"Field Work," Feature Article, Nov. 2002

field work

Smoothing the ride aims to get more out of the tractor.

This article was prepared by staff writers in collaboration with outside contributors.

Ride comfort can mean more than luxury in agricultural vehicles. There is only so much shock or vibration a driver can take, and that limits time behind the wheel and productivity. Most manufacturers have added suspension systems so tractors can run faster on the road and in the fields.

It was along these lines that John Deere developed the front-axle triple link suspension, for instance. The system connects to the chassis at three points. A panel rod link transfers lateral forces between the suspension and the chassis. A shock absorber provides another link. A spherical joint in the middle of the tractor provides the third link.

"This suspension very effectively improves ride comfort and also helps to keep the front wheels on the ground
to better transfer the steering forces," said Bernd Thomas, the leader of Deere's design analysis group in Mannheim, Germany. More recently, Deere added a cab suspension system as an option in some 6020 Series tractors, which are made in Mannheim.

John Deere engineers ran simulations in DADS software to study the cab suspension.

Deere's engineers studied several spring systems on cabs, including one developed by a German dealer for Deere tractors, Thomas said. Based on their observations, the engineers designed a system that uses hydropneumatic shock absorbers, which permit a softer suspension than is practical with coil springs, Thomas said.

In an effort to master the complexities of a vibrating system, Deere engineers performed a series of multibody simulations on a computer model of the tractor to study the cab suspension. They ran the simulations in DADS software, which the company had used in the development of the triple link suspension. The developer, LMS International of Leuven, Belgium, had added suspension features, such as McPherson struts and pivoting beams, to the software at Deere's request.

According to Thomas, one goal in tuning a cab suspension system is to give it a natural frequency compatible with that of the vehicle. The tractor chassis on its tires has resonant frequencies of about 2 Hz, he said. The cab suspension's frequency had to be as far from that as possible.

Deere's design used shock absorbers loaded with oil and nitrogen and tuned the system to a resonant frequency of about 1 Hz. The suspension is soft. The cab, which can hold a driver and a passenger, will sink about 6 cm under the maximum load of two people.

A goal in tuning a cab suspension system is to give it a natural frequency compatible with that of the vehicle.

A position sensor activates the hydraulics in the shock absorbers to return the cab to its proper level. So when the tractor is traveling, the cab suspension still has its full range of movement, plus or minus 5 cm.

Without the dynamic ability to adjust for the weight in the cab, it isn't practical to tune a coil spring suspension to 1 Hz, Thomas said. Coil spring systems must be made significantly stiffer than the tractor's natural frequency, and that means a bumpier ride. Thomas said some spring suspensions his team studied had frequencies as high as 4 Hz, 16 times stiffer than a 1-Hz system.

Deere engineers used Matlab mathematical computing and Simulink modeling software, from MathWorks of Natick, Mass., coupled to DADS to evaluate suspension control systems. They set up a multibody simulation of conditions on Deere's test track to evaluate component durability after prototyping. The simulation, which included obstacles of the same height and geometry as those on the test track, generated component loads used as input to finite element analysis programs to calculate fatigue.

The hydraulic cab suspension is available for the Premium and Premium Plus cabs on some tractors at the upper end of the 6020 Series.