Quick Mechanics question

[hlg6016]

Trick question, The threads are reversed.....Or maybe trick answer?

 

[Navy Ship]

I'm not worrying too much about it, but there was another nut/bolt question on another practice test I took that I also got wrong, so I'm trying to figure out the source of my misunderstanding.

I guess I'm being really dense on this one... Aren't A and B the ones that are equivalent? Nut stationary and bolt clockwise should be the same as bolt stationary and nut counter-clockwise, correct?

Which practice test is this question from? Can you tell me where to find the test? Thanks.

 

[Fronch]

It's the Peterson's test here: http://www.nelnetsolutions.com/dod/DODHubPage.aspx?sponsor=12894&HubPage=15150

 

[Navy Ship]

Hey guys, I originally though a question like this had to do with the way the bolt and nut were threaded.

Does the external thread have to be matched to the internal thread? Can we assume this?

 

[hlg6016]

Yes

 

[Spekkio]

Pretty sure you should both try again..not that it's critical to national defense, you understand…
A. In this instance, for a reverse-thread bolt, it will move UP.
B. In this instance, for a reverse-thread bolt, the nut will move up.
C. Only correct answer...

The bolt moving down (A) and the nut moving up (C) are the same thing, though, just with a different point of reference.

For a left-handed screw, clockwise turns would eventually remove the nut from the bolt instead of fasten it like it normally does. This means the bolt would move up and the nut would move down, answer (B).

 

[Renegade One]

The bolt moving down (A) and the nut moving up (C) are the same thing, though, just with a different point of reference.

For a left-handed screw, clockwise turns would eventually remove the nut from the bolt instead of fasten it like it normally does. This means the bolt would move up and the nut would move down, answer (B).

But NOT if the nut is "inverted", as shown in the illustration..then "perceived direction" is reversed. For reverse threads, in the configuration shown, the BOLT will always move UP if turned clockwise (righty-loosey), and the NUT will always move UP if turned clockwise (as viewed)…think the reverse of righty-loosey, which equals lefty-tighty (if you were viewing it from the nut perspective.
Hence "C".
To remind:
A. If the nut is held stationary and the head turned clockwise, the bolt will move down. (Only for standard threads…which are righty-tighty)
B. If the head of the bolt is held stationary and the nut is turned clockwise, the nut will move down. (Again, only true for standard threads)
C. If the head of the bolt is held stationary and the nut is turned clockwise, the nut will move up. (Yep…righty-loosey)

I'm fascinated that so many of us "top-notch go-fasters" have chosen to try this question on for size. Not really sure we've really helped the OP, except that I think our best advice is:
1. If an illustration is given, LOOK at it…understand that pre-conceptions don't always apply. There is an illustration for a REASON.
2. Read the question and alternate answers. Examine each thoughtfully.
3. In a multiple choice lacking "combination results (e.g., "Both A and C are correct", there will be only one correct answer.

 

[alaurin]

As Spekkio mentioned, most ordinary screws/bolts are cut with the righty-tightey/lefty-loosey thread (right handed). The only exception I know of is with bicycles (to counteract the torque from the left pedal spin, if I remember correctly). However, in the scope of this test, you can assume all screws/bolts are right handed. You can also assume that the threads must match on corresponding parts and that any bolt when fitted into a threaded hole will move down into the hole when turned clockwise (righty-tightey).

Now a word on threads:
If the threads did not match and you put a bolt with say a square cut thread into a triangle cut threaded hole (different thread profile shapes), you would end up partially stripping the threaded hole to match the bolt's threads or strip the bolt's threads to match the threads of the hole you're forcing it into. In the case of machine bolts and screws, you usually end up stripping the threaded hole since the bolt is made of harder material (but not in all cases). It's similar to that kindergarten/preschool game where you have a bench with shapes such as stars, squares, circles and triangles cut out and corresponding shaped pegs. The square peg will not fit into the triangle shaped hole unless you really force it and damage the bench or the peg.

 

[Renegade One]

...in the scope of this test, you can assume all screws/bolts are right handed.

You know what they say about "assume"…yes? Why would you assume that when the illustration is clearly the opposite?

Hint: If the illustration depicted an inverted aircraft, would it not be correct to respond that "forward stick" would increase altitude, or should one always assume that an aircraft is always upright relative to the ground below?

It's like the Lunar Module astronauts re-docking with the Apollo Command Module while looking upward through a visual port, but flying the LM with standard inputs:
1. Left roll becomes left yaw; left yaw becomes right roll
2. Right yaw becomes left roll…right roll becomes right yaw
3. And so on…only "pitch" commands remained a constant.

But the astronauts were smart…they figured it out.

I didn't think practice test question was rocket science…perhaps I was wrong.

 

[alaurin]

You do bring up a good point and maybe I was hasty to say "Assume all screws are right handed." The only assumption I'd recommend making on the test is that the average person knows how to spell their own name correctly. I know what they say about assumptions (I like to follow up with "Trust but verify, meaning I don't trust anyone I work with"). But I also took the ASTB and didn't see any left handed screws.

I did see a lot of pulleys, wedges and gears, so I'd probably waste my time reading up on those and how to calculate them rather than on bolts. There was a basic machinist's knowledge site I may have thrown up on another thread at the beginning of the year. It was pretty similar to what was on the test in terms of problems. If you run out of things to do (which I doubt will happen) you can try that.

 

[Random Task]

The bolt moving down and the nut moving up are the same thing, you're just changing your frame of reference.

Forget the picture, if you logically deduce A is right, the same logic should lead you to the conclusion that B is wrong and C is the same as A.

The picture is everything, definitely don't forget it. The test is trying to determine if you can look at something and be able to figure out how it works mechanically...at least in the mechanical part of the test. The rest is as Renegade One explained.

 

[Random Task]

Also has anyone else facepalmed yet?

 

[Spekkio]

C. If the head of the bolt is held stationary and the nut is turned clockwise, the nut will move up. (Yep…righty-loosey)

The nut is at the bottom of the picture. If the nut moves up, it is moving toward the head of the bolt. It would be tightening, not loosening.

 

[Random Task]

The nut is at the bottom of the picture. If the nut moves up, it is moving toward the head of the bolt. It would be tightening, not loosening.

Nut turning CW = Bolt turning CCW so yes...righty-loosey

 

[Fronch]

Thanks for all the replies, I hadn't considered that the bolt might be "backwards" (left-hand rule).