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Member postings for DrDave

Here is a list of all the postings DrDave has made in our forums. Click on a thread name to jump to the thread.

Thread: Con rods, stressman needed
14/07/2020 18:53:15

I missed the obvious: tensile loads at the ends of a connecting rod will want to be in the outer fibres to align with the lugs. So you have two driving criteria. Buckling gives a I- beam that is fattest somewhere between the crankpin & centre of the rod, and tension requires the ends to be the same size as the lugs. Fair it all together & you get the average conrod.

As an aside, the title of the thread is interesting. Over thirty-something years, I have worked with scores of Stressmen, but I only recall two Stresswomen.

14/07/2020 17:33:18

c99d2a10-c951-4eab-8d5f-fbbd947588bd.jpeg
To answer SoD, this is what happens when you let a Stress Engineer loose with a conrod...

Ref. http://www.formula1-dictionary.net/engine.html

14/07/2020 17:27:45

Two loads are of interest: compression due to steam pressure and accelerating the piston, etc. And transverse loads due to the mass of the con rod being accelerated up and down by the crankpin. So a classical pin ended strut with compression and transverse load.

The transverse load gives a bending moment in the rod, zero at both ends and maximum, not in the middle, but offset more towards the crank pin. Hence con rods normally being an I-beam (or H-beam on some car engines), often tapering towards the little end. For the loco rods, I dare say that asthetics and ease of machining dominate the design.

Dave

Thread: We need Pi
05/07/2020 12:13:14

+1 for the calculator.

But I must confess that, at University, I took it upon myself the learn pi to 10 significant figures, which was the precision of the calculator that I had at the time. And I can still remember it 40 years later!

Thread: Emco F1 Mill Upgrade
28/06/2020 15:12:00

I dare not connect the controller PC to t’internet! Any software that I need to put on it (I have just upgraded to Centroid’s v4.50 software, which also required .NET Framework) I download on another computer & transfer using a USB stick.

DrD

27/06/2020 20:09:08

I have done a few mods to the machine since updating the controller. I got fed up with swarf going everywhere, so a bit of bendy MDF came to the rescue. The lighting was also quite poor, so I have added a couple of led lights (they are intended to go under wall units in the kitchen) and now I can actually see what is going on!
7bf30274-2e3d-433a-9dee-4c2e5a668e1e.jpeg

27/06/2020 20:03:17

Hi Bitzer

Have you watched Matryscncgarage on youtube? I found his videos very valuable when I was setting my controller up.

Regards

Dave

Thread: What Did You Do Today 2020
26/06/2020 11:56:18

Removed duplicate posting

Edited By DrDave on 26/06/2020 11:57:08

26/06/2020 11:56:14
Posted by Nick Clarke 3 on 25/06/2020 09:25:13:
Posted by DrDave on 24/06/2020 21:36:35:

Heisenberg, my SX2 mill, was sounding unhappy, so I put a brand new end mill in and tried again.

Heisenberg?? Hopefully not because it is uncertain? laugh

You got it in one!

I never know quite how much is going to come off until afterwards...

24/06/2020 21:36:35

Bit of a mixed bag today. I thought that I was making good progress milling the excess off a block of aluminium for a new engine. Heisenberg, my SX2 mill, was sounding unhappy, so I put a brand new end mill in and tried again. Even worse. I got hold of the bed & gave it a wiggle: there is about 1 mm play on the Y-axis.

I wound the bed all the way out to see if I could tighten the anti-backlash nut only to find that the problem is that the nut has come loose from the bed. Ah, well, time to give it some of the TLC that it needs, and check to see what else has come loose.

Thread: Cloth on string that hangs down on old aircraft
05/05/2020 12:56:18

8277c614-2461-403d-bdfa-3806006ae48d.jpeg
I’ll get me coat...

Thread: Concorde model maden flight
13/04/2020 11:12:50

I used to work just down the road from Mühldorf am Inn, so I let out an involuntary “oh!” at the start of the video!

The airfield has seen other exotic aircraft, such as the Me262. There is also the site of a WWII vintage underground hanger that was being built for Messerschmitt in the woods nearby.

Thread: Model Turbines
10/04/2020 21:36:02
Posted by Turbine Guy on 09/04/2020 12:55:11:

...I also ordered a center drill with a 0.020 drill size and 60 degree included angle...

I didn’t realise that you can buy such a small centre drill “off the shelf”. Keep up the good work!

Thread: New design of mains plug?
30/03/2020 19:56:29
Posted by not done it yet on 30/03/2020 19:41:21:
Posted by herbert punter on 30/03/2020 18:10:53:
Posted by DrDave on 30/03/2020 18:01:29:

I have never understood the British love of fused plugs: most of the world seems to be happy with unfused plugs.

It’s because we tried to save copper after the war by using ring circuits for sockets. Other countries use radial circuits with the fuse in the consumer unit rather than the plug.

Bert

Quite right, too. Who really wants a 15A fuse, at the distribution panel, to protect a flimsy 3A lead from plug to equipment? I fit an appropriate fuse for the device being used. Most certainly not a 13A fuse for most things.

Question: Are all flexible leads rated at 15A in the rest of Europe? If so, that is a waste of copper!

I’ve just checked one of my Swiss socket strips: it says that it is rated at 10A max.

30/03/2020 18:34:15
Posted by herbert punter on 30/03/2020 18:10:53:
Posted by DrDave on 30/03/2020 18:01:29:

I have never understood the British love of fused plugs: most of the world seems to be happy with unfused plugs. None of the electrical equipment that I bought when I lived in Switzerland has a fused plug, for example.

Other than the regulatory requirement here (and after all it does sound like a good idea), can anyone explain why Britain and the colonies have this different plug philosophy?

It’s because we tried to save copper after the war by using ring circuits for sockets. Other countries use radial circuits with the fuse in the consumer unit rather than the plug.

Bert

Thanks for the explanation, Bert. I suspected that it might be something along those lines.

Dave

30/03/2020 18:01:29

I have never understood the British love of fused plugs: most of the world seems to be happy with unfused plugs. None of the electrical equipment that I bought when I lived in Switzerland has a fused plug, for example.

Other than the regulatory requirement here (and after all it does sound like a good idea), can anyone explain why Britain and the colonies have this different plug philosophy?

Thread: Bending and shear force confusion....
11/03/2020 19:56:39

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.

10/03/2020 21:48:05

There does appear to be some confusion about shear forces and bending moments... To clarify my earlier post, I have drawn up a pair of sketches of a bar passing through the centre boss and a similar arrangement with two stub pins. They show the shear force and bending moment diagrams for the bar and for the pins. As I said before, the peak bending moment in the through bar is very nearly the same as that for the pins (for the same force, P).

The real difference is in the pressure applied to the boss. For the through bar, this is roughly uniform at a level of, say, w1. For the pins, however, the moment is transferred by very high pressures between the pin and boss. The triangular pressure distribution required to react the moment at the end of the pin gives a peak pressure (w1 + w2) of about 6 times higher than w1. This commonly leads to a bearing failure in the boss.

There is the alternative load path for the pin-in-socket, as shown by Micheal G. I think that I have only had to use a "Desperate Dan" analysis like that once in over thirty years of doing stress analysis of aerospace structures! The pin-in-socket, below, is the standard method.

Regards

Dave

through bar.jpg

pin in socket.jpg

05/03/2020 16:22:28

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

Thread: Effect of Tensioning a Boring Bar
23/02/2020 20:01:44

As an addition, to avoid cluttering my post, above, any more:

From a reference such as Roark, the maximum deflection of a cantilever loaded at the tip is delta = force x length^3/3EI.

Here, I = pi d^4/64 = 491 mm^4. E = 210 x 1000 MPa and length = 60 mm. So force to deflect 0.05 mm is 3EI delta/length^3 = 3 x 210 x 1000 x 491 x 0.05/60^3 = 71.6 N

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