Bending and shear force confusion….

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Dave, I fail to see any opposite sliding forces in your diagrams.

Regards Nick.

[duncan webster 1Participant @duncanwebster1]

Dr Dave's diagrams look reasonable to me, what is an 'opposite sliding force'

[DrDaveParticipant @drdave]

Michael, reading many of the posts in this thread, it does appear that there is some confusion about shears and bending. I tried to clarify my earlier comment about a pin-in-socket leading to failure of the boss. I think your comment about “double shear” and Loctite suggests reacting the moment at the end of the pins by shear in the Loctite, rather than bearing pressure as I assumed in my example. I did use both assumptions the once when I was desperate to get an acceptable answer, but that is poor practice, even if I say it of myself! I hope that you did not construe this as a slight on your good self.

Nick, can you explain what you mean by sliding forces?

Dave

PS: to clarify the terminology, for those that might not be familiar with the diagrams that I showed, shear force and bending moment as used here are internal forces in a structure, not externally applied forces. The top diagram shows the applied forces; the lower two show how the shear and moment vary along the length of the beam.

[Robin GrahamParticipant @robingraham42208]

Michael -thanks for you clarification of 'bearing fit' – I hadn't come across that before. Another bit of terminology I've learned.

DrDave – thanks for taking the time to draw up the diagrams and for your subsequent clarification of the distinction between internal and external forces. I don't fully understand your diagrams yet, but I'll work on it.

When I was a wage-earner part of my job was trying to help students understand physics problems. Often they would say that they were utterly confused. Almost invariably it would turn out that their confusion was the result of trying to build an edifice on a foundation of sand – my technique was to ask them what, precisely, they meant by the words they were using. Very often that clarified their thoughts and lead them to to the answer without me having to tell them.

Embarrassingly, I haven't followed my own advice. I now realise that I said things like 'shear' and 'bending moment' without knowing what, precisely, I meant. I'll get me coat and hie to the library.

On the practical front, the latest proposal for the apple press is to replace the cross bar with a long single-ended bar, like on a one-horse mill. Hmm. I may walk away from this project now.

Robin

 

 

 

Edited By Robin Graham on 11/03/2020 23:10:54

Edited By Robin Graham on 11/03/2020 23:11:59

Edited By Robin Graham on 11/03/2020 23:13:04

Edited By Robin Graham on 11/03/2020 23:15:19

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Duncan and Dave, did you not read my post on the previous page? This is the definition of shear that I was taught during my welding and fabrication technicians course. However it seems we may be talking about two different aspects of a similar thing, or definitions may have changed.

Regards Nick.

[duncan webster 1Participant @duncanwebster1]

Nick, I've just re-read your post of 10-03, where you state that the handles are not subject to shear stress. Sorry, they are. Being relatively long, the bending stress will be the more important, but something has to resist the force being applied to the end of the handle, and bending stresses are in the wrong direction. You don't need the 'scissors effect', all beams are subject to shear stress. The best book I've ever come across to describe this is 'structures' or why things don't fall down' by JE Gordon. It should be required reading for all engineering students before they get into the more arcane stuff about elasticity

The cross pin is in double shear as you say. A simple soul like me would just divide the torque by the diameter of the screw shaft and call that the shear force, Dr Dave has taken it to a higher plane, more accurate no doubt

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Duncan, thanks for your clear explanation, I can understand Dave's drawing clearer now. This aspect wasn't explained in the lectures that I had as far as I'm aware, although we learnt about basic structures and moments of force and beams bending etc. but shear stress's were directed at the bolts, rivets and welding, which in essence, is what I was trying to say with Robins Pin. Everyday is a school day.

Regards Nick.

[Tim StevensParticipant @timstevens64731]

Can I throw in a minor comment? The device shown originally is clearly not a super-engineered product, but what might be called a cheapo press not intended for intense or industrial use. I suggest, then, that any attempt to 'improve' things by applying greater forces to parts that were only just strong enough to start with will cause a loud bang as something fails. My guess is that the cross-piece through which the screw passes is cast iron, and cast iron is not wonderfully strong in tension or bending. If it is cast aluminium much the same comments apply, but in that case the bang might not be quite so loud. I hope that these comments (along with what has already been said) will save a lot of wasted engineering effort.

Do let us know if any mods are made and how long the press then lasts – Regards, Tim

[Robin GrahamParticipant @robingraham42208]

Tim, I expect that you're right. I'm doing this work at a distance, so haven't seen the machine 'in person' , but the yoke is cast iron for sure.

Among the first responses was this from Jason:

Posted by JasonB on 05/03/2020 07:09:03:

Can't help thinking that the top yoke casting will fail at one of the ends long before any steel handle, expect its remelted scrap iron and not SG Iron.

I think Jason probably identified the source of the eventual 'bang':

However, as I think I've said before, the question I was asking in this thread wasn't really to do with beefing up the press, it was more about clarifying my ideas about bending and shearing, using this project as a practical example. Thanks to the discussion I'm getting there.

I think I'm probably done with this project – the client's response to my suggestion that replacing the crossbar with a longer single-ended handle would create more problems than it solved was muted. But if I get further feedback I shall report.

The nice brazed joint on the post is thanks to advice from this forum – the jaw marks are all my own work.

Robin.

[HopperParticipant @hopper]

I wonder if the press is like many bench vices where the handle is deiberately designed to bend before any other component fractures?

[Robin GrahamParticipant @robingraham42208]

Posted by Hopper on 14/03/2020 00:27:32:

I wonder if the press is like many bench vices where the handle is deiberately designed to bend before any other component fractures?

It would be nice to think so Hopper, but I doubt it. The reason I was asked to make a new post was that when tightening down the yoke on the post, the yoke jaw would slip out of engagement, which resulted in a thread mangled beyond redemption. Partly operator error I suspect, but I did get my hands on the original post and the metal was pretty cheesy. If they can't get that right they may not have calculated everything out…

I didn't know about that deliberate design of vices – I picked up a Record 53E for next to nothing because of the bent handle:

It was a punt, I thought it would might well be knackered after that abuse but it works fine. So maybe Record designed it that way. Quite why anyone would have to exert that sort of force on a woodworking vice is a mystery, but thanks for that nugget of info!

Robin.

[Howard LewisParticipant @howardlewis46836]

Hopefully, two short handles will encourage the user to apply the torque symmetrically rather than with one long handle which will tend to bend, even if only slightly, the pressure screw, as well as the pillars.

(That would would add to the forces involved. Bending to a pressure screw under compression, and bending to the pillars under tension. Plus bending in a second plane for the cross beam. All of which increase the risk of failure of some component ).

Don't let the client near a tube or scaffold pole!

Howard

[Gary WoodingParticipant @garywooding25363]

I decided to try the FEA facility of Fusion360 on this.

I created 2 models. Both with a 30mm diam. boss on the end of a 13mm screw thread. I didn't bother to model the thread so its just shown as a 13mm bar.

Case 1. A 12mm diam. tommy bar, 200mm long, passes through a 12mm hole through the boss.

Case 2. The hole in the boss is a 12mm socket that 'just' reaches the screw thread. The tommy bar is the same 12mm diam. It fills the socket and extends the same length as one half of the through bar.

In both cases I fixed the bottom end of the screw thread and applied a 1000N force to one end the the bar.

The pictures show the stress results. The faint dotted lines on the stress diagrams show the greatly exaggerated displacements.

Here's Case 1. (I can't put 4 pictures in, so Case 2 is the next post.)

[Gary WoodingParticipant @garywooding25363]

Here's Case 2.

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