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Everything posted by JAG Engineering LLC

" I really don't know what my duties are when I become an engineer" Welcome to the club! In my private practice I never know what is coming my way. Co Op is an excellent recommendation. It has been 30 years since my last degree and 42 since my first. I don't know how things are today in US colleges let alone your country. There is so much class work that practical office/hands on knowledge is not something made time for. If your professors have never worked in industry, they may not know what to tell you to expect. Two pieces of information that I have acquired I will pass along here. 1) Many colleges train engineers to be scientists. The disciplines overlap but they are different. 2) Approximate, Approximate, Approximate. Sort of an off shoot of number one. A scientist may require an answer to 5 decimal places but as an engineer it is unlikely. If your stress calculation indicates you require a steel bar with a cross section of 1.393739 inches, you will likely select a standard bar of 1.5 inches for most applications. If you are designing flying machines were weight is a critical factor, then you might have a bar made to 1.400+/.005 inches. A few weeks ago I was interviewed by an engineering student. He asked a series of questions and I answered more from experience than any text book, which is the intent of the interview assignment. As a follow up I directed the student to a series of events in my career I have documented and posted on a blog. http://www.jagengrg.com/blog Good luck, Joe Gulino

The word, exact, pretty much answers your question, NO! Exact implies Zero tolerance. Such a thing is not possible. Even if by chance you ended in the exact same spot how would know, how would you measure it? The best instruments will have a tolerance, with in 1/100, 1/1000, 1/10000000000000 ? close enough for any application i can think of, but not exact. A train stopping within a few inches, should be fine. Good trains drivers do this thousands of times per day in NYC subways.

Struggling with a worm gear analysis
JAG Engineering LLC replied to edave's topic in Machine Dynamics & Design Considerations
Sam, Scroll to last photo I sent. 
Struggling with a worm gear analysis
JAG Engineering LLC replied to edave's topic in Machine Dynamics & Design Considerations
Take a guess!. I am only 1/2 kidding. I did a school project so my memory is stretching back 40 + years. There are many possible solutions. Pick values and see what you get. If there is some limiting input factors given, and when you reach that point working backwards, you don't meet the requirement, select different values. Set up an Excel spreadsheet so you can do quick what if's to better see the effect of changing one parameter or value at a time. I still refer to the text I used back then. The original publication date was 1934. My 4th ed is from 1965. I don't know how this compares to any text you have now. Read sections 16.12 to end first 
Many handbooks will provide the formulas for the moment at any point for one or two loads applied at any point(s). Knowing the moment M at any point, Stress = Mc/I. Set up a spreadsheet if you are searching for some sweet spot combination of max load vs location. I assume when 10 lb was added to the 90 the beam collapsed. Not knowing any more about the situation at hand I will stop here. If this a real world application vs a lab demonstration for a class there is much more to consider.Good luck.

The new hard drives may have lost my automatic log in but I am in again. When I use any source I work through any example provided in step by step detail. If steps were skipped in the example, you may not notice the 2 in r/2 may have been dropped, just to mention one of many easy errors. I recall a professor who must have done the example in class countless times. He wrote 3x3 and came up with 12. Not that he did not know how to multiple, 12 was the incorrect answer, he just wrote down the wrong input but knew what the answer was. Proof readers can make the same error focusing on the answer and overlooking the input. If no example provided work out the units. And if possible do a simple estimate of the answer to get some idea if the refined answer is possible. I have briefly communicated with Dr D by email. In the attempt to demonstrate a basic concept the author may have concocted an impossible to build device.

Looks like I may have gotten it

I have analyzed trusses for class work, but never in professional practice. In homework I am given the geometry and the applied loads, and asked to find the loads or stresses in members, not too difficult. But coming from a blank paper where do you start to design a truss or bridge, with so many types to pick from? For a simple beam I know what to do. For a complex truss, vs suspension bridge how is this decided before all the detail analysis starts? In the attached photo the bridge over the RR tracks (right side) is a truss. Over the road way a different type. I assume the RR needs more clearance and puttinging the "depth of the beam" above the road way, provides the clearance. But why not a truss across the entire span, or the other type raised higher and extended. I assume there must be a significant cost difference but I have no practical experience to support this. Also I have seen bridges with a truss as show then other with the truss under the road bed or tracks for RR bridge. What drives the design?

Deflection of a pole with end load and UDL
JAG Engineering LLC replied to Erin HaywardLang's question in Question & Answers
The main point you are asking is can the results from different loading be added. For a static analysis, yes. Just think about applying a load at the tip and using the correct equation you will get a deflection. If you have doubled the load it would deflect twice as much. If you applied two loads each equal to the first load you would have the same deflection as a single load of twice the first. There are many reference books that will provide the equations for beams. A pole in this case is a vertical beam. Cantilever is the type of beam. As far as the taper, it could make a difference. The force at a point will decrease with decreasing diameter but the contribution to the moment at the base will increase as the distance (moment arm increases). Bending moment is very relevant. It may be the most relevant value. From the bending moment (lbft, lbin, etc) and the cross section of the pole you can determine the stress in the material. If you are addressing vibrations, Dr D. would be a good source for this info. If you are a physics major, what is your area of strength/interest if not mechanics.? 
Solution for attaching bicycle brake pads
JAG Engineering LLC replied to Zane's question in Question & Answers
First let's be clear about 6MM Is this the distance across the flats or the nominal diameter of the threaded section? 
Pendulum Hammer Impact Force Calculation
JAG Engineering LLC replied to GabeRM's question in Question & Answers
Dr D can likely tell you what to do off the top of his head. (TOLD YOU SO, JUST SAW DR D's REPLY) I would need to refresh myself on this. Conservation of momentum may be the best approach. Now that we know the anvil can move, (anvils as used by a blacksmith don't), the g question makes sense but still many issues. If the hammer is one ounce and the anvil a ton, the anvil will not move for all practical applications. Switch the weight values and the anvil will take off like a rocket. Make either out of a compliant material and the answer will once again be different. You will need to know the mass of the hammer and anvil. If you neglect mechanical friction and air drag, you will arrive a velocity greater than actual. By how much I don't know without running numbers. If you are designing to protect for max impact then that approach will provide some margin. If you must apply a minimum g load as part of a test this would not be a good approach. From the video the anvil is pretty complicated with springs. The g loading will be one value for the base being struck and less for the spring mounted upper section. Your first cut would be to use m1 x dv1 = m2 x dv2. ( for greater detail https://www.physicsclassroom.com/class/momentum/Lesson2/MomentumConservationPrinciple) Calculate the velocity of the hammer at impact. Measure the weight of the hammer and anvil. From above you get dv2 (speed of anvil) but this too has a wide range due to many variable. Assuming ideal conditions gives you a ballpark number. Depending on how close you need to be will depend on how close you need to mathematically model the system. It may be a lot easier and more accurate to use instrumentation. Or as Dr D suggested the info may exist in some spec. 
Pendulum Hammer Impact Force Calculation
JAG Engineering LLC replied to GabeRM's question in Question & Answers
The anvil is supported, but not fixed in the vertical direction? So when it is struck it will be propelled up? 
is this formula ok?
JAG Engineering LLC replied to PabLo Dal Buoni's topic in Machine Dynamics & Design Considerations
I know what you mean but that is not what I have discovered for shear strength of concrete. http://people.tamu.edu/~mhaque/cosc421/SHEAR.PDF Also attached a posted question with answers. One reply suggested the same as you. If there ever was a dimensional constant, it has been lost in time and the odd units accepted. If you have a better source I would like to see it. I was sure there was an error but found the same relationship in a few sources. Then the replies from CE's. What's up with the units in concrete design equations_ _ AskEngineers.pdf 
is this formula ok?
JAG Engineering LLC replied to PabLo Dal Buoni's topic in Machine Dynamics & Design Considerations
Dr D., For most equations if the units don't work there is an error. This is not true all the time. The strength of concrete is where I ran into this. It is because the results are arrived at empirically. I looked at several sources all had the same equation. Finally I came across a post or a book, I can't recall, that explained why. I forget if the results was allowable force or a stress. But the square root of psi does not get you to either. I get to work what are Civil projects. I find the phraseology puzzling at times. Like "the beam developing its full strength due to the load." The full strength should be X whether loaded or sitting in inventory. What this is understood to mean by civils, is the full bending strength. Or in other words will the beam experience failure in some other mode before the application reaches the allowable max bending stress with all factors of safety accounted for. But these examples aside, if the units don't work, don't assume it is as with the example above. Confirm the equations you are using from multiple sources if taken from a text or handbook. 
Solicitors assessing/adjudicating mechanical engineering disputes
JAG Engineering LLC replied to ndems's question in Question & Answers
Dr D. Pit bulls are some of the gentlest dogs unless they are trained otherwise. A lawyer like a politician only cares, or must force themselves to only care, about winning. How much is training, and how much is evil by their nature is impossible to determine.They present a set of information (I hesitate to use facts). The presented information is turned over to a third party judge/jury who come to a decision based on two or more biased and incomplete sets of information. I have sat in court knowing the full story and listening to what was presented which then becomes the only "truth" the jury knows. I have come to realize that we have a legal system and justice is an illusion. I would go one step further and say we have a decision system. As engineers we need to get as close to the truth to prevent danger to those using our designs. The legal system needs to reach a decision so the system can move on. And they are very slow to undo a decision even when confronted with information that would have altered the original decision. My advice about our legal system, avoid it like a disease. If dragged into it don't expect justice any more than you would expect a fair fight from a mugger. Good engineers can keep you out of trouble. A good lawyer can get you out of trouble. For the first case good is a complement. In the latter, it does not necessarily have the same connotation as the first. Praised be to God we made our livelihood as engineers. 
10 lý do khiến chiếc xe của bạn bị rung lắc
JAG Engineering LLC commented on EnterKnow's blog entry in Kỹ Thuật Ô TÔ
Just right click in the article and a dialog box should open with a translate option. 
Flywheel and DC motor
JAG Engineering LLC replied to Tahir Afridi's topic in Machine Dynamics & Design Considerations
Sounds like a homework assignment. Size a motor to accelerate the flywheel to a specific RPM and do so in a specific time. 
Gearbox sizing for trolley
JAG Engineering LLC replied to Xose Portela's topic in Machine Dynamics & Design Considerations
I am just thinking off the top of my head. Can you do a simple F=MA to estimate the acceleration or time to move from point A to point B under an ideal situation? If you need more power for the ideal situation or the time required to come up to speed or move from A to B is huge, that may shine some light on it. If you are sizing a system you plan to purchase the manufacturer likely has a work sheet and an applications engineer that can help. 
Gearbox sizing for trolley
JAG Engineering LLC replied to Xose Portela's topic in Machine Dynamics & Design Considerations
How do you know your calculations are wrong? 
Gearbox sizing for trolley
JAG Engineering LLC replied to Xose Portela's topic in Machine Dynamics & Design Considerations
Try this for starters.http://www2.mae.ufl.edu/designlab/motors/EML2322L Drive Wheel Motor Torque Calculations.pdf 
Try OnShape. Full cloud based. You can try full function for free as long as you are Ok with your model being open to the public.

Motor Torque Calculations
JAG Engineering LLC replied to YYSbu's topic in Machine Dynamics & Design Considerations
This may I used the attached pdf to design a manually driven work platform. Once you have the required torque you can size the motor. I see someone provided a link for robots. I suggest you read both. I find I get an understanding reading multiple sources vs just following the steps. drivetraindesign.pdf