Mono-directional and bi-directional motors are divided into three families (1,2,3) covering a range of displacements between 2.8 and 87 cm3/rev (0.17 / 53.1 in3/rev). The maximum working pressures for the motors are similar to those established for the pumps and they can deliver torque up to 250 Nm and power up to 60 kW. The run-in is the last stage of the manufacturing process and it is one of the most important operations because it permits the optimization and check of the product efficiencies.
During run-in in tests, increasingly higher pressure levels are created; the gears, inflected by the hydraulic load, act as tools machining the pump body, thus creating the best tolerances among the parts. This process is performed under computer control.
The definition of the gradual increase of the pressure is particularly important because it establishes the machining speed of the material by the gears and thus the particles dimension; these particles must be small enough not to interfere with the running of the product under testing and its future performance. Each motor of each group has a personalized pressure ramp in order that no contaminating material remains in the circuit and the pump is able to attain maximum performance levels immediately.
Reversible motors and pumps are subject to run-in procedure on both rotations. After this process product efficiencies are measured at fixed parameters.
Test data is automatically recorded to provide updated statistics on product performances, supplied on customer request.
After the run-in for gear motors another specific test follows on a dedicated test-bench, where the relative operating conditions must be reproduced:
» under braking, when the energy of the fluid is transferred to the shaft to overcome the resisting torque.
» under counter pressure, when the fluid passes through the motor with the shaft free to turn without load.
» under braking the stress distribution is similar to that which exists on the pumps: if maximum pressure exists at the inlet, and discharge pressure exists at the outlet, compensation seals and rotating parts are subjected to the maximum stress according to resisting torque.
» under counter-pressure inlet and outlet are under the same conditions: at maximum pressure the stress on the rotating parts is zero, while the flanges, body and external seals are subjected to the maximum stress. A typical motor’s working conditions are between these two situations: part of the energy is transferred to the shaft and part is used, for example, by another motor connected in series.
Therefore, on the Marzocchi motors test bench, the final control is divided in three phases:
Braking phase: at an established rotation speed a resisting torque is applied to the motor shaft. The application of this torque creates a variation in the fluid’s inlet speed and pressure; the test-bench control system stabilises the motor in fixed conditions in which running parameters are acquired, such as volumetric and mechanical performances and draining flow rate.
Counter-pressure phase: a fixed amount of oil goes through the motor without any resisting torque applied to the shaft; the outlet line is kept closed and therefore a bilateral pressure is established. The drain flow rate is measured at these conditions.
Start-up phase: without any resisting torque applied to the shaft, the start up torque is determined by measuring the minimum inlet pressure at which the motor starts running.
In the case of bi-directional motors, the three phases are performed for both rotations.
After this test the motor is delivered to the customer perfectly run-in and controlled: its extreme reliability makes it suitable for use even under extreme conditions. The following application example was developed in collaboration with the ELASIS Research Centre in Lecce and concerns the fan drive hydraulic system used to cool the engine compartment of the new Wheel Loader range by Fiat Kobelco.
To make motor maintenance easier (model W270LB Evolution is mounted with a Cummins 6 cylinder, 10.8 litre direct injection diesel, 202 kW turbo air after-cooler), the rear section of the vehicle can be opened. This incorporates the fan directly connected to the Marzocchi gear motor and the air conveyor, so permitting bilateral access to the radiant mass. The rotation direction of the fan is reversible, providing perfect self-cleaning action of the radiators.
This operation can be activated using an appropriate switch located in the cabin on the control dashboard.
The fan is driven by aluminium reversible motor (type ALM2BK1-R-20-T4-T-H, 14.1 cm3/rev displacement), equipped with support bearing and specific seals for wide temperature range.
The aluminium fan (weights 7 Kg) takes in hot hair from the engine compartment and therefore the hydraulic motor working temperature is approximately 70° - 80°C (158° - 176°F).
The hydraulic motor is also subjected to vibration due to the wide swings of the rear side of the machine, when the bucket is rapidly moving. Under extreme conditions, acceleration up to 7g possible.
Normally the motor performs with an inlet pressure of 200 bar and 50 bar in outlet pressure; to ensure the best temperature control, the FAN rotation speed is independent of the motor running speed.
For the project validation the hydraulic motor was subjected to an internal Marzocchi approval procedure (to which all new/special products are subject) and to a test performed directly by the customer.
Internal approval includes various endurance tests in which the components are checked on the test benches in the R & D department: they are subject to on-off pressure cycles at the maximum allowed pressure. Periodically, an inspection is made in order to check the components conditions.
Where necessary, tests have been performed with similar operating condition of the final application: same pressure cycles, temperature, oil type, etc.
Once the endurance test has been completed, a comparison is made between the initial and final product performances and a deep analysis of each component is performed to identify any possible failure. No failure and no performance degradation higher than the internal specifications must occur to validate the project.
The test performed by ELASIS is composed of a first cycle under continuous pressure at 210 bar (3050 PSI) at a temperature of 110°C (230°F), followed by a second cycle at pulsating pressure (0-210°C) at the same temperature conditions. Gear pumps and motors are volumetric machines widely used in hydraulic system design. They have simple construction, compact size; they are reliable, and offer a top cost/performance quality ratio. The 40+years of Marzocchi experience can support the choice of these products.
Defining the limits
Validation tests have revealed that the system is able to work on a continuous basis at a constant pressure of 30 bar (430psi) without the spilling of fluid and without the ring and the shaft exhibiting wear. Tests performed at higher pressures also demonstrated the possibility of the system functioning for short periods of time (several minutes) with over-pressures of up to 140 bar (2,030psi).
If the operating conditions must be maintained with extremely high over-pressure values (80 to 100 bar / 1,160 to 1,450psi) for prolonged periods of time (over 30 minutes), the heat generated by the friction between the ring and the shaft can result in temperatures of over 300°C. This inevitably plays a part in deteriorating the structure of the P450 material used in the seal ring, as well as compromising the ground profile on the shaft, and thereby reduces its sealing function.
In the presence of pressure peaks, for example caused by errors of manoeuvre or an incorrect timing of the valves of the hydraulic circuit, or in static conditions, with the shaft immobile, this
sealing system can bear over-pressures of over 210 bar (3,050psi) without leading to any loss of fluid or deformation of the components.
The robust and reliable characteristics of this product make it particularly suitable for applications that function as appendages of other machines without knowledge of the exact circuit characteristics.
» Link to list of displacements, working pressures and speeds of Marzocchi range of gear pumps and gear motors
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