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| Angle iron deflection | |||
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| Posted by: KevinL ® 09/27/2006, 10:14:32 Author Profile eMail author Edit |
We fabricated a conveyor support system which allowed the belt conveyor to mount to load cells mounted to the support frame. Now the customer's engineering manager is asking us to prove that the frame won't deflect too much, putting the load cells into compression or tension and affecting the weight reading. I called my steel supplier to get info on the deflection characteristics of the steel and he has no idea. The 6' long conveyor weighs 550 lbs., and the load being weighed is a maximum of 200 lbs. The conveyor is mounted front and back to pieces of 1/2" x 3" flat 27" long, which are bolted to the top of the load cells. Four Load cells (two in front and two in back) are bolted to pieces of 3 x 3 x 3/16 angle iron, 40" long. Each end of the angle iron is welded to a 2 x 2 x1/4" steel tube which is the vertical support. The angle is welded with a flat on top of the support legs, and a flat on one side of the 2 x 2. One load cell is mounted to the flat surface of the angle iron with its center 7 3/4" from one end, and the other cell is mounted 9" from the other end. The customer is concerned that the two angle iron members will twist under the massive 750 lb load, affect the accuracy, and is demanding that we show him that the deflection is within load cell requirements even though all is working well. How do I calculate this??? Modified by Administrator at Thu, Sep 28, 2006, 13:08:20 |
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| Re: Angle iron deflection -- KevinL | Post Reply | Top of thread | Forum |
| Posted by: swearingen ® 09/27/2006, 13:19:40 Author Profile eMail author Edit |
As long as the load table doesn't contact anything else due to this deflection and as long as some slight vertical misalignment of the load cells is not a problem with the manufacturer, there is no problem. To wit: If I took a steel beam 5' long, simply supported at its ends, and applied a 500lb load to a load cell in the center, the load cell would register the 500lbs. If I changed the beam to a wooden one, the deflection would be greater, but the full load still passes through the load cell and it would still register 500lbs. No calculation of the deflection is necessary. Let's say you did calculate the deflection. What would you do with that info? |
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| Re: Re: Angle iron deflection -- swearingen | Post Reply | Top of thread | Forum |
| Posted by: KevinL ® 09/27/2006, 15:52:39 Author Profile eMail author Edit |
The load cell manufacturer states that to achieve an accuracy of => .1%, maximum base plate variation can not exceed 0.2 degrees, and maximum top plate variation of 1 degree. If the deflection were to cause the mount to exceed that amount of variation, I would go back and field weld a flat 1/4" x 3" member to the 2 x 2 tubes and to the open side of the angle to prevent the twist. I don't think it is necessary since the scale is working just fine, but I have a customer who wants to see the proof before he clears me for payment. It would be simpler for me to just go in and do it, but he won't clear me to go in to his plant and cut and weld until he see's that it is necessary or not. We cut and weld steel, we don't perform FEA's. I am at my wit's end with this guy. |
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| Re: Re: Re: Angle iron deflection -- KevinL | Post Reply | Top of thread | Forum |
| Posted by: ipajewsk ® 09/28/2006, 09:35:21 Author Profile eMail author Edit |
How are the load cells bolted to the angle iron? That angle iron will twist if the force exerted by the load cell in not in line with the shear center of the angle iron. The shear center of your angle iron is about .1" from the outside flat in both directions. If you are just bolting the load cell to the flat, there will definitely be some twisting. To figure out how much, its a simple torque problem. The torque on the angle iron is simply the force multiplied by the distance between the shear center and the force vector from your load cell. From there, its a simple mechanics of materials problem. So, we need to know where (on the cross section of the angle iron) your load cells are in order to nail down a specific number. |
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