How tractor power take-off works

Power take-off or Power Take-Off (TDP) is one of the most important items in agricultural tractors. Knowing its characteristics and operation well allows for more efficient and safe handling.

To carry out agricultural activities, it is necessary to optimize operational processes and when it comes to mechanization, manufacturers seek to captivate customers by providing increasingly versatile machines and implements.

The different forms of energy transfer from the agricultural tractor allow for great versatility in pulling and activating agricultural machines carrying out various operations, including combined or combined operations. Among these forms, the power or force take-off (TDP or PTO) deserves particular attention, an external device, whose origin comes from the English expression Power take off. It is an axle with splines or slots classified according to its location on the tractor at the front or rear, being an element to transmit power in the form of rotation to agricultural machines and equipment that are coupled or not to the tractor, in this case stationary equipment.

Commonly on national tractors, the PTO axle is available at the rear, while at the front it is considered an accessory item due to the need to operate specific implements to carry out simultaneous operations.

This important and indispensable device in agricultural operations was standardized by the American Society of Agricultural Engineers - Asae with regard to its geometry and rotation regime, that is, regarding the shaft diameter and the number of splines and regarding the direction of rotation and to shaft speed, being classified into three types, according to Table 1.

The axis at the rear always rotates clockwise. The front one is made up of six grooves and has a speed of 1.000 rpm. In some brands, this shaft can have two directions of rotation and is activated through an electromagnetic clutch.

Another important aspect in relation to TDP is the way the shaft is driven, with an evolution taking place to provide more versatility to the operation, going from a totally dependent system, to a semi-independent system, to a totally independent system. The dependent system is directly linked to the engine movement and driven by a simple clutch. In this case, when the tractor is immobilized, the PTO will no longer receive movement. The semi-independent is based on the use of a double clutch – double discs – with the clutch pedal having two strokes. The first stroke only disengages the advance box and the second the TDP. The independent system is similar to the semi-independent one with the difference that there are two different levers for operating the clutch, in this case the advantage is that the PTO is turned off even when the tractor is moving and vice versa. It is generally a multi-disc mechanism in an oil bath with electro-hydraulic control.  

The transmission of rotational movement to machines driven through PTO is carried out through extension type axles, with a universal cardan joint at each end that allows the variation in angle and distance between the tractor and the implement to be accommodated.

To allow a tractor to provide two rotational speeds in the PTO (540rpm and 1.000rpm), two systems were developed: with two output shafts at the rear or a single output in which two 35mm shafts with a number of splines can be exchanged. different ones, one with six splines for 540rpm and another with 21 for 1.000rpm. It is possible to achieve this by changing the transmission ratio depending on the geometry that is inserted into the TDP output. There is currently availability of reversible shafts that have a different geometry at each of their ends.

In some cases, it is convenient to use PTO at 540rpm with a lower engine speed to drive implements that demand less power, but require a speed of 540rpm. For this, the TDP at 750rpm was created, called 540E or 540 Economic, so there will be lower fuel consumption, somewhere around 20% to 30%. In summary, the TDP has undergone an evolution in two aspects, the first in terms of drive form and, second, in terms of geometry and rotation regime.

TDP ON BRAZILIAN TRACTORS

In a survey carried out among the main tractor manufacturers that included 197 models sold in Brazil, from the brands Agrale, Budny, Case IH, Farmer, Green Horse, John Deere, Landini, Massey Fergunson, Montana, New Holland, Tramontini, Ursus, Valtra , LS Tractor and Yanmar Agritech, at first it is evident that there is no agreement between the companies regarding information about TDP. In many cases, the information is scattered and not very enlightening for the consumer who seeks to make a more appropriate analysis to support the purchasing decision.

On lifted tractors, they basically have two TDP options, proportional and constant speed. In proportional mode, the TDP shaft is driven by the transmission system's gearbox output shaft, where the rotation rate is directly linked to the tractor's travel speed depending on the selected gear. The PTO at constant speed is linked to the start of the gearbox and the rotation speed varies according to the engine speed. However, the catalogs are confusing about the type of TDP, whether in the mechanical constitution, the mechanism to activate the TDP and the form or way in which the types of TDP are activated. They generally present drive modes, whether independent, electro-hydraulic, automatic, double-stage, direct, dependent instead of TDP types.

Table 2 presents the confusion regarding how TDP characteristics are made available by manufacturers. It is a summary of how information is presented in advertising materials. In addition to many mistakes occurring in the presentation of data, others are not even made available.

Regarding the type of drive, two types are available: the independent, a model that does not require the use of a clutch to activate the PTO, and the dependent, in which it is activated by means of a double-stage clutch, mentioned previously.

Regarding the way in which the PTO is activated, three types were defined: electro-hydraulic, hydraulic, which are easily activated or deactivated by a button on the control panel, and mechanical, activated by a lever.

The tractors analyzed offer the following speeds: angular at PTO 540rpm, 750rpm and 1.000rpm. The speed options are linked to power class, where smaller tractors generally have a speed of 540rpm, while medium power tractors have both 540rpm and 750rpm, as well as the 1.000rpm option and high power tractors mainly use at 1.000rpm.

In addition to these options, some brands have an economical TDP, which is recommended for lighter operations that do not require full engine rotation, such as those carried out by mowing or spraying. The economical PTO, when activated via a lever, allows the tractor to reach 540rpm at a lower engine speed, enabling greater energy efficiency and consequently a reduction in costs, as previously mentioned. Regarding safety, there are a series of changes to the TDP on the market. The main one refers to the activation, with the need to pull and turn a lever, but it only disengages when touched, thus preventing the operator from turning it on unintentionally and allowing quick deactivation in urgent cases.

What tractor manufacturers offer in terms of power take-off in their products

- Smoothed drive system to start operation with the PTO, in this case, when activating the PTO, power is transmitted progressively, avoiding sudden load to drive the implement, thus reducing damage to the cardan shaft components and implement mechanisms.

- Independent transmission providing maximum power to carry out the operation, avoiding wear on components of the transmission system.

- External control device to activate the PTO at the rear of the tractor, on the bumpers.

- Offering rotation regimes in 540, 540E and 1.000.

- Speed ​​synchronized with the tractor’s progress.

- Use of a lower engine speed to activate the PTO with reduced fuel consumption and reduced noise in the cabin, providing a more suitable working condition for the operator.

- Possibility of selecting rotation speeds 540, 540E and 1.000 using an electronic selector in the cabin.

- Mechanical ergonomic rotation regime selection lever facilitating the activation of the PTO.

- Automatic TDP disarming system when using the tractor's hydraulic lifting system simultaneously, in this case when the hydraulic system lifting limit height is reached, the TDP disarms.

- Availability of frontal TDP for the development of simultaneous operations.

Jarbas Cardoso Araújo, Ismael dos Reis, Marcos Roberto da Silva, Hugo Souza Soares, Fabrício Pedreira Santos, Federal University of Recôncavo da Bahia

Article published in issue 156 of Cultivar Máquinas.