By: Kimberly Kayler , Constructive Communication , 614-873-6706

(Cary, NC — 08/07/2009)

The following article can be viewed on-line at http://www.sae.org/mags/SOHE/6727

Comfort is something we have come to expect in our daily lives - we like to wear comfortable clothes and shoes, sit on comfortable furniture and "comfort food" is the new culinary trend.  We want a comfortable ride in our cars, too - seats that adjust to fit our bodies and suspension systems that "glide" over bumps in the road and control vibration.

However, for those whose work environment is inside a heavy-duty dump truck, construction vehicle or tractor, ride quality is not just a comfort issue but is also beneficial for the health and safety of the operator. Using new technological developments, design engineers are now creating seat and cab suspension systems that manage vibration, rather than just control vibration. By reducing operator fatigue, the improved environment increases operator productivity and lessens health problems.

One of the companies in the forefront of ergonomic-seating design is Sears Manufacturing Company, a maker of suspension and non-suspension seating for agricultural, construction and earthmoving equipment. Since 1993, Sears has been working with LORD Corporation, a producer of devices and systems for managing mechanical motion and controlling noise and vibration, to incorporate magneto rheological (MR) technology as part of its semi-active seat suspension systems.

Magneto rheological fluids respond to a magnetic field with a dramatic change in rheological behavior. These fluids can reversibly change instantaneously from a free-flowing liquid to a semi-solid with controllable yield strength when exposed to a magnetic field. In an automotive primary suspension system, MR fluid replaces traditional hydraulic fluid in each shock absorber. As sensors monitor road and vehicle conditions, a controller modifies the damping characteristics up to 1,000-times-per-second. This enables dramatic improvements in both ride comfort and handling.

LORD Corporation's Motion Master^TM Suspension system features a MR fluid damper, a controller, a suspension position sensor and a ride mode switch. The system adjusts the strength of the magnetic field, based on real-time data such as operator body weight and changing levels of shock and load vibration, to induce the appropriate level of viscosity in the fluid. This provides variable compensation ranging from soft to firm depending on operating conditions.

In 2004, Sears introduced the VRS 2000 seating system, featuring LORD Corporation's MR technology. The VRS 2000's soft, low-friction seats improved ride performance and comfort for operators of heavy-duty vehicles. Continuing with their innovative technological collaboration, Sears and LORD recently introduced a seating system with longer stroke MR damper and improved control software, the VRS2000-LS, that is particularly well suited for very-heavy-duty applications.

"The new longer stroke damper allows us to offer the MR damper in machine applications where the terrain and ride are really tough," said Tom Brodersen, Sears' Director of Research and Development. "Applications include construction and mining vehicles, articulated dump trucks, agricultural sprayers, large tractors, and forestry equipment - anywhere there is rough terrain and higher operating speeds."

Sears is now designing several new seating products using the longer 3-inch damper, specifically for the severe construction market. They are also in the process of redesigning many existing products to accommodate the larger damper.

The VRS2000-LS is essentially a long-stroke damper equipped version of the current VRS2000. According to Brodersen, most of Sears' suspensions operate in 6-inches of travel. "So when using the 2-inch stroke damper, that equates to 2-inches of damper travel for 6-inches of motion," noted Brodersen. "That 3:1 damper ratio really taxes the dynamic capabilities of the current shock absorber."

"However, a 3-inch damper lessens the damping force required and the net damping of the suspension increases," Brodersen continued. "That might not sound like a big deal, but the damping increase is considerable to the point where this new suspension/damper technology can be used in extreme terrain applications. Prior to the development of the longer stroke damper, the VRS technology was used primarily in agricultural machines. Extensive testing on Sears' man-rated 6-axis ride simulator, using several "real world" machine ride profiles, aided in the development and tuning of the LORD controller to take full advantage of the new dynamic capabilities of the long stroke damper, especially end-stop control.

According to Brodersen, firm, positive end-stop control is key. "The better you are able to 'put on the brakes' during a severe terrain event, the softer you can leave the 'normal' ride," said Brodersen. "The end result is an overall reduction in the harmful vibration being transmitted to the operator."

The new seating designs have tested well in the Sears test labs, proving that the longer stroke damper greatly improves ride performance. The VRS2000-LS has also allowed Sears to develop a seating system that offers "soft-to-firm" controllability.  The "soft" setting delivers a good, light-terrain ride and improved rough terrain control; the "medium" setting provides a good balance between rough and light terrain both for the ride and control functions; and the "firm" setting offers excellent control over extreme terrains while accommodating a heavier-weight operator. The seat reduces vibration transfer by 40 percent, which results in less fatigue and the ability to operate equipment approximately 15 percent faster.  

Sears' VRS customers include Case-IH, New Holland, AGCO, Hitachi and McCormick. John Deere also offers a VRS Suspension Seat as an aftermarket product and Sears offers VRS seating through their own aftermarket program. Looking to the future, Sears is currently developing new seat suspension products configured around the VRS-LS system that will allow for usage in an even wider range of vehicles.

"An ergonomically designed seating system translates into a comfortable working environment for operators of heavy-duty equipment whether at a construction site, in a mine or out on the farm," said Brodersen. "With these technological advancements, operators can perform their jobs more productively, in a safer and healthier environment."