MD&M practice problem of the week

[monty01]

I'm getting 2.0 for the SF on the buckling problem assuming it's within the Euler range.  The safety factor I used was Pcrit/P.

 

[Slay the P.E.]

I'm getting 2.0 for the SF on the buckling problem assuming it's within the Euler range.  The safety factor I used was Pcrit/P.

Correct. For buckling, the Factor of Safety is Pcrit/P.

 

[Slay the P.E.]

I am getting 2.5...can you someone help to how to do? Calculate the Euler stress and compare with Sy/2?

The critical load formula is derived by assuming that buckling will occur at that load, that is when P = Pcrit. Therefore, the factor of safety for buckling is defined as  Pcrit/P. Try it this way.

 

[Slay the P.E.]

In the engine system shown, crank AB is 3 inches long and has a constant clockwise angular velocity of 2000 rpm, and connecting rod BC is 8 inches long. When angle α is 40 degrees, the velocity (ft/s) of piston P is most nearly:

(A) 3.7

(B) 21

(C) 41

(D) 44

 

[finbean5]

Most nearly D, 44 ft/sec. I had Vb=628in/sec and Vc/b=497 in/sec. Some trig happens and I'm left with 43.5 ft/sec.

 

[Slay the P.E.]

Most nearly D, 44 ft/sec. I had Vb=628in/sec and Vc/b=497 in/sec. Some trig happens and I'm left with 43.5 ft/sec.

That is correct. The actual answer is 43.5 ft/s

 

[monty01]

Here is a copy of the crank/slider solution if anyone is interested.  I definitely wouldn't consider this to be a 6 minute problem.  I had to do a fair amount of angle finding and then break up the vector equation into x/y components, being careful to keep things straight.  Hopefully somebody else has a quicker solution.

I'd have to say this one is a "C" and I'd only go back to it if I had some time toward the end.

 

[Slay the P.E.]

Here is a copy of the crank/slider solution if anyone is interested.  I definitely wouldn't consider this to be a 6 minute problem.  I had to do a fair amount of angle finding and then break up the vector equation into x/y components, being careful to keep things straight.  Hopefully somebody else has a quicker solution.

I'd have to say this one is a "C" and I'd only go back to it if I had some time toward the end.

View attachment 11954

It might be a little quicker if you approach it by finding the instantaneous center of rotation for rod BC.

 

[monty01]

Not a bad idea.  It's been a while since I've done any IC problems.  Does anyone have a solution using the IC method?

 

[nosi9wrx]

See equation 57.59 in MERM

 

[monty01]

There it is!  Nice work!

 

[monty01]

Wow, a one-liner for a crank/slider problem.  Nice!  What reference did you find that in?

 

[tx2011]

The one liner equation for a crank slider problem can also be found in the reference book by Timothy Kennedy.  Just got his book last week and its great. The equation is much simpler than the one above but it uses only x and y values instead of theta so you do have to do basic trig to figure out the different lengths. 

 

[monty01]

Ahhh, that's a time saver for sure.  I tried to get familiar with the IC method in the MERM, but it didn't really make sense to me so I moved on pretty quickly.  Maybe if i have some extra time, i'll give it another shot.

 

[Workx]

I got this from one of my friends' notes from his class. I also used to Timothy's book formula but I feel this is much easier. 

I also like the IC method. In case the question is asking other parameters, we need to use MERM. 

 

[Slay the P.E.]

We won't be adding any more problems to the "practice problem of the week" threads. However, the threads are still here in the boards and are relevant, not just because of the problems themselves but also because of the lively discussion.

 

[23and1]

Happy Thursday. Here's the problem of the week;

SPOILER ALERT: Try to solve it before scrolling down and reading the discussion.

View attachment 10959




 

Hey errrbody,

So I've been crunching on this one over and over. What is it that I'm missing with the buckling check? I keep getting a FS of 2.66. Is there an eccentric component that I'm missing to the bending (MERM equation 53.14)? I've attached my work for reference (sorry if it seems a little disorganized...) Any help would be appreciated!

View attachment Buckling FS.pdf

 

[Castaway81]

I solved for the reaction at B by taking the moment about A and got 7800 lbf.   Then your FS is 1.99

 

[23and1]

Ah, I see...
So the reaction at the column isn't R=0.375*w*L (from propped cantilever equation in MERM Appendix 51.A)? Is that because the end on the wall is pinned instead of built in?

 

[Castaway81]

That's what it looks like to me.  This would be the simple supported beam in the appendix where R=wl/2. 

I should have used this rather than the moment equation which would have been a bit quicker.