Universal joints allow shafts that are not in line with each other to bend at the joint in any direction while transmitting rotary motion. Whereas flexible couplings allow for misalignment, universal joints accommodate angles and turns. They consist of a close pair of hinges that are connected by a cross shaft at 90 degree angles to each other. Universal joints are used when the components of a drive train cannot be directly connected because of spatial issues or because the movement between the shafts is necessary.
When selecting a universal joint, the hub type, material composition and the applications for use are all important factors. Plastics are often used in the production of universal joints but steel is the most common. While plastics provide better rust and corrosion resistance and electrical and magnetic insulation, steel can handle more torque and higher temperatures. Gear couplings and universal joints are used in similar applications, most predominantly in the automobile industry.
While gear couplings have higher torque density, universal joints induce lower vibration than gear couplings. The lower torque density in a universal joint is due to the limited cross section of its cross and yoke. Other applications for universal joints include metalworking machinery, military applications, packaging systems, printing processes, medical equipment, automotive steering purposes, commercial sewing machines, food processing equipment, conveyors, cleaning equipment, surgical tables and many other applications.
All parts of a universal joint must work together properly in order for the entire device to operate effectively. One key aspect of universal joints is the drive pins. The main pin and bushing are designed for even strength and are usually ground from alloy steel. The main hub is generally available in a variety of configurations including bore, keyway, hex, square and threaded.
The cross pin keeps the assembly strong even while under stress and the central cube-shaped component that attaches the shafts is machined to minimize backlash as it provides smooth operation and control. There are two main kinds of universal joints: single joint and double joint. The single joint style has one bending joint, allowing it to function at up to a 45 degree angle. The double joint style can function at up to a 90 degree angle because it contains two bending joints.
There are other variations as well. The simplest universal joint is a Hooke joint which causes the output shaft to increase and decrease in speed twice for every one revolution of the input shaft. Other universal joints, or Cardan joints, can accommodate a large variation in the angles of the input and output shafts. As the angular difference increases, so does the speed fluctuation between them.