Cardan-coupling

1.0 GENERAL

A universal joint is a positive, mechanical connection between rotating shafts, which are usually not parallel, but intersecting. They are used to transmIt motion, power, or both.

The simplest and most common type is called the Cardan joint or Hooke joint. It is shown in Figure 1. It consists of two yokes, one on each shaft, connected by a cross-shaped intermediate member called the spider. The angle between the two shafts is called the operating angle. It Is generally, but n01 necessarily, constant during operation. Good design practice calls for low operating angles, often less than 25°, dependIng on the application. Independent of this guideli ne, mechanical interference In th e construc tio n of Cardan joints limits the operating angle to a maximum (often about 37½°), depending on its proportions.

2.0 Applications

Typical appllcatlons of universal joints include aircraft, appliances, control mechanisms, electronIcs, Instrumentation, medical and optical devices, ordnance, radio, sewing machines, textile machInery and tool drives.

Universal joints are available in steel or in thermoplastic body members. Unlversal joints made of steel have maximum load-carrying capacity for a given size. Universal joints with thermoplastic body members are used in light industrial applications in which their self-lubricating feature, light weight, negligible backlash, corrosion resistance and capability for high-speed operation are significant advantages.

Universal joints of special construction, such as ball-jointed universals are also available. These are used for high-speed operation and for carrying large torques. They are available both in miniature and standard sizes.

3.0 General Characteristics of the Cardan Joint

A basic characteristic of the Cardan joint is the nonuniformity of motion transmission through the joint. The angular-velocity ratio between input and out put shafts varies cyclically at two cycles per revolution of the input shaft. This fluctuation, which is accompanied by corresponding angular accelerations, increases with the operating angle and can be as much as 15% of peak angular velocity (in the case of a 30° operating angle). In selecting a joint the effect of these fluctuations on static torque, inertia torque and system performance needs to be kept in mind.

The non uniformity of the transmission can be eliminated by using two appropriately phased universal joints in series, as shown in Figure 2. In such cases the velocity variation induced by one joint can be made to cancel that of the ot her, thereby transmitting a constant (1:1) angular velocity ratio between shafts. The angular velocity fluctuation of the intermediate shaft, however, cannot be avoided. Two universal joints in series also permit coupling of two laterally displaced shafts (single Cardan joints are limited to intersecting shafts).