Universal joints are cost-effective power transmission shaft couplings. What makes them particularly useful is that they also compensate for significant misalignments and relative movement between the shafts they connect. Ideally, input and output shafts would be perfectly aligned with each other. In practice, displacement during operation — for example, due to technical restrictions or vibrations — is often unavoidable.
There are three types of this displacement between two shafts: parallel, axial, and angular. Compared to other compensating couplings, universal joints withstand and transmit through much greater parallel and angular displacements. Standard bellows, Oldham, and multi-disk couplings can usually compensate angular misalignments of up to 7° and, depending on their size, parallel misalignments from a few tenths to several millimeters. But these couplings lack the capability to compensate for axial shaft misalignment, except for elastomer or slotted couplings, which can compensate minor axial displacement.
Universal joints usually consist of two yokes — also known as forks — and a central, generally cross-shaped element. The precise construction of this cross or spider is not standardized and varies from design to design. It can be manufactured either as a single component or as an assemblage of several separate components. One-piece designs are found in large-volume production runs and are normally forged with machined and ground journals. The journal diameter is usually much smaller than the diameter of the corresponding yoke ear holes; this allows the cross to be fitted in the yoke without difficulty. For sufficient contact between the journal surface and yoke ears, bearing bushings or roller bearings can be fitted in the gaps between the journal and yoke ear hole.
Often, snap rings engaged within grooves machined into the pins are used to hold the pins in position. Another option is to mushroom or peen the ends of a pin or deform the middle section of the through pin, to prevent it from sliding out. A third option is to join or secure all pins in the center of the block, either through deformation, or with a firm interference fit. The fourth method is to affix the pins to the yokes instead of the block.
The bearing surfaces are then located in the block, not in the yoke ear holes. If needle bearings or bearing bushings are used, these are often secured with snap rings or screws to prevent the pins from sliding out.
When selecting a joint type for a particular application, the ease with which these variations can be installed should be considered — in addition to optimal running and transmission properties.
Plain and needle bearings
The durability of a universal joint depends on the transmitted torque and the rotational speed of the shaft to which the joint is connected. The joint's angle of deflection also comes into play: The higher the angle, rotational speed, and torque, the greater the load on the joint. An excessive torque usually causes the joint to break, while an acceptable torque at excessive speeds causes lubrication failure, which in turn leads to increased friction, heat generation, and wear.
High speeds are also problematic for joints fitted with plain bearings, even if the transmitted torque remains low. Proper lubrication of bearing surfaces can be difficult where oil-bath or drip lubrication systems are not possible.
One solution is the use of needle bearings in place of hardened bearing bushings. Although needle bearings reduce the joint's transmittable torque, they ensure that the joint can operate for longer periods of time without continuous lubrication. In addition, technical data and calculation methods for specific loads and speeds exist for roller and needle bearings. This allows better estimation of the bearing service life. (In contrast, service life can't be calculated for plain bearing joints. Too many variables exist for the mechanical environment they create.) As a general rule, roller or needle bearings should be used for frequent or continuous-duty applications with shaft speeds of more than 1,000 rev/min. Plain bearing joints can generally be operated at such speeds for short periods. However, their ability to handle these speeds for extended periods depends on the materials used, finishes, coatings and, of course, lubrication.
Plain bearings are also likely to experience increased abrasive wear on their running surfaces, causing play or backlash within the bearings. In motion control applications where precise positioning is vital, this play is not tolerable. Here, the joint must work at near-zero backlash, often for extended periods. For this reason, the low wear of the needle-bearing joints makes them the preferred choice in even low-speed motion control applications.
Joints and drive shafts
Single universal joints can compensate an angular offset of up to 45° between the input and output shaft, and this angle can be increased by installing the shaft with several U joints. Double joints can be made from two single joints by pinning or butting the hubs together, or specially designing the double joints with a single center section, to eliminate additional machining and assembling.
Drive shafts — also called articulated shafts — are shafts that include universal joints. The simplest type of drive shaft contains a joint at each end, making it essentially an extended double joint. Often, articulated shafts contain an adjustable middle element that allows the shaft's length to be varied for easy installation or to compensate axial play. This type of shaft can be used wherever a drive and its output shaft have not only parallel and angular misalignment, but axial displacement as well. Anywhere the position of the motor (drive) or the load (output) is frequently changed, an articulated shaft with a telescopic segment allows quicker, simpler repositioning than possible with a rigid two-joint shaft.
Constant-velocity universal joints were developed primarily for use in motor vehicles to overcome one drawback of articulated shafts: uneven output shaft rotation. In short, the output shaft lags behind the input shaft for part of a rotation and leads it for the other part, causing vibration. This cyclic irregularity becomes more pronounced as the angle between the two shafts increases.
Sometimes vibration can be eliminated with a double joint, whereby each joint half possesses the same displacement angle, and shares a common plane. But in many cases, notably in front-wheel-drive vehicles with front-wheel steering, even this arrangement can not be maintained. Why? The variable steer angle of the wheels has an effect on the shaft configuration. The solution here is a constant-velocity (CV) or homokinetic universal joint. In the common Rzeppa type, the pins (which form a permanent connection between the input and output shaft) are replaced with balls. Engaging in an annular shell, the balls move in a race about the joint's fulcrum. The circular shell replaces one of the yokes of the conventional universal joint, and the other joint half is formed by a second shell — which also contains sockets or races for the balls. The outer and inner races are then linked by the balls.
There are many ways in which input and output shafts can be connected to universal joints. The simplest way is a bore in the joint hub, into which the shaft is inserted and secured with a dowel pin. Alternatively, a keyway can be used to secure the shaft. A firm seat can also be achieved with square, hexagonal, splined, or threaded shaft ends. The choice of connection type depends primarily on the application and the production capabilities. In many cases, the joints are permanently welded onto the shaft.
In situations where continuous lubrication is difficult or impossible to supply, joints can be fitted with lubricant retaining boots or bellows to keep out dirt and debris. Because bellows inhibit lubricant loss, they also prevent (or at least reduce) environment contamination.
Beside the choice of materials, the joint components' corrosion resistance and required operating characteristics can be attained or improved by applying a special coating or finish, either locally or to the entire joint. For example, articulated joints are often galvanized.
Universal joints — also referred to as cardan, Hooke's, and U joints — are flexible mechanical couplings used to transmit motive power from one shaft to another. Universal joints are based on the principle of cardan suspension, as shown in the figure. The first suspensions of this kind were used for holding inkbottles. Leonardo da Vinci also used the principle in a compass to allow movement independent of surroundings. The first person known to have suggested the use of the principle to transmit motive power — and after whom the cardan joint is named — is the Italian mathematician Girolamo Cardano. He made his proposal in 1545. The Swedish inventor, scientist and industrialist Christopher Polhem later rediscovered the joint, after which it became known as a Polhem knot. A functional construction of the joint was developed in the 17th century by English scientist Robert Hooke, giving rise to the name Hooke's joint — a term still sometimes used in the UK. The most regularly used term — universal joint — was later coined by Henry Ford.