Non-Bi-Lateral Linear Devices 1

[fester225]

Consider the movement of a feed pawl on a conveyor. When it moves forward it doesn't do much, but on the return stroke it pulls the conveyor.

Is there a branch of Mechanical Engineering which deals with devices which do one thing in one direction and something else in the other?

 

[MikeHalloran]

Ratchets and pawls and such are parts of 'mechanisms', something that mechanical engineers used to study in school.

Nowadays, what with all the fancy tools to be learned, I guess that kind of stuff is too old-fashioned and primitive to bear study, so it doesn't get studied... which is too bad. 'Simple' mechanisms are left to designers who aren't capable of analyzing them, and have to build prototypes to get them to work at all. Usually they get it about right by the third generation. Unfortunately, with today's emphasis on claiming you've got it right the first time, the third generation comes after the second recall... or never, if the first recall kills the company.

Mechanisms found in modern products tend to have greatly simplified geometry, an artifact of the difficulty of drawing complex geometry in CAD and of the general lack of understanding or appreciation of the subtle details in good mechanisms, and also tend to be horribly designed, with inefficient use of material, and with overstress that often manifests itself as a search for 'better material'.

To answer your question, they don't teach this stuff in college anymore. The managers who are most affected by the problem don't know they need a 'mechanism specialist', so they look for something else. By the time they find you, their budget is long gone and their own job is in jeopardy, so they are desperately seeking 'magic bullets', which need to go into production _now_, work perfectly on the first try, and cost nothing. Accepting clients with such wildly unrealistic expectations is not good for your reputation or your bank account.






Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

And the method of analysis for mechanisms is kinematics.

Some stuff here:

http://kmoddl.library.cornell.edu/

 

[Cockroach]

You know what? Yes, yes there is and quite fascinating as a topic in itself.

I used a particular pawl in an oil & gas field operation, wireline, to grab a particular piece of equipment 1500m downhole. We dropped the device with a Sinker Bar and latched into a top end assembly. Our problem was to solve the requirement of increasing latch capacity with an increase in wireline tensile strength.

Our pawl device works like the jaw of a simple set of pliers. Increasing line tension adds "grip".

Look at the Vanoil Equipment website, Baker-Hughes wireline etc for similar devices.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada