Thread: I need some help calculating strength/torques for a new project

the answer to your query depends ...

on the longevity required from the toy,

on what sort of moving parts the toy has

on whether the toy is ride-on,

on whether if a piece broke on the toy a user be injured,

on the desired final weight of the toy,

on whether the toy includes a motor or pressure vessel,

on how good you are at making wood joints,


as a first pass, I'm thinking that calculations won't be required, thin metal sheet can be replaced by 3 mm AA ply and steel members by straight grained hardwood.

Jan,

It will not be a toy, well, maybe for big boys In short, it will be this. The plans I have call for a 38x38 mm (1,5"x1,5") iron square tube with a wall thickness of 1,5 mm (0,0625"). This tube needs be 2 metres long.
I want to replace this tube with baltic birch multiplex. I want to glue 4 pieces of 12 mm thick together, creating a width of 48 mm. I want to drill holes in the center 2 layers for weight reducement. I will make sure the holes will stay 12 mm from the sides, and 12 mm from each other, so they still support the outer 2 layers. The height, in which the direction of the force will be, will be 50, maybe 60 mm (probably 60 to be sure). Basically I create a tube of 38x60 mm with 12 mm walls. It needs to carry my weight (100 kg) + I need to be able to jump on it. Some flex over the length is allowed, but not more than say 20 mm under force of a medium jump.

If it breaks it will not hurt small children. Only my ego, and a lot of my time spent on expensive firewood.

Pogo

from what you describe, flexibility is more important than pure strength

all I have done is a google search, but to a first approximation, a 38 x 38 mm hardwood beam will be around the same strength as a 38 x 38 x 1.5 square steel tube. One reason for this similarity is that square steel section is made to dimensions that allow it to be substituted for timber (softwoood) framing.

Originally Posted by Pogo02

It will not be a toy, well, maybe for big boys

Why post this in toy making? Try reposting in a section of the Forums where those members with appropriate experience can assist you. To (medium?) jump on a 38x38x1.5 SHS that is 2m long would eventually cause metal fatigue. What would it do in timber then?

I looked up a reference on strength testing similar to what you are proposing. This excerpt is from the Journal of Structural Engineering:

"Two series of tests with different depth wood I-joists were conducted to study the effects of circular and square web openings and the placement of steel angle retrofits at openings. Finite-element analysis of all test configurations was used to improve understanding of how web openings and retrofits affect the stiffness, stress distributions around openings, and ultimate failure mechanisms. Test results show failure loads well above prescribed design loads; however, the presence of web openings affected the type of failure modes. Control joists failed at the tension flange, whereas joists with web openings failed in shear through web openings or by means directly attributable to the presence of an opening. Ultimate capacity and stiffness was generally reduced by the presence of a web opening, with openings located closer to high shear areas producing worse results. Square web openings lowered the ultimate capacity approximately 10% more than circular openings when located at the same location. Steel angle retrofits improved capacity if an adequate retrofit length was used to redistribute stress concentrations. Finite-element models accurately predicted decreases in stiffness due to web openings; however, the models predict an improvement in stiffness due to the presence of steel retrofits that was not ascertained in experimental results."

I would suggest that weight saving by drilling the two inner layers could compromise the structural integrity of the beam by cracking inside that might not be seen until the beam ultimately fails. You could forego the drilling and live with the increased weight or increase the depth of the beam and reduce the width to save weight. The strength of a beam increases by the fourth power as the depth increase, ie: double the depth of a beam and you increase its strength by 16. The width of the beam doesn't do this though.

Craig

Edit: I see ian has also suggested solid timber.

It may be worthwhile chasing up some info on how they made aircraft using glued up wooden beams.
Similarly boatbuilding using West system epoxy.
If making a laminated box beam a layer of carbon fibre might be helpful.
There is some info on the net re recent bicycle construction with wood.
Sounds like a fun project.
H.

Pogo

things that will influence how much wood you need to replicate the performance of the 38 x 38 x 1.5 square section and what type of wood include:

• the steel section the plans call fall will fail in a different way to a replacement section made of wood -- this may or may not be important but needs to be considered.
• how the steel section is anchored at each end will have an influence on the section's bending and torsional strength -- this may or may not be important but, again, needs to be considered.
• how and where loads are applied to the steel section is also a factor in selecting a suitable wooden replacement
• this link https://www.reddit.com/r/AskEngineer...al_comparison/ suggests that a 50.8 x 50.8 x 2.4 mm square tube has the same strength as a piece of 38 x 90 (nominal 2 x 4) softwood framing arranged on edge. Your tube at 38 x 38 x 1.5 is significantly lighter than this, so on that basis, a piece of 38 x 90 framing pine is too strong for what the plans envision.

Note that I've deliberately used the phrase "replicate the performance" as without knowing where the steel member is in the structure or what sorts of loadings are applied to it, it's very difficult to be more precise

I haven't read the posts in-depth but why not add carbon fibre between each layer, using epoxy, if doing a laminated section as that will help with holding it together while allowing flexibility?

Need more details of what you are going to do.

Engineers don't give out advice willingly on forums for a reason. If you are making something potentially dangerous giving advice is potentially not a smart thing for engineers to do.

Notice how many forums out there have people asking for advice yet very little professional replies? Garage forum is a great example of this.

What are you actually making? Engineers don't give any advice unless they actually know what they are making. Iron or steel tube? What grade? There's a lot of factors that make a lot of difference.

Drilling holes, laminating with carbon etc sounds great and all but it gets overly difficult to design something with confidence. And having confidence it is actually built properly.

Designing to a specified deflection is easy enough but how does one quantify a medium jump?

Make a detailed, dimensioned drawing of exactly what you are making and your chances of a professional helping are much greater.

Please don't think I am having a go at you. Asking for help and advice is obviously a great thing to do but many more details are required before you can get a meaningful response.