Thoughts on designing a hinge in steel and aluminum

[alangbaker]

Greetings,

I have a project which will require some elements connected by hinges with (I think) fairly large loads in relation to the size of hinges I can use. Basically, two elements in structural aluminum (something suitable for machining) will be connected 3 pairs of smaller elements hinged in pairs to each other and to the aluminum elements at either end.

I'm not sure of anything yet, but I wondered if anyone could help me with a couple of questions.

1. Given the different strengths of the materials involved, I'm assuming that I should proportion the "fingers" of the hinge such that the cross-section in the load-bearing area for each material is in inverse proportion to the strength of materials. i.e. If the aluminum parts are machined from aluminum with yield point 40,000 psi) and the steel parts are machined from an alloy with a 60,000 psi minimum yield point, then if I would make 60% of the shared space for the fingers of the hinge from aluminum and 40% from steel. Are there other factors that come into play here? How does the differing modulus of elasticity play into it?

2. Is it better for overall strength and stability of the joints to have more fingers, or fewer? From the standpoint of the structures design, it's probably better to make the first and last finger in the aluminum parts and have the steel fingers start on the inside of each, but... ...how many repetitions? We're talking about parts that are going to be joined over a total distance of perhaps 3" with a 1/4" hinge pin with 1/2" diameter fingers? The fewest fingers would be one Aluminum, one steel, one aluminum, but that makes the steel element quite narrow in the middle and I'd like to see the loads spread out more.

I've attached a quick and dirty snapshot of a basic SketchUp model of what I'm talking about.

Any thoughts you can offer would be very much appreciated.

 

[Tmoose]

Is the pin oriented to point vertically, like my front door ?

1 down, 19 to go.

 

[drawoh]

alangbaker,

How do you figure this thing is going to fail? Will it wear out, or will it break under some severe load? Will people abuse this thing? In my world, rigidity usually is very important. Once you have solved that, the structure is very strong.

--
JHG

 

[TheTick]

Torsion against the hinge barrels is the biggest threat. Put more material on ends to resist. Anything to resist torsion in the hinge body is good. Ultimately, all stress ends up at a singularity on the barrel.

 

[EdStainless]

If you want a hinge strong and precise it needs to be as long as possible.
As stated the outer end barrels are critical for maintaining alignment.
Given that the steel is 3x as stiff as Al (at the same thickness) defection under load will be your biggest problem.
How much twisting do you need for these to resist?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[alangbaker]

I'm working on better figures, but there is going to be torque of approximately 450 foot-pounds at the centre of a system of 3 hinges that will transmit that torque from the outer component of the system to the inner component.

The axes of two of the hinges are tangent to a 4" (approx.) circle about that centre with the third hinge tangent to a circle of about 3".

I've attached a screenshot.