Torsion Springs

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Introduction

This article takes an in-depth look at torsion springs.

Read further and learn more about:

• What are torsion springs?
• Mechanics of torsion springs
• Types of torsion springs
• Production of torsion springs
• Applications of torsion springs
• And much more&#;

Chapter 1: What are Torsion Springs?

A torsion spring is a mechanical device that stores and releases rotational energy.

The ends of a torsion spring are connected to a mechanical component. As the spring is rotated around its axis on one end, the winding of the spring is tightened and stores potential energy.

During the winding process, one end is deflected about the body centerline axis, while the other end is held fixed. As the winding gets tighter and resists more rotational force, the spring stores more potential energy.

Once a torsion spring is released, it will unwind as it makes an elastic rebound, and the stored energy is released. An equal rotational force is exerted on the opposite end of the spring, which can apply torque on the attached mechanical component. Torsion springs statically hold mechanical components in place.

The mechanics of torsion springs is based on their resistance to rotation or twisting. The mechanical energy produced by the resistance is stored and exerts torque in opposition to the twisting force that is proportional to the angle that it is twisted. Common types of torsion springs are helical, torsion bars, and spiral wound. Each of the various types are made from wire, sprung steel, or rubber.

Torsion springs are subjected to more bending stress than rotational stress as the spring is twisted to make a tighter winding. They are unlike other springs because only rotational force is involved. Linear force is not part of torsion springs, unlike compression and tension springs.

The mechanical forces of torsion springs depend on the elasticity of its material, which enables torsion springs to revert to their original winding after being twisted. Torsion springs can be rotated and apply force in a clockwise or counterclockwise direction and must be rotated in the direction of the winding to generate maximum force.

Torsion springs are found in a wide range of applications, almost in every industry. These springs come in many configurations.

Chapter 2: Mechanics of Torsion Springs

Torsion spring configurations are created to store and release energy or to hold a mechanism in place by deflecting around the axis of a body&#;s centerline. They reduce the diameter of the body and increase its length when deflected in the proper direction.

The direction of the wind of a torsion spring has to match the needs of an application. The load bearing leg must be located on the left or right side during assembly to be properly positioned. Torsion springs are supported by a mandrel that matches the hinge line of the application.

Inner Diameter

The inner diameter of a torsion spring is the width inside the helix of the coil, measured perpendicularly to the centerline axis. This dimension determines the outside diameter of a shaft or mandrel that can smoothly fit into the spring. It is recommended that the inside diameter has a 10% clearance for the inserted component to operate freely.

Outer Diameter

The outer diameter of a torsion spring is the width outside the helix of the coil, measured perpendicularly to the centerline axis. This dimension determines the diameter of the hole through which the spring is inserted, taking all clearances for the spring to operate freely into consideration.

Wire Diameter

The wire diameter refers to the diameter of the wire coiled to construct the torsion spring.

The mean diameter is equal to the outer diameter minus the wire diameter used in stress and spring rate calculations.

Body Length

The body length is the length of the torsion spring at an unloaded condition. This dimension is obtained by measuring the outer surfaces of the end coils. As torque is applied in the spring, the body length will increase as the spring diameter is decreased.

Leg Length

The leg length is the distance from the end of a torsion spring&#;s leg to the centerline axis of the coil. It determines the load or torque required to store energy in the spring. Shorter legs need higher torque input to bend the coils. The legs of a torsion spring may have different leg lengths.

Total Coil

The total coil of a torsion spring is the number of active coils present in the winding. Active coils are the turns in the torsion spring winding that twist or deflect when a load is applied to the leg and release energy when the spring is released. In torsion springs, the total coil is a fraction less than the total number of coils present in the winding because the legs are accounted for by the number of inactive coils in the value of the total coil. For torsion springs having a 00 leg angle at the free position, the value of the total coil is a whole number.

Pitch

The pitch of a torsion spring is the centerline distance between two adjacent active coils. For closely-wounded springs, the pitch is approximately equal to the wire diameter. However, closely-wounded springs produce high friction during deflection. The recommended practice is to specify the total coil and the body length of the torsion spring, rather than the pitch.

Winding Direction

The coils of a torsion spring are wound in a specific direction. Torsion springs may have a right or left hand winding, where the coils rotate in a clockwise or counterclockwise direction, respectively. The direction of the winding can be easily determined by looking at the top of the torsion spring.

Torsion springs are designed such that the load and the winding direction are the same. The load and the angular deflection must be reduced if the load and the winding are required to be in the opposite direction.

Knowing the direction of the winding is a critical aspect of its function and determines the direction it will take during deflection. The positioning of a torsion spring in an application is dependent on the direction of the winding and how the front and back legs will be placed and move.

The back leg of a right hand wound torsion spring will torque clockwise while the front leg will torque counterclockwise. This is reversed with left hand wound torsion springs where the back leg travels counterclockwise and the front leg moves clockwise.

Leg Angle

The leg angle of a torsion spring refers to the angle that the legs make when it is in the free position before any load is applied to the leg. It varies from 0°-360°. Standard torsion springs are available in stores whose leg angles are 90°, 180°, 270°, and 360°. The leg angle can also be customized by the manufacturer based on the client&#;s needs.

The leg angle varies the total coil of a torsion spring. As mentioned earlier, the total coil is a fraction less than the total number of coils present in the winding. The below equation gives the relationship between the leg angle and the total coil.

Leg Angle at Free Position = Number of Inactive Coils (fractional value) x 360°

Leg Orientation

The leg orientation is how the legs are bent with respect to the spring diameter. The sharp bends on the leg can limit the capacity of the spring since stress concentrates on the bent area. The types of leg orientations are axial, tangential, radial, and radial-tangential. The tangential leg configuration encounters the lowest stress.

Leg Style

The legs of a torsion spring can be twisted, bent, hooked, and looped to make installation and operation convenient. The common leg styles of torsion springs are enumerated below; however, leg styles can be customized upon customer request.

• Straight Legs
• Straight Offset Legs
• Short Hook Ends
• Hinged Ends
• Looped Ends

The following properties and parameters determine the performance of torsion springs:

Spring Index

Spring index is the ratio of the mean diameter to the wire diameter. It tells a lot of information about the torsion spring, including the tightness of the coils, strength, and manufacturability. To increase the strength of the spring, the spring index must be reduced. This is done by increasing the wire diameter or reducing the spring&#;s outer diameter. A torsion spring with thicker wire has higher strength compared to spring with thinner wire. Reducing the spring index makes the spring tighter and exerts more force; however, the coils bear more compressive stress. Torsion springs with low spring indexes are also difficult to manufacture because it increases tooling wear and requires additional processing to prolong service life. Springs with indexes below 4 and above 25 are not manufacturable; the ideal range of spring index ranges from 6-12.

Angular Deflection

The angular deflection is the angular distance that the leg has traveled from the free to the loaded condition.

Maximum Deflection

The maximum allowed deflection is the maximum angular deflection in which the spring can be twisted during its loaded state before it buckles due to overstressing. When a torsion spring exceeds its maximum deflection, the coils may not return to their original position after the load is released due to material yielding.

The maximum angular deflection is how much a torsion spring can be twisted during its loaded state before it buckles due to overstressing. Torsion springs with a larger diameter and a greater number of coils usually have a higher maximum deflection capacity. For example, a garage door spring can rotate several times over without yielding due to the high coil count and low design stresses.

Maximum Load

The maximum load is the maximum torque that can be exerted on the leg before buckling. Either the maximum deflection or the maximum load limits the capacity of a torsion spring.

Spring Rate

The spring rate is the amount of rotational force acting on the torsion spring per angular displacement. The following equation gives the spring rate for round wire helical torsion springs:

Spring Rate per degree (lbs-in/degree) =. PL/Θ = E x d^4 / x D x Na

Where P represents the load, L is the moment arm, Θ is the angular displacement, d is the wire diameter, D is the mean diameter, Na is the number of active coils, and E is the modulus of elasticity of the material. The constant is a theoretical factor that accounts for the friction between adjacent coils and between the spring body and the component attached.

The modulus of elasticity for common torsion spring wires used in spring rate calculations are presented in below table:

Modulus of Elasticity of Spring Wires Spring Wire Modulus of Elasticity (psi x 106) Music Wire 30 Stainless Steel Grades 302, 304, and 316 28 Stainless Steel Grade 17-7 PH 29.5 Chrome Vandadium 30 Chrome Silicon 30 Phosphor Bronze 15

The spring constant is related to torque and angular displacement, given by the succeeding equations. This relationship is used to determine how much torque is required to act on the spring for a specific angular displacement or how much angular displacement is required to generate a certain amount of force.

Angular displacement = Torque/Spring Rate

Torque = Spring Rate x Angular Displacement

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Stress

Bending stress in helical torsion springs is given by the equation:

Bending stress (psi) = 32 PLK/πd³

Where K is the bending stress correction factor. As torque is applied to a torsion spring, the inside and outside diameter will increase. This is because the bending stress in the inner surface is higher than the outer surface of the coils. For round wire helical torsion springs, the bending stress correction factor for the inside diameter is calculated by the below equation formulated by Wahl:

KID = [4C² &#; C &#; 1] / [4C (C-1)]

Where C is the index. The bending stress at inner and outer diameters can be approximated by the following equations:

KID = [4C &#; 1] / [4C &#; 4]

KOD = [4C + 1] / [4C + 4]

Torsion springs must be loaded in a direction such that the spring diameter will decrease. This is because residual forming stresses are favorable in that direction.

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Chapter 3: Types of Torsion Springs

Torsion spring manufacturers offer a wide selection of torsion springs to meet the needs of different and varied applications. The different kinds of torsion springs have made them a very useful tool for several industries since they deflect and return innumerable times without needing to be replaced.

The many types of torsion springs have found use as the clips on clipboards to the harsh and demanding conditions of construction and automobile manufacturing. In essence, the simple structure of torsion springs has made them an invaluable tool.

Single Torsion Helical Springs

Single torsion helical springs are the most common among all types of torsion springs. It is made from a wire formed into a helix. The ends of the helix are extended to form the legs wherein the load is applied to twist the spring about its axis.

Double Torsion Helical Springs

Double torsion helical springs consist of two coils, a right hand and a left-hand coil which mirror one another. The coils are wound from a single length of wire and are separated by central legs connected in an bend to minimize friction. The coils work in parallel, and the sum of the individual torque exerted by each coil is equal to the total torque of the spring. Double torsion helical springs are used to rotate, lift, neutralize, and center rotating loads.

Torsion Bars

Torsion bars are simply made of flexible and elastic straight bars which can be twisted within their elastic limit. They are subjected to shear stress about their axes when torque is applied at their ends. They are typically made of rubber and steel. Torsion bars are widely used in heavy-duty applications.

Torsion Fibers

Torsion fibers are a type of torsion bar used in light-duty applications and sensitive devices. They may require tension to exert a return torque. They are typically made from glass, silk, or quartz fibers.

Spiral Wound Torsion Springs

Spiral wound torsion springs are made from wire coiled into a flat spiral. Load is typically applied on the free end of the spiral while the central end is held in a fixed position. The coils surround each other instead of being piled up. Therefore, these torsion springs are capable of making large angular displacements of many revolutions. Angular displacements can be made without drastic variation on the torque, making spiral wound torsion springs useful for applications requiring constant energy output.

Chapter 4: Production of Torsion Springs

Materials

Torsion springs are made of steel due to its stiffness with hard drawn steel, stainless steel, music wire, and spring steels being the most common materials. When light duty springs are required, certain varieties of high strength plastics are used.. The main characteristic of torsion springs is their extremely close winding, which is necessary to create their torque.

Spring Steels

Spring steel is a group of industrial-grade materials known for its high resilience, pliability, and strength. It can be compressed, bent, extended, and twisted to its elastic limit, and then return to its original shape without being deformed. These springs also have high fatigue strength and durability and are inexpensive. Spring steels contain high carbon concentrations. The types of spring steels are:

• Music Wire
• Hard-Drawn Wire
• Oil-Tempered Wire
• Flat Cold-Rolled Spring Steel

Stainless Steels

Stainless steels have both excellent mechanical properties, like spring steel, and superior corrosion resistance. The stainless steel grades commonly used in torsion springs are:

• Grade 302
• Grade 304
• Grade 316
• Grade 17-7 PH

Alloy Spring Steels

Spring steels can be alloyed with vanadium, manganese, silicon, chromium, nickel, and molybdenum to increase the elastic limit and make them more suitable for high impact and shock applications. The common alloy spring steels used in torsion springs are:

• Chrome Vanadium
• Chrome Silicon
• Silicon Manganese

Copper-Based Alloys

Copper-based alloys have excellent electrical properties and corrosion resistance and can be used in subzero temperatures. They have high strength and ductility. However, they are more expensive than spring steel and stainless steel. The common copper-based alloys used in torsion springs are:

• Phosphor Bronze
• Spring Brass
• Beryllium Copper
• Monel 400
• Monel K 500

Nickel-Based Alloys

Nickel-based alloys have excellent corrosion resistance and can be used for elevated and subzero temperatures. They are frequently used in harsh environments. However, they have high electrical resistance and are not recommended for electrical applications. The common nickel-based alloys used in torsion springs are:

• A 286
• Inconel 600
• Inconel 718
• Inconel X-750
• Hastelloy

Round, square, and rectangular wires can be manufactured into torsion springs. Round wires are the most common and readily available. Sharp corners in square and rectangular wires are avoided since stress is concentrated in those areas; The corners are rounded to eliminate this disadvantage.

Production Process

The steps involved in manufacturing torsion springs from steel wires are the following:

1. Spring Winding

   The production of torsion springs starts with coiling a piece of wire to form the spring body. It is performed by a CNC spring coiler or a spring coiler through the help of a mandrel. Winding can be performed with the wire at room temperature (cold winding) or an extremely elevated temperature (hot winding).

   
   

   Hot winding is preferred for thicker wires and bar stocks. In this method, the wire is heated at a very high temperature to increase its flexibility and then wound over the mandrel while it is red hot. Subsequently, the wire is removed from the spring coiler and plunged immediately to an oil bath to cool and harden it at a rapid rate. The spring produced at this stage is too brittle and needs to be tempered.

   The ends of the torsion spring are bent after winding.

2. Heat Treating

   The spring winding step has generated stress within the material. Heat treatment is necessary to relieve the material from stress, restore its resiliency, and completely harden. The coiled spring is heated at a predetermined temperature and duration and then slowly cooled.

3. Shot peening

   Shot peening is a cold working process involving striking the spring with steel, ceramic, or glass shots to compress the layers beneath the surface. This process strengthens the torsion spring to resist fatigue, corrosion fatigue, cracking, galling, and erosion from cavitation. Shot peening should not be performed on small wire diameters since it can open up the spring and cause the free angle to grow.

   
   

4. Finishing

   A thin protective layer is added to the spring to prevent corrosion, increase aesthetic value, and impart special properties (e.g., enhanced electrical conductivity). The common surface finishes for torsion springs include zinc, gold, chromium, nickel, black oxide, and rubber. Finishing can be accomplished by a plating, powder coating, dip coating, or passivation process.

Chapter 5: Applications of Torsion Springs

Some of the products which utilize torsion springs are the following:

Clothespins and Clipboards

Clothespin is the simplest application of torsion springs. Torsion springs cause the prongs of the clothespin to open and grip the cloth once the finger pressure is released. The same mechanism is applied to the clips of clipboards.

Spring-Loaded Hinges

Spring-loaded hinges have a torsion spring inserted through the knuckles, and the legs of the torsion spring are attached to the rectangular plates. The torsion spring provides a self-closing mechanism on the residential, commercial, automobile, agricultural vehicles, and garage doors and compartments once the force applied on the door is released. Hence, the door remains closed once when it is not being used. The spring in the hinge may be configured such that the door is statically held to stay open.

Clock spring

Clock springs, or main spring, are a type of spiral wound torsion spring. This spring is known to provide constant force output, and it can make large angular deflections of many revolutions while having a little variation in torque. Clock springs are available in square, rectangle, and D-shaped inside diameters.

Mechanical watches are a popular application of clock springs. The clock spring stores energy as it is rotated by a knob. The stored energy in the clock spring moves the clock&#;s wheels as it unwinds until the next winding is needed. This application is adapted in the operation of clocks, watches, timers, metronomes, wind-up toys, and music boxes.

Clock Springs in Vehicles

A clock spring is usually found inside the steering mechanism of automotive vehicles, specifically between the steering wheel and the steering column. It maintains all the electrical connections of the airbag, horns, radio, and steering mechanism electrical systems linked to the steering wheel. The clock spring allows the steering wheel to be rotated many times and in different directions without damaging the electrical wiring of those systems. As the steering wheel is rotated, the spiral winding of the clock springs will coil or uncoil around a disc. If those electrical wirings are unsupported by a clock spring, they will get tangled and damaged when the steering wheel is operated.

The other names of clock springs in vehicles are spiral cables, coil spring unit, coil assembly, cable reel assembly, contact reel, and airbag clock spring (for vehicles equipped with airbags). Despite being known in several names, the function of clock springs is the same in all vehicles.

Torsion Bar Suspension

Torsion bar suspensions are torsion bars used in automobiles that support the trailing arms when lateral or vertical forces are applied to the wheels. Under such circumstances, the torsion bar twists around its axis to avoid deflection in the trailing arms.

Conclusion

• A torsion spring is a mechanical device that stores and releases rotational energy. It can be used to apply torque or statically hold a mechanism in place.
• The specifications of torsion springs are inner diameter, outer diameter, wire diameter, body length, leg length, total coil, pitch, winding direction, leg angle, leg orientation, and leg style.
• The properties and parameters that affect the performance of torsion springs are spring index, angular deflection, maximum deflection, maximum load, spring rate, and stress.
• The types of torsion springs are single torsion helical springs, double torsion helical springs, torsion bars, torsion fibers, and spiral wound torsion springs.
• The classes of metals used in torsion springs are spring steels, stainless steels, alloy spring steels, copper-based alloys, and nickel-based alloys.
• The processes involved in the production of torsion springs are winding, heat treating, grinding, shot peening, and finishing.
• Some torsion spring products are clothespins, clipboards, spring-loaded hinges, clock springs, and torsion bar suspensions.

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Posted 30 April - 17:40

I'm going to use the analytical concept "Performance Index" as described by M. F. Ashby in his book Materials Selection in Mechanical Design to describe spring performance. This analysis does not address lightweight designs for suspension links or anything else where the performance index is not the same.

A spring is a device for storing elastic energy. The elastic energy stored per unit volume when stressed to a nominal stress Sigma is (1/2 * Sigma^2)/E. If failure occurs when Sigma exceeds a failure stress Sigmaf, then the maximum energy density is given as W = (1/2 * Sigmaf^2)/E.

This equation describes an axial spring. Since solid torsion bars have a lot of material at the neutral axis that is not stressed highly, the energy density becomes W = (1/3 * Sigmaf^2)/E. Note: Ashby continues the derivation using E, the Young's modulus, instead of G, the shear modulus. For isotropic materials, G is related to E by the relationship G = E/[2*(1+nu)] where nu is Poisson's ratio. Based on this, one can see that to minimize the volume of a spring, the best material would maximize the quantity Sigmaf^2/E. In order to minimize the mass, density now enters the equation: the performance index to be maximized is now Sigmaf^2/(rho * E).

The failure stress to be used for any dynamic spring, such as suspension springs/torsion bars for automobiles and racing cars, is a fatigue strength at some number of cycles. Obtaining this type of data in the exact same format (stress ratio, stress amplitude, etc.) is fairly difficult. As a proxy, I'll use ultimate tensile strength (UTS) numbers:

Oil-tempered, Chrome-Silicon steel, Valve Quality (ASTM A 877): UTS = - MPa for a wire diameter of 1 mm.

Beta-C titanium alloy (Ti-3Al-8V-6Cr-4Mo-4Zr), solution treated, cold drawn 90% to ~ 1mm diameter: UTS ~ MPa. Subsequent aging increases UTS to MPa. This data is from SAE Techical Paper (Honda, Chuo Spring, & Suzuki Metal Ind. Co.) Beta C is usually supplied to a range of strength specified in the aerospace standard SAE AMS , but I don't have access to this particular standard, so I used the Honda data.

From this data, it is not surprising to see why titanium can substantially reduce the mass of springs-- the strength can be nearly the same, but the density is only 4.82 g/cc instead of 7.83 g/cc, or 52% that of steel.

Things to consider:
1. Steel will retain a higher fraction of its tensile strength under fatigue conditions that feature mean stress effects. The proportion isn't high enough to overcome the large difference in density, though.

2. The Performance Index method accounts for optimization of spring parameters like coil diameter, number of turns, etc. in order to minimize mass. However, placing constraints on block height (length when fully compressed, also called solid height), free height (length when unloaded), etc. can greatly influence stresses on the wire, which is why changing from steel to titanium and vice versa can result in mass savings. The article that Top Fuel posted earlier is a good example of this. Another example is from a Ford study (summary appears in issue of JOM, p. 40):

steel titanium
Rate (N/mm) 61.25 61.25
Free length (mm) 426 426
inside coil dia (mm) 102 102
max load (N) 13,642 13,642
max stress (MPa) 1,031 872
wire dia (mm) 16.95 18.00
active coils 7.90 4.97
total coils 9.09 6.16
mass (kg) 5.92 2.86

So, for a given free length, the steel can't get out of it's own way-- the stress increases too much, because it is too stiff. This is why "setting" can be such a problem with springs: stresses build rapidly as compressed length approaches solid height, which isn't a problem is you are operating in a small range of displacement, far away from solid. On the contrary, valve springs may require many excursions to solid, therefore requiring as low of a stress at solid as possible, and depending on the constraints imposed, titanium can be significantly lower in stress and mass.


3. Temperature can be an important consideration, especially for valve springs. Cr-Si steel begins to experience setting (creep) and relaxation at temperatures above 150 C. Titanium Beta-C alloy will retain a higher percentage of its strength at this temperature.

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