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Lubrication Fundamentals 2011 Part 5

Tham khảo tài liệu 'lubrication fundamentals 2011 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Figure 7.18 Two-pump circuit with unloading valve. C. Unloading Valves In some circuits two pumps may be used to meet substantially variable flow requirements. In the case shown in Figure 7.18 flow from the larger volume pump is used to ensure the rapid advance of the machine tool to a certain point but then as the work activity is performed the volume of the flow decreases markedly. During the rapid advance cycle the discharge volume of both pumps is required but while actual work is being performed only the small pump volume is required causing a rise in system pressure. As the pilot pressure rises pilot pressure set below relief valve pressure setting the unloading valve opens allowing the flow volume from the larger pump to be discharged to the reservoir at low pressure. If one constant volume pump were used most of the oil pumped during the work cycle would be discharged at full system pressure through the relief valve. Its energy would be wasted and excessive heating could occur. Use of a variable volume pump could be an alternative to the method featuring two pumps and an unloading valve. D. Sequence Control Valve In some machines two or more movements may need to be hydraulically operated in sequence. When one movement must not begin until another has ended a pressure-operated sequence valve may be used. Referring to the circuit shown in Figure 7.19 when oil flow stops at the end of the clamping-cylinder stroke pressure rises sufficiently to open valve A. Full line pressure is then available to activate the feed cylinder. Sequence valves can also be activated by pressure-sensing pilot valves or electronically by position or other pressure-sensing devices. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. Figure 7.21 Flow control valve the orifice shown in the meter-in circuit in Figure 7.20 usually will not maintain constant speed throughout the work stroke. The workload will usually vary causing the pressure drop across the orifice and flow .