MTS Series 242 Actuators - Manual (Page 17)

242 Actuator Functional Description
Series 242 Actuator Product Information Introduction 17
242 Actuator Functional Description
A linear actuator is used to push on a specimen to cause it to crush or compress
(compression testing), or to pull it apart causing it to stretch (tensile testing).
A linear actuator consists of a cylinder that contains a piston and a manifold to
mount a servovalve. An integral part of the actuator is a LVDT (linear variable
differential transformer) which measures the displacement of the actuator
(extension or retraction) and sends the information to the system controller. The
LVDT is mounted inside the actuator. It can be replaced with other specialized
measuring devices.
The double-acting, double-ended actuator operates under precision servovalve
control in MTS closed-loop servohydraulic systems. The actuator is a
hydraulically powered piston that can extend or retract (double-acting). It can
also provide equal power in tension and compression (double-ended). The
actuator includes a closed-housing linear variable differential transformer
(LVDT) which measures the displacement of the actuator.
242 Actuator Mounting
The actuator can be mounted to a wide variety of fixtures and assemblies.
• Component test systems usually have one end of the actuator connected to a
custom test frame base and the other end connected to the test specimen.
Either end of the actuator may use mounting fixtures (such as swivels) to
attach it to the specimen or base.
• Material test systems usually mount the actuator in a load unit assembly.
The actuator can be mounted below the base plate or above the crosshead.
The end of the actuator uses fixtures (such as grips) to attach the test
specimen to it.
242 Actuator/Servovalve Interface
The actuator has a built-in servovalve manifold. The manifold allows a Series
252 Servovalve to be mounted directly to the actuator. It can accept a wide
variety of servovlaves, with flow ratings up to a maximum of 57 L/min (15 gpm).
242 Actuator Operation
Actuator piston rod movement is accomplished by supplying high- pressure
hydraulic fluid to one side of the actuator piston and opening the other side to a
return line. High-pressure hydraulic fluid is ported into the cylinder through the
retraction port or the extension port. The differential pressure across the piston
forces the piston rod to move. The amount of hydraulic fluid and the speed and
direction of piston rod movement is controlled by a servovalve.
If the piston rod contacts some external reaction point, then a force is applied to
that point equal to the effective piston area times the actuating pressure. The
main criteria for selecting an actuator are the force and stroke (displacement)
required for the job.