Tractor backhoe - homemade
- Thread starter BFreaky
- Start date
- Views: 1130541
Renze
Elite Member
With that profile height i would go with 4mm side plates and 6mm horizontal, or you spend a lot of money for nothing, while reducing the capacity of the digger due to its weight.
To me this sounds like the boom of a full size 13 ton Atlas excavator
How wide is the profile ? With breadth and height, i can give you a calculation of the bend force in ton/meter it can take at various plate thicknesses, so you have a broad indication of where you're at.
Renze
Elite Member
Will get back to you tonight, i'm at work now and lunch time is over in a minute
Renze
Elite Member
OK its weekend, got time for some calculations: As a reference i take the aforementioned Schaeff backhoe, which has 125mm cylinders for boom and stick. The profile is 240x260mm, the horizontal flange is 8mm and the vertical flange is 6mm. It would have a bend resistance at yield strength of 18 ton/meter. Now we must keep in mind that both cylinders in this design (just like in your design) are both mounted on top of the boom, so they pull on the same bracket: that reduces the bending moment at the section right inbetween them. It has the boom and boom cylinder attachments spaced 40cm apart, so at 210 bar it would make 10.28 ton/meter force. the ratio between that would be a factor of 1.75 This is not an actual safety factor, as it is just indicational, because other stresses are left out of the equasion. However it gives an indication
If you want the same moment of inertia of the profile with the 200x430mm outer dimensions, 4mm would have the same moment of inertia as the schaeff backhoe with a much lower section height, but thicker plates. however because the profile is much higher and the cylinders arent spaced further from the neutral line of the profile in the same ratio, i would take 5 or 6mm for the vertical flanges and 4mm for the side plates so you have a big safety margin, and some meat to weld your cylinder attachment brackets and boom pivots to.
I havent set up the forces all up in a calculation program with consoles, and tensile forces which the cylinders apply on the profile (other than just bending) but you're wasting money when going with 8mm material, with the section height of the profile you drew
If you want the same moment of inertia of the profile with the 200x430mm outer dimensions, 4mm would have the same moment of inertia as the schaeff backhoe with a much lower section height, but thicker plates. however because the profile is much higher and the cylinders arent spaced further from the neutral line of the profile in the same ratio, i would take 5 or 6mm for the vertical flanges and 4mm for the side plates so you have a big safety margin, and some meat to weld your cylinder attachment brackets and boom pivots to.
I havent set up the forces all up in a calculation program with consoles, and tensile forces which the cylinders apply on the profile (other than just bending) but you're wasting money when going with 8mm material, with the section height of the profile you drew
I've taken everything you said into consideration and decided to go with thinner side plates so I will use 6 mm
ST 52.3. Also, I will lower the boom profile a bit to loose its weight.
Renze, thank You for your time, effort and help, I appreciate it.
ST 52.3. Also, I will lower the boom profile a bit to loose its weight.
Renze, thank You for your time, effort and help, I appreciate it.
Renze
Elite Member
Sorry i meant to say, i'd take 5 or 6mm for the HORIZONTAL (narrow) flanges and 4mm for the side plates.
The thinner plates are also much easier to clamp into position, you simply cant roll them that precise so you'd allways have to clamp them to fit.
The thinner plates are also much easier to clamp into position, you simply cant roll them that precise so you'd allways have to clamp them to fit.
Sorry i meant to say, i'd take 5 or 6mm for the HORIZONTAL (narrow) flanges and 4mm for the side plates.
The thinner plates are also much easier to clamp into position, you simply cant roll them that precise so you'd allways have to clamp them to fit.
Well, I kinda like to make things a little stronger then they have to be, so I'll put 6 mm. :-D
Renze
Elite Member
Most determining factor in the durability of the backhoe is how you lead the forces into the profile: Most excavator booms are pretty thin walled, and therefor have large overlapping brackets to which cylinders attach, do divide the force over a wide area of the wall of the boom tube. Determining the profile size is the easy part, but determining the details on how to lead in forces and where to gusset etcetera, is what makes the quality of a product... However judging from the photos of the swingframe you already finised, you've got a natural talent on that
Let's say I can "see" the whole thing in my head and can anticipate how it will behave...that's why i draw so much before doing anything. I also intend to distribute the forces on the side plates by adding large sufrace plates with circular holes to make them as light as possible. This is now the final design and it's already ready for cutting. It will look like this:
