Hydraulic power is often used for the drive train or transmission of a vehicle, especially for off-road, utility and material-handling vehicles. In this case a rotary output is required from the hydraulic system in order to drive the vehicle wheels or tracks (Fig. 2.57).
Fig. 2.57 Vehicle transmission
FULL-SPEED FORWARD
REDUCED-SPEED REVERSE
Fig. 2.58 Closed-circuit transmission DEFINITION
Hydrostatic – power transmission using the pressure of a fluid.
Hydrodynamic – power transmission using the momentum of a fluid.
This arrangement provides an efficient system, because the system flow is not restricted by flow-control or directional-control valves and the pump always produces the required amount of flow at the pressure required to move the load at any one time. It is often referred to as a closed-circuit hydrostatic transmission to differentiate it from a hydrodynamic transmission, which uses a torque converter in conjunction with an automatic mechanical gearbox.
The type of pump normally used for closed-circuit applications is the variable- displacement axial piston pump described earlier. However, the pump is modified to allow the swashplate to tilt in either direction from the zero flow position, and is therefore known as an over-centre pump (Fig. 2.59). This means that, although the shaft is always driven in the same direction (and often at a fixed speed), the output flow can not only be varied but also reversed, depending on which side of centre the swashplate is moved.
1. NEUTRAL 2. TRANSIENT
3. FULL-FORWARD FLOW 4. FULL-FLOW REVERSE
Fig. 2.59 Over-centre transmission pump (Image courtesy of Eaton Corp.) In smaller pumps the swashplate can be moved directly by a lever connected to the operator’s position by linkages or cables. Larger pumps use a power-assisted servo arrangement similar to that used for vehicle power steering. Control may still be achieved by a mechanical lever or linkage, but this now operates a directional valve spool within the pump mechanism (Fig. 2.59). As the spool is offset by an input movement, hydraulic fluid is directed to one end or the other of an internal cylinder, the piston of which is connected to the pump swashplate. The piston and swashplate then move to change the pump displacement and flow, but in doing so also move the spool valve sleeve (effectively the body of the directional valve).
When this movement corresponds to the initial input movement, the spool and sleeve are centred relative to each other, and the piston and swashplate come to rest in a position and direction directly proportional to the direction and amount of input movement. The mechanical input of the operator that controls the pump flow is therefore servo-assisted (or power-boosted), and the large control forces required by high-power pumps can easily be achieved.
DEFINITION servo – a device that
accurately amplifies the power or force of an input control signal.
As mentioned above, the rotary output required from a hydrostatic transmission drive requires some form of hydraulic motor. Where the output requirement is a relatively high speed, the type of motor used will be very similar in construction to some of the pumps already discussed. Typically these are axial or bent-axis piston units, either fixed or variable displacement. The use of a variable-displacement motor in conjunction with a variable-displacement pump will increase the practicable speed range (i.e. the effective ratio between the pump input speed and the motor output speed). Where a large ratio is not required, however, a fixed-displacement motor is a common (and less expensive) choice.
For slower-speed drives (often where the hydraulic motor is connected directly to the vehicle wheel or track), radial-piston or gerotor motors are often used. Radial- piston motors use multiple pistons acting either inwards on an eccentric cam (Fig. 2.60a) or outwards on a wave-type cam profile. In a cam-lobe motor (Fig. 2.60b) the pistons are often retractable. This provides a ‘free-wheel’ function, which may be useful if a vehicle is being towed, for example.
(a) ECCENTRIC CAM (b) CAM LOBE
Fig. 2.60 Radial-piston motors
Fig. 2.61 Gerotor motor (Image courtesy of Eaton Corp.)
The gerotor motor (Fig. 2.61) uses a star-shaped rotor that rotates in an orbital manner, again to provide a relatively slow-speed drive but with a high torque capability.
Both piston and gerotor motors use spool- or plate-type distributor valves to direct the incoming high-pressure fluid to the appropriate pistons or cavities and thus generate the torque on the motor shaft.
WARNING
As with piston pumps, the cases of piston motors should be filled with clean fluid before they are started for the first time.
Although the basic circuit shown in Fig. 2.58 has only two components and no reservoir, in practice other components will be required. The internal leakage from both the pump and motor has to be drained back to a reservoir, although the reservoir used will often be much smaller than in an equivalent open-circuit system. In order to replace the fluid being drained from the closed loop, a charge (or boost) pump is required (Fig. 2.62). This draws fluid from the reservoir and feeds it back into the loop on the low-pressure return side. Two non-return valves are required to enable this.
The charge pump itself will normally be a low-pressure, fixed-displacement pump (typically a gear pump), and so will require a simple relief valve to limit its maximum pressure.
CHECK VALVE
CHECK VALVE
RELIEF VALVE CHARGE PUMP
Fig. 2.62 Charge pump relief valve and check valves
The maximum pressure within the loop has to be limited to protect the system components in the event of the motor output being stalled. This can be achieved by the use of relief valves (as described earlier), but to take account of the bi-directional operation two relief valves are often used (one for each direction of rotation), connected across the two main sides of the loop (Fig. 2.63).
The final components required are fitted to enable a certain amount of fluid to be removed from the loop and directed back to the reservoir, where it can be cleaned and cooled (if necessary) before being replaced by the charge pump. This is often referred to as a hot-oil shuttle valve (Fig. 2.64), and consists of a simple pilot- operated directional valve spool and relief valve.
The directional valve spool is pushed across by whichever is the high-pressure side of the loop in order to bleed off flow from the low-pressure side of the loop. The relief valve ensures that a minimum pressure is always maintained on the low-pressure side (by the charge pump flow passing across it) and thus prevents possible cavitation of the main system pump.
WARNING As mentioned previously, a relief valve passing flow at high pressure will create heat.
In a closed-circuit hydrostatic system (with a relatively small reservoir) the temperature of the fluid can rise very rapidly in such situations.
DEFINITION
cavitation damage is caused primarily by the violent collapse of fluid vapour bubbles at the outlet (high-pressure) port of a pump. The main cause of cavitation is a restricted inlet flow to the pump.
Many of the components required in a closed-circuit system can be built into either the main pump or, sometimes, the motor. In Fig. 2.65, for example, the main system pump incorporates the charge pump and relief valve, the check valves and the main system relief valves, and the hydraulic motor includes an inbuilt hot-oil shuttle valve.
Such a system is shown schematically in Fig. 2.66.
Although it has only been possible in this chapter to describe a few of the components found in hydraulic systems, the basic building blocks of the components will be common to most of the others also. Hydraulic components are typically constructed from pistons, spools, poppets, springs, solenoids, etc., all of which are manufactured to tight tolerances and assembled with small clearances. To obtain an acceptable operational life from the components, therefore, it must be ensured that they are operated within the manufacturer’s recommendations for such specifications as:
MAIN RELIEF VALVES
Fig. 2.63 Main loop relief valves
LOW-PRESSURE RELIEF VALVE HOT-OIL SHUTTLE VALVE
FILTER
Fig. 2.64 Hot-oil flushing valve and filter TOP TIP
A good understanding of how hydraulic components work is essential when maintaining or troubleshooting hydraulic systems.
This chapter has considered only a very small number of the hydraulic components in use today, so always consult the manufacturer’s data sheets for operation and application information.
• operating pressure (inlet and outlet)
• operating drive speed
• supply voltage
• operating temperature (minimum and maximum)
• external shock and vibration
• fluid compatibility
• fluid type and condition
• fluid cleanliness.
MECHANICAL
SERVO VALVE MAIN PUMP
BOOST PUMP
BOOST PUMP RELIEF VALVE MAIN-LOOP
RELIEF VALVES DISPLACEMENT-
CONTROL PISTON
HYDRAULIC MOTOR
HOT-OIL SHUTTLE
VALVE
Fig. 2.65 Closed-circuit transmission system (Image courtesy of Eaton Corp.)
MAIN PUMP
CHARGE PUMP AND RELIEF VALVE
MAIN RELIEF VALVES
SHUTTLE VALVE AND FILTER
HYDRAULIC MOTOR
Fig. 2.66 Closed-circuit hydrostatic transmission circuit (Image courtesy of Eaton Corp.)
FURTHER READING For a chart of hydraulic symbols and white papers on how to select flow and directional control valves, see:
www.webtec.com/education
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