I am looking at designing a metal backhoe boom, looking for strength and
minimal weight. Rather than using a structural steel beam, I am designing a
honeycomb metal box. As only three sides can be welded with the honeycomb
structure, the fourth side being welded to close the box, I have a question
about which three sides should be welded with the honeycomb structure.
Obviously the two sides (left and right), but should the third side be the
top which tends to operate in compression or the bottom which tends to
operate in tension. If it makes any difference, one boom has a slight taper
from end to end (side view), the other a bit of a curve, curving up in the
center of the length (once again the side view). Anyone know?

Rick

I think that if you do the analysis, a honeycomb structure is not the
best approach here. A hollow box structure puts 100% of the metal as
far as possible from the centroid (center in cross section) of the boom.
A hollow box is also relatively easy to fabricate. Given the taper,
one would need to bend the plate a little before you weld it, but
heating it red hot would make that easier.

Honeycomb structures make sense for aircraft wings or boat hulls, which
have relatively thin skins which need the internal support to prevent
oil canning or excessive flex. 1/4 or 1/2 inch steel plate does not
need the internal support, especially if the width of the boom is only a
foot or so. If you had a boom that was 5 feet wide with 1/16 inch
sheet, a honeycomb might make more sense.

Richard



RJ wrote:
I am looking at designing a metal backhoe boom, looking for strength and
minimal weight. Rather than using a structural steel beam, I am designing a
honeycomb metal box. As only three sides can be welded with the honeycomb
structure, the fourth side being welded to close the box, I have a question
about which three sides should be welded with the honeycomb structure.
Obviously the two sides (left and right), but should the third side be the
top which tends to operate in compression or the bottom which tends to
operate in tension. If it makes any difference, one boom has a slight taper
from end to end (side view), the other a bit of a curve, curving up in the
center of the length (once again the side view). Anyone know?

Rick





--
http://www.fergusonsculpture.com
Sculptures in copper and other metals

RJ wrote:

I am looking at designing a metal backhoe boom, looking for strength and
minimal weight. Rather than using a structural steel beam, I am designing a
honeycomb metal box. As only three sides can be welded with the honeycomb
structure, the fourth side being welded to close the box, I have a question
about which three sides should be welded with the honeycomb structure.
Obviously the two sides (left and right), but should the third side be the
top which tends to operate in compression or the bottom which tends to
operate in tension. If it makes any difference, one boom has a slight taper
from end to end (side view), the other a bit of a curve, curving up in the
center of the length (once again the side view). Anyone know?

Rick

Have you actually calculated the saving in weight?
By going to a lighter gauge of material in your boom,
high stress points such as pivots and ram mounts may
require reinforcement of a size that may negate any
saving in the actual boom construction.

Tom

With simple box beams such as for overhead cranes the tension side is last
to go on and is not welded to internal baffles/stiffeners. The stiffeners
are stitched to the side walls and top plate.
In the case of a backhoe stick I doubt that internal stiffeners are as
important as a smooth transition to the bucket mounts and hydraulic cylinder
attachments. You might note the shape of outside doubler plates on a
commercial boom.
Randy

"RJ" wrote in message
...
I am looking at designing a metal backhoe boom, looking for strength and
minimal weight. Rather than using a structural steel beam, I am designing a
honeycomb metal box. As only three sides can be welded with the honeycomb
structure, the fourth side being welded to close the box, I have a question
about which three sides should be welded with the honeycomb structure.
Obviously the two sides (left and right), but should the third side be the
top which tends to operate in compression or the bottom which tends to
operate in tension. If it makes any difference, one boom has a slight taper
from end to end (side view), the other a bit of a curve, curving up in the
center of the length (once again the side view). Anyone know?

Rick

R. Zimmerman wrote:
With simple box beams such as for overhead cranes the tension side is last
to go on and is not welded to internal baffles/stiffeners. The stiffeners
are stitched to the side walls and top plate.
In the case of a backhoe stick I doubt that internal stiffeners are as
important as a smooth transition to the bucket mounts and hydraulic cylinder
attachments. You might note the shape of outside doubler plates on a
commercial boom.

Is there such a thing as a tension side on a backhoe? They dig/pull,
but they also lift. Unless I am missing something about the "anatomy"
(which piece is the stick, where/how the cylinders attach, etc.), either
side could be in tension or compression depending on the task. Is one
or the other set of loads severely limited such that it does not matter?

Bill

"Bill Schwab" wrote in message
k.net...
R. Zimmerman wrote:
With simple box beams such as for overhead cranes the tension side is
last
to go on and is not welded to internal baffles/stiffeners. The
stiffeners
are stitched to the side walls and top plate.
In the case of a backhoe stick I doubt that internal stiffeners are
as
important as a smooth transition to the bucket mounts and hydraulic
cylinder
attachments. You might note the shape of outside doubler plates on a
commercial boom.

Is there such a thing as a tension side on a backhoe? They dig/pull,
but they also lift. Unless I am missing something about the "anatomy"
(which piece is the stick, where/how the cylinders attach, etc.), either
side could be in tension or compression depending on the task. Is one
or the other set of loads severely limited such that it does not matter?

Bill
For all practical purposes, lift is not much more then the weight of the
booms and the material in the bucket. There will be times that objects will
be lifted rather than dug from the ground. The tension side can be assumed
to be the underside of the boom the majority of the time.
Rick

A hollow box structure is indeed the most common form for a backhoe
boom. JCB made a few with a lattice construction in the 1950s, but these
are the only exception I've seen. I know you're probably building it for
your own use, but make it strong as backhoes lead a hard life! Have you
seen an operator lift the rear of the tractor using the backhoe, then
shift it a foot or two to the side? With a strong backhoe it's a
tempting thing to do, and of course it reverses the loading on the boom.
If a boom or dipper fails, it will usually crack through the central
pivot. The pivots are the places to add extra plates for reinforcement.
The best advice I can give you is to look at some backhoes closely and
see how they're built.

Best wishes,

Chris

For all practical purposes, lift is not much more then the weight of the
booms and the material in the bucket. There will be times that objects will
be lifted rather than dug from the ground. The tension side can be assumed
to be the underside of the boom the majority of the time.
Rick

Wouldn't lift often far exceed that? I know there are times (such as
stump pulling) where I have levered the entire tractor off the ground,
only contact points being the stabilzer pads and the backhoe bucket.
Total machine weight is in the 14k lb range.

It's equipped with a thumb, and I routinely move logs 16 feet x 16 in
with it (pine).

It never fails to amaze me the amount of force that can be generated at
the bucket once you start to understand the leverage of the hoe in
different positions.

Leo