Static Calculation of Guardrails

[starscreamer]

Hi everyone,

I've been tasked to calculate the force required to push a set of guardrails using a pneumatic cylinder. The objective is to reduce the weight of the guardrails and hence, reducing the force resulting lower force required to rotate the pole of the guardrail from zero degree to 90 degree. I've attached the simple sketch of the guardrails to give better view on what i'm saying.I'm hoping that somebody can help me verify the FBD i've sketch and also help me to give some idea on how to calculate the force.

So far, i've been assuming the force of the cylinder as a moment to simplify the calculation. After calculating the moment, i've later break it into force by dividing it with the length with respect to the point of rotation which is at point A (assumption only).

Can someone help me to solve this task. Appreciate your help in advance.

Regards,

Fahmi

 

[JStephen]

For a static case, the sum of the moments about each of the base pins is equal to zero, in which case, the sums of the moments about the four base pins all added together is also zero, and that should leave you with one equation and one unknown.

If you're reducing the weight to increase the acceleration, then that's a dynamic problem, and I suspect would be best solved using energy methods.

 

[starscreamer]

Hi JStephen,

Thanks for the reply. For this case, I already look into the energy methods, but to simplify the calculation, I've decided to not use it since the acceleration does not give much impact. What i am looking for is the force required to rotate it. Meaning that the force required to hold it position at 90 degree.

Do i need to solve for each reaction force at base pins or not? Could you please explain further to me the idea to calculate the moment. Appreciate your help in advance.

Regards,

Fahmi

 

[desertfox]

Hi

From the vertical position of the guard rail does the cylinder retract or extend to move the guard rail to 0 degrees position?
Just an observation but if the horizontal guard rail is hard bolted to the vertical bars then the mechanism won't work, for the mechanism to work there must be some rotational freedom between the top horizontal rail and the vertical bars, so you need pivotal freedom top and bottom.

 

[starscreamer]

Hi desertfox

The cylinder will retract to move the guard rail back to 0 degree position. As for the connection, sorry i forgot to mention that the top horizontal bar is also being pivoted to the vertical bar. the way i pivot it is by bolted the horizontal and vertical bar trough a sleeves that will give the rotation during the system is working.

So, do you have any idea to solve this? Appreciate your help on this.

 

[desertfox]

Hi

Well firstly I would lay the mechanism down in the cylinder retracted position and whilst in that position I would calculate the centre of gravity of the guard rail, following that I would move the mechanism through thirty degrees and calculate the centre of gravity, follow that with two more rotations of thirty degrees till I got to the ninety degree movement but calculate the centre of gravity at each stage.
Now you have the loci of the guard rail centre of gravity, if I now assume the centre of gravity in the zero degree position of the mechanism lies to the right of pivot "C" (assuming the vertical links are longer than the spaces between the pivot points) then I would take moments about pivot "C" with respect to the cylinder and the mechanism's centre of gravity and calculate the force required in the cylinder to balance the mechanism from its zero position to ninety degrees.

I still think the mechanism will have a problem operating because it also relies on each of the vertical links being identical in length, I would draw a layout first of the mechanism in various positions and check that if my links aren't identical in length what would happen to the movement.