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Bending and shear force confusion....

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Robin Graham04/03/2020 23:16:56
709 forum posts
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Rather than trying to trying to frame my question in abstract terms, I'll give you the case in point.

I've got involved with making stuff for a charity. On of the projects involves modifying a fruit press:

vigopress.jpg

The latest requirement is to lengthen the arms to get more torque and hence more pressure on the 'fruitcake' to squeeze the maximum amount of juice out.

OK, slip some pipe over the handles. Eventually though the 'arms' will bend, assuming nothing else gives way first.

So to arrive at my question. If I made something like this:

presshandlemodsmall.jpg

would it be stronger than the simple 'bar through a hole in the boss on the screw'? I have a gut feeling that it would be because if the parts fit closely, the pin experiences shear rather than bending force. Gut one in my ruminations tells me that it's harder to shear steel than to bend it, but gut two says I'm getting something for nothing* so than can't be right and I'm not thinking about this in the right way.

Apologies for my roundabout approach to the question - if I had to vocabulary to express it more succinctly I'd probably know the answer.

Robin.

* First law of Thermodynamics - you can't win, you can only break even.

Second Law - you can only break even at absolute zero.

Third Law - you can't get to absolute zero.

Michael Gilligan05/03/2020 00:04:02
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Posted by Robin Graham on 04/03/2020 23:16:56:

.

 

presshandlemodsmall.jpg

 

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My first thought is that your new handles have significant stress-raisers where they enter [exit from ?] the boss ... it may be worth making the boss a fancier shape, to spread the load.

MichaelG.

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This sort of idea : https://www.enfieldelectrical.co.uk/products/die-stock-and-die-set-20mm-25mm

[ being the first suitable picture I found ]

Edited By Michael Gilligan on 05/03/2020 00:09:02

Mick B105/03/2020 06:41:12
1576 forum posts
84 photos

Where you say 'close fit on boss', do you really mean 'on pressing screw shank'?

My initial instinct would be to project the 'top-knob' above the top of the screw shank, so you could have a through-hole for both drive-pin (lower, through shank) and double-ended turning handle (higher, avoiding shank), but I dunno if there are faults with that.

JasonB05/03/2020 07:09:03
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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.

Clive Hartland05/03/2020 07:33:53
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OK, re-think the whole idea. Some years back i made a press, nice square thread and found that it did not need massive pressure on the pulp. In fact the initial pressure was enough to start the juice flow which then slowed, at this point more partial turning started it again.

If you have a massive turning pressure you will burst the bag with the pulp in it, huge mess to follow.

Actually forcing the pulp does not help it at all. What it needs is a gentle pressure because if forced the juice will be cloudy with particles which means more work later to clear.

The length of handles is not critical, just enough leverage to start the flow and to add a bit later.

Mine was for apple juice which was quite nice. I have a wide variety of apples and mixed cookers and eaters.

not done it yet05/03/2020 08:14:42
4648 forum posts
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Yet another case of attempting to over-rate a machine - to use it at a much higher power level than it was designed for. Likely end in misery.

Michael Gilligan05/03/2020 09:03:13
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I’ve just looked again at Robin’s picture of the press, and browsed for similar

... In the cold light of day, it looks much smaller than I first thought.

Very happy to accept Clive’s wisdom regarding the forces.

MichaelG.

Clive Foster05/03/2020 09:14:51
2205 forum posts
73 photos

#2 on what Clive H says.

Not much pressure is needed. Sounds like your "fruitcake" is coming up solid far too quickly. I'd look at the preparation process before you put the stuff in. Really it should never come up fully solid. Internal cohesion should always be loose enough to allow some flow.

Maybe your operators are getting in too much of a hurry and trying to squeeze the juice out in one fell swoop. As Clive says the process is squeeze a bit, let the juice flow out, when it stops squeeze a bit more. Repeat until done. Those hand presses aren't fast.

(another) Clive

Nicholas Farr05/03/2020 09:27:58
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Hi Robin, if your getting to a point where the handles start to bend, then there's the chance that the thread will be overstrained and may fail long before the life of the machine, assuming it is a good make, I'd guess it is made to function fairly efficiently as is. I don't know much about pressing fruit, but I have seen things on TV about it and the process seems to be as Clive H has pointed out.

Regards Nick.

Nigel McBurney 105/03/2020 09:50:31
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I made 100 plus handles and screws for a similar arrangement ie assembly presses and made improvements over time. first change was to make the handle from stainless steel ,initially I used en 8 but this corroded due sweat from the operators hands plus any fruit juice, the handle length needs to take into account the physical strength of the operator,is it a repeat operation or just occasional.Two handles into blind holes will eventually work loose,as the hole depth is shown as approximately shaft diameter. Make the boss longer and use a single handle in a through hole,the boss can ideally be a light press fit onto the threaded spindle with a cross pin,the pin needs to be a reasonable diameter ie about 1/4 inch and tight fit ,use silver steel, or if you have the tooling a taper pin is better,never never use roll pins,they come loose. the long top handle should be drive fit in the boss ie with a soft hammer. Now the one thing that surprised me was that I also secured the handle with a flat miled on the shaft in the centre and then locked with an hex skt grub screw, and the handles never came loose,the flat did give the grub screw a good seat and the screws had the ring type end and not a point which always seem to come loose. If used a lot those plastic handles on the end of the handle will work loose and be forever be coming off ,I have a Meddings drill with this type of handle end,and when I bought it s/hand three of the handle plastic ends were missing and another one loose,so I made turned aluminium handles secured with grub screws. The real way to pee off an operator is to have handles & knobs that are forever coming loose and falling off and have handles that make your hands sore ,dont knurl handles, years ago I found that assembly operators will cover any knurling with masking tape.

DrDave05/03/2020 16:22:28
197 forum posts
40 photos

Robin,

Coincidentally, I have been looking at a very similar problem at work today. The difference is that the contact pressure between the "bar and the boss" is nearly 6 times higher if you have two bars in the boss, rather than a single bar passing straight through. This causes failure in the boss if two bars are used. The bars have similar strengths for either case.

Dave

Robin Graham08/03/2020 00:58:44
709 forum posts
167 photos

Thanks for replies. Interesting as always, but no one has addressed my question directly - quite possibly because it was a daft one.

I'll try to express my confusion in another way. Scenario A. You have an infinitely hard and rigid boss cross drilled to diameter D. You bung a bar through the hole - the bar is idealised only to the extent that it also has diameter D, so a perfect fit. Otherwise normal metal. You put torsional forces on the bar and eventually it fails in the sense that it it becomes permanently deformed into some sort of S shape.

Scenario B. You make something like the sketch in my original post. Everything fits perfectly, and the only thing that can fail is the pin which also has diameter D. The pin is subjected to pure shear force in this scenario. I think! In this case 'failure' is any sort of plastic deformation of the pin.

Is there anything to choose between the two scenarios? I now think not, but still not sure!

Thanks for the practical advice about apple presses - I shall pass that info on.

DrDave - sounds interesting but I can't understand the relationship between what you're working on and my problem from your brief description. I'd be interested to hear more - it sounds like it might have been a surprising result.

Robin.

Michael Gilligan08/03/2020 07:53:18
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Posted by Robin Graham on 08/03/2020 00:58:44:

[…]

I'll try to express my confusion in another way. Scenario A. You have an infinitely hard and rigid boss cross drilled to diameter D. You bung a bar through the hole - the bar is idealised only to the extent that it also has diameter D, so a perfect fit. Otherwise normal metal. You put torsional forces on the bar and eventually it fails in the sense that it it becomes permanently deformed into some sort of S shape.

Scenario B. You make something like the sketch in my original post. Everything fits perfectly, and the only thing that can fail is the pin which also has diameter D. The pin is subjected to pure shear force in this scenario. I think! In this case 'failure' is any sort of plastic deformation of the pin.

Is there anything to choose between the two scenarios? I now think not, but still not sure!

[…]

.

So far, so hypothetically good, Robin ... except to note that that if ‘everything fits perfectly’ then the load on the pin is indistinguishable from the general internal loads ... The nearest practical implementation probably being to secure all those elements with something like Loctite 638 retainer.

If, however, your boss is actually a ’bearing fit’ on the shaft ... yes, your pin will be in double shear.

Given suitable choice of size and material for the pin: It is likely that either of these is simply transfer the problem area up to your new handle-fixings [which is where I came in], or [as Jason wisely noted] to the support structure.

MichaelG.

Perko709/03/2020 02:10:27
328 forum posts
24 photos

If I am understanding this correctly, the original has a full length bar of the same diameter as the pin in Scenario B, but the bar bends where it enters the boss. If that is the case then replacement of the bar with the pin and separate handles will achieve greater pressure, provided that the handles have greater resistance to bending than the original bar. In the original, if the bar is a loose fit in the boss, the bar is subject to both bending and shear, with the resistance to bending being much less than the shear strength. If the pin in Scenario B is a press fit, then it will only be subject to shear stress. Theoretically you will get greater pressure in the press, until you either exceed the shear strength of the pin or the bending strength of the separate handles, or the tensile strength of the boss where the handles are pressed into it.

As others have said, be careful that your improved design in one part does not overtax other parts not designed for the additional forces you are now able to generate.

Steviegtr09/03/2020 03:26:22
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Drill a bigger hole & stick a bigger bar through. Job done.

Steve.

Martin Kyte09/03/2020 09:40:20
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It's not really in shear is it?

To my way of thinking if the entire thing was solid the end of the handle where it enters the boss is in tension at the top and comprssion at the bottom and it's this couple that has to resist the force on the end of the handle. The failure would be a tear starting in the tensioned part of the handle where it enters the boss.

Clive Heartland has pointed out the primary problem which is a kind of hydraulic lock when you try and exceed the maximum rate at which the juice will escape. There is no point in trying to overcome this you will just break something.

regards Martin

Michael Gilligan09/03/2020 09:59:35
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Posted by Martin Kyte on 09/03/2020 09:40:20:

It's not really in shear is it?

[…]

.

Not sure if that’s a response to my post, or a general observation, Martin

If the former ... May I, for the avoidance of doubt, repeat what I wrote:
.

[quote] If, however, your boss is actually a ’bearing fit’ on the shaft ... yes, your pin will be in double shear. [/quote]

.

MichaelG.

Robin Graham09/03/2020 23:11:14
709 forum posts
167 photos

Thanks for further replies.

Perko7 - you have correctly understood my question. There was an implicit assumption that the new 'handles' would be more rigid than the originals in my framing of the question, but perhaps I should have been explicit. Thank you for your answer.

MichaelG - I think you said the same thing , but I didn't understand what you meant by 'bearing fit'. Presumably the same thing as Perko7's 'press fit'? Thanks for your response.

Thanks also to others for general advice about the design and operation of the press, which will be digested and passed on.

I was actually sceptical about the proposed modifications to the machine for the same sort of reasons that have been put forward here - something else will break. What happened was that in thinking about it I realised that I didn't understand the relationship between bending and shearing - hence the title of this thread. I think I might be getting nearer some sort of explanation in my own mind though.

Robin

Michael Gilligan09/03/2020 23:27:08
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Posted by Robin Graham on 09/03/2020 23:11:14:

[…]

MichaelG - I think you said the same thing , but I didn't understand what you meant by 'bearing fit'. Presumably the same thing as Perko7's 'press fit'? Thanks for your response.

.

Not quite, Robin ... ‘bearing fit’ [relating to the fit of your new boss onto the feed-screw] was meant to describe something which slides together, as distinct interim between ‘loose’ and ‘press fit’ [i.e. the fit you would expect to find in a plain bearing] : This is the circumstance in which the pin could most reasonably be considered as being loaded exclusively in shear.

... It matters not.

MichaelG.

.

Edit: for clarity ... your sketch uses the term ‘close fit’

Edited By Michael Gilligan on 09/03/2020 23:32:28

Nicholas Farr10/03/2020 08:35:02
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Hi Robin, shear stress, or shear strain is when something has two opposite and parallel sliding forces acting upon an item, like a pair of scissors or a guillotine. Your pin will have shear stress on it, but your handles won't.

shear stress001.jpg

The double shear mentioned, will be the same forces on the other side, but in the opposite direction. Your pin will only have bending forces on it if it is a loose or sloppy fit, but they will be small and limited by the clearance in the holes.

Regards Nick.

Edited By Nicholas Farr on 10/03/2020 08:49:12

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