Air Springs Meet High Comfort Demands

For a long time, tractor developers focused mainly on meeting Tier 4 final requirements. Now it is time to again provide tractor operators with noticeable benefits when it comes to comfort and working conditions. Trends are moving toward multimedia-based workplaces. EU directives and provisions from professional associations have raised noise and vibration requirements. Vehicle manufacturers are battling for customers by offering new comfort concepts. In this market environment, spring-loaded cab concepts are catching on.

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For a start, 2-point suspensions with two spring/damper elements at the rear of the cab are being offered. The front of the cab is connected to the chassis via rubber-metal elements. This cost-effective concept is already bringing significant improvements because the rear cab connection is located more or less directly over the unsprung rear axle. In the case of 4-point suspensions, each corner of the cab is mounted on a spring/damper module. The cabs are controlled via swingarms or panhard rods. Spring/damper elements are generally used to influence the cab movements (stroke, pitches, rolling) depending on the selected suspension architecture and vehicle manufacturer requests - with the aim of creating good vibration characteristics that offer relief both to the driver and the vehicle.

Spring/damper units in cab suspension

The springs and dampers are connected in a coaxial arrangement and form one unit. It is essentially the spring characteristics, the damper's line of force and how they are matched with each other that influence the vibration characteristics of the cab.

The use of coil springs is considered a good compromise between function and costs and is then used if no good compressed air supply or the required installation space conditions are not available. Natural frequencies between 1.8 and 3.0 Hz offer a good comfort level. Different cab weights can partly be compensated for by 4-stage manual spring adjustment elements.

Clear trend toward air suspension

Air springs are being used more and more often in modern tractors. Low natural frequencies between 1.0 and 1.4 Hz allow for outstanding ride comfort. Air suspension systems are also connected to the driver's cab level control system. A prerequisite for the level control system is air pressure control. The system considers three basic states and connects them to corresponding level control actions:

1) Cab is too high: system releases air 
2) Cab is too low: system admits air
3) Cab is at the right level: system holds air.

This is achieved by means of level control technology which is often located outside of the air spring. A mechanical valve moves over a linkage and adapts the air pressure according to the load. Valve and air spring are connected to each other by hose assemblies and linked to the vehicle air supply.

Frequently, the external ride-height control is prone to disruptive factors, such as dirt and damage. For that reason, manufacturers have increasingly opted to use air springs which integrate all critical elements. The internal level control valve works according to the same principle, but is now installed in the air spring, the same is true of the valve actuator that moves the valve. This solution has been implemented in the ZF product called CALM (Cabin Air Leveling Module) (Fig. 17). It makes the overall system considerably more robust. And, by now, it has proven worthwhile hundreds of thousands of times in commercial vehicles. Thanks to its compact design, it saves a lot of installation space and is easy to repair.

The spring/damper units are designed for as large a stroke as possible. Nevertheless, it is important to alleviate the reaching of the damper end stops using suitable rebound and compression stop buffers. This assures a soft use and prevents comfort sacrifices when reaching the limit stops of the damper stroke.

Releasing air does not equal leakage

The noise of air being released by the spring is often falsely interpreted as leakage. When the driver leaves the cab, the cab weight is reduced which triggers the opening of the valve so that excess air can be released in order to restore the cab to a level position. If the air spring is at head level, the noise caused by this discharge is sometimes falsely interpreted, which can lead occasionally to complaints regarding defects.

Adjustable damping systems are trendsetting

For a while now, some vehicle manufacturers have used semi-active damping systems instead of passive shock absorbers. The goal of this is to optimally adapt the damping force of the dampers to the respective driving condition. Based on the Skyhook control strategy, the use of a Continuous Damping Control system (CDC) (Fig. 18) allows for considerable cab dampening as well as isolating the individual axis excitations. Both air and steel spring applications are possible with adjustable damping (Fig. 15 and 16).

Conclusion

Air-sprung cabs, in particular, offer a significant increase in comfort. Depending on the availability of compressed air, there is no technical reason not to use air spring systems. Field experience has shown that more than anything the integrated CALM solution offers a robust product for high comfort requirements.