M&R Spring Blog
Design Tips: Coil Count
It may be simplest feature to measure, but the number of coils accounts for an astonishing number of errors in drawings and in new product development. Counting them is easy enough – every rotation of 360 degrees equals one coil(90 degrees = ¼ coil, 180 degrees = ½ coil). However, the coil count can be critical and effects every functional aspect of a spring.
If a rate or load is specified coil count must be reference only
Spring rates, and by extension all spring forces, are controlled by 4 variables: coil count, spring diameter, wire diameter and material. Under most circumstances the coil count is the only variable the spring manufacture has control over. Tensile strength in wire can vary as much as 10%. Because of this, two identical springs can have two different spring rates. In order to hit a specified spring rate the manufacturer must be able to change the number of coils. Putting a tolerance on both the spring rate and the coil count is spring maker’s equivalent of putting a square peg into a round hole.
Compression Springs and active coils
The number of coils on a compression spring are divided into two categories, active and inactive. The active coils do all the work and handle all of the stresses. Use the active coils when doing any calculations for stresses or loads.
The inactive or dead coils to not support any of the load. They are usually the close wound coils used to square up the ends of the spring. Additional dead coils can usually be added to increase the solid height, to assemble a spring attachment or to decrease tangling.
Torsion springs also have an active coil count that may be different from the total coil count. When the torsion legs are long and the moment is applied a significant distance from the centerline of the spring the wire in the torsion legs must also be considered as active coils. Use the formula below to approximate the effect of the torsion legs on the active coils.
Active coils = Total coils + Leg Lengths / (Pi x Dmean)
Torsion springs present another challenge because the coil count also dictates the free position of the spring. If you want the legs at 90 degrees, you need ¼ of a coil. This limits the manufacturer’s options as far as controlling the torque and spring rate. Under most circumstances it is not possible to put a tolerance on spring rates of torsion springs.
The number of active coils on an extension spring is almost always equal to the total number of coils. But, in addition to the spring rate, the number of coils directly effects the free length of an extension spring. For normal loops the free length is defined by the following formula;
Free Length = (wire diameter x coil count) + (2 x inside diameter)
It is possible to cheat on this formula. Shorter half loops can be formed to somewhat reduce the free length and extended hooks can be added to lengthen it.
It has been quite a year here at Murphy and Read. There have been many improvements, new equipment, a lot to be thankful for and a lot we would like to talk about. But we will get to all of that next year. For now the biggest change we have had in a long while, Karl Seemuller, part of the Murphy and Read family for over 36 years and President for over 20 years, is retiring beginning in the New Year.
Karl’s attention to detail and ability to focus on a project is something that can be seen throughout Murphy and Read. You cannot find a piece of equipment in the shop he has not sweat, bled and yelled at. And his work ethic has rubbed off on all of us here as we value quality and craftsmanship over all else. It is rare these days to see the president of a company so directly involved in its operations and the training of its employees. This dedication to work is what has made us, and many other small businesses, so successful. And it is a tradition we intend to continue.
I am sure this is not the last Murphy and Read will see of Karl. He is often found in the shop after hours working on wooden stools for his Grandkids, machining awards and targets for his shooting club and bullets for his rifles(which is always frightening to new employees). One thing is certain, everyone at M&R owes Karl a debt of gratitude for his long years of hard work.
Happy Holidays everyone. See you in 2013!
It is certainly an exciting time for science, and Murphy and Read is proud to be involved in at least a little part of its history. In the 1970’s Murphy and Read worked with NASA to manufacture a spring that was to be outfitted on NASA’s Voyager 1 spacecraft. According to NASA’s website, the Voyager 1 spacecraft now may be nearing the edge of our solar system! Recent signals from the spacecraft show a rapid increase in high level cosmic ray particles. Scientists say this indicates they are entering a “new region at the edge of the solar system.” The new data is different than what scientists anticipated and it is unclear what to expect once the ship completely leaves the protection of our suns magnetic field. You can find out more about the Voyager program at NASA’s website.
With recent breakthroughs such as entering this new frontier, the discovery of the (likely)Higgs boson and the engineering marvel that just landed on Mar’s, it is an exciting time to be alive.
Welcome to the Murphy and Read Spring Blog! You can check back here for all things spring; company related updates, industry news, design tips or really anything we feel like talking about. We also have facebook and Linkedin pages you can follow for updated news and pictures.
It has been a busy year already. We have attended two successful Tradeshows; The Design Part II in Secaucus New Jersey, and the Atlantic Design and Manufacturing show just across the river from us at the Convention center in Philadelphia. Both shows offered great opportunities to catch up with some of our current customers, meet some new people and get a few leads on potential suppliers. You can check out the shows and listings at www.AtlDesignShow.com and www.d2p.com.
And a quick thanks to b2b design & development for getting this page up and running, and Arizona Designs for the design of our new show backdrop.
Murphy & Read Spring is proud to announce the launch of their new blog. Check back here soon for future updates related to our company and our industry.