Evaluation of large-scale load-carrying structures of machines with the application of the dynamic effects factor
(*) Tekst artykułu w polskiej wersji językowej dostępny w elektronicznym wydaniu kwartalnika na stronie www.ein.org.pl in the construction industry which are usually designed to transfer only static loads and thus only need to fulfill the ultimate strength requirement. Dynamic loads accelerate the progressing degradation of the load-carrying structure, which is manifested in an increasing number of fatigue cracks. Fig 1 shows an example of a fracture and its location. Such faults are difficult to spot and if the fracture is located in a critical area, it can grow uncontrollably and lead to a catastrophic failure. A fracture similar to the one in Fig. 1 caused the catastrophe of the KWK 1400 excavator (Fig. 2). Such load-carrying structures are most often found in the mining, rock processing or bulk material handling industry. The situation in this case is even more special because some of the basic technological processes (e.g. excavating, crushing) are very often subject to large dynamic loads [15, 16]. Due to the environment in which such machines most often operate, it is difficult to use solutions that are effective at reducing vibrations but are not susceptible to damage [2, 48]. In addition, the design of load-carrying structure can be prone to excitation [7,17] (e.g. long-span superstructures of surface mining machines). Such machines are still being designed in accordance with standards from the 1980s, which define the dynamic effects factor, which, in turn, is used in calculations of fatigue life. Both the definition of this factor and its assumed values, in accordance with the standards, do not reflect the actual operating conditions. This is manifested in the occurrence of numerous cracks in superstructures [1, 12, 37] and undercarriage structures[1, 4, 13, 46]. Over the years several studies have been carried out aimed at solving this problem
Tie-rods of the bucket wheel excavator slewing superstructure: A study of the eye plate stress state
Abstract Tie-rods of the bucket wheel and counterweight booms are vital parts of the bucket wheel excavator slewing superstructure. This paper is dedicated to the problem of identifying the stress state of the critical cross-section ('net section') of the eye plates on these tie-rods. It presents the results of the experimental and numerical investigations carried out on the eyebar model. The results show the appearance of a plastic deformation in the zone near the contour of the hole even when the eye is subjected to the loads within the range of the permissible loads prescribed by the standard DIN 22261-2. Plastification in the eye net section occurs across the full thickness of the plate, but it does not jeopardize the integrity of the structure. The stress value in the eye net section, calculated in accordance with the standard DIN 22261-2, is significantly lower than the actual stress values around the hole and serves only as a comparative value for the static strength proof for the eye.
Mechanism of the depressor response to dopamine in the rat treated with phenoxybenzamine.
Alterations of hypotensive responses to dopamine by antagonists were characterized in alpha-blocked, anaesthetized rats. Responses were not affected by d-propranolol (0.1 mg/kg) whereas d,1-propranolol (0.1 mg/kg) or haloperidol (1.0 mg/kg) attenuated them; higher doses of inhibitors (1.0 mg/kg; 5.0 mg/kg, respectively) failed to produce a higher inhibition, but combinations of low doses abolished the depressor responses. In adrenalectomized rats, hypotensive responses decreased; haloperidol always attenuated the responses while d,1-propranolol became ineffective. Dopamine produced an enhancement of plasma adrenaline and noradrenaline levels, which was decreased by d,1-propranolol and increased by haloperidol. The data suggest that in rats the depressor component of dopamine is due to activation of both dopaminergic and beta-adrenoceptors. The beta component appears to be due to the release of adrenaline. The results also support the concept of the existence in sympathetic nerve endings and adrenal glands of stimulatory beta-adrenergic and inhibitory dopaminergic prejunctional receptors.