1,720,961 research outputs found
Flow-Forces Effect on the Behavior of an Open Center Hydraulic Distributor
No full textThe effect of steady-state axial flow-forces on the motion of an open center hydraulic distributor notched spool is considered. More in details, the spring driven spool centering is considered, in which undesired axial flow-forces could overcome the spring action. A steady-state analysis of the forces influencing the spool motion is reported, calculating the axial flow-force for different positions of the spool. Different notched shapes of the metering edge are considered, and an optimized solution for the notched edge of the spool is proposed at last
Modellistica Dinamica di un Attuatore Elettroidraulico
No full textGli attuatori idraulici comandati elettronicamente sono utilizzati in molte applicazioni nelle quali è necessario controllare elevate forze e potenze non ottenibili convenientemente attraverso dispositivi elettrici. In questo articolo si propone un modello dinamico dettagliato di un attuatore idraulico con elettrovalvola corrente-pressione per frizioni in bagno d'olio. Il modello proposto è stato validato mediante misure sperimentali e permette di simulare il comportamento dinamico del sistema idraulico di attuazione al fine di valutare a priori l'effetto di modifiche strutturali o di verificare nuove strategie di controllo
Vehicle rigid motion effects on whole body vibration
Full text linkThe vibrations transmitted to the driver are dangerous to his safety because they can cause temporary and permanent injuries to the body. Tractors are characterized by vibrations with high intensity and low frequency that increase the difficulties in the design of solutions able to reduce vibration transmissibility to the driver. The new suspension systems diffused in recent years have been designed to reduce the vertical acceleration, despite the fact that numerous studies have highlighted how the most critical directions are represented by the horizontal ones. This is caused by the angular accelerations, amplified in modern tractors with high dimensions, by the high distance from the seat and the tractor rolling axis. Despite manufacturers’ efforts in the last decade, the level of the vibration transmitted to the driver in different tasks exceeds the exposure levels established by the standards. The goal of the paper is to evaluate the influence of each vehicle’s rigid motion (pitch, roll and bounce) on the vibration level transmitted to the driver in a tractor of about 170 kW power with axle suspension, cab and seat suspension. Two inertial measuring units have been fitted on the cab and frame of a tractor to measure the roll angular velocity and the bounce motion respectively of the cab and frame. A triaxial accelerometer has been fitted on the driver seat to measure the driver acceleration. The tractor has been used in different working conditions to evaluate the influence of the mass distribution on each rigid motion and with different driving velocities. The power spectrum densities have been analyzed to evaluate the predominant frequencies for each mode. These tests have permitted a better comprehension of the whole body’s vibrations caused by a tractor. In particular, the distribution of the acceleration on the three axis has been defined with respect to different working conditions. In the on road transport the longitudinal acceleration is prevalent. The vibration level on the road is more influenced by the pitch and the bounce. The suspension systems and, in particular, the front axle suspensions are able to reduce the pitch of the tractor, but do not reduce the bounce due to the absence of the rear suspensions
Performance of an agricultural tractor fitted with rubber tracks
No full textAgricultural tractors can have different types of undercarriage such as two wheel drive, four-wheel drive, and steel tracks. Despite a higher tractive performance and lower soil compaction, steel-tracked tractors are not popular due to their complexity and the difficulties of moving steel-tracked vehicles on roads. Recently, rubber belt tracks have become a notable solution for agricultural tractors, because they unite tractive performance and lower soil compaction with a better trafficability. Triangular rubber belt systems have been developed to replace the conventional wheeled configuration found on combine harvesters, and these can also be used on tractors. The purpose of this paper is to evaluate the performance of these solutions on a standard agricultural tractor. In particular, a tractor equipped with standard tyres, water-ballasted tyres, half-tracks, and fully-tracked was analysed. Drawbar power tests on asphalt and on soil were carried out, and compaction tests were performed on soil. The results show a better tractive efficiency and a reduced soil compaction for the solution with four rubber tracks with respect to the others. In addition, a reduction of the tractive efficiency with the increasing of the slip was highlighted in the half-track solution caused by an unfavourable weight distribution on the axles
Characterisation of the Behaviour of a High Power Tractor on the Road
No full textIn recent years the use of the tractor on the road has increased also due to a higher maximum velocity allowed. An increase in velocity has highlighted certain limits of the vehicle in terms of driveability and comfort. A lot of studies and standards on the behaviour on the road exist in the automotive sector. On the contrary, there are no studies on the tractor behaviour on the road but only off the road with the objective of developing steering systems that facilitate the driveability of the vehicle compared to similar on the road studies. The objective of this paper is to design tests that allow to characterise the functionality of the steering system and the tractor behaviour on the road. The tests were executed on two high power tractors. The tests were carried out using the standards of the automotive sector with some changes: a decrease of the travelling velocity, the lateral acceleration and the maximum frequency of the steering action. Different parameters characteristic of the steering system were analysed in function of the lateral acceleration of the tractor while steering to take a turn with a constant velocity and in function of the oscillating frequency of the steering wheel with a constant velocity. The results are a global behaviour of the tractor on the road and in particular the characterisation of the steering system’s quality
Evaluation of a Tractor's Driving Performance on the Road
No full textIn recent years, tractor use on the road has increased, in part due to a higher maximum velocity allowed. An increase in velocity highlights certain limits of the vehicle in terms of driveability and comfort. Numerous studies regarding the standards of on-the-road behavior exist in the automotive sector. On the contrary, there are no studies concerning the on-the-road behavior of tractors but, rather, only off-the-road studies with the objective of developing steering systems that facilitate the driveability of the vehicle. The objective of this article is to discuss tests that allow the characterization of a tractor's steering system and its behavior on the road. The tests were performed on two tractors with the same power rating but different steering systems. The tests were carried out using the standards of the automotive sector but with some changes: a decrease of the traveling velocity, the lateral acceleration, and the maximum frequency of the steering action. Different parameters characteristic of the steering system were analyzed under lateral acceleration of the tractor while steering to make a turn with a constant velocity and when the system was excited in a transient manner. The global behavior of the tractor on the road and, in particular, the characterization of steering system quality are presented
Evaluation of the Frequency Response of Tractor Cab Angular Movements
No full textThe mission profiles of tractors have been modified recently due to a rise in their use as transport vehicles. This use of the tractor together with an increasing maximum velocity on the road emphasizes the containment problem of the vibration transmitted to the driver. In particular, a significant contribution to the increasing global vibration level transmitted to the driver is given by low-frequency movements. Recent work has demonstrated the capability of the ISO standards used for tests in the automotive sector to characterize the dynamic behavior of tractors. In this work, one test, derived from the ISO standards mentioned above, was used to evaluate the capability of the suspension system to isolate the cab from the angular oscillations coming from the driveline. In particular, the frequency response of the cab angular movements was measured on two tractors, with and without an implement connected to the three-point linkage, and was compared with that measured on the tractor's driveline. The results showed a considerable amplification of the roll from the driveline to the cab, particularly with an implement, and a negligible amplification of the yaw
Frequency roll response of farm tractors’ cabs
No full textThe increase in the use of the tractor as a transport vehicle has emphasised the containment problem of the vibration transmitted to the driver. The rolling movement at a low frequency increases the transversal acceleration and influences the global vibrations’ level transmitted to the driver. Recent works have demonstrated the capability of ISO standards, used for handling tests in the automotive sectors, to analyse and characterise the dynamic behaviour of tractors. The frequency sweep test was used to analyse the frequency response of the cab’s rolling movement, which in turn was compared with that measured on the tractor’s driveline. The aim was to evaluate the capability of the suspension system to isolate the cab from the roll oscillations coming from the driveline. The results show a high amplification of the roll from the driveline to the cab, in particular, in the condition with implements connected to the three point linkage
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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