Machine Design

Is your design “reasonably safe?”

The whole thing took only a couple of seconds. One moment, the farmer was mowing the slope of a hill. The next, his tractor was on its side, leaving him dangling from his seat belt, stunned but not seriously injured. Thanks to the roll bar built into the tractor, his biggest worry was how to explain to his wife that he would need help righting his machine.

Fifty years ago, that farmer likely would have been dead.

While product safety is a fundamental and critical objective, no product can be considered perfectly safe. The legal system recognizes this, and one of the things that a manufacturer must often show in a lawsuit is that the product was reasonably safe for its intended purpose. The challenge is determining what “reasonably safe” means in the context of a specific product design. The meaning tends to shift over time as a result of changing technology, design improvements, user experience, competition, evolving industry consensus standards, government regulations, and litigation. Proposed design improvements for safety must also be technically feasible and commercially viable. These measures also tend to change over time.

An example of how expectations evolve comes from the history of rollover-protection structures, or ROPS, used on agricultural tractors and construction equipment. Before the 1960s, farm tractors rarely incorporated roll bars or roll cages.

Farm tractors tend to have a relatively high center of gravity and are prone to rollover on uneven ground. Although tractor rollovers are usually to the right or left sides, rear rollover or flip is also a significant risk when implements exert high forces on the drawbar and cause higher traction forces at the tire/ground interface. Rear rollovers have been the source of numerous deaths in tractor-pull competitions, where power and load were unusually high.

As evidence mounted showing that tractor-rollover events claimed many lives, manufacturers and third-party suppliers developed ROPS. ROPS became mandatory for new agricultural tractors over 20 hp in the late 1970s with the issuance of OSHA regulations for agriculture, 29CFR1928. Today, OSHA regulations and industry standards, such as ASAE S383.1 and SAE J2194, specify in great detail how ROPS for these tractors must be designed and tested.

There have been substantial reductions in tractor-rollover fatalities, and ROPS have become standard in other types of off-road equipment, such as loaders and bulldozers. One of the most valuable functions of ROPS on tractors is to limit the roll to no more than about 90°. When a tractor rolls 180° or more, the operator may be trapped under the weight of the machine, which is often fatal. Limiting rollover to 90° greatly reduces the risk of serious or fatal injury. Seat belts are required on all tractors equipped with ROPS. But many farmers fail to use the seat belt. In a rollover, an operator not belted in may be thrown into the path of the rolling tractor or even be injured by the ROPS itself.

Even the best of safety features can’t guarantee absolute safety. Consider the case of a small utility tractor mowing an empty lot next to a drainage ditch. The tractor was equipped with ROPS and the operator used the seat belt. When the soil at the edge of the ditch gave way, the tractor slid into the ditch and, because the ditch was deep, rolled nearly three-quarters of a full revolution before coming to rest. During the roll, slack in the seat belt let the man’s head strike the ground, and he died before he was found. And in some cases, objects such as large rocks or tree branches can intrude into the operator’s space, causing injury.

Like any safety feature, ROPS is imperfect but is part of a “reasonably safe” tractor design. Its use on tractors has greatly reduced the risk to operators, and it is likely that injuries and fatalities from tractor rollovers will continue to drop in the future.


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