H-Frame Press - weld or bolt?

[hopefuldave]

At last a sunny day, started tidying the patio and came across a machine pallet and.got thinking, as you do...

It's built from hefty (4mm) steel sections, a pair.of 1150 long 100 x 45 U channels, a pair of 1150 long 60 x 60 angles and another three 1200 long 60 x 60 angles, so there's a fair bit of steel in it! I think I can get a H-frame 1150 tall, 575 wide, about 450 between verticals, max height after bottle jack and horizontals about 900mm, so a sensible size? Using the angles with flanges "outwards" I'd have room to pass anything up to 100"" wide from side to side so all looks good so far

Now to what has me scratching the old noggin... I could stick weld the frame together, which would be good rigidity wise, but then it would have to be cut apart if anything bent / broke; bolting up wouldn't be as rigid but would mean it could be dismantled - and it could be squarer, as no welding distortion! Any thoughts?

Next ponderable, what would be a suitable jack, in terms of lifting power? I've seen a couple in the 10 to 20 ton range on eBay in my price range, is that too much for the 60 x 60 x 4mm angles (four of 'em) to handle? Are welds more.likely to fail than (e.g.) 16mm bolts or 25mm pins through crush tubes?

Any advice or thoughts welcome!

Dave H. (the other one)

[Lew_Merrick_PE]

Dave,

I would vote for bolting the frame together.  It can be disassembled for transport or repair.  There is no question as to the strength of the connection.  And, if a component starts to fail, it's being able to pivot as it fails is no mean safety advantage (in most cases).  Assuming you are using metric bolts, I would consider an 8.8 grade (SAE Grade 5) as a minimum here.

The thing about the size of the hydraulic ram to use is that this should really be determined by the beams against which it is pressing.  (1) I normally work in pounds & inches, so my metric is not as readily off the top of what's left of my mind for standard values such as the tensile modulus and the like; and  (2) I am not sure how much you know about things such as area moments of inertia and the like.  However, the real decision here should be made based on: (A) the stress level in the beams at full load; and (B) allowable deflections for the tasks you will undertake.

The work platform beam(s) and ram reaction beams will be loaded in bending.  Just about everything else in the press will be loaded in tension or compression which is much more straightforward.  Bending is usually the primary failure mode to consider.  If you wish to e-mail me (tangent@olympus.net) with sizes/shapes and dimensions, running those numbers only takes a couple of minutes.

[hopefuldave]

Thanks Lew, you're a Gent...

I found my old copy of Machinery's and a calculator, ran the numbers and the uprights have an UTS of around 20 tons each, no big issue there, assuming the formulae hold as long as I'm using the same units all the way through? I substituted mm and Newtons for inches and pounds, finally converted to Pascals to get "metric" stresses.

Bending the crossbeams, however... 20 tons puts the stress in the U channels up around 4x mild steel's tensile strength,  so either I need a rethink on the crossbeams or I get a bit less of a jack! I have more of the channel available or some hefty box section,  I'll think of.ways to use the existing material to better.advantage first, e.g. having the U channel with its longer axis (the web) in the plane of the forces and doubling up - time to run the.numbers again!

Thanks for the offer of calculations, I might run 'em by you once I've had a go - my brain could use the exercise...

Dave H. (the other one)

[hopefuldave]

OK, ran the bending stress again with the beam rotated, a single crossbeam comes close to the limit at 20 tons ( 370 Newtons/mm2 vs 420 safe stress)... So double up on the crossbeams? And get a 10 Ton jack? I'm not sure I have the deflection calculations right yet, I got a very small deflection...

The crossbeams are open U section ( no lips, square outer corner, about 4mm / 0.160 radius inner), 100mm / 4" web in the vertical plane, two sides of 45mm / 1.75", supports call it 500mm / 19" apart with a central 20 Ton load. The idea at the moment is to sandwich a 100mm x 50mm 2mm wall box (an old Volvo towbar!) between the channels, seam-welding the "outer" corners to give a "flanged box"  with a 4" wide, 2" deep channel on the tension face - on the 'moving beam' the box would leave a 100mm / 4" gap at the centre to allow pressing shafts etc. through, and stop short of the ends to leave room for the uprights.

Hopefully this will be strong enough...?

Dave H. (the other one)

[Lew_Merrick_PE]

Dave,

It sounds as if you are using American Institute of Steel Construction (AISC) C4 X 6.25 lb/ft channel.  That is a u-shaped section with the "base" being 4 inches (101.5 mm) wide with the "legs" being 1.77 inches (45 mm) tall.  They would have a (parallel to the 4 inch base) Area Moment of Inertia (I) of 4 in^4 (about the centroid -- what you are using here) and a Section Modulus (Z) of 2.43 in^3 (also about the centroid).  (Almost everybody in the world uses AISC rolled steel sections because they defined it all back before WWI.  So-called "metric" sections are just AISC sections dimensioned in mm.)

They are almost always made from ASTM A36 steel which has (minimum properties of): Tensile Modulus = 29,000,000 psi; Yield Strength = 36,000 psi; Ultimate Tensile Strength = 58,000 psi; Shear Strength = 30,000 psi; Shear Modulus = 11,500,000 psi; and Elongation = 20%.  (1 psi = 6894.8 Pa.)

Rectangular sections have an Area Moment of Inertia given as I = bh³/12 where b is the "base" perpendicular to the load and h is the "height" parallel to the load.  The Section Modulus would be given as Z = bh²/6.  The "answer" will be in whatever units you are using -- i.e. mm^4 or m^4 or Barleycorns^4 for Moment Area of Inertia or mm^3 or m^3 or Barleycorns^3 for Section Modulus.  Just make sure you do not change units moving from Moment to Load and Stress!

Does this help?

[hopefuldave]

Thanks Lew!
I've been scrupulous about sticking to the same units (mm and Newtons) throughout, so I guess my results should hold true - I think the channel may be the bastard offspring of US and Chinese standards, as the web thickness is 4mm, about 160 thousandths rather than 248, which puts it at about C4 x 5.4... But thinner walls and broader flanges...  This may explain the unexpectedly low deflection I got, it's stiff (high section modulus) but not strong, as using a single beam with the web vertical it isn't *quite* at the elastic limit at the projected load!
I think I'll definitely double up on the crossbeams, probably weld in some braces between the flanges to prevent buckling, and bolt it up with strategic crush tubes - I may sacrifice some convenience for strength by making the "work" beam bolted rather than sat on cross-pins.

Thanks again,
Dave H. (the other one)

[Lew_Merrick_PE]

Dave,

Yeah, I have seen major corporations having to deal with Chinese versions of standards.  I sat and watched as supposedly ASTM A569 low carbon sheet steel broke when folding a hem.  My spark test analysis suggested that the steel had somewhere in the 0.55% carbon content (and actual tests later confirmed it was an 1158 type of steel).  Until you have seen what passes for certificates of compliance from Chinese steel mills, you don't know what satire is!

Chinese industry cannot be as bad across the board as my (roughly) 3 dozen experiences suggest.  But I cannot say that I have ever had a good experience when dealing with major corporate outsourcing to China operations.

[hopefuldave]

That's the joy of Standards, so many to choose from... Or to ignore.
I think your impression based on experience is about 105% accurate - I'm really not a fan of Chinese "engineering", seen too many imported machines etc that haven't even tried to meet basic safety standards and had to put 'em right... As for machine inspection reports with the handwritten measurements printed along with the form...

Dave H. (the other one)