Gating(redirected from cardiac gating)
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the system of channels or elements through which the melt passes into the working cavity of a casting mold from a ladle or other pouring device. The function of the gating is to generate optimal conditions and duration of pouring in order to produce castings with sharp edges and contours, to prevent nonmetallic inclusions from entering the melt (during casting from a tilting ladle), and to supply melt to the casting to prevent the formation of contraction cavities during hardening. The elements of a gating are divided according to function into runners and feeders (in certain special cases, such a distinction does not exist).
The runner elements of gating include the pouring basin, riser, throttle, whirl gate (collector or sprue), and feeder. The pouring basin is a receiver for the melt, which should contain an adequate quantity of melt for operating convenience and retention of slag, and also to prevent air from being sucked in. The riser is a vertical (rarely inclined) channel connected to the basin. The throttle consists of a narrow channel (or several channels), which is usually located at the base of the riser, which is a local hydraulic resistance element designed to control the rate of pouring and to eliminate vacuum in the riser. The whirl gate is a channel, usually of elongated trapezoidal cross section, that is located behind the throttle and that supplies the feeders with melt and retains nonmetallic impurities. More complete retention of slag is achieved by the addition of local expansions to the whirl gate and by the use of centrifugal whirl gates and filtration screens (made from ceramic firebrick or core mixtures for use in the casting of iron, from thin sheet steel for use in the casting of nonferrous metals, and from silica fabric for use with all alloys that require casting temperatures up to 1350°C). Whirl gates are not required in casting articles by pouring the melt from bottom-pour ladles (the slag remains in the ladle) and in cases when the density of nonmetallic inclusions is close to that of the melt (in the case of certain nonferrous alloys). In such cases the channel called the collector, or sprue, is used only to distribute the melt. The feeder, which is a channel connected to the slag catcher, is usually of rectangular cross section, through which the melt passes into the working volume of the mold either directly or through a feeder head.
The dimensions of the runner elements are determined mainly by hydrodynamic factors, such as the design of the gating, the hydrostatic head, and the flow rate of the melt.
The feeder elements of gating include the side riser and neck. The side riser is a compact riser pad on the side surface of the casting. It is designed to act as a feeder during the cooling and hardening of the melt. The neck is a narrow portion of the riser and is designed to connect the riser with the casting. The feeder elements should harden more slowly than the casting. Their dimensions are determined by thermal factors (thermophysical properties of the melt and the mold), by the casting properties of the alloy, by the weight, wall thickness, and configuration of the casting, and by the requirements made of the casting (such as mechanical properties and airtightness).
The feeders are usually sufficient for feeding thin-walled castings produced from eutectic melts, such as gray cast iron, which have short cooling cycles. In such cases special feeder elements are not required, and the gating consists only of runner channels. If a small volume of the melt is required for feeding, the system contains feeder-runner elements in addition to the runner elements. For example, the whirl gate may act as a feeder head at the same time, whereas a feeder may act as a neck.
Gating is divided into side, top, shower, siphon, multilevel, and slit systems, depending on the method and place of connection. According to the molding method, a distinction is made between gating with feed channels located on the horizontal parting line of the mold and gating with feed channels located either on the vertical parting line or outside the parting line.
REFERENCESDubitskii, G. M. Litnikovye sistemy. Moscow-Sverdlovsk, 1962.
Rabinovich, B. V. Vvedenie v liteinuiu gidravliku. Moscow, 1966.
Basic Principles of Gating. London, 1967.
Le Remplissage des empreintes de moules en sable. Paris, 1956.
Holzmüller, A., and L. Kucharcik. Atlas zur Anschnitt- und Speisertechnik für Gusseisen. Düsseldorf, 1969.
B. V. RABINOVICH
a method of selecting some interval on, for example, a time axis or a frequency scale to increase the probability of detecting a desired signal against the background of noise.
Gating is used primarily in radar. It finds application in search systems and in tracking systems used to determine range or angular coordinates. Gating is also employed in determining target velocities. Suppose, for example, a pulse radar is used to determine the range of a target. If the time interval tp during which the pulse reflected from the target arrives at the radar set is known (that is, the position of the target is known with an accuracy of tp), then it is sufficient to receive the reflected pulses, or signals, only during this time. For this purpose, the receiver input is brought into operation by means of a gating pulse of duration tp and does not operate during the remaining time. As a result, the total effect of the noise is considerably reduced, and the noise immunity of the system is improved.
Gating is also used in other systems where it is necessary to select a signal against a background of natural or man-made noise and where the distortion of individual characteristics of signals must be eliminated. For example, gating is used in television and computer hardware.
B. V. REPIN