Licensed Professional Engineers
A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 6: Vol. 1 March 2004
Lawn Mower Accidents
By L.D. Ryan and John L. Ryan
Numerous studies have been conducted on lawn mower accidents. One source claims that lawn mower accidents account for 75 fatalities each year in the U.S., as well as 1200 serious injuries annually. Two sources claims that a serious lawn mower accident involving a young child occurs every day. The Insurance Information Institute has found that about 75,000 people each year require emergency room treatment for lawn mower accident injuries. Yet another study says that about 9400 children receive emergency care each year in the U.S., with 25% of these injuries occurring to children younger than five years.
Types of Accidents
There are four basic types of lawn mower accidents which cause most of the injuries. These include: contact with a rotating cutting blade, overturning of the mower, running over a person with the mower, and injuries caused by objects being propelled by the spinning mower blade.Running over a person
These accidents are often the most severe. The most common scenario is a child getting run over with a riding mower when the operator backs the mower up and doesn’t see the child. People cannot see everything in front and behind of them. This is due to our optical arrangement, and is widely variant for different individuals. There will always be a way for a child or person to approach the mower from behind without being seen by the operator. Other accident scenarios include pushing or pulling a walk-behind mower over the operator’s or someone else’s foot, and driving a riding mower forward over a person, usually a child.
Contact with a mower’s blade occurs in running over accidents as well as when someone reaches in the cutting zone of the blade to clear a blockage or remove an obstacle such as a rock.Overturning of the mower
This occurs usually when riding mowers are used on steep slopes or embankments. The operator can become pinned under the mower, causing crushing injuries, or the operator may come in contact with the mower blade in an overturn. Overturns occur most commonly when a mower is being driven across a slope, instead of mowing with the slope (up or down-hill). Mowing perpendicular to a slope should only be done with a walk-behind mower.Propelled Objects
The spinning blade of a mower can propel rocks, glass, sticks, and other obstructions found in lawns, to deadly speeds. Injuries from propelled objects range from blindness to death. One study showed that small riding mowers propelling debris can generate three times the muzzle energy of a .357 Magnum.
Causes of Accidents
Many medical experts claim mower injuries are mostly due to human error. People make errors in judgement and reach under mowers, people fail to check behind them and back over children, people fail to pick up rocks in the yard which could produce deadly projectiles. The American Society of Mechanical Engineers published a document entitled “An Instructional Aid for Occupational Safety and Health in Mechanical Engineering Design”. It states that “Some injuries may result from human error, misjudgment or omission (pushing the wrong button at the wrong time, forgetting to put a guard back in place, etc.), while others are due to design error…. Both kinds of errors must be avoided by the designer who must include design features which will prevent adverse results from ensuing due to such errors.” This clearly puts the burden of human error on the designer. This statement coincides with the hierarchy of design, an established guideline for designing hazardous products. The best thing a designer can do is to eliminate the hazard. The danger in a spinning lawnmower blade cannot be designed away, but the lawnmower blade can be covered with a guard or be made to stop automatically before a person can contact the blade.
The Problem & Solution
The problem with lawn mower design is the failure to provide adequate safety systems which will prevent injuries from occurring. These include effective guards, operator-presence control switch, and no-mow-in-reverse feature.
All mowers should come with adequate guarding. The guards used currently by most mower manufacturers do not provide adequate protection. There are at least 2 different types of guard systems that could be adapted for both walk-behind mowers and riding mowers, that would effectively guard the hazard of the mower.
Blade Cage Guard
The first guarding solution is a steel-rod screen that completely covers the base, along with the spinning blade (see Figure 1). This screen forms the base of a cage that encloses the blade. This guard prevents contact with the blade even if the victim is completely run-over. Buchele and Baldwin developed this design in the 1960’s. It was tested extensively and was found to effectively guard the mower blade.
Figure 1: Blade cage guarding solution
Rear Bumper with Collision Sensor
Another guarding solution was developed at the University of Iowa. It incorporates a rear bumper and a blade brake and clutch. If the mower bumper contacts a child, the blade clutch disengages and the brake stops the blade. The co-author developed a similar system. This system consists of a bumper/sensor which attaches to the rear of the mower. When the mower backs into something, the engine is immediately stopped, and the driver can attend to the source of the collision.
Figure 2. Prototype of Bumper/Sensor Attachment.
Figure 2 shows the bumper/sensor mounted on a lawnmower. This bumper is a hollow tube that can detect the occurrence of a collision by sensing the increase in pressure caused by some object pressing against the bumper. This increase in pressure will send an electrical signal to the circuit board of the device. The pressure-increasing signal triggers another switch on the circuit board that shuts the engine down. The electronics design opens the shut-off switch for 5 seconds, which kills the engine. After the timed out 5 seconds, the open switch closes and the mower can be restarted. This technology could be used on the front of mowers also. If the mower hit an obstacle, it would shut the engine down, which could reduce the severity or prevent altogether a run-over mower accident.
Operator Presence Switches
The next feature needed on all mowers is an operator-presence switch. With a walk-behind mower, this entails a lever which must be held in order for the mower to be operated. As soon as the lever is released, the mower is shut off. Federal regulations were imposed about ten years ago requiring all walk-behind mowers to have this safety feature. On a riding lawn mower, an operator presence switch turns off the mower or disengages the blades when the operator of a riding mower gets up from the seat. The federal government has yet to require this feature standard for riding mowers.
The American National Standards Institute (ANSI) publishes a standard that requires the use of operator presence switches. This standard is ANSI B71.1 standard, Walk-Behind Mowers and Ride-On Machines With Mowers - Safety Specifications. This standard calls for an operator presence control that will stop the lawnmower blades if the operator lets go of the mower handle (walk-behind), or if the operator leaves the seat (riding lawnmower). The standard requires the blades to come to a halt within five seconds, which means that mowers have to use a brake to stop the blades quickly. This standard also calls for increased safety measures to prevent injuries due to propelled objects.
“No Mow In Reverse”
The last major safety feature necessary for lawn mowers is known as “no mow in reverse”. Many tragic mower accidents occur when a child approaches a riding mower from behind. Operators fail to see the child, and back over the child with the mower running. “No mow in reverse” entails equipping a mower to disengage the drive shaft when it is shifted into reverse, and braking the blade once the clutch has disengaged the drive shaft. This is an effective means of isolating the hazard in a back-up scenario. This technology has been available since at least 1977, evidenced by U.S. Patent #4,016,709, “Safety Systems for Lawn Mowers”, issued April 12, 1977. This patent is a mechanical system that automatically disengages the blades if a riding mower is shifted into reverse. On October 12, 1976 Allis-Chalmers
Corporation was issued a patent for a mechanical system (U.S. Patent # 3,984,967, “Mechanical safety interlock for preventing mower operation during reverse travel”) that prevented mowing in reverse by preventing the shifting of the transmission into reverse if the blade is engaged, and by preventing the blade clutch from being engaged if the transmission is in reverse. Another embodiment of this technology is found in some mowers, where the blade clutch must be deactivated before shifting into reverse. If the blade clutch is not deactivated, the engine will be shut off. There are currently only a few manufacturers of riding mowers who utilize “no mow in reverse” technology. The federal government has yet to require this essential safety feature on lawn mowers. The ANSI B71.1-1998 standard, Walk Behind Mowers and Ride-On Machines with Mowers – Safety Specifications, does not specifically require the use of “no mow in reverse”. This standard clearly recognizes the inherent danger ofmowing in reverse. The standard states on page 59: “Do not mow in reverse unless it is absolutely necessary. Always look down and behind before and while backing.”, and on page 60: “ Before and when backing, look behind and down for small children.” The ANSI standard committee obviously knows of the hazards and the solutions. Why the standard doesn’t reflect current technology is a mystery to me. This would be the first step in a federally mandated “no mow in reverse” law.
Figure 3. Production Mower Equipped with “No Mow in Reverse”
Operator controls are very important to safety. The operator must be able to quickly disengage the blade, shut off the mower, turn the engine off, and be able to control the mower’s movement. We are currently working on a case involving a run-over accident on a mower with unconventional controls. The steering and forward and rear motion of the mower is completely controlled by two lever arms. An accident occurred when a child was run over. The unconventional controls prevented the operator from reacting in time to prevent injury to the child. If mower designers intend to break convention with operator interface, the designers should ensure that the operator will be receiving adequate training to overcome past conditioned responses.
Lawn mowers are tools that nearly every household in our country has, and are causing thousands of accidents annually, many of them involving children. The technology to make lawn mowers much safer has been around for over thirty years. Until the necessary safety requirements are made mandatory by the government, lawn mower manufacturers will continue to produce unsafe products, and people will keep getting hurt.