Lesson 13. Study of tractor suspension system

The suspension system supports the vehicle, allowing the wheels to move up and down over irregularities in the road. It cushions the ride for the frame, engine, transmission, and passengers, while keeping the tires in firm contact with the road under all conditions. Suspension system parts include springs, dampening devices (shocks), ball joints, steering knuckles, and spindles or axles. Two types of front suspension systems are widely used in today’s vehicles: the Mac-Pherson strut suspension and the short/long arm (SLA) suspension. Early automobiles have a straight axle (I-beam) front suspension, and up until recently some vehicles were having with a variation called a twin I beam suspension. Many rear suspensions still use a straight axle.

1. Objective of Suspension System:

Ride comfort is considered as the first objective of the suspension systems of vehicles including tractors. Ride comfort is an important characteristic of vehicles that indicates how much riding is comfortable for passengers. Ride comfort is very important for agricultural tractors also, because the acceleration transmitting to the driver compared with other vehicles is very high due to more undulations in the agricultural fields as compared to the smooth road. In addition, the operators of agricultural tractors spend many hours in the field during peak working seasons. These conditions can affect the comfort, efficiency, alertness, and health of the operators.

In order to quantify the ride comfort of a vehicle, vibrations of the vehicle body should be measured in two directions of vertical and horizontal (i.e. roll, pitch and yaw). The most commonly used measurement methods is the RMS acceleration. Root Mean Square (RMS) acceleration is defined as:

    131.png                                           ... (13.1)

Where T = Total sample time,

           a = spring mass acceleration,

           t = Time.

In order to evaluate the ride comfort, the RMS accelerations of both the vertical and horizontal directions is measured.

Handling is a characteristic of a vehicle that provides stable and safe driving that can be created via a steady contact between the tires and surface. Handling is also called road holding, ride stability, and driving safety, implying the same meaning. The handling capability of a vehicle is important during maneuvers such as at turning, braking, or accelerating. In these extreme situations, weak handling reduces the control ability of the vehicle and can affect the safety of the passengers/drivers. Due to this fact, handling is considered as an important capability for vehicles, and beside the ride comfort, it is considered as the main target of using the suspensions in vehicles.

The early agricultural tractors had no suspension systems, and different types of suspension systems were employed in them gradually with time. The primary systems were seat suspensions, which were used with the aim of improving ride comfort of tractor drivers. Along with the development in tractors technology, chassis suspensions were also used for these vehicles. These systems were able to improve the handling of tractors besides the ride comfort of them. This provided more safety and stability for tractors, and their travel speed was increased This begins with characteristics of tires, and suspensions of seat and cabin. Then, the chassis suspension systems of tractors including front axle and full suspension were introduced in the tractors.

2. Suspension Characteristics of Tires

When pneumatic tires were used first time for agricultural tractors, ride comfort and handling were improved, and the travel speed of tractors was increased upto 20 km/h (Hoppe-01, 2004). Tires are the first elastic elements between a vehicle and the ground surface. For that reason, the suspension properties of the tires have an important role in the dynamic behaviour of tractors, particularly for tractors having no other suspension systems.

The suspension characteristic of a tire can be demonstrated by a simple model, which is constructed of a spring and a damper in parallel (Fig. 13.1). Based on this model, the measured stiffness and damping characteristics of a typical tire for agricultural tractors are presented. The stiffness and damping coefficients of the tire are correspondingly too high and too low than typical suspension characteristic needed for a tractor. Therefore, tires are not able to work lonely as a proper suspension system, because high stiffness of tires is equal to a very hard suspension that is unable to provide good ride comfort. On the other hand, with very low damping capacity of tires, they are not able to provide effective control on vibrations. In addition to the improper value of stiffness and damping coefficient of tires, these characteristic are not constant and depend on the inflation pressure and speed of tires. All these factors cause a poor dynamic behaviour for the conventional tractors that have no primary suspension,

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Fig. 13.1: Single point contact modeling of tires (Courtesy Shahriar Sarami, 2009)

3. Types of suspension systems

(i) Seat Suspension:

The first system used for the tractor was suspension for the seat of the operator. This system is placed directly between the driver seat and tractor body, and it affects directly driver comfort and reduces the vibration experienced by him. Seat suspensions are not so complicated, inexpensive and have a robust construction. For these reasons, they are used on all modern agricultural tractors (Fig. 13.2). A suspension seat is made of typically a foam cushion suspended on a parallel spring and damper set. Type of suspension systems may be mechanical, pneumatic, hydraulic, hydro-pneumatic, or a combination of these systems.

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Fig. 13.2 Modern seat using “active seat” technology (CourtesyJonhDeere, 2013)

Air–ride seat from New Holland is another active seat system, constructed based on a pneumatic suspension. This system uses an adaptive control system that adjusts the parameters of the suspension relative to the driver’s weight. Another example of the modern seats is the semi-active system produced by the tractors manufacturer of Valtra. This system is equipped with a pneumatic spring and a MR damper, which works as the actuator of the system. Besides the seat suspension, which has major role in providing ride comfort for the driver, some other characteristics influence on the ride comfort. For example, size, design, and materials of a seat have important role in sitting perception.

(ii) Cabin Suspension

In modern agricultural tractors, a cabin is used in order to isolate the driver from the outside, and it protects the driver from the annoying environmental conditions, dust, and noise. Using a suspension system for the cabin, driver place could be isolated from the tractor vibrations as well. A cabin suspension can offer more benefits than a seat suspension and provide better ride comfort for the driver. Cabin suspension decreases the structurally transmitted noise to the drivers. Since the mass of a cabin is greater than a seat, the natural frequency of cabins is lower than the one of seat suspensions. The manufactured cabin suspensions of the agricultural tractors can be categorized in two groups of semi suspended and fully suspended. In the first group, the rear side of the cabin is connected to the chassis by means of two suspension units in its corners. In spite of this, the front side is connected to the tractor chassis without suspension and only via a joint-type link. This type is often used for the tractors with front axle suspension. In this system, body vibrations appear on the cabin as pitch rotational movement, and response of this system to the roll movement of the tractor body is very limited.

(iii) Hitch Suspension

During transportation, tractors mounted with implements, change largely the mass characteristics of the tractors, and influence mainly the dynamic behaviour of the tractors. Without the primary suspension, the tires are the only elastic elements that affect the handling of the tractors, whereas the tires have no required suspension properties. Under these conditions, the dynamic behavior of the tractor becomes worse under the influence of the attached implements, particularly the bounce and pitch movement of the tractor are increased. This situation causes a reduction in the control ability of the tractor, especially during high-speed transports which leads to unsafe modes.

In order to reduce the effect of the mounted implements on the tractors and in order to control their vibration, a suspension system can be used in the connection between them and the tractors. Since the implements are normally mounted on a tractor via the three-point hitch, this suspension is applied to this mechanism and called “hitch suspension”. Such a system that employs a hydro-pneumatic suspension in damping the pitch vibrations of the implement. A hydraulic actuator is used instead of the upper link in this system (Fig. 13.3). This actuator is connected to a hydraulic accumulator through a throttle valve and works as a hydro-pneumatic suspension.

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Fig. 13.3: A hydro-pneumatic shock absorber system applied to the tractor three-point hitch (CourtesyGoehlich, 1984).

(iv) Front Axle Suspension

When, four Wheel Drive (4WD) tractors became popular due to the better traction performance. The front wheels acquired a new role in the traction capability of the tractors and also in the steering ability of them. In this condition, there is a typical problem of the pitch movement of the tractors, because it creates load variation on the front tires and cause the problem of bouncing, particularly during of the pulling heavy loads by tractors. This produces a traction variation effect, called “power hop,” that is not only uncomfortable for the operator, but also causes a major loss in the traction efficiency of the tractors. In order to overcome this problem, the front axle suspension was used for tractors. This suspension provides a stable contact between the front tires and ground that leads to a significant improvement in the traction and steering capability of tractors. Because of this advantage, front axle suspension has become a common option in 4WD tractors now. In addition, the front wheels are the navigating wheels. They take the major steering and brake loads of a tractor. Front suspension by keeping front tires in firm contact with the ground, allows better steering control and brake efficiency. This promotes the handling capability and increases consequently the travel speed of a tractor.

However, influence of the front suspension on ride comfort of tractors is not notable as much as the influence of this system on the handling of tractors.

(v) Full Suspension

Higher travel speed is taken into consideration for the modern tractor design (Fig. 13.4). This led to try for development of high tractor speeds, which are able to cover the transportation needs of agricultural applications (Goehlich, 1984). As mentioned above introduction of the front suspensions in tractors provided a major benefit regarding the high travel speed. However, front axle suspensions can be sufficient just to reach to a limited maximum speed. In order to achieve a higher speed, the full suspension is needed for tractors. A full suspension tractor is a tractor equipped with both rear and front axle suspensions. These tractors can provide improvement on both driving behaviour and ride comfort, promising the possibility for the rise in the driving speed.

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Fig. 13.4: Full frame construction of fully suspension “JCB-FASTRAC” tractor (CourtesyFASTRAC, 2013).

Tractors with only front suspensions cannot present a perfect dynamic characteristic. The front axle suspension affects negatively the vibration behaviour of the unsuspended rear axle, which keeps more than half of the load of the tractor, because by using front axle, rotation axle of the pitch body oscillation shifts to the rear axle. This increases the bounce of the rear wheels and decreases consequently the tires contact to the road surface. On the other hand, the front suspensions have significant effects only on the pitch and longitudinal vibration of the tractors, whereas the rear suspension influences the vibrations in most of the directions, particularly in the important vertical direction. After all, using the rear axle suspension besides the front suspensions can end these problems by creating an essential contribution in vibration isolation at both the rear and front axles. In this way, a full suspension system provides both safe driving and good ride comfort for the tractors. Despite of the advantages of the full suspension tractors, adding the rear suspensions to the conventional tractors is not as simple as the front suspensions. This is originated from the typical structure of the conventional or standard tractors, which are about 80 % of the whole agricultural tractors (Kaplick C., 1995). These tractors have a similar construction and called block construction. These tractors have a block construction, instead of the chassis construction. This construction is formed by connecting the three blocks of engine, transmission, and rear axle unit. Since primary types of these tractors have just rear drive wheels, in order to achieve optimal traction, at least 60% of their weight stands on the rear axle, and the rear wheels are selected bigger and heavier than the front wheels.

In the way of modifying the block-type construction of the standard tractors, finally, the tractors with full frame-type construction were presented. These tractors, similar to automobiles have a chassis that all the components of a tractor such as motor, transmission, cabin, and axles are installed on it. These tractors are four wheels drive with the usually same size of the rear and front wheels. Weight of the tractor is distributed on the wheels almost equally, causing optimal traction. Position of the driver seat and cabin is shifted from the rear part of the tractors to the mid-point of them between two axles. This leads to provide better ride comfort, even if the tractor is equipped just with front axle suspension. As an individual characteristic of the full frame tractors, the rear axle is separated from the chassis, and it is connected to the transmission via a universal shaft. With this construction, these tractors can be equipped simply with the rear axle suspensions. Therefore, along with presenting the full frame tractors, the idea of the full suspension tractors gets more practicable.

Summary of all these suspension systems used for tractor are mentioned in Table 13.1

Table 13.1: The capability of different suspension systems used for agricultural Tractors(Shahriar Sarami, 2009)

Suspension

Influence on

 

Ride comfort

Handling

Expenditure

Seat

Middle

slight

Low

Cabin

High

low

Middle

Hitch

Low-middle

Low-high

Low

Front axle

Low

high

Middle

Full Axle

High

High

High

 

Last modified: Monday, 7 April 2014, 6:12 AM