Explanation Of Torsion Springs: Torsion Spring Design
Explanation of Torsion Springs
Definition: (Tor-si-on Spri [ng]-s)
Torsion springs are an example of a helical spring made to generate rotational and torsional forces. Torsion springs come in all shapes in sizes such as small springs, large torsion springs, double torsion springs and extended length torsion springs. A door hinge is a prime example of small torsion springs, this is because it always causes the door to return to its original position. Every time you saw a bad guy bust through the saloon doors in an old western movie, it was the torsion spring that created the dramatic effect of those doors bouncing back behind him.
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Applications:
There are many applications to which a torsion spring can be attributed. The applications range from Levers, Fulcrums, Switches, Hinges, and grips. Wherever torsional or rotational force is needed, torsion springs can be applied.
How do torsion springs work?:
Before designing a torsion spring you must first understand the laws that govern them. In order to design one we must consider the following design characteristics.
A: How much room do we have to place a torsion spring in our product below.
1. I.E: Max OD, Max body length , Max Leg length etc.Leg Bends etc.
B: How much force do I need to generate? This question is more easily answered by thinking in terms of Inch, Lbs/degree of movement. All torsion springs generate inch or Ft lbs of torque/degree of movement.
1. Example: If I have a door that moves 90 degrees of travel or deflection (open & shut), then I can gather that what I need is at least 90 degree of travel/deflection from my torsion spring.
2. Next, I need to figure how much force is required to make the door close. Lets assume the door weighs 90lbs, then the torsion springs formula is easy. I need the torsion spring to have at least 1in/lb of torque per 1 degree of radial movement. If I move 90 degree of movement I get 90 in/lb of torque. One must remember that all torsion springs have a rate force per 360 degree of movement. But in this example my rate would be 360in/lbs of torque. This gives me 1 in/lbs of torque per degree of movement. (Please keep in mind that the example above was purely hypothetical, in real time applications one must consider the weight of the door plus the angle of the door if the door is laid horizontally on the ground.)
C: I need Preload & How do I get that? Preloading on a torsion spring is the act of deflecting the spring a certain amount of distance in degrees of travel. Preloading your spring allows you to get the right amount of pre-loaded torque at the beginning of your travel. This is necessary because torsion springs are used to open and shut many mechanical devices.
D: I want many cycles of life (High Repeat Ability). How do I achieve this? In order to do this, one must understand the Logics of Torsion springs.
The Force Chart
For basic spring design consideration
To get more force you can have a smaller OD, less coils, thicker wire, or more travel. To get less force you can have a larger OD, more coils, thinner wire, or less travel
These are the basic design considerations. It is important to understand all of the above design considerations because they all work together.
Basic Design Formulas:
Rt = Ed^4 / 10.2 DN
S = 10.2 M / d^3 (correct for D/d)
Explanation of symbols:
d = wire size (inches)
D = Mean Diameter (inches)
N = Number of active coils
Rt = Rate
S = Stress (lbs / square in)
P = Load (lbs)
M = Moment (inch-lbs)
D / d = Index correction
Modulus of Elasticity for common spring wires.
PSI x 10^6
Music Wire = 30 psi
Stainless Steel = 28 psi
Chrome Vanadium = 30 psi
Chrome Silicon = 30 psi
Phosphor Bronze = 15 psi
Modulus in Torsion for common spring wires
PSI x 10^6
Music Wire 11.5 psi x 10^6
Stainless Steel 10 psi x 10^6
Chrome Vanadium 11.5 psi x 10^6
Chrome Silicon 11.5 psi x 10^6
Phosphor Bronze 6.25 psi x 10^6
Example: If my torsion spring's OD is too small I may get a lot of torque, but my torsion spring will become too stressed out thus taking a set in FP, or worse yet breaking all together.
Torsion Spring Index:
Here is where my spring index plays a huge part in Torsion Spring Stress. To figure out spring index get OD - 1 WD = mean diameter (MD). MD divided by WD = Spring index.
Example: .500 OD - .050 WD = .450 MD divided by .050 WD = 9 to 1 index (this is a good index). It is not recommended to go below a 5 to 1 index for a good torsion spring design. It is important to be careful on your index because the smaller you go on index the tighter your spring displacement is. Thus adding more stress on your design. Stress is detrimental to your torsion spring because stress and fatigue will cause presetting of your torsion spring. Example: you have a spring that measures .500 OD with a .100 WD, .500 OD - .100 WD = .400 MD divided by .100 WD= 4 to1 index or 4 wire size to1 wire size. A 4 to1 index is low index. However, a 4 to 1 index will work if you need to generate a strong amount of force with a small amount of travel. But in the case where you need the same amount of force along with a good amount of travel or deflection (say 90 degrees to 360 degrees) a 4 to 1 index will not be sufficient. In order to correct this problem your index will need to be opened. If the design permits you , you should make your torsion spring OD bigger. An 8 to 1 index is a better index number to achieve a good amount of force along a greater amount of travel with less stress.
Number of Coils in Torsion Springs:
The number of coils in your Torsion spring is another aspect of Torsion Spring design that is greatly connected to the spring index. Remember the chart above: Smaller OD = more force (MF), Larger OD = Less force (LF). The same is true with the number of coils in your spring. Less coils = a stronger Torsion spring, more coils = weaker torsion spring. At the same time less coils = more stress and fatigue, more coils = less stress and fatigue. The perfect balance on your torsion spring design is the one that combines the right amount of coils for how many degrees of travel (with preload included) you want to achieve with the correct amount of force needed to do the job. The correct index to attain a low fatigue and stress level is also important. Your torsion spring will have millions of cycles of life. This should all be calculated to fit in the space of your application.
Body Length (BL) or Solid Height (SH):
Torsion spring body length or solid height is defined as the vertical measurement of the number of coils in the length of your torsion spring. To determine your body length or SH the following formula is required.
Example: 10 total coils (t/c) using .050 WD (10t/c X .050= .500 + 1 wire size = .550 SH) Why add 1 wire size? You need to add one wire size because your torsion spring has 10 coils, but the 1st coil you count is really the second wire. A coil on a spring is not a complete coil until you follow it around a complete 360 degrees, thus the need to start counting your coils on the second wire. Please be very clear on this so you may understand how to obtain the correct Body Length (BL) or Solid Height (SH). Many engineers fail to understand this simple concept, this causes them to end up with incorrect prints.
Torsion Spring Pitch:
A torsion spring pitch is closely related to a BL or SH. Pitch is the distance, measuring vertically or horizontally along the torsion spring body, from the center line of the wire to the proceeding center line (C/L) of the following coil (similar to pitch on a screw). Many torsion springs need pitch in order to fill a certain amount of distance between post or pins, that they work on. A good example of this is the torsion spring on a spring hinge. The spring needs to have a good amount of pitch so that it can distribute its force along a greater area of distance, which in this case is the length of the spring hinge.
Pitch Applications:
May include using a torsion spring for its radial force as well as the benefit of having a built in compression spring. Your torsion spring/compression spring can now do the function of two springs, giving you linear force (compression spring) as well as torsional force (torsion spring). This definitely expands your possibilities for using torsion springs. If you would like more help on the spring pitch, Please visit the Compression Spring Design Page.
Torsion Spring Leg Configurations:
Torsion springs have legs that transmit force to your product. From an economic manufacturing point of view, you as a designer/engineer should avoid having many complex leg bends so your end torsion spring design will be economical as well as functional. Many designers/engineers never take this into account, thus ending up with complicated leg configurations. Please do not limit yourself either, you should consult a spring engineer to asses your design criteria. In many cases the prototype or product is finished then the engineer figures out the torsion spring design as an after thought. A good torsion spring design needs to be done while the product or prototype is being developed before molds and tooling are made. I have seen this happen many times and the engineers torsion spring designs are very complex and cannot be manufactured. This is why it is so crucial to consult a professional spring maker before attempting the design on your own because he or she may see something that is not obvious to an untrained eye. They can point out great ways to assemble springs to your products with very few bends. Remember a great spring is all in the design, by using all the above factors we take into account to get there. Sometimes less is more.
Torsion springs can have offset bends coming right off the body that give you your choice of different radiuses that can be put on your spring's legs. Example: 90 degree, 45 degree, 25 degree, just about any kind of angle can be manufactured into your torsion spring. Your bend angles can be on different planes as well as different radiuses. The legs on a torsion spring can have hooks, loops, u-turns, and even additional springs can be made from a leg length such as a smaller torsion spring of a leg from the bigger torsion spring.
Unique Application for Torsion Springs:
Torsion springs can be used for gripping around a shaft.
Example: Have you ever bench pressed free weights at the gym? Chances are that you used a torsion spring to grip around the weight bar to keep the dumbbells from falling off the bar. Torsion springs can also be used as hose clamps. Open the hood of your vehicle and you will notice small round wire clamps that grip your rubber hoses. Those clamps are torsion springs in disguise. For a spring maker to create a hose clamp he has to make it the way a spring maker normally makes a torsion spring. Its that simple. If you are looking to determine the binding in spiral torsion springs you have to remember that as a torsion spring turns, the legs will increase the free length of the spring. Some people will forget to allot for that extra length and they will reach the binding hight and the torsion spring will no longer fit in the application.
Another example: The Heli-Coil is a torsion spring made of diamond shaped wire that helps repair worn out and stripped threads in a hole. The logic is, as you wind a torsion spring clock-wise (to the right), the torsion spring will decrease its inner diameter and outer diameter thus allowing the spring to enter into your worn out hole or bore. Once the spring or Heli-Coil is set to it's specific depth then you release the spring and it will return to its original ID and OD giving you a tremendous amount of grip and biting force on the inside diameter of the hole or bore. Just think of how much force a threaded hole needs to have so it can keep the screws tight. That's a lot of force.
How to Determine if Torsion Spring is L/H Left Hand and R/H Right Hand Wound:
Hold your torsion spring as if you were looking down the barrel of a gun. Now look at the back leg, which is the leg further away from you. Take the back leg and put it at 12 o'clock ( on the top part of the coil) "See figure Above". If the leg faces to the right it is a R/H spring. If the leg faces to the left it is L/H spring. The logic here is simple just make sure you are holding the spring like the above example and you will be fine.
Torsion Spring ID (Inner Diameter):
Torsion spring ID contract (Shrink) when deflected. Torsion springs ID's need to have sufficient clearance over the shaft they are working on to prevent the torsion spring from griping the shaft they are working on. A good rule of thumb is to leave a 20% clearance from the torsion springs ID and the OD of the shaft.
Example: My torsion spring will be working over a .500 shaft so I need a .600 ID torsion spring to work safely over a .500 shaft. Keep in mind that 20% is only a rule of thumb. If your spring has a lot of deflection (travel) your torsion spring ID will contract more, and if you have a large index with a fine (or thin) wire size your torsion spring will contract even more. Keep all these points of concern in mind when designing a torsion spring.
Torsion Spring Material:
Torsion springs can be constructed from many types of materials. You can have all types of metal springs from a stainless steel torsion spring to music wire springs Phosphor bronze springs as well as many others. The most important aspect of choosing the right material for your torsion spring is identifying your torsion spring's environment then coupling it with the correct type of material to do the job correctly.
Example: If your torsion spring will be in salt water, then your choice would be 302-SS. If your spring environment is indoors or enclosed in a housing then Music Wire will be sufficient. If your torsion spring is subject to high temperatures, above 800 degrees, then your obvious choice is Inconel X 750. If you are looking for conductivity then use Phosphor Bronze with gold electroplating. These are just a few examples that I have covered. For more information go to the Properties of Common Spring Materials page.
Plating and Coating:
When coating a spring the following types of platings are available. Zinc Plating, (white, blue, gold, & black are available) offers corrosion resistance, Nickel Plating (gives a very bright chrome looking finish or you can have black finish) offers good corrosion resistance, Black Oxide, Shot peering to reduce stress of fatigue, powder coating to give corrosion resistance and gives you the options of choosing just about any color under the sun. Electro Gold plating offers your springs conductivity if used in electrical applications. Galvanized wire is great economical choice for a spring, it offers the user a pre-galvanized wire so that the spring maker purchases this wire with the galvanization already on it, they do not have to send it out for a secondary process after the spring is made, this saves the buyer a substantial amount of excess work and money. A word of advice about material selection: Take the time to ask a spring engineer his or hers opinion of your particular spring environment. They can professionally asses the torsion spring environment and recommend the correct material for the job.
Word for Thought:
Once you understand these basic principals for creating and engineering your unique torsion spring you will be better prepared for spring design. Remember, a great torsion spring is one that will function properly in the confined parameter of your product with low stress and fatigue, and high cycles of life.
Calculating spiral torsion springs:
If you are looking to design your torsion spring online Planetspring.com has just the thing. We have one of the few torsion spring calculators available on the web today. With three simple inputs you have a spring rate calculation, spring constant, as well as 11 other values. You can also see an example of a torsion spring if you are not sure what one looks like. If you would like to see a torsion springs video please visit our home page and click on the "spring videos" link
by: Ashley Hughes
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