Design Tripod to Support 20kg Mass at 2m: A Student Guide

[Tim Allen]

Homework Statement

I am designing a tripod which is for supporting a mass of 20kg at a height of 2m from the ground. The legs are cylindrical and hollow, dimensions are not set in stone, and would be made from an aluminium alloy. I do not want the legs to be bound at the bottom, they should be free to move so the tripod can be adjusted in height.

Could someone please advise how I begin? I know that my 20kg mass will exert a downward force of 196.2N due to gravity, but I don't know what the next step should be. I suspect I need to calculate the bending moment but I am not certain how to go about this. Furthermore, once I know the bending moment due to the force, how do I then calculate the stress which is acting on the legs? I have come across Euler's Formula for the max axial load for a long, slender column and I think I need to use this at some stage but not sure when exactly. Excuse my ignorance, I'm only a student!

Homework Equations

The Attempt at a Solution

 

[Nidum]

If the legs are to be moveable then you need to design this tripod as a mechanism first . What is the practical construction going to be ?

Post a sketch of what you think is a plausible design and we'll have a look .

The actual strength calculations are relatively simple and we can deal with these later .

 

[berkeman]

Homework Statement

I am designing a tripod which is for supporting a mass of 20kg at a height of 2m from the ground. The legs are cylindrical and hollow, dimensions are not set in stone, and would be made from an aluminium alloy. I do not want the legs to be bound at the bottom, they should be free to move so the tripod can be adjusted in height.

Could someone please advise how I begin? I know that my 20kg mass will exert a downward force of 196.2N due to gravity, but I don't know what the next step should be. I suspect I need to calculate the bending moment but I am not certain how to go about this. Furthermore, once I know the bending moment due to the force, how do I then calculate the stress which is acting on the legs? I have come across Euler's Formula for the max axial load for a long, slender column and I think I need to use this at some stage but not sure when exactly. Excuse my ignorance, I'm only a student!

Homework Equations

The Attempt at a Solution

What constraints do you have for how the mass attaches to the tripod? Is there a formal problem statement, or some other guidelines? What does the mass look like?

What are your initial thoughts for how you will attach the mass to the tripod. Depending on the constraints that you have been given, there may be an innovative way you can attach the mass to the tripod to maximize stability and strength of the overall structure...

 

[Tim Allen]

OK guys, thanks for the feedback.

To give you a bit more info, essentially what this is is a method for making a laser scanner mobile, so I'm wanting to design a tripod system that is mobile. The laser scanner attaches to the top of the tripod using a standard Tribrach attachment. The mass itself is a laser scanner, it's essentially a rectangular box that weighs approximately 9kg, but I wanted to design the tripod to support more weight than that.

I want to design something that can make the tripod mobile, so it needs to have casters or wheels but also it must have a method of immobolising so that it can be held in place whilst a scan takes place. My thoughts are either to use a simple braking caster system or a pin that lifts the wheels from the ground temporarily.

Sketch attached - it is by no means perfect, it gives a rough idea of what I'm thinking.

 

[berkeman]

essentially what this is is a method for making a laser scanner mobile

Ah, then my idea won't work. I was going to suggest mounting the mass underneath the apex of the tripod, to add stability and strength. That won't work for a laser scanner.

How do you plan to level the scanner?

 

[Tim Allen]

Ah, then my idea won't work. I was going to suggest mounting the mass underneath the apex of the tripod, to add stability and strength. That won't work for a laser scanner.

How do you plan to level the scanner?

The scanner can be levelled on the Tribrach attachment, it allows a 3 point adjustment.

Any advice on how I can calc the bending moments and stresses?

 

[berkeman]

The mass itself is a laser scanner, it's essentially a rectangular box that weighs approximately 9kg, but I wanted to design the tripod to support more weight than that.

I want to design something that can make the tripod mobile, so it needs to have casters or wheels but also it must have a method of immobolising so that it can be held in place whilst a scan takes place.

I'll leave the help on the calculations to others like @Nidum but I wanted to ask about the vibration tolerance of the laser scanner you are planning on using. Were you planning on rolling the whole assembly around on casters with the laser scanner still in place? That might be pretty harsh on the scanner unit. Or are you planning on detaching the scanner any time that you are going to start wheeling the tripod around.

If you want to wheel the whole assembly around, you should check the vibration and shock specifications of the scanner. It's probably not designed to take a lot of vibration without losing calibration and accuracy...

 

[Tim Allen]

I'll leave the help on the calculations to others like @Nidum but I wanted to ask about the vibration tolerance of the laser scanner you are planning on using. Were you planning on rolling the whole assembly around on casters with the laser scanner still in place? That might be pretty harsh on the scanner unit. Or are you planning on detaching the scanner any time that you are going to start wheeling the tripod around.

If you want to wheel the whole assembly around, you should check the vibration and shock specifications of the scanner. It's probably not designed to take a lot of vibration without losing calibration and accuracy...

Thanks berkeman for your advice.

Yes, the theory is that the whole unit will be wheeled around to speed up the process of laser scanning a vast area such as a car park or onshore gas installation. The company that I work for specialise in laser scanning so I will double check with them the limits on vibration for throwing out the calibration. I was certainly going to take into account that vibration might affect the actual scanning process if vibrations are present in the local environment due to plant or other machinery nearby.

Perhaps involving some sort of dampening might be an added advantage in the design. Thanks

 

[berkeman]

Perhaps involving some sort of dampening might be an added advantage in the design. Thanks

Yes, maybe some dampening, and probably best to use soft tires (not hard casters) with a reasonable radius.

 

[Tim Allen]

Can anyone help please? @Nidum ? I have changed the design slightly from what I posted originally. The legs are now braced at the bottom. I need some assistance in completing force diagrams/stress analysis... HELP

 

[Nidum]

Please post a drawing of the structure that you now propose using . Preferably a proper technical drawing done using CAD but a good hand sketch will do pro tem .

 

[Tim Allen]

Please post a drawing of the structure that you now propose using . Preferably a proper technical drawing done using CAD but a good hand sketch will do pro tem .

 

[Tim Allen]

@Nidum for some reason I cannot see your reply, it says 'error' when I click on the attachment..

 

[Nidum]

(1) Assume that the column , legs and cross struts are all pin fixed to each other .

(2) The column is not supported at the lower end by contact with floor or by rigid connection to the cross struts .

So how does the weight load from the instrumentation unit distribute between the column and the three legs ?

 

[Tim Allen]

I think the weight load from the instrument and self weight load of the columns is transmitted through the 'locking mechanism' (a collar which is tightened) when the tripod is extended at height, and from here spreads down the legs and column by means of a head piece which supports the connections for the top of the legs. Is this what you mean or have I misunderstood your question?

 

[Nidum]

Essentially correct . The instrument and column weight loads sum at the connecting collar and are supported by the legs only .

So what does the idealised structure and loads diagram look like for anyone leg and cross strut ?

 

[Nidum]

Do you want some help to draw the diagram ?

 

[Tim Allen]

Do you want some help to draw the diagram ?

Yeah, I'm not certain where to start. Can it be based on the drawing that you referenced in your reply above? That's what I had in mind...

 

[Nidum]

ok. Get back to you asap .