A NEW WAY FOR FLUID DYNAMICS IN HYDRAULIC TRANSMISSIONS


EXTRAORDINARY EFFICIENCIES IN HYDRAULIC PUMPS AND MOTORS


The big efforts made by hydraulic components manufacturers to realize an hydrostatic transmission in the past decade, have been not enough to satisfy the market demand in terms of efficiency, speed and reliability. The Ecotec company has developed a new volumetric pump-motor system called "Poweroll" which makes a big step ahead in this field.


Gabriele Pecorari
Ecotec

 

Basic concept of the new sistem

  • Rotary sliding friction has been completely eliminated and replaced with rolling friction

  • The remaining sliding friction is alternating and has a high degree of hydraulic balancing

  • As the sliding friction is proportional to the displacement, at zero displacement there is only rolling friction

  • There are no alternating masses

Introduction

A new design of pump-motor enables very high performance to be achieved in terms of efficiency and power density, in particular due to the much higher speeds which can be reached, operating pressures have increased to a smaller extent. This has been achieved by replacing soft anti-friction material, such as bronze, with roller bearings. The oil flow sections have also been increased with the use of a new radial distributor. It is a generally accepted fact that hydraulic components are much less reliable than mechanical components. The drastic reduction in the number of parts operating with sliding contact brings this design much closer to a mechanical concept rather than a hydraulic one; this represents a big step forwards in resolving the problem of reliability of operation, which can now be compared with that of an epicyclical gearbox.

Pump-motor volumetric system with 7 pistons


Mechanical friction

The new system can be described as follows: "THE ACTION AND REACTION FORCES OF THE PISTON ARE ABSORBED BY ROLLER BEARINGS, CONTACT BETWEEN SLIDING PARTS ARE COUNTER-BALANCED AND WITH LOW SPECIFIC LOADS"

Only a minimal part of the mechanism consists of surfaces with sliding contact; these are case hardened to 60 HRC thereby also ensuring optimal resistance to fluid contamination. This is also possible because the sliding speed is very low even when the unit is operating at high speed, making the use of antifriction materials unnecessary.

This is also possible because the sliding speed is very low even when the unit is operating at high speed, making the use of antifriction materials unnecessary.



This component may also be defined as follows: "HYDRAULIC COMPONENT CONSISTING MAINLY OF ROLLER BEARINGS, WITH PISTONS PLACED BETWEEN THEM". It is worth noting that a roller bearing has an efficiency of approx. 99.8%;


Mechanical force generator
 
Hydrodinamic friction
Compared to motors of the same size the fluid flow section has been increased by a factor of 3-4 times. As the flow resistance is proportional to the square of the flow velocity, such an increase in the section means a significant reduction in the pressure drop due to flow resistance.


The graph shows the same flow with about 16 times higher pressure than the black curve.


The dead volume between distributor and piston as been practically eliminated: it may be quantified as approx. 3-5% of the maximum displacement. This factor is of vital importance in order achieve good performance at minimum displacement avoiding problems of stick-slip.

The reduced quantity of oil between distributor and pistons is also important in reducing noise levels, vibrations, optimising bearing lifetime and improving motion control.

The large flow sections and the straight and short oil channels are fundamental in improving the hydrodynamic characteristics, which combined with the reduced mechanical friction ensure a very high overall efficiency.

Main characteristics

  • The concept is equally suited for use as hydraulic pump and as motor, there is no preferential condition

  • The unit can vary its displacement from +100% > 0 > -100%, and as such is perfectly compatible closed circuits.

  • Limited vibrations due minimal alternating masses

  • There is no bronze or other low friction metals

  • All moving parts are hardened

  • Adjustable distributor timing to anticipate or delay opening of oil channels

  • Adjustable distributor timing to adapt it to sudden changes in pump/motor operating conditions of the circuit

  • Possibility of operating with negative pressure; often pumps and motors may be damaged by cavitation due to loss of mechanical contact or hammering effect between moving parts. With this design no damage is caused.

  • Particularly resistant to contaminated fluids

  • Much wider operating temperature range

  • Much wider operating fluid viscosity range

  • Very wide displacement range: from 1 cc to over 30 litres, with power ratings not normally achieved in hydraulic components

  • Torque oscillation approx 2.4%

  • Starting torque of over 95% at 250 bar with a 50 cc motor

  • Starting pressure with no load less than 2 bar

  • Stalling displacement 5% of the maximum displacement

  • Good heat dispersion as the oil path passes through the centre of the unit

  • Extremely high max/min speed ratio; with a 50 cc motor it is 1 : 5000

  • Extremely advantageous pump/motor combinations for torque converter applications



Applications

Its favourable size-power ratio, combined with its efficiency provides the designers some interesting opportunities. In today's increasingly competitive markets, companies are forced to seek solutions which provide greater performance and economy; one of the shortest routes towards this objective is compactness.

In many cases this design is capable of achieving a space advantage of approx 30% compared to other components of equivalent power rating; obviously this advantage can be added to the characteristics mentioned previously.

The applications possible are essentially the same as those which currently use hydraulic components, but there are also a large number of other applications for which the hydraulic transmission has not been used yet due lack of efficiency. More to the point, the main problem is not so much low efficiency, caused by friction, but the heat which this generates which is often impossible to dissipate, at least locally.

A new field of application is certainly that of transport, vehicle transmissions.

The possibility of combinations for use as torque converters has already been mentioned.. The car has evolved enormously in many aspects, but the transmission has remained largely the same. In motorcars the need for torque converters is becoming increasingly important and this is confirmed by the increasingly frequent reports of important research and development projects being carried out in this area.

The transmission of the vehicle has a big influence on fuel consumption and consequently on pollutant emissions. The use of a torque converter with a very wide speed range and with a high overall efficiency enables the engine to operate at a more stable optimum speed thereby enabling it to reduce consumption and emissions.

The torque converter may not be able to compete specifically with a manual gearbox in terms of efficiency, but taking into consideration the engine and continuously variable transmission as a whole then there are considerable advantages. The specific efficiency is certainly higher than that of an automatic gearbox, which has the disadvantages of the manual gear change as well as extra power absorption and complexity inherent in these gearboxes

The range of applicable sizes is extremely wide which means that as a torque converter it can equally well be applied to small applications such as motorbikes, as to the much higher power ratings required for tractors, lorries and other even larger machinery.

In the plant and machinery sector the applications are unlimited, ranging from agricultural machinery, civil engineering machinery, mining machinery, to marine applications such as winches, haulers, thrusters and energy applications such as wind mill generators. All of them need better efficiency. The graph below can easily give an idea of the improvement.


Comparison between two variable displacement motor cc.64
 
A: axial piston motor
WITH ORDINARY POLLUTED OIL or however when the bronze components are filled in by iron particles, which cannot happen in Poweroll
A: axial piston motor
WITH CLEAN OIL
B: Poweroll
WITH ORDINARY POLLUTED OIL


Working conditions:

area 1 and 2:  
mechanical efficiency at 250 bar.
Costant displacement and pressure
Variable RPM and flow
 
area 3,4 and 5:  
total efficiency at 250 bar.
Constant pressure and RPM
Variable displacement and flow
 

Resulting

Area 1:  
A losses 30% efficiency because hydrodinamical effect has not yet started, B losses 8% efficiency only thanks to the bearings.
 
Area 2: 
the two systems have substantial balance of efficiencies.
 
Area 3: 
it is stressed higher efficiency of B.
 
Area 4:  
it shows a growing gap in favour of B.
 
Area 5:  
gap in favour of B is now very much visible.

The above graph shows Poweroll much more efficient than axial piston motors. An axial piston pump doesn't suffer hydrodinamically as a motor consequently Poweroll improvements are not enormous, but the advantages are very important in a closed loop. Really Poweroll request smaller charge pump and lower feeding pressure because of the radial architecture. This new transmission improves considerably the overall efficiency making an important fuel consumption reduction.


ECOTEC s.r.l. are interested in setting up Joint Ventures in all the various fields of application, (torque converters for vehicle transmissions, industrial machinery, etc) as well as with manufacturers of standard hydraulic components.


ECOTEC s.r.l.
Via Galileo Galilei 216/C
41100 Modena, Italy

Tel: Fax: +39 059-2929466

www.ecotecsrl.net

www.pump-motor.com