What is meant by jerk and how it can be measured

[indianmc]

Main Question or Discussion Point

dear sir i want to know clearly about jerk and it's measurement. what i undersatnd is jerk means rate of change of acceleration. if it is measred in terms of g then plot against time. i want to konw is there is any instrument is there to measure it.

thanking you

 

[Astronuc]

In physics, jerk (in British English, jolt), also called surge, is the derivative of acceleration with respect to time (or the third derivative of displacement). Yank is mass times jerk, or equivalently, the derivative of force with respect to time. Jerk is a vector, and there is no generally used term to describe its scalar value.

- http://en.wikipedia.org/wiki/Jerk

One can measure the instantenous acceleration using an acclerometer, and then by taking the difference with respect to time, calculate the time rate of change of acceleration.

A simple accelerometer is a mass-loaded spring for which the spring constant is know. It's displacement and rate of change can be measured electromagnetically.

As rockets consume propellant and lose mass, the acceleration increases with time. Variable thrust rockets (e.g. SSME) obviously will cause time-varying acceleration.

 

[indianmc]

sir , i want tknow in depth regarding this . how to measure jerk in case of vertical machining centers.

 

[Astronuc]

sir , i want tknow in depth regarding this . how to measure jerk in case of vertical machining centers.

Please elaborate on vertical machining centers. Do you mean Computerized, Numerically Controlled (CNC) Milling or Drilling Machines? The displacement of the tool head (bit) can be monitored electronically with a high sampling rate. The CNC must know the precise position within the manufacturing tolerances, so the sampling of that alone is extensive.

The work plane (plate) may have a load cell, as well as any load cell in the tool positioning system.

This is all controlled by software and various controllers.

BTW, I have seen bad software in action. I once watched a new program in action on a prototype investment casting. The operator was 'stepping through' the program, and suddenly the drill bit came down hard on the casting destroying the casting and tool bit! It would have been wise to use a piece of forged plate instead of the prototype casting, which cost hundreds or perhaps thousands of dollars. The responsible engineers were not present when this happened, and the operator, who was looking at the printout of the machine instructions, had no clue what when wrong!

And this happened at one of the top manufacturing companies in the world!

 

[FredGarvin]

The operator was 'stepping through' the program, and suddenly the drill bit came down hard on the casting destroying the casting and tool bit! It would have been wise to use a piece of forged plate instead of the prototype casting, which cost hundreds or perhaps thousands of dollars.

And this happened at one of the top manufacturing companies in the world!

And here I was thinking that we were the only ones that did things like that...it makes me feel better. Ahh the joys of working with prototypes.

As Astronuc already mentioned, jerk is going to be a calculated data. You can measure the acceleration directly and then differentiate that data to give you the jerk.

 

[Astronuc]

And here I was thinking that we were the only ones that did things like that...it makes me feel better. Ahh the joys of working with prototypes.

LOL. No Fred, it happens everywhere.

And what's sad is when it happens again, and possibly again.

Another time, at a different shop, on a similar product, holes were being drilled in an investment casting (hundreds of dollars per part). The holes (you know, simple cylindrical geometry) were being drilled oversized or slightly off-center. Why? Because the shop was resharpening their drill bits, and the sharpening tool machine had worn so much that the drill bit would chatter on the sharpener (grinder)! I asked why the shop supervisor had not insisted 1) on using better drill bits, or 2) getting a new sharpening tool machine. He said it was because of budget constraints!

At a close out meeting, I insisted that 1) the tool shop supervisor push for better drill bits and/or tool sharpening machine, and 2) that management be more proactive. And I was telling people with 10+ more years experience than me at the time!

The company was trashing parts, each worth hundreds of dollars, and they could have spent the amount of one or a few parts to buy decent equipment!

It was @#$%& unbelievable!

 

[Danger]

It was @#$%& unbelievable!

And some people think that 'Dilbert' is outlandish.

 

[Astronuc]

And some people think that 'Dilbert' is outlandish.

NO! Dilbert (Scott Adams) is spot on!

I worked for the pointy-haired guy.

You'd be surprised and shocked by what I have seen.

Besides, just look at all the recalls for defective automobiles, or the recall by Guidant because of short circuits in cardiac pacemakers.

And remember the DC-10 (UA232) which crashed in Sioux City, Iowa ( http://en.wikipedia.org/wiki/United_Airlines_Flight_232#Causes ). The cause was attributed to failure of the fan disk - and poor maintenance practices were cited as a factor - however, it also goes back to manufacturing and the failure to detect a very small inclusion in the disk bore. That inclusion should have been detected and the part scrapped. It should never have been put in an engine. :grumpy:

 

[FredGarvin]

And remember the DC-10 (UA232) which crashed in Sioux City, Iowa ( http://en.wikipedia.org/wiki/United_Airlines_Flight_232#Causes ). The cause was attributed to failure of the fan disk - and poor maintenance practices were cited as a factor - however, it also goes back to manufacturing and the failure to detect a very small inclusion in the disk bore. That inclusion should have been detected and the part scrapped. It should never have been put in an engine. :grumpy:

That disc, IIRC made it through about 10 years of service. I remember seeing a show on the falure analysis of the crash. That inclusion was extremely small too. I faulted McDonald-Douglas for designing a triple redundant hydraulic system that had a line from each system meeting at a location that was clearly in the plane of rotation of the engine.

NO! Dilbert (Scott Adams) is spot on!

It's scary sometimes, isn't it? How he can be so right.