"Lifting frame" - Sept 2017

Observation of a workplace lead to conjecturing this alternative lifting equipment.
The "floor-loading" is so restrictive that any proprietary counterbalance lift-truck uses-up just about all the load-bearing capacity, and leaves a tiny payload.
Most applications can have the load within the wheelbase - unusual but is the case in this application.
So here is what I ended up with.

It would have two fixed wheels at the front and a rotating powered wheel at the back.
That power drive could be something like a "Honda GX-series" petrol / gasoline engine, maybe with centrifugal clutch. The distances travelled are trivial, and low weight of petrol wins over low per-hour fuel consumption of diesel in this application...

The weight would be in the low 100's of kg, and free up most of the weight of a commercial counterbalance lift truck as "payload" for this lifting-frame.

For its application of lifting, a strop can be wrapped over the horizontal top beam at any location, supporting any generic hoist - eg a chain-block (manual or electric).

My "safety-case"

the legs and beam have thickness enough to make them "plastic" (you could bend this structure to the ground but it will never buckle and collapse)
the 3-way intersection can take as much as the 2 legs and beam
the intersection is Finite Element modelled (here) showing ability to take load and low stress-raising to limit fatigue-cracking vulnerability
a prototype should be tested to destruction in various overloads
a prototype should be fatigue-tested far beyond any plausible lifetime number of load cycles
the frame is lightly pressurised with air and has pressure gauges(s) - which are checked before each shift and after any suspected untoward event - the lifting frame being removed from service if ever the pressure gauge(s) show zero pressure

Explaining the latter point more:
this is a "non-destructive test" for a tubular structure which is hermetically sealed with regard to the outside environment but fully-cross-connects so all parts of the structure share the internal pressure. Which is introduced through a valve similar to a tyre-inflation-valve.
Typically drilled holes at necessary locations enable gas to flow between the tubular members at welded intersections.

Pressurising a tubular structure is a fatigue-crack detection technique used on aircraft with tubular lattice frames - in the USA on "experimental" (home-made) planes and reputedly quite extensively in the former Soviet Union and Eastern Europe.
Typically the pressure-gauge would be in the cockpit, and seeing a positive pressure exists in the tubular structure is part of the pre-flight checks.

This is a first-principles obvious method of warning if there is a through-thickness fatigue crack somewhere. It doesn't tell you where, but it has a 100% probability-of-detection of telling you if there is a through-thickness crack somewhere - irrespective of paint and coatings, location, accessibility, geometry, etc

The pressure would typically be something like 1Bar/1Atmosphere.

Obviously, knowing there must be a crack somewhere, the lifting-frame is immediately taken out of service, and whatever detection / inspection method(s) will find it are applied.

The possibility to use this technique of pressurisation as a fitness-for-service indicator comes from using Structural Hollow Sections for the entire structure.

Units shown in the solutions

These are SI units.

lengths - metres (m) - example 0.00123 == 1.23mm [deformations]
rotations - radians [deformations]
stresses - example: 1.234E+08 == 123MPa, etc [all stresses - Principal; von Mises]

Line-element representation of overall 200mm square frame

This line-element model prefectly well shows the overall stress-state along the lengths of the tubular structurals.
The imposed loads and constraints can, and do here, exactly match and represent the real loadings the lifting-frame is intended to bear

The imposed load here on the top-beam is 4.5Tonnes-force
(which is many times higher than the current payload with a lift-truck(!))

The model

The stresses

Shell-element representation of 3-way tube intersection

This complex computationally resource-intensive model has to detail solely the intersection area.

The finite element analysis model is a longitudinal half, as the component has a longitudinal-vertical plane-of-symmetry, and symmetrical loading is satisfactory.
Which enables a plane-of-symmetry to be applied in the finite element analysis model. Halving computer resources.

By the way - this model took 8 minutes to run - about the maximum beyond which the resources required crashes my computer, and much more than the usual under-a-minute entire elegant solution times for my typical shell-element solutions.

The model

Meshing (fine!)

with loads (green arrows at lowest part of leg - 2 tonnes-force rearward and sideways spreading the structure) and constraints (all the red "stuff" - fixed support for beam-end and plane-of-symmetry down axial centre)