A spline coupler is a device used to affix two splined shafts together. The typical splined coupler represents the female connection, and looks like a short piece of pipe with splines or grooves cut into the inside; a common splined shaft represents the male connection and features splines or grooves cut into the outside of the shaft. There are two basic styles of spline coupler: same spline to same spline coupler and different spline to different spline coupler. Many couplers are designed to allow a different type of component to attach to another type of component. This is usually to complete a modification intended to aid a racing or performance issue with a vehicle.
For some of the most demanding components, such as a transmission or four-wheel drive transfer case, the spline coupler is the only method that can be successfully used time and time again to join two independent components and form one drive line. Unlike a universal joint that places the torque and power on two points, the spline coupler uses multiple splines to place the torque and power against as many as 40 points inside the coupler. With a spline, the strength comes in the number of splines that are being used in the coupler. Similar to a drive axle, the higher the spline count, the stronger the axle and the lesser the chance the axle will slip inside of the differential.
To give the coupler strength, it is common for the splines inside of a spline coupler to first be cut and then hardened by heat-treating the entire coupler. Some couplers are machined from pieces of pipe that are cut to size, however, the most exotic metal couplers are typically machined from a block of billet steel or aluminum. Billet steel and aluminum are renowned for their superior strength and uniform grain created by the forging process. This type of material is often used in extremely close tolerance and high-performance applications due to its ability to be machined to blueprint plans.
Technology has allowed composite materials, such as carbon fiber, to be used in the manufacture of spline couplers. This typically involves the use of a machined die in which the carbon fiber material is applied over and worked into the machined splines in the surface of the die. This intricate work commonly equates to higher prices and longer turnarounds in production. The composite coupler is typically found in aviation, space craft, satellites and high-performance vehicles. Drag racing vehicles as well as Indy®-type cars frequently feature the lightweight and strong composite couplers in a variety of applications.