Evaluation of the Hydro—Mechanical Efficiency of External Gear Pumps

Abstract

This paper proposes and describes a model for evaluating the hydro-mechanical efficiency of external gear machines. The model is built considering and evaluating the main friction losses in the machines, including the viscous friction losses at the tooth tip gap, at the bearing blocks-gears gaps, at the journal bearings, and the meshing loss. To calculate the shear stress at each gap interface, the geometry of the gap has to be known. For this reason, the actual position of the gears inside the pump casing and consequent radial pressure distribution are numerically calculated to evaluate the gap height at the tooth tips. Moreover, the variation of the tilt and reference height of the lateral gaps between the gears and the pump bushings are considered. The shear stresses within the lateral gaps are estimated, for different lateral heights and tilt values. At the journal bearings gaps, the half Sommerfeld solution has been applied. The meshing loss has been calculated according to the suggestion of the International Standards. The hydro-mechanical efficiency results are then discussed with reference to commercial pumps experimentally characterized by the authors in a previous work. The average percentage deviation from experimental data was around 2%, without considering the most critical operating conditions (high delivery pressure, low rotational speed). The limits of this approach are also explained. Finally, the role of each source of loss is discussed, considering different operating conditions and two values of fluid viscosity. Lateral gap losses and meshing loss are much more relevant in determining the hydro-mechanical efficiency variation in the pump’s operating range, especially at a low delivery pressure. Moreover, while lateral gap losses increase with the rotational speed, the meshing loss shows the opposite behavior. The tooth tip gap losses are never as relevant, but they increase at high pressure. The journal bearings losses become comparable with the lateral and meshing ones at high delivery pressure values. Considering the pumps analyzed and the operating range of delivery pressure values and rotational speed values, the meshing loss made the mechanical efficiency vary in a percentage range of ±7%, with lateral losses in the range of about the ±15%, when also considering the extreme operating points (low speed, high pressure; high speed, low pressure). The weight of the lateral losses slightly reduced when we analyzed the higher temperature results, while the meshing losses slightly increased.

The Influence of Water and Mineral Oil on Mechanical Losses in a Hydraulic Motor for Offshore and Marine Applications

“…The viscosity, density, and lubrication properties of these liquids differ [15]. These parameters have a significant impact on the size of mechanical, volumetric, and pressure losses in hydraulic machines [30][31][32]36]. These losses have an impact on the energy conversion efficiency in these machines [1,16,21,23,29,34,36].…”

“…These parameters have a significant impact on the size of mechanical, volumetric, and pressure losses in hydraulic machines [30][31][32]36]. These losses have an impact on the energy conversion efficiency in these machines [1,16,21,23,29,34,36]. Furthermore, the design parameters of hydraulic components have an influence on energy conversion efficiency [2,3,[10][11][12][18][19][20]32,38].…”

Theoretical and Experimental Fatigue Strength Calculations of Lips Compensating Circumferential Backlash in Gear Pumps

“…Conditions (12) and (13), with the assumption that σ m = σ a = σ max /2, may serve to define the maximum stresses possible in the lip restraint cross-section:…”

“…The flow velocity of the working fluid significantly influences the losses observed in the ducts, as well as in the hydraulic and fastening elements. The energy-consumption of a system is also significantly influenced by the total efficiency of a displacement unit [9][10][11][12][13]. Total pump efficiency describes the relationship between the output (hydraulic) power N h and the input (mechanical) power N m or is defined by the product of the volumetric efficiency η v and the hydromechanical efficiency η hm .…”

Acoustic tests of type KPF1 high-pressure external gear pumps

“…▶ a sudden increase in pressure between the suction and discharge area of the pump, ▶ flow ripple and the resulting pressure pulsation, ▶ the phenomenon of liquid capping in the notches of gears, ▶ cavitation, ▶ variable loads resulting from the liquid impact on the gears. The development of modern gear units is currently taking the following directions: increase of working pressures (Osiński, 2013), improvement of total efficiency (Zardin, Natali, Borghi, 2019), reduction of the occurrence of flow ripple and dynamic loads (Svishchev, Aistov, 2015), cavitation (Battarra, Mucchi, 2018;Stryczek et al, 2015), the minimisation of mass and the level of noise emitted by the pump (Rodionov, Rekadze, 2017;Zhao, Vacca, 2018).…”