Cardan-articulated transmissions, or simply cardan shafts, are mechanisms used to transmit movement between rotating trees whose geometric axes intersect at any angle. The term “tree”, although apparently strange, is used in the field of mechanics for rotating shafts that have elements such as gears, pulleys, flywheels or others mounted on them.
The cardan shaft, like the hydraulic system of tractors, is one of the main ways of transferring power from the tractor to the remote operation of agricultural machinery. While the hydraulic system uses oil flow under pressure, the cardan shaft uses mechanical energy to transmit power (torque and rotation) at an angle to agricultural implements.
The cardan shaft connects the power take-off shaft (TDP) of the agricultural tractor to the power input connection shaft (CEP) of the implement.
CARDAN SHAFT COMPONENTS
The cardan shaft basically consists of two universal joints joined by two solid and/or tubular telescopic shafts (Figure 1). Transmission takes place through the so-called “universal joint” or cardanic joint, composed of two terminals (also called forks or sleeves) coupled to each of the trees (of the tractor and the implement) joined by a crosshead in which the movement is articulated in two perpendicular planes happens.
UNIVERSAL JOINTS
Universal joints (Figure 2) can be “single” or “double”, better known as homokinetic. The simple joint is formed by two terminals/forks and a crosshead, and is recommended when the PTO axis and the CEP axis are angularly aligned at the same height. The constant velocity joint is formed by a central component and two crossheads, being indicated when the two axles are at different angles and not at the same height, ensuring equality between the instantaneous output and input rotations. This type of joint can briefly reach a working angle of 80º, facilitating headboard maneuvers.
When we only have a universal joint with an operating angle other than zero, the instantaneous output speed, in most cases, is different from the input speed. With two joints it is possible to obtain an output speed equal to the input speed as long as the angles at the two joints are equal. With constant velocity joints, uniformity of output speed is guaranteed in practically any angle condition.
CROSSES
The crosshead plays a fundamental role in the transmission of force through the cardan shaft, as transmission takes place through the nuts and rollers that make it up. The crosshead is formed by two cross-shaped axes, the ends of which are called trunnions. In each of the four trunnions of the crosspiece there is a nut (a type of cover) articulated, the space of which is occupied by rollers, through which forces responsible for the movement are transmitted (Figure 3). The reciprocating movement in the crosshead penalizes the frictional surfaces of the journals and nuts, leading to wear and even premature “failure” of the component when operating conditions are adverse.
TELESCOPIC AXLE
The telescopic shaft may have a round, triangular, square, oval, star, grooved or other shape, aiming to reduce contact pressure and reduce vibrations.
For the transmission to function properly, the eyes of the intermediate shaft forks (telescopic tube) must be contained in the same plane, that is, in phase. In square profiles, for example, a 90° offset is possible between the fork eyes; in striations, various angularities can occur, depending on the number of striations out of phase; in rectangular and oblong cases, assembly can only be carried out in two positions, one offset by 180° in relation to the other, which is equivalent to 0°, in ideal conditions; The triangular ones are constructed in such a way that assembly is possible in only one position. To ensure that, on square profiles, the forks have their eyes aligned, the male and female components may contain grooves and guide projections. For example, at high speeds (1.000rpm) the ribbed profile is more suitable as it reduces gaps between tubes.
Circular tubular profiles have the most favorable transmission capacity/mass ratio among all others and are, therefore, preferable in high-speed drives such as motor vehicles. In this case, it is usual for one of its ends to have an extension in the form of a short, ribbed spike, which fits into a sleeve that allows telescopic movement.
In agricultural drives, with low operating speeds, in addition to circular profiles, others such as square, rectangular, oval and triangular profiles work satisfactorily and are widely used.
The tubes are sized for maximum safety torque and are often superficially treated to minimize telescopic pressure. One of the treatments, called "Rilsan", reduces the coefficient of friction by 50% compared to normal contact between two metal surfaces. If working conditions are highly abrasive, the pipe may be heat treated to increase its hardness.
CARDINAL AXLE CONFIGURATIONS
The ISO 5673-2:2005 standard presents three possible arrangements (Figure 4) involving simple universal joints and constant velocity joints that operate together in agricultural transmissions, always with the aim of standardizing the rotational movement at power input and output. Since the irregularity of the rotational movement generates noise and vibration, which can reduce the useful life of its components, it is important to observe the best configuration.
Configuration A is composed of two simple cardan joints; at small and equal angles, this transmission compensates for variations in angle and length of the telescopic tube, ensuring uniform transmission of the rotational movement.
Configuration B has a simple joint and a constant velocity joint; in this way it is possible to obtain practically uniform rotational movement as long as the simple joint operates in a straight line or with an angle below 10o.
Configuration C is composed of two constant velocity joints; It is possible to always obtain uniform final rotation, even with different angle values between the joints, and can achieve misalignment of 40º. In this situation, the operator can perform the work under normal conditions, even when maneuvering curves, without requiring greater attention or effort.
CLASSIFICATION OF CARDINAL SHAFTS
Although in practice each manufacturer has its classification established in “series”, where each series has its torque capacity, they should correspond to the categories established by the American standard Asabe/Ansi S331.5 (2010). This standard establishes eight static torque capacity categories for cardan shafts, each with two subsets of connecting members (shaft itself), one for heavy duty, the other for normal duty. Table 1 presents the categories of the American standard depending on the requirements of static torque.
Therefore, it is expected that cardan shaft components are interchangeable within the same series or equivalent series when dealing with different manufacturers.
Table 1 - Static Torque Requirements
| Categories | Fork, crosshead and trunnion set | Connection members | |
| Normal service | Hard work | ||
| _______________ Nm ______________ | |||
| 1 | 1.130 | 565 | 791 |
| 2 | 1.808 | 1.130 | 1.582 |
| 3 | 2.486 | 1.582 | 2.147 |
| 4 | 3.955 | 1.921 | 2.938 |
| 5 | 5.649 | 2.938 | 4.180 |
| 6 | 7.344 | 3.728 | 5.423 |
| 7 | 9.604 | 4.858 | 7.005 |
| 8 | 12.428 | 6.779 | 9.039 |
Categories
Fork, crosshead and trunnion set
Connection members
Normal service
Hard work
_______________ Nm ______________
1
1.130
565
791
2
1.808
1.130
1.582
3
2.486
1.582
2.147
4
3.955
1.921
2.938
5
5.649
2.938
4.180
6
7.344
3.728
5.423
7
9.604
4.858
7.005
8
12.428
6.779
9.039
GIMBAL MAINTENANCE
Lubrication of the cardan components is essential for its durability. The main point is the lubrication of the cardan's moving parts – crossheads and male and female parts – which must be done in accordance with the manufacturer's recommendation and with the appropriate lithium soap grease (EP 2 - Extreme Pressure with consistency level 2 ), resistant to high temperature and water. Silicone-based products, graphite greases and chassis greases are not recommended, as they only provide superficial protection. In addition to reducing friction, the lubricant protects components against rust or corrosion.
Cardan shafts without lubrication, or lubricated with an inappropriate product, show premature wear of moving components. As a result, temperatures of around 200ºC can be reached, a situation in which any lubricant that may still be available becomes fluid enough, escaping from the places where it should act, further shortening the useful life of the component. Grease carbonization may also occur in the lubricant ducts, blocking its passage.
The lubricant applied by means of a manual pump, through a grease fitting, is stored in the lubrication channels of the crossheads and conveyed to the nuts by the action of centrifugal force, therefore it is essential that they are not obstructed. On the intermediate tree, the lubricant is applied between the male and female parts, using a brush.
Adequate lubrication for the proper functioning of the transmission varies with use. When the component is used in light duty, at small operating angles, permanent or long-term lubrication may be appropriate. When transmissions are used at large operating angles, high speeds, excessive loads and the presence of intense dust and humidity may require daily lubrication.
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