How to get sense on magnitued of calculations? 7

[Azzazil]

Hello all, when you calculate some force, moment or temperature how did you get sense on magnitude of your calculations what is big value and what is not?
Example 1) (Torque value)
I have M6 screw and I defined tightening torque to be 20Nm how to know if this value is OK or not if I did some mistake on my calculation, and how to visualize which value of torque is too big and which is small?
Example 2) (Heat/Temperature)
If I get on calculation that I will have some mechanical element that will be heated during his work to 80°C will you be able to touch it by hand or you will get burn.
Example 3) (Force)
I have 2 plastic parts which are joined by cantilever snap fit connection, assembly force is 50N. How to know if this is too big or not if I never assemble two halves of plastic parts to join them?

Basically my question is, are there some methods that you can get theoretically sense of magnitude you get by calculation or all of them I need test empirically?

 

[BrianPetersen]

There is no substitute for real world experience.

Get out in the workshop, and build or fix some stuff.

 

[SnTMan]

Get a bathroom scale and weigh some stuff. Get a fish scale and measure forces.

Regards

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[GregLocock]

I suspect most companies involved in designing stuff for people to use, or make, have collections of photocopied notes of typical values. One almost completely useless number I mentioned is that if you twist a knob between your finger and thumb, you can exert about 1.4 Nm. You don't want people applying more than about 50N once a minute. And so on and so forth.

Cheers

Greg Locock


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

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[JohnRBaker]

Back in the days when generally the only thing you had was a slide-rule, you had to learn quickly how to estimate what the order-of-magnitude of your expected answer was going to be since the slide-rule left the tracking of decimal places to the user.

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[hydtools]

I, at one time, would reference a Mil-Hdbk volume for human factors.

Ted

 

[dik]

Catch thread...

thread404-367235

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[danw2]

>Example 2) (Heat/Temperature)
If I get on calculation that I will have some mechanical element that will be heated during his work to 80°C will you be able to touch it by hand or you will get burn.

You'll get burned by temperatures over 50 Deg C.

 

[JStephen]

Pretty good article on heated surface here:

https://www.linkedin.com/pulse/protecting-against-burns-from-hot-surface-richard-palluzi

 

[drawoh]

I, at one time, would reference a Mil-Hdbk volume for human factors.

MIL-STD-1472. I believe it is at revision[ ]G at the moment.

I keep what looks like a fish scale in my briefcase. I bought it at a scientific store and it is calibrated in Newtons and kilograms.

--
JHG

 

[Azzazil]

OK there is no other solution than to empirically get this experience , thanks on advice of using fish scale I will buy one soon.

 

[dvd]

As far as bolt torque goes, there are tabulated data and calculators -

https://www.portlandbolt.com/technical/bolt-torque-chart/

I am a fan of the Skidmore-Wilhelm torque tension testers for the final word on fastener torques.

With regard to heat and fluid flows, it seems like a good check is to look at mass flowrates.

A lot of machine design stress analysis is more dependent on deflection than stress - it is possible to have fairly low stresses and end up with a machine frame, component, etc. that has way too much deformation. There are also simple resonance calcs that can be made based on deflections.

 

[Stick.]

You need to get some "landmarks" for the values that you commonly work with so that you can take your answer and compare it back to one of these "landmarks" as a sanity check. This is also how I approach working in metric since it's not my native unit system. For instance, your 50N assembly force doesn't mean much to me, but when I convert it to 11 lbf, I can start to get a sense of it. At the same time, I haven't worked much with low forces like that, so I'd be more comfortable with a different application where the force was in the range of 100 kN, since I can convert that to 11.2 short tons and compare that to machinery I've worked with in the past. Similarly, I can say that your 80°C surface is too hot both from my experience cooking but also since I am familiar with NFPA 86 and know that's above the 71°C it allows for exposed surfaces. For your torque reference, I don't usually work with bolts that small, so I'd have to check a torque table and go from there as a sanity check. If you were asking about M12 bolts I'd have a better sense since that's the size of the lug nuts on my car and I have that torque spec memorized.

 

[Gr8blu]

Azzazil There is no substitute for doing something yourself - at least once. Build or test something to get a first-hand grasp of what can (and cannot or maybe more importantly should not) be done.

With regard to your specific examples:
1. Torques - know your materials, your material grades, and your available material specifications. Grade 8.8 bolts size M6 have a max torque of 11.8 Nm. All this type of data is tabled in the appropriate standards. How much is 11.8 Nm on an M6 bolt? From my own experience, a reasonable estimate of "finger tight" is around 2 Nm.
2. Heat effect - again, know your materials and characteristics. And the standards. Water boils at 100 C (at elevations below 1000 m) - so 80 C is pretty hot. Chances are your home hot water tank is set for 55 to 60 C temperatures. At 60 C, bare skin will get a third degree burn in 3 seconds of contact. As a point of interest, if you set your oven to "warm" to keep something warmed up for later (or maybe slow-cook some ribs), the setting is roughly 95 C. For most of us, an insufferably hot day will have an air temp of about 40 C. Some desert dweller might handle 50 C, but that's pretty rare.
3. Force - snapping plastic together. Material properties and geometry, of course. Some will withstand it, others won't. You think you need 50 N? That's about the same force as a large bag of rice or potatoes (11 pounds actually, for those of us with Imperial measurements rolling around) would exert on the floor - or the amount of force you'd need to lift it up.

Converting energy to motion for more than half a century

 

[Ng2020]

I once had a colleague who described load in terms of Qty. XYZ Cadillacs. E.g. "5 kip = 1 Cadillac". It helped maintain some perspective. They probably weigh a bit more than that now.

 

[Azzazil]

Gr8blu thanks for your advice , btw. great post.

 

[LittleInch]

One which crops up in valves is the force on a valve wheel is normally 50N Max when a human tries to turn a wheel to do something (open a valve in this case).

for torque one other version is think of your car wheel nuts. Nominally about 100Nm. Think how hard it is to take them off sometimes with a small wrench....

Normally companies look at insulation for personnel protection at 60C. You can withstand that for enough time (1-2 sec) to pull your hand away without getting burnt.

100C plus all you can do is deliberately tap or swipe the surface without getting burnt.

The snap together bit is harder as it depends on the geometry of the part and how it fits into your hands. I think 50N can be ok if this is a squeezing action. but having two parts to push together held in your hand might be harder. also easier to push together than pull apart.

Remember - More details = better answers
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