ABSTRACT Mining injury statistics show that a significant number of back, neck, and head injuries are linked to exposure from vehicle vibration. Use of a suspension seat is a common way to isolate the vehicle operator from the adverse effects of vibration exposure. Thus, researchers at the National Institute for Occupational Safety and Health 1 -Pittsburgh Research Laboratory (NIOSH -PRL) performed laboratory studies on four passive and two semi-active seat suspension designs. These are typical of seat suspensions commonly found on large off-road heavy surface mining, construction and agricultural vehicles as either replacement or original equipment manufacturer (OEM) systems. One included a pneumatic (air bladder) spring mechanism. The fifth and sixth suspensions were a NIOSH magnetorheological (MR) semiactive damper design based on the pneumatic (air bladder) and one of the coil spring suspensions above. These suspensions were modified with a commercially available MR damper substituted for the OEM damper. These six seat suspension systems were tested and analyzed, for vertical vibration only, using the ISO 5007 Standard Results for suspensions 1 through 3 showed frequencies of isolation from 2.1 to 3.0 Hz using the 40-kg (88-lb) mass and from 1.65 Hz to 1.8 Hz using the 80-kg (176-lb) mass. Suspension #4, in tests with only the 80-kg (176-lb) mass, showed an isolation frequency of 3.7 Hz. With the MR damper added to seat suspension #4, the peak transmissibility was lowered from 1.3 to 0.95 and showed a corresponding downward shift in frequency from 2.25 Hz to 1.4 Hz. In fact, the results for suspension #5 (the MR damper added to seat suspension #4), using test #3 conditions of the programmed control algorithm, showed isolation (attenuation of transmitted vibration) throughout the test frequency range from 1.0 to 6.0 Hz. INTRODUCTION NIOSH mining vehicle seat and vibration research is dedicated to reducing the risk of injuries to the back, neck, or head for vehicle operators through improved mining vehicle seat designs. In this regard, injury statistics for mobile mining equipment operators from the Mine Safety and Health Administration (MSHA) showed incidences of exposure to whole-body vibration (WBV) and mechanical shock (vehicle jarring or jolting). These injuries can be described as acute and chronic musculoskeletal disorders affecting the back, neck, and head. For example at surface mining operations, Wiehagen et al. One way to lessen the adverse effects of vehicle vibration to the operator is through the use of a seat suspension system. Suspension seats are included in most industrial vehicles and are passive in nature in that they consist of a damper and some form of a spring. Moreover, during operation, these vehicles are subjected to significant energy in the 2 to 4 Hz region where conventional seats tend to amplify vertical vibration. A common method used to reduce vehicle operators' exposure to vehicle vibration is to design the natural frequency of the seat suspension so that it is considerably lower than the typical operating frequency generated by the vehicle. However, a large decrease in seat stiffness ("softer" or lower spring rate) is required to achieve this. Considering equatio
