Springs are used in machines, from everyday items to heavy machinery. They function as mechanical energy storage, akin to batteries for electrical storage. The use of springs dates back to the 1400s with spring-driven clocks. Despite technological progress, modern devices like the Apple Watch require daily charging, unlike the efficient winding of clocks. My interest in springs began with toys like Slinkies and evolved to deconstructing mechanical pencils. Now, I value sophisticated applications, such as my car’s coil spring suspension providing a smooth ride over potholes.
Classification of Springs
Springs are typically classified based on the applied load. Here are the primary classifications of Springs:
- Compression Springs: Function under compressive loads and are present in shock absorbers, spring mattresses, mechanical pencils, and retractable pens.
- Extension Springs: Operate under tensile loads, exemplified by items like Slinkies. They are also used in luggage scales and garage door mechanisms.
- Torsion Springs: Function with torque or twisting force and power everyday items such as clothespins and mouse traps.
Types of Springs
Each spring type has additional classifications. Now, let’s explore how force and displacement relate in springs. There are three classes:
- Linear (constant rate)
- Variable rate
- Constant force springs
Let’s discuss each one by one.
Linear Springs
Linear springs follow Hooke’s Law, stating that the force required to extend or compress the spring by distance x is proportional to x, as long as it stays within the elastic limit.
Hooke’s Law for Linear Springs:
F = -k * x
Where:
( x ) = distance or (L2 – L1)
( F ) = force needed to extend or compress the spring by distance ( x )
( k ) = spring constant or spring rate
The negative sign in Hooke’s Law signifies that the restoring force opposes the applied force; pulling the spring down results in a downward extension but an upward restoring force.
Torsion Springs
Torsion springs follow a variation of Hooke’s Law, where θ represents the angle of deflection or twist. In both instances, ( k ) remains the spring rate, and it remains constant regardless of the spring’s deflection. This is why linear springs are often referred to as constant-rate springs.
Hooke’s Law for Torsion Springs:
F = k * θ
Where:
( \Theta ) = angle of deflection or twist
( F ) = force exerted due to torsion
( k ) = spring constant or spring rate
The negative sign in Hooke’s Law indicates that the restoring force opposes the applied force during torsional deflection.
Variable Rate Springs
A variable rate spring does not maintain a constant spring rate throughout its axial length; in other words, ( k ) is not constant. The spring rate can exhibit either a progressive change or a more abrupt variation—refer to the diagram below.
Cone-shaped compression spring
A well-known example of a variable rate spring is the cone-shaped compression spring, often used in battery boxes. The fully compressed height may be as low as one wire diameter. Variable rate springs offer the added advantage of being laterally stable and less susceptible to buckling. For further conical spring calculations regarding stiffness and allowable working stress, refer to the additional resources provided.
Constant Force Springs
Constant force springs have a distinct structure, composed of pre-tensioned metal strips instead of the standard wire used in typical springs. As implied by its name, these springs necessitate nearly the same force, regardless of the extension length. Commonly referred to as clock springs, they are typically coiled ribbons of spring steel employed in applications like counterbalancing for monitors and clocks.
It’s worth noting that the term “constant force spring” can be slightly misleading. In reality, the spring’s full load must be overcome by extending it to 125% of its original diameter before a nearly constant force can be applied to further expand the spring.
Manufacturing Springs
Springs can be categorized based on their manufacturing methods, as various techniques exist for spring production. The most widely recognized is the metal coil spring, also referred to as a helical spring. However, numerous other spring types exist. Surprisingly, even an elastic band qualifies as a variable rate spring, given its ability to store mechanical energy.
Coil Springs
To craft lightweight coil springs, metal wires are shaped using a CNC coiling machine. The multi-axis CNC control offers the flexibility to design variable pitches and end conditions, with possibilities limited only by creativity. Springs produced by coiling machines initially lack springy properties. To imbue them with shape memory, they must undergo heating to a high temperature (typically 500 degrees Fahrenheit or more) to relieve stress, followed by quenching.
Flat Springs
Flat springs exhibit diverse sizes and shapes, encompassing spring washers, PCB spring contacts, and retainer clips. These springs, including coiled variations like clock springs and volute springs, are essentially sheet metal components produced through stamping. It’s essential for all flat springs, regardless of type, to undergo heat treatment for shape memory.
Disk Springs
Belleville washers and conical springs are interchangeable terms referring to disk springs, which are disk-shaped with a concave surface. Typically produced by stamping, plasma cutting, or blanking a flat metal sheet, the concave shape is often machined. While disk springs may resemble distorted metal washers, they serve significantly more intricate purposes.
Machined Springs
Machined springs and die springs are employed in heavy-duty applications demanding high strength and precision. As implied by the name, machined springs are crafted using CNC lathes and mills.
Molded Springs
Plastic or composite springs are prevalent in corrosive environments like food production, medical, and marine applications. Due to susceptibility to creep, they are suitable for intermittent cycles only. Plastic springs are relatively recent additions to the field compared to metal springs, and their supply is not as abundant.
Conclusion
That’s all about ‘The Main types of Springs’. If you have any questions, make sure to write us in the comments.
