Setscrews

Setscrews are semipermanent fasteners that hold collars, pulleys, or gears on shafts. They are categorized by drive type and point style.

An important consideration in setscrew size selection is the holding power provided by the clamping action. Some additional resistance to rotation is contributed by point penetration. Cup-point and cone-point setscrews are used without a spotting hole. They penetrate the shaft deeper than oval-point or flat-point setscrews. Holding power is generally specified as the tangential force in pounds, since design considerations may cause different sizes of shafts to be used with a particular size of setscrew.

Setscrew selection is based on the fact that setscrew diameter should be equal to roughly one-half shaft diameter. This rule often gives satisfactory results, but its usefulness may be limited. Manufacturers' data or data supplied by standard texts give more reliable results.

Seating torque: Torsional holding power is almost directly proportional to the seating torque of cup, flat, and oval-point setscrews.

Point style: Setscrew point penetration contributes as much as 15% to the total holding power. When the cone-point setscrew is used, it requires the greatest installation torque because of its deeper penetration. Oval point, which has the smallest contact area, yields the smallest increase in holding power.

Relative hardness: Hardness becomes a significant factor when the difference between setscrew point and shafting is less than 10 Rockwell C scale points. Lack of point penetration reduces holding power.

Flatted shafting: About 6% more torsional holding power can be expected when a screw seats on a flat surface. Flatting, however, does little to prevent the 0.01-in. relative movement usually considered as a criterion of failure. Axial holding power is the same.

Length of thread engagement: The length of thread engagement does not have a noticeable effect on axial and torsional holding power, provided there is sufficient engagement to prevent thread stripping during tightening. In general, the minimum recommended length of engagement is 1 to 1.5 times the major diameter of the setscrew for threading in brass, cast iron, and aluminum; and 0.75 to 1 times the diameter for use in steel and other materials of comparable hardness.

The lengths of engagement specified are for full threads engaged, not overall screw length.

Thread type: A negligible difference exists in the performance of coarse and fine threads of the same class of fit.

Drive type: The shape of the screw head affects the seating torque that can be attained because it determines how much torque can be transmitted to the screw. Less torque can be transmitted through a slotted setscrew than a socket head setscrew. Therefore, holding power of the slotted screw is about 45% less.

Square-head setscrews can be tightened with a wrench until the screw fails. Recommended seating torques for square heads are about twice that for socket setscrews, giving a proportional increase in holding power.

Number of setscrews: Two setscrews give more holding power than one, but not necessarily twice as much. Holding power is approximately doubled when the second screw is installed in an axial line with the first but is only about 30% greater when the screws are diametrically opposed. Where design dictates that the two screws be installed on the same circumferential line, displacement of 60° is recommended as the best compromise between maximum holding power and minimum metal between tapped holes. This displacement gives 1.75 times the holding power of one screw.

Torque force: The compressive force developed at the point depends on lubrication, finish, and material.

Plating and lubrication: Most unplated setscrews are used with a thin film of rust-preventive oil. For added corrosion protection, setscrews can be plated with cadmium or zinc. An increase in the holding power for the same tightening torque results when the screw is plated or used with a suitable thread lubricant. However, the addition of these lubricants to the screw can reduce resistance to loosening.

Setscrews and keyways: When a setscrew is used in combination with a key, the screw diameter should be equal to the width of the key. In this combination, the setscrew holds the parts in an axial direction only. Torsional load on the parts is carried by the key.

The key should be tight fitting so that no motion is transmitted to the screw. Under high reversing or alternating loads, a poorly fitted key will cause the screw to back out and lose its clamping force.