This article will take an in-depth look at compression springs.
The article will look at topics such as:
This chapter delves into the fundamentals of compression springs and outlines crucial considerations for selecting the right type.
Coil springs, known as compression springs, serve as energy storage mechanisms when compressed. These helical, open-coiled springs resist compressive forces and, when compressed, shorten to absorb substantial potential energy.
Upon reduction or removal of the load, compression springs regain their initial shape and length due to the energy storage. When loaded, the springs become more compact, differing from extension springs where non-stressed coils remain uncontacted.
For designers and engineers, understanding spring mechanics, particularly Hooke's Law, is critical, as it dictates that deformation force is proportional to deformation amount. This principle applies to the need for increased force when compressing a spring.
The spring constant, indicated in Newtons per meter or pounds per inch, dictates the deformation force needed. A higher constant implies a stiffer spring. It is influenced by wire diameter, coil diameter, free length, and number of active coils.
Comprehending the spring constant is vital for manufacturers to achieve optimal spring performance. If the constant is overly high or the wire excessively slender, the spring risks failure. Precision in large-scale spring manufacturing ensures stability and prevents damage due to carefully calibrated spring coiling machines.
When selecting compression springs, consider various factors:
Compression springs feature diverse end types that are standard or custom-designed, such as open, closed, ground, and ungrouded ends. These configurations can impact the spring rate, despite constant factors like wire size, coil count, and outer diameter (OD).
Closed-end compression springs stand upright on flat surfaces due to closed terminal coils, favored for their simplicity and cost efficiency, as they demand less processing. For springs with high slenderness ratios, additional rod or shaft support may be necessary.
Ground end compression springs, a variant of closed-end springs, have precisely ground ends to align with spring dimensions. This precision comes with increased production time and costs. The ground ends ensure proper slenderness ratios, allowing effective operation without extra rod or shaft support.
Double closed end compression springs boast two closed terminals, similar to closed and squared ends. Produced akin to extension and torsion springs, all coils are in contact. These ends enhance stability, offering higher slenderness ratios, necessitating reinforced ends to prevent buckling, often at lower costs than their closed or ground counterparts.
Open end compression springs are less common due to stability concerns without rod or shaft support. With open and spaced coils, these springs suit applications prioritizing minimized solid height.
Paired with closed ends, open ends enhance load distribution and mitigate buckling risks. Ground ends, however, increase manufacturing costs due to additional processing.
Not all producers carry closed and ground ends as standard inventory, an essential distinction. Custom end varieties may include expanded coils for ring grooves, offset legs for alignment, or reduced coils for screw attachment.
Popular spring materials include carbon steel and exotic alloys. Music wire, high carbon steel, is commonly used, while stainless steel 302, though less strong, offers superior corrosion resistance.
Nickel alloys are selected for extreme temperature tolerance, corrosive resistance, and non-magnetic properties, available under various brand names. Copper alloys, such as phosphor bronze and beryllium copper, are prized for excellent electrical conductivity and corrosion resistance.
Outer Diameter (OD): For springs inserted into holes, consider the outer diameter. If surrounded by internal components, measure their dimensions. OD expands under compression, essential to consider if used in tubes or bores. The OD measures across the outer coil edges.
Manufacturing processes can restrict spring OD, impacting required assembly space. Manufacturers offer work-in-hole diameters based on anticipated OD expansion and tolerances, crucial when ordering custom springs or choosing from catalogs.
Inner Diameter (ID): For springs fitting over shafts or mandrels, account for the inner diameter. A minimum clearance of ten-thousandths of an inch prevents friction. The ID calculates by deducting twice the wire diameter from the OD.
Free Length: Ensure the spring's free length exceeds available space for a preloaded state, maintaining position. Free Length, the spring length without compression or force, measures from end to end or tip to tip.
Solid Height: Wire diameter and total coil count dictate solid height, crucial to ensure loaded heights do not exceed or undercut the solid height.
Environmental factors, temperature, and moisture exposure influence spring performance. Costlier materials withstand higher temperatures, increasing spring expenses.
Spring Pitch: Pitch refers to space between adjacent coils, from center to center. Calculate by measuring coil gaps and adding wire thickness.
Active Coils: These are the coils in compression springs that compress and deflect under load, contributing to the spring's movement.
Total Coils: This includes both active coils and non-deflecting, closed coils devoid of pitch.
Noting total and active coil counts is important. Closed or ground-end springs have inactive ends, whereas open-end springs boast fully engaged active coils.
Considering a spring's load or travel is imperative. The spring rate or spring constant indicates necessary compression force per unit length, typically in pounds per inch (lbs/in). Designers use this to anticipate spring travel under loads. Advanced compression adds stress, potentially leading to spring set, affecting original length recovery post-load, though they may still operate based on application.
Appropriate wire diameter and material selection is vital to align with load, travel, and environment requirements. The Rockwell scale gauges wire hardness, providing insights into its flexibility and rigidity. This assessment measures indents from a specific load, elucidating durability and stress performance.
| Compression Spring Wire Diameters | ||||
|---|---|---|---|---|
| Music Wire and Stainless Steel Wire Diameters | Chrome Silicone Wire Diameters | |||
| English Units | Metric Units | English Units | Metric Units | |
| 0.008 in | 0.203 mm | 0.08 in | 2.032 mm | |
| 0.009 in | 0.229 mm | 0.091 in | 2.311 mm | |
| 0.010 in | 0.254 mm | 0.098 in | 2.489 mm | |
| 0.011 in | 0.279 mm | 0.105 in | 2.667 mm | |
| 0.012 in | 0.305 mm | 0.118 in | 2.997 mm | |
| 0.013 in | 0.330 mm | 0.125 in | 3.175 mm | |
| 0.014 in | 0.356 mm | 0.135 in | 3.429 mm | |
| 0.015 in | 0.381 mm | 0.148 in | 3.759 mm | |
| 0.016 in | 0.406 mm | Up To | ||
| 0.017 in | 0.432 mm | 0.162 in | 4.115 mm | |
| 0.018 in | 0.457 mm | 0.172 in | 4.369 mm | |
| 0.019 in | 0.483 mm | 0.187 in | 4.750 mm | |
| 0.02 in | 0.508 mm | 0.312 in | 7.925 mm | |
| 0.021 in | 0.533 mm | 0.343 in | 8.712 mm | |
| Up To | 0.375 in | 9.525 mm | ||
| 0.362 in | 9.195 mm | 0.394 in | 10.008 mm | |
| 0.375 in | 9.525 mm | 0.406 in | 10.312 mm | |
| 0.394 in | 10.008 mm | 0.437 in | 11.100 mm | |
| 0.437 in | 11.100 mm | 0.453 in | 11.506 mm | |
| 0.453 in | 11.506 mm | 0.468 in | 11.887 mm | |
| 0.468 in | 11.887 mm | 0.5 in | 12.700 mm | |
| 0.5 in | 12.700 mm | 0.532 in | 13.513 mm | |
| 0.562 in | 14.275 mm | 0.562 in | 14.275 mm | |
| 0.625 in | 15.875 mm | 0.625 in | 15.875 mm | |
This chapter will explore the various manufacturing techniques employed in the production of compression springs, as well as the materials utilized in their creation.
The production of compression springs involves several manufacturing methods, which include:
In the coiling process, wire is first straightened before being fed into the coiler. The accuracy of the final product improves when the wire is properly straightened before entering the coiler. During coiling, CNC machinery with preset configurations adjusts various parameters, such as the spring's free length, pitch, and number of coils. High-speed cameras capture images of the coiling process, enabling precise measurement and adjustment to ensure that the spring meets tolerance specifications. After coiling, the spring is cut from the wire and then undergoes a stress-relief process.
During the coiling process, the wire's material undergoes stress, making it prone to brittleness. To address this, the spring is heated in an oven, allowing it to stabilize in its new shape and form metallic bonds. The oven maintains a consistent temperature for the coil for a specific duration, ensuring proper hardening before gradually cooling the coil.
After undergoing the stress-relieving process, the wire may undergo various finishing operations based on its intended use. These finishing steps transform the spring from its initial state into a refined component optimized for specific applications. The following are some of the finishing procedures involved:
Various steel materials are suitable for manufacturing compression springs, including stainless steel, hard-drawn steel, steel music wire, and spring steel. Generally, springs with larger wire diameters can endure more significant forces compared to those with smaller diameters. In essence, thicker wire typically results in a stronger spring, and reducing the coil diameter can also enhance its strength.
Stainless steel is particularly advantageous due to its corrosion resistance, even in environments frequently exposed to moisture and chemicals. Its durability ensures that it can withstand prolonged use without failing.
While spring steel is commonly used, other steel types and even plastic materials can also be utilized for making springs. However, improper selection of a spring for a specific application can lead to premature failure, potentially causing damage to surrounding components or posing safety risks.
Selecting the appropriate material for springs is essential for optimizing their performance and durability. Steel alloys are commonly used for spring manufacturing, including low carbon, high carbon, stainless steel, chrome silicon, and chrome vanadium alloys. Occasionally, metals such as titanium, phosphor bronze, and beryllium copper are used, while ceramic materials are employed for springs in high-temperature settings.
Music wire, known for its high carbon steel content, is suitable for demanding applications such as gym equipment, lawn and garden tools, and home improvement products. It exhibits an elasticity modulus of 30,000 psi and minimum tensile strengths ranging from 230 to 399 psi.
Hard-drawn wire, a medium carbon steel, is commonly used in springs found in everyday items like pens, office supplies, toys, and other products designed for indoor use. These springs can be customized to various needs due to their diverse hardness levels, with Rockwell hardness ranging from C31 to C52.
Various kinds of compression springs are available, including:
Convex springs, often referred to as barrel-shaped springs, have a unique design where the coils are wider in the middle and taper towards the ends. This design allows the coils to nestle into each other when compressed. Specifically, in a convex spring, the outer diameters at the top and bottom are smaller compared to the diameter at the center. These springs are commonly used to produce linear force.
Barrel springs are versatile and can be manufactured in various diameters, offering numerous design possibilities. They are often favored over standard compression springs due to their ability to save space, reduce buckling, and adapt to different shapes to fit specific needs. These springs can be designed to either telescope or remain non-telescoping. In applications requiring enhanced stability and resistance to movement when compressed, manufacturers prefer convex springs. They are commonly utilized in industries such as toys, furniture, and automotive.
Conical springs, shaped like a cone, have a diameter that tapers from one end to the other, with coils gradually decreasing in size along the length of the spring. This design allows for a progressive change in the coil diameter, enabling the coils to nest within each other. Such springs enhance stability while reducing the solid height. Some conical springs are designed to achieve a telescoping effect when compressed, where the tapered shape allows the coils to collapse into a smaller diameter, increasing the available travel or deflection. This makes them an ideal choice for applications requiring significant compression travel.
Conical springs offer enhanced stability compared to conventional compression springs. Their tapered design, with a larger outer diameter at the bottom, ensures improved stability and reduces the likelihood of buckling. This design helps maintain balance and performance even under compression.
The image below illustrates a Belleville spring, which is characterized by its coned disk design. Invented by Julian Belleville and patented in France in 1867, this spring's load-deflection characteristics vary depending on the (h/t) ratio. Belleville springs are widely used in applications such as plate clutches, brakes, relief valves, and various types of fastened connections.
Here are some advantages of using Belleville springs:
Concave springs, also referred to as hourglass springs, feature a design where the coil diameter is reduced in the center compared to the ends. This symmetrical design helps to maintain the spring's position and minimizes space usage while reducing the risk of buckling. The broader end coils contribute to an uneven pressure distribution, enhancing overall stability. With a variety of configurations available, concave springs can be tailored to fit many specialized applications.
Straight coil compression springs feature a uniform outer and inner diameter along their entire length.
Each coil in a straight coil compression spring maintains a consistent diameter. The ends of these springs can be ground or closed, offering a 270° bearing surface. Their cylindrical form distinguishes them from tapered compression springs, which have a conical shape.
A volute spring features coils with a cone shape rather than a round, oval, or square cross-section. Much like a conical compression spring, it functions in a similar manner. Instead of being compressed into a smaller space, the cone shapes nestle over each other. As a result, a volute spring typically achieves a lower solid height compared to a non-conical compression spring of the same length.
Variable pitch springs feature coils with varying spacing along their length, with some sections having wider gaps and others being more tightly wound. The term "pitch" refers to the distance between adjacent coils. In variable pitch springs, these intervals differ along the spring’s length, allowing for a range of performance characteristics.
Magazines utilize compression springs with oval or rectangular coils to propel cartridges or bullets into a handgun's chamber. These springs must be produced with high precision and stringent quality standards. Various design options are available, including differences in length, coil count, and force requirements. Given that magazine springs often operate near their solid height, the spring rate is a critical factor in the design process.
A torsion spring is a mechanical tool that stores and releases rotational energy. The torsion spring is attached to a mechanical part at each end. The winding of the spring is tightened and stores potential energy when it is turned around its axis at one end. As the other end is kept fixed, it is deflected about the body's centerline axis. The spring stores more potential energy as the winding becomes tighter and resists more rotating force. The spring will unwind as it performs an elastic rebound after being released, releasing the tensioned energy.
Torsion springs generate an equal and opposite rotational force at their ends, which can apply torque to the connected mechanical components. These springs are designed to hold mechanical parts statically in place. As the spring is twisted to tighten the winding, it faces more bending stress compared to rotational stress.
In contrast to compression and tension springs, which deal with linear and rotational forces, torsion springs are unique in that they operate solely under rotational forces. They rely on the material's elasticity to return to their original winding position after being twisted.
Torsion springs can exert force in either a clockwise or counterclockwise direction, depending on the direction of the twist. To achieve maximum force output, the spring should be twisted in the direction of its winding.
Torsion springs are utilized across various industries and come in numerous variations to suit different applications.
Tapered compression springs feature a cone-shaped design, with a larger outer diameter at the base and a smaller diameter at the top. This tapered form enhances stability, reducing the risk of buckling compared to standard compression springs. These springs are known for their low solid height, which improves stability and resistance to surging. The solid height of tapered compression springs can be as minimal as the diameter of a single wire. They are designed to resist compression forces and store energy when compressed.
This chapter will explore the various applications and advantages of compression springs.
Compression springs are utilized in a variety of applications, including:
Pens: A compression spring can be observed by examining a ballpoint pen. This spring enables the pen to write while exposing the tip and then shields the tip inside the housing to prevent the ink from drying out. This makes it possible to use pens without cumbersome and easily lost caps.
Firearms: Whenever considering tension, consider compression springs. Take into account the strain needed to fire a bow and arrow. The crossbow is a much simpler weapon if the human component is replaced with a compression spring. Technological advancements continue with the modern semi-automatic handgun, which uses a compression spring to absorb the energy produced by recoil and then redirect it to advance the slide or bolt and reload the weapon for the subsequent shot.
The benefits of compression springs are as follows:
Putting a component back in the right position: Door latches on both automobiles and building doors are an additional advantage that demonstrates how frequently utilized and essential compression springs are. Imagine raising a handle to open a door to get the greatest understanding of how a spring operates. The lock mechanism's compression spring would restore it to the locked position if the motion was used without pulling the door open. The spring can be compressed by tugging or turning the device; if it retains its position, the spring will stay compressed; otherwise, it will latch once more.
The drawbacks of compression springs are as follows:
Typical issues related to compression springs include:
When one end of a helical spring is placed on a rigid support and the other end is abruptly loaded, the coils do not deflect evenly. This is because it takes time for the tension to propagate along the spring wire. Initially, the coils at the end closest to the applied load absorb most of the deflection before passing some of it to the adjacent coils. A compression wave travels through the spring towards the supported end and then reflects back to the deflected end. This behavior is similar to a disturbance traveling through a closed water body before returning to its origin. This compression wave can continue traveling along the spring. If the applied load varies and the intervals between load applications match the time it takes for the wave to travel from one end of the spring to the other, resonance occurs. This causes the coils to experience high strain and significant deflections, potentially leading to the spring's failure. This phenomenon is known as surge.
The following methods can be employed to prevent spring surge:
Experimental data suggest that when the free length of a spring (LF) exceeds four times its mean or pitch diameter (D), the spring acts like a column and may buckle under relatively low loads. The following formula can be used to calculate the critical axial load (Wcr) that leads to buckling.
To prevent buckling, consider the following measures:
Compression springs can store mechanical energy when they are compressed. These open-coiled, helical springs provide resistance to compressive loading. When these springs are subjected to a compression load, they compress, grow shorter, and absorb a large amount of potential force. The springs are forced back to their original lengths and forms after the load is reduced or eliminated by the stored energy.
Thus, the selection of compression springs has to be made in consideration of the intended application, characteristics, benefits, and disadvantages of compression springs.
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