ROPS DESIGN AND TESTING FOR AGRICULTURAL VEHICLES
Dr. Paul Ayers, Professor
Department of Biosystems Engineering and Environmental Science
2506 E.J. Chapman Drive
University of Tennessee
Knoxville, TN 37996
Tractor rollovers cause a large number of agricultural work fatalities each year. ROPS (Roll Over Protective Structures) can protect the operator from major injury if a tractor rollover occurs. Tractors are not the only agricultural vehicles involved in rollover fatalities. Large lawnmowers, ATV's and off-road utility vehicles (ORUV) are involved in fatal agricultural vehicle rollovers. ASAE Standard S547 "Tip-Over Protective Structure (TOPS) Protective Structure for Front Wheel Drive Turf and Landscape Equipment" was recently approved in December 2002 for ROPS design for front-drive mowers. The S547 Standard lateral upset test is designed to limit the vehicle continuous roll behavior down a steep slope. The standard also includes a new modeling component that would allow ROPS design without actual field upset testing. A Deere F925 front drive mower was used in the evaluation of the continuous roll field testing and the proposed model. Eight continuous roll field upset tests were conducted in accordance with ASAE S547 to evaluate the rollover protective structure (ROPS) continuous roll performance. The results reveal the proposed continuous roll model does not take into account the influence of the front mower deck and can inaccurately predict continuous roll behavior. Based on these observations, the ROPS manufacturing industry has been notified to discontinue ROPS design using the existing model. Continuous roll model modification is underway to include the influence of the front deck on the continuous roll behavior.
Background
A new standard developed for ROPS design for front drive mower has been approved in December 2002. It includes the continuous roll test that can be modeled on field test. The model addressed originally developed for narrow tractor, therefore, the applicability for mower with deck needs to be evaluated. ASAE Standard S547 “Tip-Over Protective Structure (TOPS) for Front Wheel Drive Turf and Landscape Equipment” was recently approved in December 2002. The S547 standard lateral upset test is designed to limit the roll angle during the side upset of a self-propelled ride-on machine (ASAE, 2002). Within this standard for testing there is two methods in which a machine may be tested. The first of which is the lateral upset in which the vehicle is tipped down a slope. The second is a demonstration of prevention of roll over by calculation through a model taken from the Organization for Economic Co-operation and Development (OECD) Standard Code 6. Only one of these testing methods needs to be done to determine if the vehicle has a continuous roll down a slope. If a continuous roll is observed, the tip-over protective structure does not pass the standard test. Also section 1.2 of the ASAE S547 states that any vehicle that meets the requirements of ASAE standards S383 and S478 meets the performance requirements of this standard.
Existing ASAE S547 Continuous Roll Model
The ASAE S547 Standard also includes a new modeling component that would allow ROPS design without actual field upset testing. The model described in the Standard is from the OECD Standard Code 6. The existing model only evaluates the rotation of the tractor around the longitudinal axis.
The Standard allows assumptions of the ROPS/slope deformation (0.2 meters) and the slope coefficient of elasticity (0.2) (embedded in the model). However, the mower deck sizes are ignored by the OECD CODE6, i.e. the influence of the mower deck to determine the vehicle/slope contact points is not considered by the existing model.
Inputs of OECD CODE6.
The inputs of the existing model include:
1 Height of the center of gravity (H1)
2 Horizontal distance between the center of gravity and front axle (L3)
3 Horizontal distance between the center of gravity and rear axle (L2)
4 Height of front tires (D3)
5 Height of the rear tires (D2)
6 Horizontal distance between the center of gravity and the rear corner of the engine bonnet (L7)
7 Horizontal distance between the center of gravity and the leading point of intersection of ROPS (L6)
8 Width of ROPS (B6)
9 Height of ROPS at the point of impact (H6)
10 Width of the engine cover (B7)
11 Height of the engine bonnet (H7)
12 Front tire width (B0)
13 Rear axle swing angle (D0)
14 Height of the rear-axle pivot point (H0)
15 Rear track width (S)
16 Tractor mass (Mc)
17 Moment of inertia about the longitudinal axis through the center of gravity (Q)
The governing equation of the existing model is shown below:
The existing model uses conservation of energy principles to determine the angular velocity at
the positions except position 3 and 5.
KE= PE
Where
KE = Kinetic Energy Change (J)
PE = Potential Energy Change (J)
Where
J = moment of inertia about the longitudinal axis through the center of gravity (kg•m²)
w = angular velocity (rad/sec)
m = mass of the mower (kg)
g = gravity acceleration (m/s²)
h = height of center of gravity (m)
The outputs of the existing model are described below:
The outputs of the existing model are roll behavior and the angular velocities at the critical vehicle positions.
The figure shows 6 critical vehicle positions as defined in the existing model.
Outputs of OECD CODE6.
If the angular velocity is great than 0 at position 6, then the vehicle rolls over itself. ROPS fails. In addition, the existing model uses conservation of momentum principles and Newton’s Second Law to determine the angular velocities at the position 3 and 5.
Test Site Construction
As stated in the S547 Standard, the lateral upset test shall be carried out on a natural earth slope or a test ramp at least four meters long. The slope of the surface shall be 33 degrees +5/-0. The surface shall be covered with a minimum of a 180 mm layer of material, which when measured in accordance with ASAE Recommendation R313; has a cone penetration index of A (235 ± 20) or B (335 ± 20) psi.
Ramp for testing the non-continuous roll
Roll Test Video
A front drive mower has been provided for model evaluation. The mower has a 16kW Yanmar Diesel Engine, a 72 inch wide front deck and has a mass of approximate 900 kg. The mower has a ROPS design to satisfy OSHA 1928, which does not have a continuous roll criterion. The mower is used in model evaluation with the original ROPS height of 1.92 m, and then the ROPS height is extended to 2.20 m with an adjustable fitting.
Field tests for the front drive mower were conducted to evaluate the accuracy of the existing model. The continuous roll behavior was observed with original ROPS height of 1.92 m as predicted by the model. The model predicts a non-continuous roll while a continuous roll was observed with extended ROPS height of 2.20 m. Field tests results also show that deck influences the roll behavior due to the axes change of rotation. Field test results with the inverted ROPS showed a continuous roll with ROPS height of 1.92 m, while the model predicts a CRH of 1.41 m. Therefore, the OECD CODE6 needs to be explored prior to implementation for ROPS design. The existing model for the mower needs to be modified to include the mower deck dimensions.
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Roll test video 1 Continuous roll
Roll test video 2 Noncontinuous roll
New Model Development
Modify the Model to Include Deck
The modifications of the existing model will include 3 procedures. First, the mathematical model of continuous roll prediction will be developed for mower with a front deck. Second, the computer program derived from it to simulate the roll behavior of the mower with ROPS will be developed. Finally, this model will be programmed in MATLAB. The details of the revised model are discussed below:
Revised contact point location
Inputs of the proposed model to include the deck. Three parameters will be added into the model: 1) the horizontal distance between the contact point of the deck and the center of gravity, 2) the height of contact point of deck, and 3) width of deck.
Revised tipping axes
Because of adding the deck size to the model, the tipping axes will change. The modified tipping axes are described below: Three versions will be defined in order to distinguish the roll behaviors.
1. If the leading point of intersection of ROPS is in front of the longitudinally unstable equilibrium point, the mower will roll back. This is defined as B1VERSION.
2. If the leading point of intersection of ROPS is near the longitudinally unstable equilibrium point, the mower will directly roll over the top of ROPS. This is defined as B2 VERSION.
3. If the leading point of intersection of ROPS is behind the longitudinally unstable equilibrium point, the mower will roll front. This is defined as B3 VERSION. Tipping axis 2 and 3 will be changed from dashed line to solid line by adding the mower deck. The longitudinally unstable equilibrium point is the intersecting point between the line passing through the center of gravity and perpendicular to the tipping axis and the horizontal plane passing through the impact point of ROPS.