVDT, or a dial gauge. A dial gauge is a great option if you can figure out a way to mount it. They're very cheap and require little if any other equipment.

 

[Fowzan]

the deflections i am measuring are usually within 15mm. some are just 1mm or lower. but i do not require precise measurements as my work is only to check if the beam deflection is within the limit.
@dauwerda. this is exactly what i am doing. you got it to the point.
pros for this technique
1. the only instrument required is a $30 laser measurer
2. all measurements can be done at floor level and do not require access to crane beam
3. quick procedure
cons for this technique
1. difficult to virtually mark a reference point on the beam (centered horizontally and vertically) when taking the readings.
2. tedious to point the laser at the same spot on the crane beam when taking the readings at an angle.
3. very difficult to be precise

if i am not able to find another technique i would improvise on mine by
1. using a tripod to make the laser measurer stable (handheld laser measurements are a big pain if the object is far and even more if the reading is to be taken at an angle) and easier to point at the reference point. a cheap tripod for smart phones shall do the trick
2. getting a laser measurer with angle measurement. this would require me to take only 2 measurements. i would get 2 sides of the triangle and the included angle and get the deflection (3rd side of the triangle). Didnt know these instruments existed till i started researching since yesterday on this topic. Or i could just rubber band the angle measurement instrument i already have to the laser measurer and get the included angle when taking the 2 measurements.
deflection = √((unloaded beam distance[sup]2[/sup])+(loaded beam distance)[sup]2[/sup] – 2x(unloaded beam distance)x(loaded beam distance)xCOS(included angle))
this should give a bit more accurate result if i am not wrong.

 

[PEinc]

Remember, the crane beam will deflect and its support beams or rails will also deflect. Total deflection will vary depending on the location of the hoist on the crane beam and with the location of the crane beam on its support beams or rails. If you measure from the floor, the floor may not be level so you would need to take measurements with the crane located above the same spot on the floor. The higher the crane beam, the more difficult it will be to measure the deflection. There are many ways to make this measurement. Pick the one that best meets the site conditions.

http://www.PeirceEngineering.com

 

[Tomfh]

I'd do it like this. That way you measure it directly.

 

[PEinc]

If the crane beam is not too high, you could also use a survey level and an upside-down survey rod (or hang a tape measure as Tomfh said).

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