Mechanical Engineering

# G B Reid MIMechE, SIMarEST

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1. ## Temperature compensation formula

Its worth just trying out a standard thermal expansion coefficient calculation. (You have not stated the material that you are measuring) Take the material thermal expansion coefficient of the material you are working with and calculate its diameter for 30 degrees C...then calculate the same for 60 degrees C...I think you'll find a good correlation. Then just use the 60 degree figure for your measurements. Hope this helsp
2. ## How to Measure Torque Required for motor to spin a rotating shaft?

With any motor, the power is from P = IV The power (in Watts) used is proportional to the voltage (in Volts) AND the Current (in Amps).....If you up your voltage, the ampage should drop significalty...as you've experimentally discovered, if the voltage is too low, the ability to supply a sufficient current (ampage) becomes the limiting factor. New (DC) motors are rated for a no load speed at a specific VOLTAGE...the current increases subject to the load to match the power requirements...and CRITICALLY the wire requirements are rated on CURRENT. If you run your motor at too low a voltage, your wires will overheat - both externally and internally - the varnish will evaporate from the winding wires inside the motor and the motor will either burnout through a resulting internal shortcircuit or catch fire! Hope this helps I shan't - at the moment - expand further as your experimentally discovering lots on your own which is by far the best way to learn....but I'll be happy to look again for additional questions. Good luck!
3. ## Mechanical Numerical of the day

Okay...enough is enough...my turn! Assuming the chain links form a cyclic and therefore have a natural load-spread between both legs and no stress concentration: Stress = F/A Stress = 75 000 000 N/m^2, F = 50 000 N A = 50 000 / 75 000 000 = 0.000 667 m^2 for both legs A = (pi r^2)/2 per leg so r = root (0.000 333 / Pi) = 0.010 301 m d = 2r = 0.020 601 m = 20.601 mm - always round to the safety side....so: Therefore use 21mm dia stock minimum!
4. ## Shaft Design & Analysis

I'd agree with DrD....that said, ball-bearings are inappropriate in this application....I'd use taper roller as it has both an axial and radial component of force, otherwise - due to the mass of air, mass of the assembly and the resultant action - you will have premature bearing failure!
5. ## Mechanical Numerical of the day : May 18 2021

Marin; NEVER feel ashamed for attempting something and making an error....being ashamed should be left to those who do not try - and I include myself in that (ie I have not tried and that IS shameful!!!). I've been an Engineer now for 30 years, and the one thing I can say without fear of contradiction is you NEVER learn anything from getting things right...that just generates hubris...it reassures that the calculations are done correctly, but that can occur without the deepened understanding needed to be successful in the field! Celebrate mistakes, especially if you learn from them! And to tackle my "Shame": Using the H7/u6 details on the quoted text (reference docs not available) Generally on a "nominal" 120mm dia shaft, allow for a "runout" of 0.1, so all dimensions should be a minimum of the runout on both sides (120 - (0.1*2)) = 119.8 - ie the maximum diameter of the shaft MUST be below this value. Therefore assuming a "base" of 119.6mm to allow for the +.166 of the shaft... Therefore shaft at u6 = 119.744 - 119.766 (on a "base" of 119.6mm - +.144+.166) Hole on H7 = 119.6 - 119.635 (on a "base" of 119.6 - +0.000/+.0.035) It's definitely an interference fit...now, where did I leave the nitrogen...? I may be wrong; I look forward to Saurabh's answers! Warmest wishes! Bruce
6. ## Eco-cooler

There are a couple of different types of "Eco Cooler" design....one utilising water - sometimes known as an evaporative cooler works by taking advantage of the energy of evaporation of water....basically as water (or any liquid for that matter ) evaporates, it absorbs a certain amount of energy... If this utilises natural airflow through an opened window, it will have the effect of cooling the air enterring, but will increaee the humidity within. The other form (inverted cones) takes advantages of the ideal gas laws...it effectively forces a minor pressure drop in the air which reduces the air temperature, but this may not be sufficient to actually notice. Both of these do work to some degree, however if utilising a powered driver there will be energy consumed there. The best form of heating/cooling is that of a "heat pump" which moves heat from one area to another and only "cost" is the power to do so...they can have an energy multipier effect - sometimes of as much as 8x - where for a power input of - say - 1000 Watts - a heating/cooling effect of 8000Watts can be achieved.
7. ## Mechanical Numerical of the day : May 18 2021

Just perusing the answers to this quiz...I find them...well....."interesting" answers to say the least and...er.."worryingly" consistent. If the shaft starts at 120mm you need to think on the maximum size of the shaft available....are you going to spray-weld the shaft to increase its size...before you even begin? If you look at the question it states the SHAFT diameter is 120mm....
8. ## Mechanical Numerical of the day

Balaji, you have rounded down - ie to the "unsafe" side. In fasteners of any kind, you would always round up...always "enhance" safety, not detract from it.
9. ## Differential Pressure measurement

I may no t be understanding you r question properly, but it looks like you may be misunderstanding what the "static pressure" actually is.... When the flow increases through an enclose space - be it pipe or venturi - the presure reduces proportionally to the velocity of the flow. Basically, the faster the flow rate, the lower the pressure is. This flow rate presure (was historically....there are now sensors that can....)/(is) difficult to measure directly, so there's a "trick" that can be used to infer - with a very high degree of accuracy - the velocity of the fluid - be it gas or liquid - in the pipe....that is the measurement of the static pressure. Effectively, the static pressure is the pressure to which the internal flow presure drops. This is from memory, so I may get it upside down, but it still works anyway as it is a ratio of proportionality....it's: rho1/v1 = rho2/v2 The static presure measurement is the measurement of the rho(1 or 2). It's basically the mass flow calculation at two different speeds for the same mass...rho = m/v This then becomes rho1/v1 = m and rho2/v2 = m and, as m is the same, rho1/v1 = rho2/v2 Therefore, if the flow velocity increases, the pressure drops - this is measured by a static tube measuring hte drop in the static pressure as a result of the change in velocity - so therefore any change in velocity DOES affect the static pressure. Hope this helps
10. ## Alternative for thermal spray welding in scarcity of oxygen

Now this is entirely theoretical!! It may be possible to use "arc-spatter" to advantage. (My welder's knackered at the moment so I cannot try it to see...) If a (semi-sacrificial) earthed-point is located immediately proud of a MIG-tip, and the power and gas-flow ramped up to "silly levels", the spatter from the feed wire "should" spray on to the target metal. It will likely take some fine tuning, but - despite the increased use of gas - it may be a workable solution. Basically, the "earthed-point" will spark the arc...if the power is high enough, the metal should near-vapourise, forming a fine spray, being easily blown - by the shielding gas - onto the workpiece where it will solidify. If it works, it's an oxygen free solution. Please do feed-back if this works - good luck! My thoughts and best wishes are with all in India at the moment....and indeed all who have been either directly or indirectly affected by this terrible disease wherever they are in the world!
11. ## Cannot figure out why I get this error in COMSOL

Hi there "Alwayslearning"! I've had a similar error in the past using ANSYS (or may have been NASTRAN...?)...when I gave up after banging my head against the wall for about two days, I got on to the Technical support guys... ...it turned out in my case that it was a migrogap between two edges (where there should not have been a gap). This resulted in the model be discontinuous from the analysis perspective. The model was redrawn and the problem solved. Hope this helps.
12. ## Force applied to fulcrum

Hi there...it is just a simple moment....on the fulcrum itself, it is the total force applied in the down direction. I'd calculate the moment on one side then the other, then total the vertical forces and the "reaction" is the sum. I would do both as - occasionally - the moment forces can exceed. typed in a rush sop hopefully makes sense - good luck!
13. ## This bad boy can run!

This made my day Dudley! The kid in me: - Fantastic!!!! The Engineer in me: - It may be able to run, but would you want it to...? The way I (have always) see(n) it is it has far too high a centre of gravity, far too narrow a track and - from the animation - a hyper extending knee joint, so if it did run, the would likely break its front legs, or fall over which - even stationary - is not a "nice" concept, but at speed... I never could "square" the design being an "all terrain attack transport".......Now the Walkers on the other hand... (AT-ST)...seem a far more "considered" design! Thank you for sharing this, you really have put a broad grin on my chops! Bruce
14. ## Seaengine design to use combined rising air bubbles as force

I believe this is known as a "hydraulic bouyancy lifting engine" (the Hindenburg was an Air-buoyancy lifting engine) The easiest way to calculate the usable force is to try it out with a tube set vertically in water and catch the water that comes out over a set period of time (say 10 seconds)...the usable energy (Work) can then be calculated back from the volume of water that has been ejected. Work done is Force times distance (W = Fs). The volume of the water lifted would be linked to the mass of the water x G...this would be calculated as energy gained (PE) = mgh. With regards to the amount of force each balloon applies, it is a buoyancy, so it is the volume of the balloon that displaces water - the volume of water displaced (less the mass of the balloon and whatever is tethered to it) is the upward force. Compared to air, the water is very viscous, so the terminal velocity of the rising balloon will be limited suite quickly by the resistance of the water it is pushing through (your diagram states 3ft per second...?) The energy for the lowest balloon will be near as dammit equal to the pressure at the point of origin - very slightly higher - though it may have to be significantly higher depending on the elasticity of the balloon...the "danger" would be the balloons bursting as they near the surface due to the expansion of the air as the pressure drops. For reference "Electrical answers " is not quite accurate. Energy is measured in Joules. Watts is a measure of energy applied per second, so 1 Watt would be one Joule per second. 800 Watts is 800 Joules per second. in my first paragraph, the "mgh" is measured in joules. In the air-con unit, it benefits from a "free" energy boost as a result of an evaporative (or condensing) phase resulting in a significantly greater usable energy output than that input. Work done is also measured in Joules. The difference between the usable work done and the energy used is the efficiency. (Note for equations "s" is "Spacial displacement" - ie distance. Time is always "t". I know many new to the field get "s" and seconds confused, so its worth mentioning.) Unfortunately, this is nowhere near enough information for you to go on - I understand - but hopefully it is a start in the right direction. Unfortunately, there is only so much time that can be spent on "interest items" when work beckons! If I can spare some more time and expand on this, I will do, but my visits of late tend to be sporadic and short-lived due to other commitments, so there are no promises made! Good luck...
15. ## Seaengine design to use combined rising air bubbles as force

This is a system that has been used for many years to provide an upthrust in dredging. This displaced water at the top provides a negative pressure at the bottom which is automatically pressure adjusted, and very - often dangerously - powerful! The pressure needed to create the bubbles at depth is slightly greater than the depth of the water, so there is still a net loss...but that does not mean that the system cannot be utilised do a specific end...but it should not be seen as a route to perpetual motion...e.g....if you can get compressed air bottled at a sufficiently low cost, you could use this system to displace water to power a hydro turbine. There are methods that do produce a net gain in energy utlilsiable...heat pumps utilise a phase transition to give an energy boost and a "net gain" against input energy. e.g. an aircon unit is an example of a heat pump...to quote one I've just looked ups...it takes an ~800W input and can give a heating or cooling output of about 3.3-3.4kW, a significant net gain! {Note: Not typos...British-English spellings!}
16. ## Swimming pool pump sizing

Pool circulation requirements are well documented in various standards, and vary depending on the geographic area...basically, UV light exposure (and intensity) of the water has a significant affect on the requirements. Once you have the recommended pool water circulation/filtration rate, it is a relatively straightforward calculation. I think from memory (so may be completely wrong!) for a low use pool in the UK there was a minimum filtration rate of 3 x volume per 24 hours...but (again from memory) on my kids (very large) paddling pool I ended up going for 8 x volume due to surrounding foliage. First port of call the local regs!
17. ## Graduated in BS Mech E, did not enjoy any class in special. Not sure what field to pursue/specialization. How did you overcome this challenge?

Bob I have been an engineer for the last 30 years, and I still do not specialise...for me, there's just too much I find interesting! (Certainly in the UK,) many companies offer "Graduate Schemes" to help address exactly the predicament you find yourself in. These take you into the real world and offer additional training where you will utilise your degree skills and knowledge and develop the necessary skills to develop in engineering. You may find this helps, or you may even find yourself being pulled in other directions. One of the absolutely wonderful things about Mechanical Engineering (as opposed to other fields of study) is its value in so many fields - interdisciplinary within engineering itself (aero, thermal, hydro, statics, civils, chemical etc) but - even outside of the engineering field the training is highly valued...so you may benefit from widening your view...? Good luck!
18. ## Speed Transmitting Between Motor Gearbox and Gears

Hi there I'll try to help out without giving you the answer...(if I do, you'll be no better off than before!) Most of the information you have is superlative! You know the chain pitch, and the chain length. You also know the length of the drive wheel and the speed with which the drive wheel is rotating (from your text 93.33 rpm). As you have calculated the rpm of the T20 wheel, and you know the diameter of this wheel and the pitch of the chain, can you think of any way to translate this into the speed of the chain across the wheel...? If you can figure this out, everything should fall into place! Good luck!
19. ## Design working period calculation of cranes

Do you mean the "Design Life"...?
20. ## Ball Screw Vs Roller Screw

I would definitely head for the roller screw option - they completely eliminate backlash which - for your apparent application - would be very important. The above said, I agree strongly with DrD....discuss with your suppliers and bear in mind that their answers will be loaded heavily to get you to buy; not necessarily what is best for you, but what will be best for them. If you always have that at the back of your mind, it should enable a good, informed decision. Good luck!
21. ## The question is about set screws differences

In short contact pressure. The Dog point you would tend to want to either locate in a hole, or provide resistance with minimal surface damage (such as on a retractable height adjuster on a stand...it will grip, but not damage) The cup point will dig in to the surface a little...providing a little firmer grip at the "cost" of damage to the surface. Generally more used for semi-permanant fixing. There are various other ends available too!
22. ## making a pendulum car as fast as possible

Interesting project... ...I would be inclined to use the pendulum to build energy in a store - such as a spring or elastic band - in order that it acts as a sink and releases. You use the basic F=ma...so you can increase the F.... the trick would moderate the release so that you don't loose it all in wheelspin... Thinking aloud....you could use the pendulum to ratchet a gravitational potential accumulator which would naturally release the energy in a controlled fashion giving maximum acceleration while preventing wheelspin...? Good luck!
23. ## Can vibrations be created due to large pressure differences in air

Of course they can cause vibrations...it is exactly the same as a flute! the AMPLITUDE of the vibrations, however, is going to be so low as to make them negligible! It is likely to whistle, but the only ay it could be anything other than a minor peculiarity is if it hit a resonant frequency...which SHOULD be checked for as a matter of course... (Tacoma Narrows Bridge)...check your shell frequencies and you should be fine.
24. ## Aircraft Architect

The beauty of Engineering is its cross-disciplinary nature. I wouldn't suggest an architect design an aeroplane, but a structural engineer could!
25. ## Drag Coefficient on Right Angled Isosceles Triangle

I had a look round and I can't find anything definitive. I always used to find the GRC a great resource, but its become a nightmare to navigate...this may be a start... https://www.grc.nasa.gov/www/k-12/airplane/shaped.html I would suggest it's a hybrid. On a flat plate, you have the face effects, and then the vortices draging against the back. This will have a "resolved face effect" and the same rear vortices. You may be able to find a reasonable facsimile by combining the two..? Good luck!
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