How to Use Hexagon Socket Set Screws 

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Shaft Diameters and Set Screw Sizes

When selecting the set screw size, you should select a size where the indentation on the end of the screw is apparent on the cylindrical surface of the shaft. The following figure shows the correlation between the tightened shaft diameter and the set screw (half point).

Indentation on the Screw End

Correlation between the set screw and the shaft diameter

Shaft Diameters and Shaft Holding Power

A limit where the shaft and hub or flange can be held together (which is called a shaft holding power) is determined by the friction coefficient between the end of the set screw and the shaft. The following figure shows the limit of the practical shaft holding power obtained by experiment results.

Diameter of the Tightened Shaft and
   Shaft Holding Power (Half Point)

*The shaft holding power of the set screw is related to the tightened shaft diameter size.

If the Set Screw Size Cannot Be Increased

Note that even if two set screws are used for applications that require a great shaft holding power, the shaft holding power is not necessarily doubled. The reason is that the shaft holding power varies depending on the opening angle between the two set screws. The following figure shows this relationship.

Opening Angle of the Set Screw and
   Shaft Holding Power

Hardness and Shaft Holding Power

The greater the tightened shaft hardness, the lower the shaft holding power. The following figure shows this relationship.

Set Screw and Shaft Hardness and
   Shaft Holding Power

Set Screws and Fit Length

With an increasing use of materials such as zinc die-cast and ironbased sintered alloy in the female screw parts, the allowable load of the female screws decrease resulting in problems. This problem can be solved by increasing the thickness of the female screw part. The following figure shows the relationship between the fit length of the set screw and the strength of the female screw material.

Female screw strength and fit length of
   set screw

Parallel Misalignment Amount of Female Screw Hole

If the female screw hole is displaced from the center of the tightened shaft, the shaft holding power decreases. The following figure shows this relationship obtained from an experiment with the M4 set screw.

Parallel Misalignment Amount and
   Shaft Holding Power of Female Screw

Shaft and Hub or Flange Hole Fit Accuracy

The shaft holding power does not decrease very much until the shaft accuracy h9 for the hole standard as shown in the figure below. Since the fit accuracy is expected to have considerable influence in a dynamic operating environment, sufficient attention needs to be paid to the fit accuracy.

Fit Accuracy of the Bushing Hole and
   Shaft Holding Power

Reference:Socket Screw Technology Group
"How to Select and Use Hexagon Socket Set Screws"

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