Machine Design

Semipermanent Pins

Pins are typically used under primarily shear loading. They are separated into two groups: semipermanent and quick release.

These general design rules apply to all types of semipermanent pins:

  • Avoid conditions where the direction of vibration parallels the pin axis.
  • Keep the shear plane of the pin a minimum distance of one diameter from the pin end.
  • Allow pins to protrude the length of the chamfer at each end for maximum locking effect in applications where engaged length is at a minimum and appearance is not critical.

Removal and installation of semipermanent pins require the application of pressure or the aid of tools. The two basic types of semipermanent pins are machine pins and radial-locking pins.

Machine pins are separated into four categories: dowel, taper, clevis, and cotter.

Hardened and ground dowel pins are standardized in nominal diameters ranging from 1/16 to 7/8 in. Standard pins are 0.0002 in. oversize on the nominal diameter; oversize pins are 0.001 in. oversize.

Standardized pin lengths vary with nominal diameter, ranging from a minimum of 3/16 in. (1/16-in. size) to a maximum of 5 in. (7/8-in. size).

Some dowel pins are threaded and tapered for use with a tapered sleeve. The threaded section allows the pin to be pulled from the sleeve, and a special sleeve puller removes the sleeve. This reusable dowel-and-sleeve combination can properly align holes as much as 0.001 in. oversize. It is said to speed machine-shop operations by eliminating knockout holes, eliminating the need to turn dies to remove dowels, and enabling machinists to remove and replace die sections while the die is in the press.

Taper pins have a taper of … in./ft measured on the diameter. Basic dimension is the diameter of the large end. Diameter d of the small end is given by d = D - 0.02083 L where D = diameter of the large end, in., and L = pin length, in.

The standardized series of numbered pin sizes ranges from No. 7/0 (0.0625-in. large end) to No. 10 (0.7060-in. large end).

Clevis pins have nominal diameters from 3/16 to 1 in. Corresponding shank lengths vary from 19/32 in. for 3/16-in. size to 2 5/8 in. for 1-in. size. Standard material is steel, either soft or cyanide hardened to meet service conditions.

Cotter pins come in a number of point styles and have been standardized into 18 sizes with nominal diameters ranging from 1/32 to in. Available materials include mild steel, brass, bronze, stainless steel, and aluminum.

Radial-locking pins include grooved and spiral-wrapped types. Grooved straight pins have a locking action provided by parallel, longitudinal grooves uniformly spaced around the pin surface. Rolled or pressed into solid pin stock, the grooves expand the effective pin diameter. When the pin is driven into a drilled hole slightly larger than the nominal pin diameter, elastic deformation of the raised groove edges produces a secure force fit with the hole wall.

Standard groove-pin sizes cover a range of nominal diameters from 1/32 to in. with lengths from 1/8 to 4 in. Materials include cold-drawn, alloy, and stainless steels, and copper alloys.

Low-carbon-steel groove pins have load capacity in single shear from 110 lb for a 3/64-in. size to 10,300 lb for a -in. size. Higher shear strength may be obtained from heat-treated pins.

Locking force developed by a groove-pin assembly is a function of pin diameter and effective length of engagement. Best results under average assembly conditions are obtained with holes drilled the same size as the nominal pin diameter. Undersize holes must be avoided. This practice can lead to deformation of the pin in assembly, damage to hole walls, and shearing off the raised groove edges, thus reducing holding action and preventing reuse of the pin.

When the part material is appreciably harder than that of the pin, chamfered or rounded hole edges should be specified to avoid shearing the expanded pin section. Shearing also may be avoided by using through or case-hardened pins. Hardness of the pin and the metal fastened should be equal.

Spring pins use the resilience of hollow cylinder walls to hold in place. The two main types are spiral-wrapped and slotted tubular pins. Both pin forms are made to controlled diameters greater than the holes into which they are pressed. Compressed when driven into the hole, the pins exert spring pressure against the hole wall along their entire engaged length to develop locking action.

Spiral-wrapped or coiled pins come in standard sizes that cover a range of nominal diameters from 1/32 to 3/4 in., in lengths from 1/8 to 6 in. Metric diameters range from 0.8 to 20 mm. Standard materials are: 70/30 brass, copper alloy 25, heat-treated 1070 to 1095, carbon steel, stainless steel (302 and 402), and 6150 alloy steel.

Locking force of a spiral-wrapped pin is a function of length of engagement, pin diameter, and wall thickness. Pins are available for light, medium, and heavy-duty applications.

Slotted tubular pins come in standard sizes from 1/16 to in. and lengths from 1/8 to 5 in. Standard materials are heat-treated carbon steel, corrosion-resistant steel, and beryllium copper. Readily adaptable to manual assembly techniques, these pins offer a tough, resilient, self-locking fastener that can withstand high shock and vibration loads.

Spiral-wrapped pins have an advantage over slotted pins in automatic assembly because they cannot interlock during feeding. They may also resist vibration and absorb shock better than slotted pins because the coil design can flex after assembly, while the slotted version cannot flex after the gap is closed.

For maximum shear strength, the pin should be assembled so that the gap is in line with the direction of load and 180° away from the point of application. The maximum shear strength value provided by this orientation represents an increase of about 6% over the minimum value.

TAGS: Fasteners
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