Stress question to help streamline component

[adamgnt]

Hello all,

I've inherited a project designed by a previous engineer (which we all love!) and am taking a second look at it, attempting to streamline some geometry. I've attached some images of a clevis hook used to gain a mechanical advantage by use of its lever arm. The hole shown is for a half inch bolt and the green arrow represents an external holding force. When the holding force is released it (should) allow the clevis to pivot counterclockwise and let go of the pin cradled between the hook and another vertical fixture.

My question concerns the bulk of material to the right of the pivot point. I don't see it adding anything structurally. I have seen similar geometry added in an attempt to balance the moment of inertia around the pivot point, however, in this application this is unnecessary. If anything I would want to it to naturally hang open (cradle facing down) if released with no external downward load. I did a real quick Solidworks stress test (which I realize is by no means a full FEA analysis) and it seemed to confirm that the material to the right is not being taxed by the downward load when in equilibrium (unreleased). Why should I keep this material in? Just looking for another set of eyes to help catch any oversights... Thanks!

 

[drawoh]

adamgnt,

Could you please put your graphic up in some format other than DOCX? A lot of us cannot read this format. GIF, PNG, JPEG and PDF are reliable formats for graphics. DOCX is the latest version of Microsoft Word.

JHG

 

[PetieSmo]

adamgnt,

Are you able to get documentation and/or speak to anyone who was involved in the design/fab/test of this part? That may give you insight into the design intent of the previous engineer.

Depending on your application, I might be concerned about fatigue. Based on the image you sent (and I realize it is scaled), the hook is deflecting quite a bit - compare the degrees of rotation in the hook compared to the degrees of rotation in the lever arm. The "extra" material could have been the engineer's attempt to control this deflection. You could experiment with reallocating some of the "extra" material from the East side of the hook to the SouthEast quadrant of the hook.

Peter

 

[MikeHalloran]

I like the hook as is.
I think if you start raping material off the right side, you will have stability problems in the material that remains.
I'm not sure that SW's simplified analysis will help you evaluate that, unless you put some out of plane asymmetry into the problem.



Mike Halloran
Pembroke Pines, FL, USA

 

[CoryPad]

Do you need the material on the right for kinematics? A constant radius for constant action? Otherwise, it appears that it is not required.

 

[corus]

I'm a little suprised there isn't more stress around the hole as you should have some bearing stress there in the ligament above it. I suspect that you've just fully restrained the hole and so have masked that. An alternative way of loading the part is to restrain it at the hook and apply the loads in the holes, perhaps as some sinosoidal function around the circular hole. I think SW can apply loads as per 'pin' loading in a hole anyway.

Deformation is a little misleading as I suspect that SW has scaled the deformation so it's more clear as to how the part deforms under load. The problem is more of fatigue damage, as others have said, and the main concern is at the hook radius. Removing material to the right of that would increase stresses there by increasing the bending stress component. You could of course have the part thickened locally to the hook radius and then remove material to the right of that without affecting stresses at the hook radius.

Incidentally, docx is ok for me as Word just converts the document so it can be read in older versions.




corus

 

[GregLocock]

I agree with corus, the stress distribution shown doesn't tie up with the description of the function of the thing. Therefore criticizing the current design seems redundant.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[adamgnt]

First off, thank you all so much for your prompt replies.

I've attached a JPEG of the hook as it appears in the assembly.

I made my first post rather hastily, and I see how it came across a little misleading. I can't find how to turn off the scaling in SW, but the image was indeed scaled is excess of 300x. I think attaching the stress analysis did more harm then good submitting this to you guys (who are always so helpful). Ignore the stress graphic, and just take a look at the new assembly image. To give some sense of scale, the circular portion of the hook is about 5" in diameter. It pivots about a 1/2" bolt, capable of supporting a load of 2500lbs.

@corus: Good point about switching around the fixtures/loads in the model. Believe me I by no means plan to use any "express FEA" tool to base design off of, it was more a quick litmus test to help visualize where the stresses were being distributed. I switched them around, fixing the cradle surface, and distributing the load proportionally, "x" to the end lever and "5x" to the pivot hole. The results gave the same ridiculous scaling, except this time the pivot hole simply stretched upwards. The stresses were still concentrated between the pivot point and the cradle, barely anything towards the right side of the piece.

@CoryPad: This was my point, there are no kinematic benefits (should have been my word choice) to the extra weight on that side. If the material isn't essential to preventing structural failure, we could REALLY use the cost savings and improved versatility of the set-up.

This product works as is, we've tested it as a prototype, however that bulk of material makes the geometry difficult to implement in other applications, i.e. one variation where two are placed back to back. I don't plan on cutting off the whole end and expecting it to work, it just seems to me like there is disproportionate amount of material placed in the wrong area. But I'm going on MY logic and a gut feeling. There are so many years of tangible experience out there in this forum, I truly enjoy the communities input

Thanks so much,
Adam

 

[adamgnt]

Here's another image zoomed out. Maybe it better shows how the "hump" could be considered significant enough to be at least addressed.

 

[CoryPad]

As a ballpark, you should be able to use the distance from the hole to the edge and keep that constant as the ligament that remains after removing material from the right side of your part. Have a look at the attached image.

 

[rstupplebeen]

I would come up with a couple of loadings including some all or more of these (ultimate load, out of plane, fatigue, tolerance stack up...). Obtain the appropriate allowables preferably from internal testing. Determine a desired FOS and then let the analysis determine what material is required. I hope this helps.

Rob Stupplebeen

 

[corus]

I'd reduce the amount of material under the hook. The amount of material should be about the same as that around the top hole as the load there must be similar (as it's almost directly underneath). If you look at the top clevis it has a big radius. Aim for that and undercut the horizontal line between the holes (see sketch).

You also need some limit on the stresses you'll get with any new design. To be conservative a limit for fatigue should be for an infinite life, or approximately half the UTS of the material. Also make allowances for surface finish.

corus

 

[cpretty]

I assume you are using the FEA express in SW by your comments on how you set it up, and the look of the pictures. This is totally inadequate to give a good indication of any stress. You can't constrain it properly, and it is greatly simplified.

A quick thought on any analysis. The hook needs to be loaded at it's very end. When releasing, at some stage the end of the hook will be taking the full load (momentarily).

As it appears to be a lifting device with quick release function, I would make sure you know exactly which standards the design needs to meet. If it is for marine, or lifting of humans, then you will need to be very confident in any stress induced.

To my mind the shape looks pretty good, and the amount of work to streamline it is significant. I would just make a number, and overload in different scenarios looking for where and how it fails.

Sometimes one can spend too much time 'improving' a design instead of 'proving' it.



Craig Pretty
Tru-Design Plastics

 

[flash3780]

I wasn't able to open your FEA because I cannot read DOCX format. However, a few comments:

I assume the cylinder on the LHS is a release mechanism, and that the lip engagement into the bolt retains the hook from falling off.

1. What's the advantage of removing the material? Are the production quantities of this item sufficient to justify the risk of removing material? The current configuration of the rotating bit is quite strong.

2. Do you really HAVE to pivot on a bolt? Could you use a rivet or welded in pin or something? Bolts have a nasty tendency of backing out when you twist them. If you have to use bolts to pivot on, consider safety wire or some sort of thread locking mechanism.

3. The arm poking out to the left looks disproportional to the rest of the part (on the top and bottom). Can they be made thicker?

4. If you were to remove material from the RHS, be sure to reanalyze. I don't know your loads, deflection limits, etc, but you'll want to make sure that nothing fails until well after the release cylinder drops the hook.

Good luck.