Читать книгу Fundamentals of designing hydraulic gear machines - Jarosław Stryczek - Страница 6
Оглавление1. Definition, systematics and methodology of designing hydraulic gear machines
1.1. Definition and systematics
The concept of ‘hydraulic (fluid power) gear machines’ (HGM) refers to such a type of machines utilized in the fluid power drive and control systems, where gears constitute the basic system both in terms of the principles of design and operation.
Figure 1.1 presents the simplified diagram of HGM. The machine consists of a gear system and a housing. Between the gear teeth, the intertooth displacement chambers are created, and in the housing the channels and the clearances are formed.
Fig. 1.1. Simplified diagram of the hydraulic gear machine (HGM)
The intertooth displacement chambers co-operate with the channels and clearances system, transferring the working medium from the inlet side towards the outlet side of the machine. The HGM can perform two basic functions:
– the function of a pump, in which mechanical energy from the motor is transformed into hydraulic energy accumulated in the discharged working fluid – see the solid line in Figure 1.1.
– the function of a motor, in which hydraulic energy accumulated in the working fluid supplied to it, is transformed into mechanical energy of the rotational motion of the shaft driving the working unit – see the dashed line in Figure 1.1.
The design solution of HGM results from the selection of two basic systems, that is a gear system and a system of channels and clearances created in the housing of the pump.
In the HGM, the following systems are mainly employed:
– the involute and cycloidal systems,
– the internal and external gearing systems,
– the fixed or the moveable rotation axes systems.
Gear systems can co-operate with:
– a fixed system of internal chambers and channels,
– a moveable system of internal chambers and channels.
Taking the basic function performed by a machine as well as the design solutions of a gear system and an internal channels and clearances system as the criteria of division, Figure 1.2 presents the systematics of hydraulic gear machines. Four groups of machines are distinguished.
Fig. 1.2. Systematics of hydraulic gear machines (HGM)
Machines of the first group include pumps or motors featuring the external involute gearing, fixed rotation axes and the fixed channels and clearances system. Of all the HGMs, machines of the first group are most frequently applied in practice [68].
Machines of the second group include pumps or motors featuring the internal involute gearing, fixed rotation axes and the fixed channels and clearances system. Machines of that group are of more compact design, smaller size and mass than the machines of the first group [66, 67].
Machines of the third group include pumps or motors featuring the internal cycloidal gearing, fixed rotation axes and the fixed channels and clearances system. Machines of that group are of an even more compact design, smaller size and mass than the machines of the second group. They are often referred to as ‘gerotor machines’ [80, 82, 83, 84, 85].
Machines of the fourth group are predominantly low speed, high-torque machines with the external or internal involute or cycloidal gearing, moveable axes and the system of moveable channels and clearances. Machines of that group are of small size and mass relatively to the power generated. They are often referred to as orbital or planetary machines [80, 82, 83, 84, 85].
That group also includes the orbitrol control units and the multifunctional hydraulic gear machines.
The systematics of the HGM can also be carried out depending on other criteria of division. Regarding their displacement (capacity), the HGM can be divided into the constant and variable displacement (capacity) machines.
From the viewpoint of independent streams flowing through the machines, the single- or multi-stream units can be distinguished.
Regarding the way the machines are connected, the single-stage or multi-stage machines can be distinguished.
Hydraulic gear machines are designed in a systematized way. Basing, among others, on the source [60, 62], in Figure 1.3 a diagram of methodology for designing and studying hydraulic gear machines is presented. The methodology includes eleven stages of design and study. Each of the stages is symbolically illustrated by a diagram of a machine, which shows what design problem is solved within its framework. The problem is marked in the diagram with a solid line.
At the first stage of design, a concept of a particular design solution for an HGM is developed. Within the framework of that concept it is necessary to select a machine type out of four machine groups shown in Figure 1.2, and consequently, the following:
– the basic function performed by the machine,
– the design solution for the gear system,
– the design solution for the system of the channels and clearances.
It is also necessary to determine the values of the basic technical parameters which the machine is supposed to feature.
At the second stage, an analysis of energy transformation in the HGM is carried out, which Figure 1.3 depicts with the arrow-head line on the inlet and the outlet, which goes through the inside of the machine. This stage consists in developing an energy model of the machine, analysing its energy balance, and in developing anticipated characteristics, which are related to the technical parameters of the machine assumed at stage 1.
At the third stage, a gear system of the HGM is designed, which Figure 1.3 presents in a form of a circle with the blackened intertooth displacement chamber.
The designing includes the geometry and kinematics of the gear system, in order to minimize the dimensions of the gear system and to maximize the volume of the intertooth displacement chambers.
At the fourth stage, the system of channels and clearances of the HGM is designed, which Figure 1.3 presents as lines edging the gear system. The task of the system is to supply and discharge the working fluid to and from the intertooth displacement chambers while the machine is operating. As it has already been mentioned above, it can be either a fixed or a moveable system.
The system consists of:
– a channel, a chamber and an inlet bridge,
– a channel, a chamber and an outlet bridge,
– an axial and radial clearance.
The questions of the primary concern at the designing of the internal chambers and clearances system, are to secure continuity of the flow, to minimize the flow resistance, to eliminate the cavitation phenomena and to minimize internal leakage.
At the fifth stage, the displacement (capacity) Q and of the displacement pulsation (capacity) ΔQ of the HGM are calculated, which in Figure 1.3 are presented in a form of a reference mark directed towards the outlet port of the machine. The calculations are carried out basing on the gear system with its geometry and kinematics defined at stage 3. According to the designed gear system, specific formulae are applied. The formulae allow to determine the effect of the gear system on the displacement (capacity) and on the pulsation of the displacement (capacity).
At the sixth stage, a theoretical analysis of pressure p and pulsation of pressure Δp in the intertooth displacement chamber of the HGM in the full working cycle is carried out, which Figure 1.3 presents in a form of a reference mark directed towards the outlet port of the machine. The analysis is carried out to test the accuracy of co-operation of the gear system and the system of the internal chambers and clearances.
Fig. 1.3. Methodology of design and research of hydraulic gear machines (HGM)
At the seventh stage, a visual study of the flow processes and phenomena in the channels and clearances of the HGM is conducted, which in Figure 1.3 is presented in a form of a reference mark directed towards the inside of the machine. At the beginning of that stage, an experimental machine is constructed and a test stand equipped with a fast camera for photo recording is prepared.
Basing on the results of the designing work obtained in stages 1– 6, an experimental machine with a technical glass housing is constructed. The operating machine then is monitored and, by means of the fast camera, the flow processes and phenomena occurring in the machine are recorded. By changing the design solutions and the operational parameters of the machine, it is possible to monitor their influence on the processes and phenomena inside the machine. Based on the analysis of the processes, it is possible to correct the design solution of the entire HGM.
At the eighth stage, an experimental research into the pressure in the channels and clearances of the HGM is conducted, which Figure 1.3 presents as a reference mark directed towards the inside of the machine. Similarly to how it is carried out at stage 7, an experimental machine and a test stand are constructed, equipped with a system for the measuring of the dynamic pressure in the intertooth displacement chambers during the operation of the machine. The pressure curves are then drawn. By changing the design solutions and the operational parameters of the machine, it is possible to monitor their influence on the processes occurring in the mesh of the gear system and the system of the internal chambers and clearances. Based on the analysis of the processes, it is possible to correct the design solution of the entire HGM.
At the ninth stage, the HGM housing is designed, which in Figure 1.3 is marked with a bold line edging the gear system and the system of channels and clearances.
To design the housing the following steps need to be taken: determining the basic shape of the housing and conducting the strength analysis by means of FEM, modifying (correcting) the basic shape of the housing and carrying out the strength analysis utilizing FEM, and finally, accepting the final shape of the housing.
At the tenth stage, the axial clearance compensation system of the HGM is designed, which Figure 1.3 presents in a form of an oval compensation element working with the gear system, edged with a bold line and hatched with oblique lines. The designing process starts with the selection of the shape and size of the compensation element. Next, the pressure research results obtained at stage 8 are implemented, on the basis of which the resultant repulsive force working on the compensation element and on its point of contact are determined. Finally, on the outer surface of the compensation element, a surface is formed, which is influenced by the working pressure. As a result, the resultant pressing force is generated, which should be greater than the repulsive force. The point of contact of the pressing force is also determined. It should be placed as close to the point of contact of the repulsive force as possible.
At the eleventh stage, the final design solution of the HGM is developed based on the results obtained at the ten preceding stages, their synthesis is carried out, and the design documentation of the HGM is prepared.
