Pressure Measurement
(1) Bourdon tube pressure gauge: this gauge is the most popular for visual inspections. Its case is usually filled with glycerin to protect the gauge from vibrations in hydraulic systems.
(2) Pressure gauge with an electric contact: the following types are available.
i . Hydraulic piston type: this type turns a micro switch on and off by pushing a spring.
ii. Bellows type: this type turns a micro switch on and off by pushing a bellows (as in the hydraulic piston).
iii. Bourdon tube type: it turns a micro switch on and off in the same way as a pressure gauge.
iv. Pressure gauge with semiconductor
(3) Pressure sensor: this sensor outputs continuous signals for hydraulic system monitoring or servo system feedback.
Heater
An electronic heater is widely used as a general heating tool. For large reservoirs, a steam heater is employed to deliver heat through pipelines to reservoirs.
Coolers
Water-cooling heat exchangers are the most popular because they provide the highest cooling efficiency. However, in recent years, air-cooling heat exchangers, which do not require cooling water, have become popular for compact units in machine tools. Heat exchangers with circulating coolants are widely employed in machine tools because of their high accuracy.
(1) Tubular heat exchanger: this exchanger delivers a working fluid through many copper tubes to accomplish heat exchange between the working fluid and the cooling water.
(2) Plate heat exchanger: this exchanger consists of many thin plates, which dissipate heat with the cooling water.
(3) Air-cooling radiator: this exchanger has many fin tubes to cool the working fluid with air flow through the tubes.
(4) Refrigerant heat exchanger: this exchanger uses a coolant gas and a compressor, similarly to home air conditioners, to dissipates heat from the working fluid. The unit is comprised of a hydraulic pump for circulation, a motor, and thermo control equipment such as a thermostat. This heat exchanger is used for applications where highly accurate temperature control is required.
Accumulators
Usage
1. Accumulation of energy: Achieving a large flow rate and compensating from internal leakage
2. Absorption of pulsations: Reducing noises and pulsations
3. Absorption of shocks: Used as a shock absorber Type
1. Bladder type: Separates gas from oil by a rubber bladder.
2. Diaphragm type: Sometimes used as a small
accumulator.
3. Piston type: Shaped in the form of a cylinder without a rod.
4. Spring type: Often used to prevent pulsations.
5. Weight loaded type: Designed for large-scale machinery and consisting of a cylinder and a plummet.
Figure 11.10 shows the accumulated condition of a bladder-type accumulator. V3 – V2 is equal to the output volume.
Cautions on Usage of Accumulators
(1) Accumulators should be vertically positioned so that their oil ports face down.
(2) Pre-charge pressure should be approximately 85 to 90 percent of the minimum working pressure, and should not be less than 25 percent of the maximum working pressure.
(3) Inert nitrogen gas (N2) should be used, while oxygen is strictly prohibited.
Changing Conditions of Gas
Changing conditions of gas under accumulation can be classified into isothermal, adiabatic, and polytropic changes.
(1) Isothermal change
When accumulation and output flow take a long time, heat generated in the action is completely absorbed into the surrounding atmosphere. As a result, there is no noticeable temperature change. Such volumetric change in accumulation, in association with temperature, is called isothermal change. In this case, the polytropic index is one.
(2) Adiabatic change
When accumulated gas is flashed out instantly, the gas expands or contracts with no heat exchange with the surrounding atmosphere. This type of volumetric change is called adiabatic change. Its polytropic index is 1.4.
(3) Polytropic change
No heat transfer takes place in adiabatic change, while all generated heat is transferred in isothermal change. In reality, pressure accumulation and discharge are accompanied with gas movement. The gas temperature, thus, changes after these phases with heat exchange. Temperature change caused by accumulation and discharge is not parallel with external temperature change. The gas temperature increases as the pressure accumulates, resulting in larger energy accumulated than in the case of isothermal change. The gas temperature, on the other hand, decreases as the pressure is released, generating a smaller amount of output flow. Polytropic change is generally obtained by the mean working pressure and the passage of time.
Other Cleaning Equipment
(1) Air breather: An actuator causes the working fluid and air to flow in and out of a reservoir. An air breather serves as ventilation for air intake and discharge, filtering out dust in the air. The air breather is installed on the tank-top. An appropriate air breather should be selected according to the Fire Service Law, if applied.
(2) Oil filling port with an air breather: This filtration tool combines an oil filling port and an air breather.
(3) Oil filling port: A working fluid is supplied through this hole to a reservoir. The hole is equipped with a filter so that no dust is allowed to enter the reservoir.
(4) Magnet separator: The magnet separator keeps a working fluid clean by absorbing iron powders in the reservoir. It is often used in servo systems.
Off-Line Filters
These filters clean working fluids in a reservoir by using a dedicated pump and a filter separate from the line. They are employed when a higher cleaning level is required.
Line Filters
A reservoir filter with a higher filtration rating results in higher suction resistance. To avoid such a rise of suction resistance, line filters are required.
Line filters protect hydraulic components by removing contaminants from working fluids and keeping them clean. Filter selection generally depends on the working pressure, flow rate, and filtration rating. The filters may be installed with a line connection or a manifold. There are also a line filter type that is equipped with a stop valve, a check valve, a relief valve, and an electric switch.
Reservoir Filters
Two types are available. One protects the pump by installing filters at the pump suction port. The other, called the return filter, cleans the working fluid returning to the reservoir.
(1) Suction filter without a case (FT): It consists of a core rolled up with a filter. This filter equipment is called a strainer and submerged in working fluids for filtration. Typically, it uses a 105-^m mesh filter.
(2) Suction filter with a case (FS): It has a filter element in a case. One type employs a differential pressure indicator to indicate that the filter element is clogged. Another type is equipped with a by-path valve. Typically, a 105-^m mesh is used as the filter element.
(3) Return filter: There are two types of the return filters. The tank-top type is installed on the reservoir, and the in-line type is within the lines. The latter type employs a low-pressure line filter.
Figure 11.3 shows the tank-top type.
As shown in Table 11.1, the filtration rating should be decided with consideration given to the filtration conditions.
Oscillating Motors
Oscillating motors are actuators that generate rotation at a certain angle with a hydraulic pressure. They include the vane and piston types. They can be easily mounted because they do not require a link mechanism. Also, these motors are compact and provide a high torque.
(1) Vane type oscillating motor: single vane and double vane types are available. The single vane type operates with a rotation angle of 280 degrees or less, while the double operates with 100 degrees or less, giving the torque twice what the single can produce. Compared to the piston motors, the vane type oscillating motors have simple structures and provide high response. Due caution should be paid to internal leakage inherent in the motor design. Fig. 10.10 shows the vane type oscillating motor.
(2) Piston type oscillating motor: The rack and pinion type and helical spline type are available. Fig. 10.11 shows the rack and pinion type, which uses an opposed single acting cylinder to drive the rack for pinion rotation. Fig. 10.12 shows the helical spline type, which has a piston with a screw mechanism to rotate the shaft. Piston oscillating motors allow the oscillation angle to be adjusted with the cylinder stroke length; therefore, careful consideration of the oscillating angle is not required. They also have very little internal leakage.
Piston Motors
Piston motors are available in axial and radial types. Also available are fixed and variable displacement axial piston motors that operate at a high, medium, or low speed with a bent axis or swash plate. The axial piston motor has a higher volumetric efficiency than other types of motors. Radial piston motors generally operate with a high torque at a low speed.
(1) Bent axis fixed displacement piston motor: This motor is based on the same structural principles as the bent axis fixed displacement piston pump. Variable displacement type is also available. The motor can have a bent-axis angle of up to 40 degrees and is generally suitable for high-speed operation.
(2) Swash plate type piston motor: The high-speed type of this piston motor is based on a structure similar to that of the fixed displacement swash plate type pump. Fig. 10.7 shows an example of the low- /medium-speed type. Its cylinder block has two pistons and swash plates facing each other. It provides good low-speed performance at a time of the motor starts.
(3) Eccentric radial piston motor: This motor is widely known as the star-shaped motor. It has several fixed cylinders perpendicularly placed on the output shaft in a radial pattern. The flow is fed or discharged to/from the cylinders through a distributor valve connected to the shaft with a swivel joint. When a hydraulic pressure is applied to the piston, it reciprocates and pushes the eccentric cam on the shaft to produce torque. This motor is designed for low-speed high-torque applications.
(4) Multi stroke radial piston motor: This motor has a piston placed in the direction of the radius of the shaft with a cam roller. The piston moves on the patterned cam ring to produce torque. This motor can offer a larger displacement capacity than that of any other hydraulic motors. It is often used in winches and crushing machines.
