Vane Motors
(1) Low-speed high-torque vane motor: This motor has a relatively large capacity and operates with a high torque at a low speed. The motor shown in Fig. 10.6 has four motor chambers and can change the torque control mode to 1/3 and 2/3.
(2) Medium-speed vane motor: This motor has a structure similar to that of the pressure-balanced vane pump, the capacity of which is relatively small. It is provided with a vane lift mechanism (spring) to ensure that contact between the vane and the cam ring is always maintained from the time of the motor starts.
Gear Motors
(1) External type: This motor is simple, compact, lightweight, inexpensive, and capable of relatively high-speed rotation. It has a cross-sectional structure similar to that of gear pumps. It requires a drain to be installed for oil seal protection.
(2) Internal type: This motor is compact and relatively inexpensive, and operates with a high torque at a low speed. It consists of an internal trochoid gear motor and a differential speed reducer. Fig. 10.4 shows the cross section of this motor.
Considerations on Yuken Cylinder Selection
(1) Models (JIS B 8367: 1999 Mounting dimensions for hydraulic cylinders)
®7HT: For 7 MPa (1 015 psi), Rectangular Cover, Tie-rod
©10HS: For 10 MPa (1 450 psi), Built-in Cylinder ©10HT: For 10 MPa (1 450 psi), Rectangular Cover, Tie-rod
©14HT: For 14 MPa (2 031 psi), Rectangular Cover, Tie-rod
©16HT: For 16 MPa (2 320 psi), Rectangular Cover, Tie-rod
©16HR: For 16 MPa (2 320 psi), Round Cover, Flange-Welding or Screw-In
©21HT: For 21 MPa (3 046 psi), Rectangular Cover, Tie-rod
®25HR: For 25 MPa (3 626 psi), Round Cover, Flange-Welding
(2) Cylinder Speed
Generally, a cylinder speed of 15 to 300 mm/s (.59~ 11.8 in/s) is recommended. An excessively high or low speed leads to rapid packing wear, causing fluid leakage from the rod seal or internal leakage. When the cylinder speed is too low, a stick-slip phenomenon may affect smooth cylinder operation. Special caution must be paid to the packing and the sliding parts when operating the cylinder beyond the recommended speed range.
(3) Cylinder Mounting
The cylinders can be mounted as shown in Table 10.1. They should be mounted so that the pressure is always applied in the moving direction with the minimum radial load, depending on the load characteristics and cylinder motion.
(4) Cylinder Bore and Rod Bore
Table 10.2 lists rod bores corresponding to cylinder bores. The rods are classified according to the area ratio between the head and rod sides.
(5) Rod Buckling
When a rod is subject to tensile stress, only its tensile strength need be considered. On the other hand, a long rod subject to a compression force may bend and easily break with a small stress. This phenomenon is called buckling, and the buckling strength of the rod must be determined, based on the load, rod bore, the rod-end coefficient (depending on the cylinder mounting type), and rod length. The buckling strength is an essential factor in determining the maximum stroke length. The safety factor is normally set at 4.
(6) Minimum Operating Pressure
The operating pressure is defined as follows: 0.5 MPa (72.5 psi) or less for U-packing, X-rings, O-rings (JIS B 2401), and combination seals (S), and 0.25 MPa (36.3 psi) or less for piston rings. Standard cylinders operate with 0.3 MPa (43.5 psi) of the working pressure.
(7) Cushion
The piston in a cylinder, which moves in the reciprocating motion, contacts the cover at the stroke end to produce shocks. To minimize the shocks, the cylinder should be provided with a cushion mechanism to reduce the piston speed at the stroke end. However, the cushion mechanism generates a higher internal pressure as inertial force increases.
(8) Packing Materials and Hydraulic Fluids i . Nitrile rubber (NBR):
Standard fluids and other than phosphate ester
ii. Hydrogenated rubber (HNBR):
High-temperature fluids
iii. Polytetrafluorethylene (PTFE):
High-temperature fluids
iv. Fluorinated rubber: Phosphate ester
Telescopic Cylinder
This cylinder uses a multistage tube as a piston rod to obtain a long stroke. It is useful for pistons requiring a more compact housing. Single and double acting types are available.
Single Acting Cylinder (Ram Cylinder)
This is a cylinder that pushes the piston in one way (extending direction). The piston rod is returned by gravitation or a mechanical force.
Double Acting Cylinder
This is a standard cylinder that produces reciprocating motion. It is available in single and double rod types. Fig. 10.1 shows the single rod cylinder. This cylinder is provided with a cushion mechanism, which uses a cushion sleeve to close the fluid return port as the piston rod retracts. Therefore, the fluid is discharged through a restrictor located in parallel with the return port, resulting in a higher piston back pressure and a slower piston speed.
Energy Saving Hydraulic System with Rotational Frequency Control
Conventional servo systems using servo valves generally operate with hydraulic power sources that produce excess load flow and pressure. Therefore, such systems involve heat and power loss caused by excess flow, as well as considerable pressure loss through the valves. On the other hand, systems with rotational frequency control regulate the motor speed so that the flow rate and pressure are optimized for load requirements. Systems with rotation control can minimize pressure loss and greatly reduce the power consumption for pressure holding.
Table 9.5 shows the specifications of the IH servo drive pack.
Description of IH Servo Drive Pack
The IH servo drive pack drives the pump with an AC servo motor, and the pump supplies pressured oil bidirectionally.
This results in a simple hydraulic control system where the load cylinder is simply connected to both outlet ports without control valves. The pump draws oil by using the self priming valve to compensate oil in
the cylinder.
The deviation of the control signals and sensor signals from the host is sent to the servo driver to drive the AC servo motor, consequently forming a feedback loop. The sensor signals come from the position sensor on the cylinder and the pressure sensor on the servo drive pack.
Servo Systems with Pump Speed Control
Simple servo systems, which do not use hydraulic servo valves or high-speed proportional valves, can be made by controlling the pump rotation speed with a servo motor. Fig. 9.13 shows an example of a simple servo system with pump speed control (IH servo drive pack). The IH servo drive pack is a compact and energy-saving hydraulic power source consisting of an AC servo motor, piston pump, reservoir, and closed hydraulic control circuit. This unit controls the pump discharge and pressure by adjusting the pump rotation speed. It can be combined with a sensor-equipped cylinder and a dedicated controller to facilitate the configuration of a position, speed, and pressure control system.
Servo Amplifiers
Servo amplifiers drive servo valves, based on the same principles as the amplifiers for proportional electro-hydraulic control. Table 9.3 lists the functions and features of the servo amplifiers and related devices. Fig. 9.10 and Table 9.4 give a circuit example and specifications, respectively.
