The use of various types of circulation in the design of excavator hydraulic drive

This article is devoted to the review, study and analysis of the existing types of circulation used in the design of hydraulic drives of construction machines. Three main types of circulation are distinguished: closed, open, and semi-closed. A comparison is given and the characteristic features of each of them are described. Particular attention is paid to semi-closed circulation because it is under investigated. The sequence of movements of the equipment elements during the work cycle is shown. The results are presented, in the form of a diagram of flow rates in the actuators of an excavator planner, made by semi-closed circulation. This work will be of interest to specialists in the field of designing volumetric hydraulic drives of construction vehicles.


Introduction
Excavators are multi-engine machines with a rigid suspension of working equipment, in which a hydraulic volumetric drive is used to transfer power from the engine to the working mechanisms. Compared to mechanical, hydraulic excavators have a wider range of interchangeable work tools, a greater number of main and auxiliary movements of the working equipment, which significantly expands their technological capabilities.
The hydraulic drive allows [1,2]: to significantly simplify the kinematics of the transmission and working equipment; expand the range of interchangeable work tools; reduce the size of the machine; rationally combine work operations; maximize the power of the power plant; increase the mobility and versatility of machines and improve the quality of work performed; ensure smooth movement and precise orientation of the work tool; realize great digging efforts; improve machine performance. Fig. 1 shows various types of circulations used to create the hydraulic drive of construction vehicles. When creating hydraulic circuits for hydraulic machines, the first two types of circulation are widely used [3].
A feature of the closed circulation circuit is the work on the hydraulic mechanism, which has the same volume of cavities, in the case of using a hydraulic cylinder with a two-sided rod or a hydraulic motor. The flow rate diagram for a circuit made by closed circulation is shown in Fig. 2a. Make-up is provided by a make-up pump 1, this pump also compensates for leakage in the hydraulic machine and the actuator. When the rod moves, the flow displaced from the cavity of the hydraulic cylinder enters the suction of the pump minus volume losses in the pump [4,5].  Figure 1. Types of circulation a) -closed circulation; b) -open circulation; c) -semi-closed circulation;1 -make-up pump; 2 -the main pump; 3 -make-up valves;4 -a hydraulic cylinder; 5 -safety valves; 6 -pump compensating for the lack of working fluid in the cavity of the differential hydraulic cylinder A feature of the circuit with open circulation is the difference in the volume of the piston and rod cavities of the hydraulic cylinder, which is determined by the coefficient: where, P S -the area of the piston cavity of the hydraulic cylinder; R S -the area of the rod cavity of the hydraulic cylinder. The pump draws liquid from the tank, and the liquid is also drained into the tank, so this scheme is most often used for machines in the hydraulic drive of which several differential hydraulic cylinders are used. For example, in excavators, where differential hydraulic cylinders are used in the mechanisms of the shovel boom, handle and bucket: in the drive of the mechanism for lifting the shovel boom and telescoping. In the hydraulic drive of excavators, secondary valves (Fig. 1b, pos. 5) are used to protect the drive and metal structures, which protect the drive and the metal structure from overload, in the case when, for example, the excavator handle mechanism loads the locked cavity of the boom or bucket hydraulic cylinders while digging [6,7]. When the valve is activated in the rod cavity, the piston moves and the rod extends, since the fluid is drained through the valve. Since the hydraulic cylinder cavities are locked, the piston cavity is fed by gravity from the tank through the check valves Fig. 1b pos. 3, due to the difference between the pressure in the tank and the pressure in the cavity.
The flow rates diagram for the scheme made by open circulation is shown in Fig. 2b. When the rod is extended, the pump feed enters the piston cavity, the discharge rate from the rod cavity is less than the pump feed, due to the difference between the areas of the piston and rod cavities [8]. When retracting the rod, the flow rate entering the drain is greater than the flow rate entering the rod cavity. 1c shows a circuit made by semi-closed circulation. The pump, designed to work in closed circulation, is used to drive a mechanism with a differential hydraulic cylinder. And the difference in the volume of the piston and rod cavities necessitates the introduction of additional devices [9,10].
When the rod is extended, Fig. 2c, a pump feed is supplied to the piston cavity, and a smaller volume of liquid is supplied from the rod cavity to the suction, the difference in these rates is compensated by an additional pump operating in open circulation and performing, in addition to compensation, other functions in the drive. When the rod is pulled the pump feed is supplied into the rod cavity, and a larger volume of liquid is drained from the piston cavity, the difference in these rates is dumped into the tank so that the balance of rates on the pump operating in a closed circulation remains unchanged.
Despite the complication of the hydraulic circuit used for semi-closed circulation, it is used, for example, on Slovakian excavators, which are widely used in the CIS [11,12].
To control the drive speed in open circuits, throttling hydraulic valves are used; in semi-closed circuits, speed is controlled by changing the pump's working volume, which is more energy-efficient than throttling [13,14].  The sequence of movements of the elements of the working equipment during the work cycle In the work cycle, the sequence of operations of the excavator planner can be divided into two groups of operations. Digging: by turning the bucket or moving the telescope. Transport operations: lifting the working equipment from the bottom, turning the bucket to keep soil in the bucket, turning from the bottom, unloading, turning into the bottom, lowering the working equipment, installing the bucket at a new digging point [15].
Duty cycle operations can be performed without combining the movements of mechanisms or with combining, for technological reasons, as well as to reduce the duration of the cycle. Some operations, for example, planning slopes or planning the bottom cannot be performed without combinations. Combining operations gives the operator more opportunities, and therefore, when creating hydraulic circuits for excavators, the need to combine the movements of mechanisms is always taken into account.
The table below lists possible combinations of movements of mechanisms of six elements in two, which usually tend to ensure the use of a dual-pump circuit [16]. Telescope "+" "+" "+" "+" Bucket "+" "+" "+" "+" Boom "+" "+" "+" "+" Rotation "+" "+" "+" "+" Movement "+" "+" "+" "+" When digging, the movements of the bucket and telescope do not combine, since before digging, when turning the bucket, the cutting angle is set, which does not change during digging. If digging is done with a bucket, it is also possible to combine with a boom to reduce the thickness of the chips. And it is also possible that, when digging, the movement of the telescope is combined with the turn of the bucket for processing slopes.
The movement of the boom is combined with rotation during transport operations, with the movement when pulling the machine from the ground when working in viscous water-saturated soils.
Excavation of the soil with telescopic working equipment with a backhoe bucket can be done both by turning the bucket and retracting the movable boom sections.
With above information, the retraction mechanism is considered the main, and the bucket rotation mechanism is considered auxiliary. The latter can dig weak soils, cut the bucket when digging the calculated soil, but its main purpose is to fix the bucket relative to the movable section when it is retracted.
All possible combinations of six elements of two can be used in the work cycle. In some cases, it becomes necessary to combine three movements, for example, when planning the bottom of the trench. Any sequence of movements in the work cycle for digging and transport operations can be accepted. Fig. 3 shows a cycle in which there is a combination of three movements when digging and when performing transport operations [17].
When controlling mechanisms in the work cycle, it is desirable that the speeds are not dependent on the load, because operations often must end at the same time. For this, volume control of the speed of movement of the actuators and semi-closed circulation of the flow can be applied.
An example of such a solution is the hydraulic circuit of the UDS-214 excavator. In this hydraulic circuit, some of the mechanisms are powered by a liquid in a closed circulation circuit, and some are separately in an open circulation circuit.  Figure 3. Excavator work cycle with backhoe work equipment 1 -operations performed by energy; 2 -short-term operations; 3 -operations performed without energy consumption (the boom lowers under its own weight, the telescope lowers the bucket without soil into the face) Having data on the flow of pumps, the parameters of hydraulic cylinders and hydraulic motors, as well as assigning the stroke of the hydraulic cylinders and the angles of platform rotation to unload and to the bottom, you can determine the flow rate flowing into the hydraulic mechanisms and out of them during uncombined and combined operations. Using these data it is possible to determine the required make-up for pumps operating in a closed circulation, as well as the flow rates discharged into the tank when there is an excess of flow to the suction of pumps operating in a closed circulation.
The tables below show the parameters of pumps and hydraulic motors obtained from the UDS-214 operating manual. The parameters of the hydraulic cylinders are also given and the costs in the piston and rod cavities, as well as in the cavity of the platform rotation hydraulic motor, are calculated.   The following is a typical work cycle of an excavator with telescopic equipment for two main operations: during digging and during transport operations. It should also be noted that the work cycle and sequence of actions may be different, and depends on the type of operator control and depending on the operations performed.