At what load will this bracket start to un-bend 2

[PaulCTEC]

.125” Mill Aluminum 5052
1T bend radius's

I have put my entire weight on the assembly at 220 lbs (using 6063 T5 extruded) with no visible deformation. Just curious at what load they will unbend with the 5052 (without spring-back).

Trying to decide between .125" 5052 or 12Ga CR steel. Corrosion resistance vs strength. There should really never be any more than 100 Lbs of dynamic load on the pulley but curious as to what load will destroy the brackets if abused. If I had a pull scale I could test but I don't.

The reason for the 90's vs a shallower z like 45's is to keep cables from getting between the bracket and the spool.

Thanks

 

[berkshire]

Paul, Welcome back I have not seen you here in a few years. 5052 condition 0 yields at 13,000 psi, you do the math.
B.E.


You are judged not by what you know, but by what you can do.

 

[racookpe1978]

The limit is the deflection needed to "bend" (partially-straighten) the two bends of the bracket from the very short axle going through the two bracket holes. Could be right side, left, or both (each deflecting to different lengths).

If your axle were restrained at both ends (even a mechanical cotter pin), then there'd be no failure at even 3x load.

 

[PaulCTEC]

racookpe,

I did a poor job of illustrating the clevis. I should have drawn a snap hook or shot a pic when I get to the shop this morning. I agree in that if the top ends cannot spread, they will be much stronger. the snap hook I am using will allow very little spread under load and in fact should force the ends together the higher the load. the "math" i kept coming up with was crazy high in the range of 600 lbs, which can't be right.... 5052-H32 has a 28000 PSI tensile strength btw. I'm just going to bolt one bracket to the wall and pull straight down til it fails, then double that weight for my number.
I was just sitting around Saturday and couldn't figure it out using formulas.

Thanks,

 

[btrueblood]

Seems like the first (lowest load) failure mode is when the two sides holding the axle deflect inwards under load, and start scraping against the pulley.

 

[Tmoose]

"5052-H32 has a 28000 PSI tensile strength btw."
The (ultimate) tensile strength would be the number to use if outright breakage from one big yank was the concern.

Like Btrueblood said, I'm thinking the First failure mode is when elastic deformation (temporary) lets the two legs bow inward and scrape the pulley.
That would depend on geometry and "E" modulus of elasticity.

You asked about when the " bracket starts to un-bend" . If the concern really is permanent deformation/unbending, then the material's yield strength would rule, modified with Bauschinger effect, work hardening, and blah blah blah.

With the .62 " radius pulley/spool groove, what size cable are you using, that it could sneak beside the pulley?


There is a reason Google only finds a few square shouldered pulley "blocks".

http://imavex.vo.llnwd.net/o18/clients/runyonrental/images/7600.gif

http://www.hiseamarine.com/photos/Aluminum-alloy-pulley-block-151019155306.jpg

https://sc01.alicdn.com/kf/HTB1mQMikx3IL1JjSZPfq6ArUVXa0/5-Ton-Three-Sheave-Snatch-Pulley-Block.jpg

https://opimedia.azureedge.net/-/me...o-move-the-big-stuff/block-and-tackle-jpg.jpg

 

[PaulCTEC]

Hi Tmoose,

Cat5 network cable at avg OD of .20, Cat6 is closer to .25
In a couple early prototypes using 30 degree bends, when pulling multiple cables around a horizontal direction change, it was possible that one or more cables would be less taught than others and fall into the gap. This is exacerbated by the fact that the pull is not constant, it's a lot of pull - relax - pull type of thing. To solve that with the 30 degree Z, I could pop rivet cone washers to block the gap, but that's one more step. If I could make the 90 degree Z work, that'd be ideal.
And yes, pulling hard enough to deform the bends to the point of scraping is/was the concern. Typical Cat5 has a max pulling tension of 25lbs. I designed the spool to accept up to 25 cat5 cables, but in practice, I find pulling 10 or 15 at a time is about all I can manage efficiently. So.. with that said, in most cases if there is enough load to bend the brackets, the cables have been over-stressed.

As I mentioned earlier, I was shying away from the 90degree Z until I tried it and could not deform the 6063 brackets by lifting myself off the ground. No one is going to want to work that hard to pull cables....But that doesn't mean that someone won't figure out a way to overload it.

What's the old engineering adage... If you make something idiot-proof, someone will just make a better idiot.

Conical washers i was considering with the 30 degree bends

just for FYI

 

[SceneryDriver]

The climbing industry already makes what you're looking for. It's called a knot passing pulley; it allows a knotted rope (or two, tied end to end) to pass through the pulley. It's also a split sheave, so it's easy to get off the cables you just pulled.

http://www.rescuetech1.com/extra-largeknotpassingpulley.aspx

SceneryDriver