centre of gravity, load and footprint

[c0rd0bes]

Hi folks, unsure if this is the correct forum.

We have a piece of equipment that accepts a load 158 kg max..

The equipment has a footprint area and a maximum height at which the load is placed upon. i.e. used in its accepted way the unit is stable.

Question. If we were to move the weight above its nominally accepted standard height via a height extension, in order to keep the unit stable what would the reduction in weight be to keep the unit stable and within the equipment parameters to ensure stability.

Am I correct in presuming this just a ratio of the height extension to the reduction in load in order to keep within the equipment stability.

many thanks

 

[hydtools]

Please define your terms 'stable', 'equipment stability'. Conditions of stability.

Ted

 

[c0rd0bes]

Sorry In talking stability here I am not talking about the shape size or strength here all this is all assumed.

I am merely trying to convey to stop the unit from falling (or tipping) over due to its own defined footprint as we increase the height of centre of gravity by extending the load height (reducing its weight as we increase the height) by the minor application of a horizontal moment force at the max length.

I think I have worked it out, simple really. just want to check with you guys

would I need to decrease the load by the percentage of the amount we increase the height?

so if I increased the height by 25% above the nominal height I would need to reduce the max load by 25%

 

[GregLocock]

That's the easiest answer. CGZ*mass*g/footprint is the schoolboy physics, and does at least give the right trend.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[3DDave]

There is no general formula. If you want the same stability the CG of the item cannot move. Changing the CG location changes the stability.

 

[c0rd0bes]

Thanks everyone.

This is for 3DDave (Aerospace)

The equipment does crane up and down so the Centre of gravity does fluctuate through using the equipment. What I am trying to do is extend the height range.

So assuming no tipping (stability) with a load at the maximum height. If I increase that height I reduce the load to stop it tipping .. correct?

 

[robyengIT]

only if you have a horizontal force and/or if the "column" is bending

 

[ornerynorsk]

A diagram or sketch would be very helpful.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[drawoh]

c0rd0bes,

Draw free-body diagrams of your equipment.
[ul]
[li]Draw a free-body diagram of your equipment with the 350lb weight at the maximum height. Examine all the conditions under which your equipment must be safe. This means acceleration, and forces people apply to it.[/li]
[li]Draw a free-body diagram of your new weight and your new elevation, and try out all of those dangerous external conditions.[/li]
[/ul]

Would you willing to stand next to this thing? How acceptable is it for this thing to tip over?

--
JHG

 

[rb1957]

ask the equipment manufacturer, if you want to operate outside of his envelop.

another day in paradise, or is paradise one day closer ?

 

[handleman]

With a given base geometry size, the height of the CG of the entire system must remain the same to keep the same stability. Therefore, you have to take into account the weight of the base/structure. If your base weighs 10,000, increasing the height of the 158kg load will not move the cg much. If the base weighs 100kg, moving the load will significantly change the CG.

Of course, this assumes that stability is is defined as energy input to cause it to tip over. Energy to tip only depends on the total mass and the location of the CG relative to the tipping point. I suppose if you keep the CG the same by reducing the load, the total mass does decrease somewhat such that the tip energy required is reduced. How much this is a factor depends on the ratio of the base weight to the load weight.

If your stability is defined as tipping force applied at the highest point, then you need to consider both the CG and the increased moment arm available at the higher extension.

But it's all sophomore Statics.

 

[77JQX]

For similar stability against tipping due to a horizontal disturbing force applied at maximum extension (OP post of 10 May 06:52), I think the weight needs to INCREASE to proportional to the height of the horizontal disturbing force.

 

[chicopee]

I am still trying to figure out the OP and part of a statement starting "The equipment does crane up and down...." So does the problem involve a crane lifting a load and you are trying to find the C.G. and still maintain crane stability as the load is raised or lowered?

 

[c0rd0bes]

I cant get a chance to return on this until after the weekend.

for chicopee (Mechanical): sorry, as we all know this medium can at times be confusing or maybe it just me yes it is just me There is no overhead crane involved.

The equipment is actually a telescopic column section with a pneumatic internal ram for balancing the load ( so it can be easily moved up and down), on a base with the load placed at the top. Obviously, the centre of gravity changes(or does not really change much depending on the base weight etc..) but used within the manufacturer's parameters the unit is safe, no tipping etc..

Now I know I can contact the manufacturer but a) the equipment is old and b) I know they will say; you are using outside the parameters we don't condone such actions etc...

We do have and have used height extensions which bolts onto the unit and extends the height. Trying to keep it within the manufacturer's design spec ( which should have a factor of safety built in at the design stage) I am reducing the Max load by the % increase in height over the max height.

so a load of 158 kg at max height 1.5m, if I want to go say 10% higher, I would reduce the load by 10% ? (any weight up to the max weight can be balanced with increasing or decreasing gas pressure in the ram)

just looking for confirmation of my thoughts.

btw any horizontal sideways force is just user movement as the equipment can be moved around while in use, i am just tryimg to stop any potential dangerous situation which could cause the equipment to tip over

 

[jlnsol]

In our company we have a rule that, in order to avoid a machine tipping over, the COG always need to be within a triangle with a top corner 40 degrees and a basis equal to the footprint width of a frame. Footprint width is mostly the machine feet pitch distance in our case. So theoretically one can tip the machine over for an angle of 20 degrees when before it arrives at the unstable go/no go position between falling back or forward.
In your situation the COG is a combination between the equipment COG and the load COG placed on top of it. The latter COG is moving up and down I understand. So it would be a sum formula with at least two multiplications in it to find the overall COG and then check if it is still in that safe triangle I mentioned above. My two cents...