Aligning with hollow dowels

May 1, 2012
On motion designs, there are many ways to align components. One useful approach is to drill twin holes into mating parts, and then use a pin or dowel

On motion designs, there are many ways to align components. One useful approach is to drill twin holes into mating parts, and then use a pin or dowel to fill and align the holes. Here, the most common (and often most expensive) alignment dowels are solid, precision-ground “parallel” pins manufactured per ISO 8734. These deliver high positional tolerance, though require extremely tight hole tolerances to do so. In addition, these dowels are typically through-hardened for strength, or case hardened for wear resistance. However, if the pin's sole purpose is for alignment, the heat-treating adds cost without any functional benefit.

As an alternative, some ground hollow dowels are designed to be a direct replacement for solid ground dowels in alignment applications. These lighter subcomponents provide the same positional accuracy as solid ground dowels, and require the same hole tolerances for proper installation and retention. However, some varieties are 50% lighter than a solid dowel — useful where weight reduction is a design objective.

Other options are spring alignment dowels, dowel bushings, and coiled spring pins. None are precision ground, but all can accurately project hole position for the purpose of alignment. Each of these options is designed to be larger than the hole in which it is retained: The mechanism of retention is compression rather than interference. The flexibility of these parts provides much lower insertion force than that required for ground solid or ground hollow dowels. In addition, the spring characteristics of these parts absorb wide hole tolerances, reduce manufacturing costs, and trim component costs.

This month's handy tips provided by Christie Jones, SPIROL International Corp., Danielson, Conn. For more information, call (860) 774-8571 or visit


Why use hollow dowels?

Hollow alignment dowels reduce weight and cost, plus simplify assembly. They also maintain precision alignment, ease insertion, absorb hole tolerances, and prevent interlocking — the hooking together of traditional rolled dowels (also called slotted pins) when they're stored in a bulk bin. The latter is particularly problematic in situations where the insertion of the pins is automated from a hopper.

Where are interference-type dowel and pin options appropriate?

Spring dowels are generally recommended for applications requiring a dowel with a length to diameter ratio of 1.5:1 to 4:1. In contrast, coiled pins are generally recommended in applications with greater length to diameter ratios — between 4:1 to 10:1.

In applications requiring bolts for axial retention, further savings can be achieved by passing the bolt through the inside diameter of a dowel bushing. This eliminates the cost associated with drilling a separate hole for the bolt. Dowel bushings are also hardened to protect the bolt from shear loads, and isolate the forces on the bolt to tension loading to ensure joint integrity.

How do challenges in shock and vibration affect which pins are most appropriate for a given application?

Hole material is normally softer than the pin, so any transmission of shock and vibration through the pin can cause hole elongation or enlargement. Then the fit between the hole and the pin loosens, increasing the impact force and accelerating the rate of hole damage. The inevitable result is premature failure of the assembly.

In applications that exhibit such heavy shock and vibration, coiled pins shine. Because they serve as shock-absorbing elements — coiled pins provide for compression of the pin into the hole and for continued flexibility after insertion — they damp and therefore diminish the amount of applied load that is transmitted to the hole wall.

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