How do I calculate the Max bending force for a metal plate? 1

[MohammadB84]

Hi,

I am in the process of testing a very heavy mechanical patient lift. The lift has to be placed on the surface of a sloped metal plate. The weight of my lift(load) is known (~ 1040 kg). The metal plate that i will be using is a carbon steel plate with the dimensions of 96"X60", but I am not sure what would be good choice for the thickness of that plate.

Unfortunately, I don't have any background in mechanical engineering but I am sure there should be a formula to calculate the maximum load a metal plate with certain dimensions can handle before any permanent deformation.

So as a summary, I would like to know how thick my carbon steel plate should be to handle the max load of 1040 kg positioned in the middle before it deforms permanently.

Sorry if I can not explain any better but let me know if you need more explanations.

Any help would be truly appreciated.
Mohammad

 

[KENAT]

Better stress engineers than I will proably have all sorts of input but a few points:

If you don't have the training you may want to sub the work out to someone that does, as this may not be as simple/trivial a question as you think.

A few points to consider:

What grade of steel? Different grades have different properties.

What angle of plate re the load.

How is the plate supported.

There are standard Engineering textbooks that may give you an equation you can use, but using it without having a basic understanding probably isn't a good plan.

KENAT, probably the least qualified checker you'll ever meet...

 

[MohammadB84]

Hi KENAT,

I have all the properties of my metal such as tensile strength and so on. The plate is fully supported at both ends and it has an angle of 10 degs. Can you name one those textbooks please?

Thanks,
Mohammad

 

[Gymmeh]

I can't think of my old statics or dynamics books but one book that is easy to find is the "Mechinery's Handbook" under stength of materials. I have the 10th edition from 1941 which has all the information needed to solve your problem, I hope you can get your hands on a newer verison.

If you have an understanding of statics and dynamics, "Mechanical Engineering Design" by Shigley. covers Load and stress anaylsis, solving for saftey or design factors.

 

[40818]

Your thickness is directly related to your plate stresses, which in turn are checked against the allowable stress.
The stress level at which permanent deformation takes place is called the yield stress.
You need to be more specific as KENAT says to get an actual numerical answer if thats what your after.
Simply put, if your looking for the stress on a rectangular cross section, the extreme fibre stress is calculated by (6 times the bending moment) divided by the (width times the thickness squared)
stress = 6M/bd^2

so you can see that as you double the thickness your stress is quartered, and vice versa.

 

[MohammadB84]

Thank you 40818 and Gymmeh for the book name!

 

[rb1957]

adding to 40818, it's onlt the load normal to the plate that is bending it, so if P is the total weight (acting down) and the plate is inclined at theta to the horizontal then the normal force is P*cos(theta) and if the plate is length L (the edge distance of the plate), then the maximum moment is P*cos(theta)*L/4, and if the plate is width W and thickness t the bending stress = P*cos(theta)*L/4*6/W/t^2 = 1.5*P*L/W/t^2*cos(theta) and you should apply some safety factor (maybe 5?).

 

[GregLocock]

Can I point out that your first real life design exercise should NOT be a device that is used for lifting people.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MohammadB84]

I am designing the required safety tests for the lift according to CSA. The tests are not going be done on people, we are using sandbags as weights. There are so many tests to be passed before any human trial.

 

[GregLocock]

OK. Well I suppose that was obvious. Nonetheless there are a lot of gotchas when designing stuff that has to be failsafe.

A plate that will support that load will be very heavy, usually we would design a frame and then use a thin plate on top of that for the wheels to run on.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

as you say greg, a flat steel plate is what you get when you don't have structures people designing the structure. maybe weight isn't a significant design parameter ?

if the ramp is powered, then being heavy means that it will need more hydraulic power.

maybe, as others have posted, you need to involve some structures people in your project ? just trying to help.

 

[Kwan]

Don't forget about fatigue. If you design that plate to almost yeild you will be fine statically. However, how many times can you lift people before it fails??

Are the edges fixed or pinned. They are probably somewhere between the two. Are all edges supported or just two?

Just my 2 cents.