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Thrust 1 - Efficiency


1F.1: Variable Displacement External Gear Machine


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 one: Formulate a new design principle for VD-EGM
Objective two: 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

External gear machines (Figure 1) are one of the most widely used positive displacement machines in numerous hydraulic applications. The strong-point of these machines lies in their good reliability, operating efficiency, compact size and low cost.  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
Variable displacement in EGMs can be obtained by proposing an innovative concept of variable timing of connections between the displacement chambers (tooth space volumes, TSVs) and the inlet and the outlet ports. A schematic representation of this idea is shown in Figure 2.


Figure 2: (A) Slider placed on the bearing block for the proposed VD-EGM. The position of the slider determines the timing of the connections of each TSV with inlet/outlet ports (Patent pending for the design of the gears and recesses in the slider for the proposed VD-EGM). (B) Position of the slider to achieve 100%. (C) Position of the slider to achieve 65% displacement.

In this design, the delivery and the suction grooves are machined on a slider which is a part of the lateral bush as shown in Figure 2. Displacement variation is achieved by moving the element. When the movable element is moved to the left, the volume of the fluid which is trapped between the contact points of the teeth of the gears is connected to the delivery for a larger interval of time as shown by the timing diagram.