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Thrust 1 - Efficiency


1F.1: Variable Displacement External Gear Machine


Project Overview Sheet

1F.1 Variable Displacement Gear Machines (PDF)

Project Leader

Prof. Andrea Vacca (Purdue)

Statement of Project Goals

The primary goal of this project is to formulate and develop a unique concept for variable displacement external gear machines (VD-EGMs). The new innovative design of the machine will preserve the well-known advantages of current fixed displacement EGMs such as ease of manufacturability, low cost high pressure range of operation and good operating efficiency. To reach the primary goal, the project also proposes a general and innovative design method for EGMs that surpasses the current empirical design approach used to design such units. Particularly, the project will take into consideration unconventional designs, such as non-involute or helical gear profiles. Therefore the goals of the project can be mentioned objectively as:

  • Objective 1 (O1): Formulate a new design principle for VD-EGM
  • Objective 2 (O2): Propose a novel and general design methodology for EGMs

Project's Role in Support of the Strategic Plan

The proposed research directly addresses the technical barriers “efficient components” and “efficient systems” by introducing a new concept for a VD hydraulic machine. CCEFP is extensively researching new system concepts to minimize energy consumption of the fluid power applications, and many solutions are based on the potentials of VD units. However, the diffusion of efficient system layout architectures based on VD units is not as broad as it should be, due to the inherent high cost factor associated with VD pumps and motors. Therefore, research toward more cost effective solutions for VD units is needed in the fluid power field. By proposing a new VD design concept, this project will support the ongoing research on novel architecture and will permit a wider diffusion on more efficient systems also in low cost fluid power machines. With a strong fundamental component on the approach for designing EGMs, the research aims to surpass the current empirical methods that limit the possibilities of formulating new design concepts for EGMs. 

Description and Explanation of Research Approach

The well-known advantages of external gear machines (EGMs) such as low cost, compactness, reasonable operating efficiency and good reliability make them as one of the prominently used components in fluid power. Despite these advantages, EGMs are fixed displacement and they cannot be used as primary energy conversion units in modern energy efficient layout configurations based on variable flow supplies, such as in load sensing systems, hydrostatic transmissions or in displacement controlled systems.

External Gear Machine

Figure 1: Parts of an external gear machine

With the exception of cases where the unit operates at fixed pressure and flow rate, the energy consumption of fluid power circuits based on fixed displacement units can be as much as 70% higher than standard VD system layouts. For this reason, both industry and academia have been dedicating effort in formulating VD design solutions for EGMs, with the aim of preserving the advantages of limited cost (about 10 times lower than existing VD units with the same capacity) and reliability.  Representative of the past efforts are given by references. All these past effort share the idea of realizing an axial or radial relative motion between the gears to obtain a variable output flow. However, the motion of the gears, which are the most loaded elements in an EGM, involves major problems such as: sealing the tooth space volume; guaranteeing a smooth meshing process and a good balance of the gears avoiding contacts. A good solution for mentioned aspects generates complexities which increase the cost of the unit and penalize its reliability. For these reasons, none of the solutions proposed for VD-EGMs have found successful commercial application.

The proposed solution for VD-EGM
The novel idea for achieving Variable displacement in EGMs can be obtained by introducing an optimal concept of variable timing of connections between the displacement chambers (tooth space volumes, TSVs) and the inlet and the outlet ports. 

The variation in the timing of the connections is achieved by the introduction of a movable element called the slider as shown in Figure 2(A). The position of the slider determines the amount of flow displaced by the unit per revolution, for both the cases of pumps and motors.

Figure 2:  (A) Slider placed within the bearing block of the VD-EGM (B) The progression of TSV as a function of shaft angle.  The meshing process realized the displacing action in the angular interval θ, for a portion of the meshing process (between D-S), the volume is trapped between points of contacts.

In order to achieve max displacement, the commutation between of the TSVs between inlet and outlet groove (shown in Figure 3(A)) is realized when the volume is at its minimum (represented by “M” in Figure 2(B). Therefore, the max volumetric capacity of the machine is utilized since the TSV is connected to the inlet and outlet for equal intervals of time. A variation of the displaced flow can be achieved by positioning the slider closer to the inlet side as represented in Figure 3(B). In this configuration, each TSV is connected to the outlet for a larger period of time as shown in Figure 2(B), thereby a part of the fluid already delivered to the outlet is taken back into the TSV. Therefore, an effective reduced flow rate is displaced to the outlet.

Figure 3: (A) Position of the slier to achieve maximum displacement (B) Position of the slider to achieve minimum displacement