Linear hydraulic actuators

Hydraulie aetuators are employed in sueh areas as the aerospaee industry beeause they possess a good power to weight ratio and have a fast response. 

The following analysis will be linearized for small perturbations of the spool-valve and aetuator. 

2in 2oui = (rate of change of chamber volume) + (rate of change of oil volume) 

In equation (4.30), the rate of change of chamber volume is due to the piston movement, i.e. dVldt. The rate of change of oil volume is due to compressibility effects, i.e.  

Bulk Modulus of oil, [3 = Volumetric stress/Volumetric strain 

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Mechanical force can be more easily controlled using fluid power. The simple use of valves and rotary or linear actuators control speed, direction and force. The simplicity of hydraulic and pneumatic components greatly increases their reliability. In addition, components and overall system size are typically much smaller than comparable electrical transmission devices. 

The energy within a hydraulic system is of no value until it is converted into work. Typically, this is accomplished by using an actuating device of some type. This actuating device may be a cylinder, which converts the hydraulic energy into linear mechanical force a hydraulic motor, that converts energy into rotational force or a variety of other actuators designed to provide specific work functions. 

There are many different t)rpes of actuators. They are defined by their power source and range of motion. Actuators typically use electric, hydraulic or pneumatic power sources. The range of motion is typically linear, partial turn or multi-turn. 

Hydraulic and pneumatic piston and diaphragm actuators provide a linear output. These may be integrated with scotch yoke or crank arm mechanism to provide a quarter turn output. These two mechanisms have variations in the output torque over the stroke of the actuator. When integrated with a rack and pinion mechanism they may provide up to a full turn output. 

As with pneumatics, hydraulics uses an actuator called a cylinder to convert its fluid power into mechanical power in the form of linear motion. The advantages gained by fluid power cylinders apply to both hydraulic and pneumatics. 

Oil hydraulics has become firmly established as the drive system for the vast majority of injection molding machines and until recently was almost unchallenged as the power source. Put at its simplest, the injection molding machine contains a reservoir of hydraulic oil which is pumped by an electrically-driven pump at high pressure, typically at up to 14 MPa, to actuating cylinders and motors. High and low pressure linear movements are performed by hydraulic cylinders, and rotary movements for screw drive and other purposes are achieved by hydraulic motors. Hybrid machines, in which the screw is driven by electric motor while the linear movements remain hydraulically powered, are not uneommon. 

Actuators. The problem of radiation damage to hydraulic fluids, elastomers, or electrical insulations is avoided by utilizing pneumatically powered metallic bellotvs for remote actuation of the valves. The actuator is a simple linear device which can be controlled with standard pneumatic controllers or regulators. The bellows may also be stacked to multiply the forces available. In the HRE 2, pneumatic actuators develop up to 5440 lb force. 

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