Introduction to Spring Pins

Spring pins, commonly known as slotted pins, rolled pins, or tension pins, are hollow press-fit fasteners with chamfered ends. They are crafted from alloy steel, carbon steel, and stainless steel and feature spring-like elastic properties. They are often used as locking devices, positioning points, anti‑chafing surfaces, shafts, or sleeves for wiring.

Spring pins are typically split into two types: slotted and coiled. Within the slotted group, there are those with smooth slots and those with toothed slots. This guide talks about the different types of spring pins, how they work, and the design elements that make them suitable for different application environments. First, we’ll discuss how a spring pin works and the benefits they offer.

A copy of the infographic below can be downloaded here: Spring Pins Infographic

How a Spring Pin Works

Spring pins compress when inserted into a hole. Due to their spring like nature, the diameter decreases as it is being inserted into the hole. After installation, elastic forces return the pin to its resting state, forcing the outside face against the inner wall of the mating hole. The frictional force between the two surfaces hold the pin in place, preventing it from falling out or becoming inadvertently dislodged. 

Slotted Pins

Coiled Pins

Why Use Spring Pins?

Spring pins offer several benefits: 

• Their elastic nature allows them to fit into holes without reaming.
• Slotted pins can double up to increase shear strength.
• They are functional in both blind and through-hole applications.
• As long as they are in good condition and maintain their spring nature, they are reusable. 

Additionally, these pins are easy to install and simply require a straight drilled hole. Installation can be either manual or automatic.

What are the Different Types of Spring Pins?

There are three main types of spring pins: slotted, slotted toothed, and coiled.

Slotted Spring Pins

Slotted spring pins are hollow, single‑turn, formed pins with one straight parallel slot running the entire length. The width of the slot is narrower than the wall thickness of the pin, which prevents interlocking or nesting with other pins. The hollow design and slot give them their unique self‑locking capabilities.

They are manufactured to a controlled outside diameter that is slightly greater than the hole in which it is meant to be installed. Installation compresses the pin and applies continuous pressure to the host component. The flexibility of a slotted pin allows it to be inserted in holes with slightly wider tolerances than solid pins, reducing manufacturing costs.

Orientation and Stress

It is important to consider orientation when installing slotted pins, as stress is concentrated directly opposite the slot. 

After installing the pin in a hole, the slot must remain open slightly to ensure that applied forces are absorbed. If fully closed, the slotted pin will transfer loads to the host material and damage to the hole or assembly may occur. 

Doubled-Up Installation

For additional shear strength, two slotted pins may be used in combination to construct a single assembly made up of two different slotted pins (one pin inserted inside of a second pin). It is important to have the slots staggered by 180° degrees, and it is also recommended to make sure the hole size is on the high side of the tolerance.

Strength

Standard ASME B18.8.2‑2000 slotted tension pins are stronger than standard duty ASME B18.8.2 coiled pins. This is also true when comparing standard ISO 8752 slotted pins to ISO 8750 coiled pins. In fact, standard slotted tension pins are stronger than the mild steel solid pins, taper pins, or grooved pins of like diameter, and can handle the most demanding applications. More strength, versatility for hole design, and ease of installation make slotted pins a "go to" for many applications.

Toothed Spring Pins

Super-duty ExpanPin toothed spring pins are hollow like standard slotted spring pins; however, the slot on toothed pins features staggered teeth. These teeth are spaced to interlock when the pin is compressed. This alignment helps retain the roundness of the pin while minimizing lateral movement in vibratory environments to prevent walkout.

Slotted toothed pins can serve a dual purpose in certain applications, like when both a mechanical stop and a spring retainer is needed. In addition, they can double up to add shear strength by positioning the slots at an angle between 90° and 180° to each other.

There are several benefits that these heavy-duty pins offer. They provide:

• Solid pin advantages but with less weight and less chance of jamming in automatic feeds
• High seating capacity
• Ease of installation and removal with the lightly oiled finish
• RoHS compliance
• Custom length options
• Rockwell C hardness of 42-50 to offer high wear and shock resistance 

Coiled Spring Pins

Coiled spring pins, or spiral pins, are rolled forms made with 2‑1/4 turns of steel. They are made in light, standard, and heavy-duty styles, distinguished by the material thickness. The coiled design allows stress to be evenly distributed throughout the pin's cross section and provide 270° of surface contact to the host component.

Coiled spring pins’ flexible design absorbs shock and vibration to prevent damage to the mating component while maintaining shear strength, regardless of the direction of force. The design also forces the outer edge of the pin to rotate inward upon compression. As pressure is relieved, the outer edge rotates outward, returning to its resting state. This cycle repeats itself countless times in high vibration environments.

A unique benefit of these pins is that, because of the coils, they conform to the shape of out-of-round holes. This is especially helpful in applications that are deformed or when the exact shape of the hole is unknown before installation.

Design Considerations When Choosing a Spring Pin

Some design considerations to keep in mind when selecting a spring pin include material, material grade, finish, and fit.

Material

The material from which a spring pin is crafted is always an important consideration. Spring pins are made from alloy, carbon, and stainless steel. Carbon steel and alloy steel are both common economical materials. However, they provide little corrosion resistance without a supplemental plating or other finish. Pins crafted from stainless steel boast superior corrosion resistance without the need for additional plating. However, they are weaker than pins made from high carbon steel.

Material Grade

Material grade is an important element when evaluating application requirements. If the spring pin will be subjected to temperature extremes, an alloy steel may be appropriate. However, if the pin will be exposed to corrosive elements, stainless steel should be considered. Below is a list of common spring pin materials.

• 1070 to 1095 series carbon steel spring pins are considered a high carbon steel spring pin. High carbon steel spring pins have high strength and resistance to wear as well as extreme hardness and moderate ductility; however, they do require added plating or other protective coatings to guard against corrosive elements.
• 300 series stainless steel is austenitic and hardened only by cold working. 300 series stainless steel provides the best corrosion protection. However, since it is not heat‑treated, it is not as strong as 400 series stainless steel.
• 400 series stainless steel is martensitic, hardened through heat‑treating. It presents high strength and high wear resistance comparable to high carbon steel but is more susceptible to corrosion than 300 series stainless steels.
• 6150 alloy steel spring pins feature high tensile strength and toughness as well as high resistance to vibratory stress and high torque strength. For added resistance to corrosive agents, 6150 alloy steels should be coated with a phosphate finish or zinc coating. 

Different types of materials have unique mechanical properties that make them behave in their own ways. To learn more about these properties and what they mean, check out our Introduction to Mechanical Properties of Metals.

Finishes

Spring pins are offered in a variety of finishes, each with their own benefits. Common finishes include lightly oiled, phosphate, zinc plating, cadmium, and passivated stainless steel.

Lightly oiled finishing provides protection against oxidation of hardware during transportation and storage. It should not be considered a permanent finish. Some plain alloy steel spring pins are offered in this finish.

Phosphate coatings are made of thin crystalline layers of phosphate compounds and tend to make a suitable base for applying paints. Sealing oil can also provide further corrosion resistance. Phosphate coatings are used on low alloy and high carbon steels and should not be relied on in marine or highly corrosive environments.

Zinc platings provide good corrosion resistance and are a popular choice for protecting carbon steel fasteners against corrosion. Since zinc plating is popular, the cost to apply it is relatively low compared to other plating solutions. It also features high ductility and can tolerate temperatures up to 120°. While zinc plating offers greater corrosion protection compared to phosphate coatings, it should not be used in marine applications or other highly corrosive environments. 

Passivated finishing is a passive oxide layer on stainless steel that provides additional protection from corrosive elements. While stainless steel is naturally corrosion resistant, this oxide layer provides additional protection preventing surface corrosion on stainless steel parts. 

Huyett also offers high-carbon spring pins with a cadmium plating finish. While this is sometimes the preferred option for salt water and aerospace applications, it is listed on Huyett’s Toxicity and Materials of Concern Compliance page. Environmentally friendly finish alternatives may apply for specific applications.

To learn more about how metal finishing options work, visit our Introduction to Types of Metal Finishes.

Fit

Fit refers to how a spring pin fits in a desired hole. Interference fit and clearance fit are two forms of fitment. Interference (friction) fit provides the highest level of accuracy in alignment. However, it should not exceed a light press for seating or be confused with press fit that is experienced with solid pins.

Clearance (free) fit is desirable for ease of assembly as pin surface contact pressure is considerably less than interference fit. Clearance fit is ideal when a spring pin is used in a hinge application where unbinding movement is required.

Looking For More?

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