simple spring inspection

[joebk]

We spec springs on drawings leaving the initial tension (if it is a tension spring) and the free length as ref only dimensions. We give two lengths and two loads (with tolerances) as well as other information to specify the spring.

We are not a spring manufacturer so we do not have the inspection equipment or expertise typical of a spring shop. Our QA department is fighting me claiming that the free length must be controlled for them to inspect the parts as they come in. But to me (the design engineer), the two working loads at the specified heights are the most important and the free length can be wide open to obtain

My question is: how are springs usually inspected when drawn in this manner? I assume a relatively simple (and hopefully inexpensive) machine capable of measuring loads at certain points is required?

Thanks in advance for your help!

 

[TVP]

http://www.sastesters.com/

http://www.larsonsystems.com/applications/ext_spring_1_point.html

 

[sbozy25]

I work for a large spring company and we have many different types of equipment we use to measure. But, if you are looking for a real cheap way to check the part. Buy a push/pull force gauge, hook the spring to something, place a ruller under the spring, and pull to the desired lengths. This will give you a pretty close reading of your forces.

Also, just a side note.... Be careful when putting your tolerances on the loads. We typically have our own tolerances we like to go with depending on type of spring, wire size, etc. Since as you put it you are not spring experts, you could make the part harder and more expensive than you need. Also, your thought that free length is not an issue... you are correct, we will match the loads and try to get the length close. The only things you need to worry about is if the part works, and how well it works.

 

[israelkk]

Your QA department is wrong and you are correct. The free length has no importance. The two load points at specified heights is the functional and correct way to test the springs. You can have the same length and have completely different loads depends on the initial tension in case of a tension spring. in case of a compression spring it depends on variations in wire tolerances, mean diameter tolerances, number of active coil tolerances. Your QA department doesn't have to actually test the spring. They need to receive a testing report from the manufacturer. This report/test can be suprvised in person by your QA department during manufacturing if they are paranoids, or your QA can approve the manufacturer testing department once and use periodical visits to make sure the manufacturer is still good as before.

http://israelkk.googlepages.com/home

 

[sbozy25]

hahahah, Our inspectors would flip a lid if some other company came to observe them testing. I could see grievance after grievance getting thrown at hr over that.

 

[israelkk]

sbozy25

This is common practice in the aerospace and military field. You have to be approved by the buying company. Although I am not a QA engineer (and probably because I am not) I have "caught" spring manufacturers "cheating" on number of preset (setremove) cycles, time, and temperature. I have found nicks and cracks on tension spring hooks that resulted in catastrophic failure. I have found improper heat treatment and coating procedures, etc.

No well repected aerospace and military company will order parts from a supplier without reviewing and approving their manufacturing procedures to their special spec in addition to the ISO QA requirements. I know companies that are approved by General Gynamics, General Electric, Martin Marriata and many more had to go the whole process of approval with every one of them.

 

[sbozy25]

Yeah, we do a lot of military and aerospace projects. Honestly, if you have found a company "cheating" I would think that they weren't a very good company. We take all our stuff serious, and have strict procedures to follow. However, like all companies we do have the ocasional bad lot slip by and make it to the customer. But, that is nothing a corrective action can not handle.

The problem I have encountered with military projects, is the buyer not passing my "Engineering notes and reccomendations" to the engineers. There are some times that I know we would not be able to meet a tolerance, or we can't supply a specific coating on the part, etc. So I make a note on the quotation stating what is wrong and why we can't do what they are asking. Then a couple of weeks later, we get the P.O. and begin supplying parts. Within a month we then start recieving angry e-mails and phone calls from the engineers because are parts are "bad" to them...

"Blame the purchasing agent" - motto to live by!

 

[TVP]

Always blame Purchasing...

 

[MikeHalloran]

Within a certain range of sizes, an automotive valve spring checker will work well enough. There are equivalent checkers for smaller springs based on force gages. You might have to write a procedure on how to use them and what to measure.

From my perspective, the only reason to have a drawing for a common spring is so that QA can check that what's in the bag is what you ordered and not something from the next bin. For that purpose, you could have a loose tolerance on free length, and less loose tolerances on OD and wd and Nt. Maybe you could mark them as 'identification attributes'.



Mike Halloran
Pembroke Pines, FL, USA