TORQUE TENSION: Socket Set Screws Critical Features for Optimum Performance
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Fastener manufactures are being called upon to educate end users regarding items to consider before purchasing fasteners since, most often, the cost of fasteners is minor in relationship to the cost of an entire assembly. This cannot be emphasized strongly enough, especially for socket set screws. The decision to purchase a socket set screw should be based on performance alone. Knowing how well a set screw performs can only be determined by what a fastener manufacturer considers to be important.
A set screw is designed to hold parts together under a compressive force and keep them from coming loose. A typical application involves holding a collar on a shaft. A large percentage of set screw usage involves situations where moving parts are utilized. If a set screw(s) fails, damage to expensive equipment could occur, not to mention downtime and production losses.
There are, basically, three key elements: fastener design, heat treatment, and installation consideration.
FASTENER DESIGN:
Within the design of the socket set screw are three important features: socket, threads, and point style. Proper socket depth and key engagement is essential to achieve proper torque. If the socket depth is too deep, this could weaken the socket. Also, the width of the socket across the corners and across the flats of the socket must be within specification to ensure proper fit with the hex key. If the socket width is too large, this could cause the socket to ream. Each diameter set screw must withstand a specified tightening torque without any evidence of the socket reaming or bursting.
THREADS:
Threads also play a vital role in the performance of a set screw. Whenever possible purchase set screws with a rolled thread because the grain flow is unbroken and makes for a stronger thread. However, this is not always possible due to some very small diameters in which the threads must be cut.
POINT STYLE :
Point style is yet another critical area. When the set screw is tightened, a large force is achieved at the point. This force should be sufficient to inhibit motion between the mating parts. Therefore, the point must be capable of providing the necessary holding power. There are several different point styles available, each designed to fit the needs of a specific application.
HEAT TREATMENT:
Since socket set screws are a compression fastener, they are heat treated to a high hardness range of (Rc45-53) and maintaining this hardness range is critical because if the set screw is too hard, the socket or the point could crack when installed. If the set screw is too soft, the socket could ream, or the point could deform and not supply the required holding power.
A precise heat treat balance ensures that:
- The socket can withstand full-wrenching capability.
- Threads do not shear when the set screw is properly tightened.
- The point does not distort or crack, enabling the load to be transformed to the mating part.
INSTALLATION CONSIDERATION:
There are two basic considerations, the hex key wrench used to tighten the set screw and proper usage. The hex key like the set screw, must also be within specification. It needs toughness necessary to twist without fracturing prematurely and at the same time be strong enough to transfer the required torque to the set screw. If the key is not capable of providing the required torque, then, the point of the set screw will not deliver the force needed to keep the assembly properly tighten.
Keep in mind that a set screw is just that, a set screw. It is not designed or intended to be used in a tensile application, reference ASTM F912. Although end users are often tempted to design it as such. Utilization as a stud is not recommended because it generally entails running a nut on the set screw. This places tensile stresses on the set screw which invites cracking and eventual failure due to the high hardness.
Regarding one specific type of fastener-the socket set screw-knowing what dictates proper performance is essential if an end user is to make an intelligent purchasing decision. The major areas have been identified as being critical to the socket screw design.