Calculating Steel Tube Size for Wrecker Boom: 5Ton SWL

[Dman1801]

I have a question about how to calculate what size of steel tubing to use in building a boom for a wrecker. Tentatively the compressed length will be 12, and extended I would like to be 28' as this will be a 3 stage boom. I would like to have a safe working load limit of 5Ton at full extension. I need to know tube diameter and wall thickness to attain this. If further specifications are needed, just let me know.

 

[Baluncore]

It seems similar to a 5 tonne crane with a telescoping boom. Or maybe a JCB Loadall.
There are many solutions. They depend largely on mounting, control and attachments.

There are a few complexities. Is the “5 ton” a static load or an impulse load. Will the surface of the boom ever come into contact with any other object. Does the boom move quickly in azimuth and so generate side forces.

The machine would require several features to comply with workplace safety rules. For example duplication of some critical components and hydraulic cylinder lock valves.

Dismantling structures, (or trees), is very dangerous because an unknown load suddenly appears on the boom. For that reason cranes are not used to hold things while they are dismantled. The part being removed must be supported independently during separation, before being lifted by the crane. That permits the required crane instrumentation warn of and avoid excursions from the safety envelope.

 

[Dman1801]

5Ton load would be static for the most part, in that it would be lifting vehicles, not tearing them apart or recovering them. The hyd side movement of the boom would be limited to be very slow as to keep control of the load. And yes, would have cylinder lock valves. It is basically going to be a very small 'Rotator Wrecker' when finished. Meant for and only used on cars and light trucks that weigh less than 8000lbs.

 

[Dman1801]

That's not to say there would not be impulse loading, and therefore a certain safety margin needs to be built in.

 

[Travis_King]

What is this going to be used for? I'm pretty sure that in most countries there are various, and numerous, certification requirements which must be met before a crane design can be used. At the very least you'll need to conform with your country's material standards, welding standards, Health and Safety standards, etc. And generally require certification by a Professional Engineer.

Here's an OHS regulation set from Canada

http://www.workcover.nsw.gov.au/formspublications/publications/Documents/cranes_hoists_winches_guide_4422.pdf

 

[Dman1801]

I am going to build a wrecker (tow truck) ground up. Light duty unit, with a rotating boom. This is not a backyard, weld some old junk together project. This will be a very detailed, planned and designed piece of equipment. There is a huge gap in the light duty market, and the ultimate goal is to start building them for sale to the public. But for now, I'm covering the details first. And ya, certification is on the list. Plan on building one boom, and failing it to see what the limit it. Than address any weakness issues.

 

[Travis_King]

You might find this useful:
Whiting Crane Handbook

And a guide to american standards for crane design

 

[Baluncore]

I do not believe that Dman1801 appreciates the technical and legal challenges here. The only way to design and build a successful and safe prototype, is to look at how others do it, or to carefully refine and evolve a design over 10 years.

Weight is critical in mobile applications because the available payload is reduced as the crane weight increases. There are now many folding arm crane designs available on the market, they are colloquially called HIABs. They weigh significantly less than telescopic booms.

 

[nvn]

Dman1801: For sizing this particular round steel tube, I guess you could use a yield factor of safety of FSy = 1.50. That seems to be what section 25.41(a) at this link implies. (I am referring to structural steel, not forged steel parts.) However, check the yield factor of safety in the crane design code for your jurisdiction, and adjust the FSy value accordingly.

I am assuming your round steel tube is ASTM A500M, grade B, steel, which means tensile yield strength is Sty = 290 MPa, in which case you would need to use a tube thinness (n1) not exceeding 48, where n1 = d2/t1, d2 = tube outside diameter, and t1 = tube wall thickness.

It sounds like your given applied load is P1 = (4540 kg)*g = 44 537 N. Your boom fully-extended length is L1 = 8534 mm.

You must install a braking mechanism that limits braking deceleration to a maximum of 1.35 g, for both vertical and horizontal deceleration. You must install motor limits to limit load acceleration to 0.35 g, for both vertical and horizontal acceleration. Thus, when you combine vertical and horizontal acceleration or deceleration, I think a combined dynamic amplification factor of daf = 1.50 is allowed.

Therefore, using all of the above data and assumptions, and solving for d2, the necessary tube outside diameter at the base of the first tube segment would be, d2 ≥ 4.022*[FSy*daf*L1*P1/(290 MPa)]^(1/3) = 4.022*(1.50*1.50*8534*44 537/290)^(1/3) = 576.8 mm. And the tube wall thickness would be t1 ≥ d2/n1 = 576.8/48 = 12.02 mm.

Note that the above d2 equation is not a general formula, and therefore does not apply to any other scenario.

 

[SteamKing]

A circular section crane tune may not be the best use of the material. Most telescoping booms use deep rectangular tubing sections to better resist bending loads.

If the proposed wrecker unit is going to rotate, then some consideration must be given to the stability of the vehicle when it is lifting, particularly over the side. Tires by themselves are unstable, which is why you see large mobile cranes which are equipped with outriggers. The outriggers support the entire vehicle on mats while it is lifting to ensure that it does not overturn with a suspended load.

This is not a simple project, especially for the shade tree mechanic.

 

[Dman1801]

It will be square tubing, and out-riggers will be used for stability.

 

[nvn]

Dman1801: In post 1, you said you wanted to know the tube diameter. Round tubes have a diameter. Square (or rectangular) tubes have a width, not a diameter.

Because you now changed the cross-sectional shape in your question, it invalidates the second and fifth paragraphs of post 9. For square (or rectangular) tubing, those two paragraphs no longer apply.

Are you sure you want square tubing? Or would you rather have rectangular tubing? If rectangular, what cross-sectional depth (h) to width (b) ratio would you want? Something like h/b = 1.5?

 

[nvn]

Dman1801: Using a square tube, I currently get a tube width of 450 mm for the first square tube segment, and a wall thickness of 15.0 mm. But check the crane design code for your jurisdiction.