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Coiled spring pins were designed for assembly. Compared to other pins, the square ends, concentric chamfers, and lower insertion forces ease installation. Additionally, coiled pins can accommodate wider hole tolerances better than any other pin type because they compress during installation and conform to the shape of the hole. Coiled pins are the easiest type of pin to install and are designed to be serviceable.

The coiled pin’s nominal diameter represents the recommended hole size for the product into which it’s being installed. Coiled spring pin design guidelines provide the recommended hole tolerance range. The coiled pin’s pre-installed diameter is greater than the hole size, and the chamfer diameter is smaller than the hole. During installation, the swaged chamfer aligns the coiled pin with the hole and facilitates compression of the coils as it’s driven into position.

Installation options

Most manufacturers prefer to use a hammer when only a handful of prototype assemblies are needed. When the pins are too small to hold while hammering into the hole, some opt for a pin-driving chuck so that they don’t have to hold onto the pin while it’s being inserted. Automatic pin inserters are preferred for high production volumes because they provide a significant return on investment (ROI) and are also preferred for difficult-to-handle small diameter pins.

Hammer – For the simplest coiled-pin installation method, align the coiled pin up to the hole by hand and hammer the coiled pin into the hole like a nail. Drive the pin to the desired insertion depth and be careful not to damage the host.

Using a hammer is great for prototyping a handful of assemblies or trying out a coiled pin for the first time. This installation method is not recommended for short pins or pins with small diameters because it can be difficult to hold them in place by hand.

Manual press or air hammer with pin driving chuck – Secure the pin driving chuck into the press or air hammer, then manually insert the coiled pin into the end of the driver. Next, place the exposed end of the pin into the hole and complete the installation by advancing the press handle or actuating the air gun.

Heavy, standard, and light duty coiled spring pins.
Photo courtesy of Spirol Corp.

Presses and air hammers with pin chucks allow for better axial alignment, control, and quicker cycle times compared to a hammer. These are great solutions for small- to medium-volume production. Pin-driving chucks are cost-effective and versatile tools allowing manufacturers to control alignment and insertion depth. Also, the chuck will hold the pin securely in place prior to and during installation. The pin-driving chuck has an internal punch with a diameter smaller than the hole but greater than the pin’s chamfer diameter, critical for effective installation.

Automatic installation equipment – Ideal for efficient, high-volume production, automated solutions are completely self-contained. The fastener is oriented in a vibratory feeder and fed to a shuttle mechanism that positions the fastener directly underneath the insertion quill. The shuttle is mounted on a retracting mechanism that positions the pin exit bushing as close to the assembly as possible for insertion, then retracts for unobstructed loading and unloading.

Installation is simple with an automatic pin inserter. The operator will load the assembly onto the fixture, activate the machine to insert the pin, then remove the assembly from the fixture. The operator doesn’t handle pins during any part of the installation process; all that is required is to pour the pins from the shipping container directly into the feeder bowl when the quantity in the bowl needs replenishing.

Many options are available to customize the automatic inserter such as adding adjustable workstations, infrared safety light curtains, force monitoring, distance monitoring, part presence sensing, rotary index tables, feeder bowl level monitoring, drilling and pinning combinations, and multiple pin insertions per cycle. The feeder bowl can also be designed to sort out errant product or debris.

The coils on coiled spring pins compress during installation.
A pin driving chuck.
Figure 1
An automatic pin installation machine.
Photos courtesy of Spirol Corp.

Additional considerations

Fixturing – Devices that hold, support, and align components during installation are critical to the performance of the installation equipment and the quality of the final product. Effective fixturing also improves cycle times, reduces the risk of scrapped assemblies, and poka-yokes the beginning of the assembly process.

Spring-loaded alignment pin – Recommended for assemblies with through-holes, spring loaded alignment pins ensure alignment is maintained between the pin and the holes of the assembly components throughout the installation process. A spring-loaded alignment pin affixes to the installation table and aligns the assembly prior to and during installation. Spring-loaded alignment pins (also known as locating pins) have spring mechanisms so they retract as the pin is inserted into the assembly (Figure 1, below). Fixtures using the periphery of the assembly components as datums rather than a disappearing pin require tighter tolerances on the parts to maintain proper alignment, increasing manufacturing costs without adding value compared to using a disappearing pin for assemblies with through-holes.

Spirol Int. Corp.

About the author: Jeff Greenwood is a product sales engineer at Spirol Int’l Corp. He can be reached at jgreenwood@spirol.com

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