The number of fatalities caused by tractor rollovers has decreased in recent years, but the number of fatal tractor rollover accidents with a folded-down rollover protective structure (ROPS) has increased. Operating a ROPS-equipped tractor in low overhead clearance zones is difficult and sometimes impossible. The foldable ROPS (FROPS) was designed to solve the rigid ROPS problem, but lowering and raising a conventional FROPS is a time-consuming and strenuous process. After operators fold down a FROPS to pass a low overhead clearance zone, some prefer to leave it in the folded or inoperative position, increasing the risk of a rollover fatality. The actuation forces for raising and lowering a FROPS are not well known and may be influenced by actuation speed. A completely randomized block design with two blocks, five levels of speed, and multiple replications was conducted to investigate the effect of speed on actuation torque. The blocks were two sizes of tractor FROPS. The test included five levels of speed, including two levels of static measurement and three levels of dynamic measurement. A variable-speed motor system was used to control the speed for raising and lowering the FROPS. The actuation torque is a function of the FROPS upper part shape, dimensions, material density, turning acceleration, and friction. A theoretical model was developed to predict the actuation torque based on the FROPS shape, dimensions, and material density. For one ROPS, due to friction, the dynamic actuation torque was greater for raising and less for lowering than the theoretical torque. Indicator variable regression was used to analyze the effect of speed on actuation torque. Results showed that speed had a significant (p > 0.05) effect on actuation torque. Although there were statistically significant differences between the dynamic actuation torques, these differences were relatively small and negligible compared to the differences between the static torques.

Tractor rollovers are the major cause of occupational death in U.S. agriculture (

Overhead obstacles were reported as the most important reason for the operator not to install a ROPS (

However, the FROPS only partially solves the problem of low clearance applications, and recent surveys have revealed a new issue. The number of fatal accidents and severe injuries in tractor rollover accidents with folded-down FROPS has increased in the last few years (

One possible explanation for leaving the FROPS in the folded-down position is that raising and lowering the FROPS is a time-consuming and strenuous process. After lowering the FROPS to pass an obstacle, some operators prefer to leave the FROPS in the folded-down position.

An OECD working draft is being considered to regulate rear-mounted FROPS actuation forces (i.e., the forces for raising and lowering the FROPS). Based on the OECD working draft, the maximum actuation force should be less than 100 N, but this criterion could increase by up to 50% for some points and for lowering the FROPS (^{−1} (3.3 rpm) has been recommended for testing an automatic locking system (

Current FROPS actuation forces are not well known.

The aim of this study was to evaluate the effect of rotational speed on rear-mounted FROPS actuation torque. It was hypothesized that the raising and lowering speeds may affect the actuation torque. The effect of rotational speed on the actuation torque was investigated by actuating the FROPS at five speed levels that included two static actuation torque measurements. A theoretical model was developed to predict the actuation torque based on the geometry and material density of the FROPS.

The initial goal of this study was to measure the actuation torque as a function of the FROPS turning angle. The torque was measured at five speed levels that included two static torque levels. The procedure for the test included two steps: (1) developing the measurement setup, and (2) conducting the experimental tests to evaluate the influence of rotational speed on actuation torque.

In the first step, a measurement system was developed to measure the actuation torque and the angle. Based on the OECD working draft, the actuation force can be determined by measuring the actuation torque and then calculating the force at the grasping area (

The actuation system was composed of a motor, platform, fork, speed controller, switch, and battery. A reversible gear motor (Groschop model PM801-PL73) was used to turn the upper part of the FROPS. The motor was mounted on a platform that was attached to the fixed section of the FROPS. The motor applied torque with a fork that gripped the upper part of the FROPS. The motor shaft was collinear with the pivot point of the FROPS (

The measuring system included angular displacement and torque measurement sensors. The turning angle is the relative angle of the upper part of the FROPS to the horizon, which is the

The angle of the upper part was measured with an accelerometer (Crossbow model CXL04LP3). The tilting angle of an object can be measured using an accelerometer because objects are subject to gravitational force (

? = tilting angle (degrees)

_{out} = accelerometer output (V)

_{0} = accelerometer output when sensing axis is horizontal (V)

?^{2} m^{−1})

^{−2}).

The sensitivity of the sensing axis (^{2} m^{−1}), and _{0} was 2.527 V. The sensor was attached to the top of the FROPS with a magnet. The magnet did not affect the sensor output.

A reaction torque cell (Omegadyne model TQ420-2K) was used to measure the torque. The torque transducer had a maximum limit of 225 Nm. The torque transducer was attached between the motor and the fork. A data logger (Campbell Scientific model CR23X) read and saved the accelerometer and torque transducer outputs at 20 Hz sampling frequency.

The experimental tests were based on a completely randomized block design. The test included two blocks with five levels of speed and multiple replications within each block. The blocks were two different FROPS models. The FROPS were selected from two different weight categories of agricultural tractors. The first FROPS was Deere & Company model Se1 0095, which was designed for John Deere tractor models 4120, 4320, 4520, and 4720. The second FROPS was FEMCO model 301013466, for use on John Deere tractor models 2210 and 2305. The weight of the upper part of the Deere and FEMCO FROPS was 219.8 N and 109.8 N, respectively.

The five speed levels included three dynamic levels and two static levels. The speed levels were defined based on the motion of the ROPS. For the dynamic levels, the actuation forces were measured while the FROPS was continuously raised or lowered. For the static levels, the actuation forces were measured when the FROPS was stopped at a point or started moving from a static position.

The dynamic speed levels for each FROPS are listed in

Two concepts of static actuation torques were defined: holding torque and initiation torque. Holding torque was measured while the upper part of the FROPS was held at certain angles for at least 3 s. As the FROPS started its transition from static to dynamic movement, a sharp change in the torque values around the measured holding torque value was initially apparent. The initial value of the torque in the transient step was recorded as the initiation torque as the upper part of the ROPS was raised or lowered from a static position to 3.3 rpm (20° s^{−1}).

A mathematical model was developed to determine the theoretical actuation torque for the Deere and FEMCO FROPS. The FROPS actuation torque is a function of the weight, center of gravity (COG), turning acceleration of the upper part, and friction. The weight and COG of the upper part of the FROPS can be calculated from the shape, dimensions, and density of the upper part. The acceleration affects the inertial force and consequently the actuation torque. The friction force depends on the coefficient of friction and the normal force. The coefficient of friction is not a constant value and depends on several factors, such as the movement condition (static or dynamic) and contact surface properties. The model was developed based only on the shape, dimensions, and material density of the upper part of the FROPS.

The measured actuation torques for raising and lowering the Deere FROPS are shown in

The upper part of the Deere FROPS leaned 12° rearward from the vertical in the upright locked position. Therefore, the lowering process started around the upper lock point, which was 78°, and moved down to the lower locked position at −71°. The raising process started at −71° and rotated with a constant rotational speed up to 78°. During lowering, the actuation torque for the Deere FROPS was negative from 78° to 67° and then became positive. The point at which the actuation torque was equal to zero (about 67°) was called the breaking point. Before this point, the fork pushed the FROPS to overcome the friction. After the breaking point, the fork held the FROPS from folding. The peak point was defined as the angle at which the maximum torque occurred. The peak point for the Deere FROPS was approximately −13°. At that FROPS angle, the COG of the upper part was horizontal with the pivot point, as determined by the theoretical model.

The theoretical and experimental test results for the FEMCO FROPS actuated at low rotational speed are shown in

During the raising of the FROPS from a static position, a peak of initial torque above the holding torque was apparent as the FROPS started its transition from static to dynamic movement. As the FROPS was lowered from its holding position, a substantial lowering of the torque was seen (^{−1}).

The results of the holding and initiation torque measurements for raising and lowering the Deere and FEMCO FROPS are shown in

The static holding torque includes the moment of static friction and the weight of the upper part around the pivot point. The holding torques had a good agreement with the theoretical curves, considering the effect of friction (

An indicator variable regression was used to analyze the effect of speed on the actuation torque (

Although there were statistically significant differences between the dynamic actuation torques, these differences were relatively small compared to the differences among the static torques, especially for the initiation treatments (

A measurement setup was developed to measure the actuation torque and turning angle of the upper part of FROPS. The influence of rotational speed on the actuation torque of FROPS was investigated. Actuation torques were measured for three dynamic levels and two static levels for two different FROPS. The static levels included the initiation and holding torques. Experimental test results showed that the dynamic actuation torque for raising the FROPS was greater than when lowering if frictional resistance existed. A mathematical model was developed based on the dimensions, shape, and material density of the FROPS upper part. The developed model can predict the dynamic actuation torque for FROPS that have little friction. With friction, the theoretical torque at a given angle was between the measured raising and lowering torques. The static initiation torque was higher for raising the FROPS than for lowering. The initiation torque values for raising the FROPS at the peak points were 30% and 19% higher than the holding values for the Deere and FEMCO FROPS, respectively. The initiation torque values for lowering the FROPS decreased by 33% and 25% for the Deere and FEMCO FROPS, respectively, compared to the static holding torque.

Regression analysis of indicator variables showed significant differences (p > 0.05) between quadratic regression parameters for the five speed levels. The torque-angle relationships were modeled using nonlinear regression lines. Although the results showed that speed had a significant effect on actuation torque, the differences between the three regression lines for dynamic actuation torques were relatively small. The static and dynamic levels were apparently different. The static initiation torque included the inertial force and was distinctly different from all other speed levels.

We thank Dr. Arnold M. Saxton for assistance with the statistical analysis that greatly improved the manuscript.

FROPS positions: (a) FROPS in the raised or protective position, (b) FROPS in the horizontal position, and (c) FROPS in the folded or inoperative position.

Measurement setup.

Actuation torques for raising (speed = 2.6 rpm) and lowering (speed = 2.5 rpm) for three replications with the Deere FROPS and the theoretical torque for raising and lowering.

Actuation torques for raising (speed = 0.9 rpm) and lowering (speed = 0.7 rpm) for three replications with the FEMCO FROPS and the theoretical torque for raising and lowering.

Actuation torques for raising and lowering the Deere FROPS at three speed levels.

Actuation torques (Nm) for raising and lowering the FEMCO FROPS at three speed levels.

Static holding and static transient torques for raising and lowering the Deere FROPS.

Static holding and static transient torques for raising and lowering the FEMCO FROPS.

Regression lines for lowering the Deere FROPS

Regression lines for raising the Deere FROPS.

Regression lines for lowering the FEMCO FROPS.

Regression lines for raising the FEMCO FROPS.

Three levels of actuation speed (rpm) for raising and lowering the Deere and FEMCO FROPS. Values are means (standard deviations are shown in parentheses).

FROPS | Raising | Lowering | ||||
---|---|---|---|---|---|---|

Low | Medium | High | Low | Medium | High | |

Deere | 2.6 (0.4) | 4.7 (0.1) | 6.7 (0.7) | 2.5 (0.2) | 4.4 (0.1) | 6.1 (0.9) |

FEMCO | 0.9 (0.3) | 7.1 (0.7) | 9.3 (0.3) | 0.7 (0.3) | 4.6 (0.1) | 7.3 (0.3) |

Mean comparison of peak torques values.^{[a]}

Dynamic Speed | Deere | FEMCO | ||
---|---|---|---|---|

Lowering | Raising | Lowering | Raising | |

Low | 57.4 a | 70.7 b | 33.3 a | 33.9 a |

Medium | 56.7 a | 72.4 a | 32.5 b | 33.2 b |

High | 57.3 a | 73.0 a | 31.9 c | 33.4 b |

Values followed by the same letter are not significantly different (p > 0.05).

Regression equations for lowering the Deere FROPS.

Treatment | Equation | R^{2} |
---|---|---|

Static holding | ^{2} - 0.1547? + 59.95 | 0.96 |

Static initiation | ^{2} - 0.1290? + 41.33 | 0.98 |

Dynamic low speed | ^{2} - 0.1928? + 55.69 | 0.98 |

Dynamic medium speed | ^{2} - 0.1768? + 55.33 | 0.97 |

Dynamic high speed | ^{2} - 0.2317? + 55.54 | 0.98 |

Regression equations for raising the Deere FROPS.

Treatment | Equation | R^{2} |
---|---|---|

Static holding | ^{2} - 0.240? + 61.565 | 0.98 |

Static initiation | ^{2} - 0.2614? + 80.574 | 0.95 |

Dynamic low speed | ^{2} - 0.1826? + 69.743 | 0.96 |

Dynamic medium speed | ^{2} - 0.2007? + 70.954 | 0.93 |

Dynamic high speed | ^{2} - 0.2217? + 71.949 | 0.95 |

Regression equations for lowering the FEMCO FROPS.

Treatment | Equation | R^{2} |
---|---|---|

Static holding | ^{2} - 0.0862? + 31.51 | 0.99 |

Static initiation | ^{2} - 0.0663? + 24.51 | 0.96 |

Dynamic low speed | ^{2} - 0.0979? + 32.34 | 0.99 |

Dynamic medium speed | ^{2} - 0.1120? + 31.64 | 0.95 |

Dynamic high speed | ^{2} - 0.1157? + 31.46 | 0.93 |

Regression equations for raising the FEMCO FROPS.

Treatment | Equation | R^{2} |
---|---|---|

Static holding | ^{2} - 0.1397? + 31.80 | 0.99 |

Static initiation | ^{2} - 0.1584? + 37.74 | 0.97 |

Dynamic low speed | ^{2} - 0.0959? + 33.14 | 0.99 |

Dynamic medium speed | ^{2} - 0.0987? + 32.63 | 0.98 |

Dynamic high speed | ^{2} - 0.1053? + 32.63 | 0.96 |