MTS Series 248 Actuators - Manual (Page 16)

Series 248 Actuator Product Information 16
Series 248 Actuator Functional Description
Introduction
Series 248 Actuator Functional Description
The double-acting, double-ended, heavy-duty 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). The
actuator provides displacement of (or force into) a specimen or structure for
testing. It can also provide equal power in tension and compression (double-
ended). The actuator includes an LVDT which measures the displacement of the
actuator.
The actuator is designed specifically for low friction, low distortion, fatigue and
vibration testing of structures and components.
The actuator is typically configured with a pedestal base and internally mounted
linear variable differential transformer (LVDT) for free-standing actuator
applications. The actuator may be equipped with an open housing LVDT
assembly (typical for platen mounted actuators). The optional static support
assembly provides an additional pressure vessel that can be pressurized to
support a substantial mass with minimal pressure differential across the actuator
piston.
Extension port/
Retraction port Ports allow fluid flow up to 945 L/m (250 gpm) for high velocity or high frequency
operation. Internal passages are designed to minimize fluid flow restrictions.
High-pressure hydraulic fluid is ported into the cylinder through the retraction port or
the extension port. The flow is regulated by a servovalve. As hydraulic pressure is
applied to one port, the other port is opened to a return line causing the actuator to
extend or to retract.
Pedestal base The pedestal base allows the actuator to be mounted to a reaction mass for free-standing
vibration testing.
Drainback port The drainback port allows fluid throughout the bearing area to be routed out of the
actuator. This prevents pressure pockets from interfering with the actuator performance.
Hydrostatic
bearings Each end cap contains four hydrostatic bearing pads. High-pressure hydraulic fluid
within the pad cavity prevents rod-to-end cap contact. Hydraulic fluid is metered into
each pad by a small fixed capillary. Pressure in each pad is controlled by the clearance
between the pad and the piston rod. Due to carefully controlled clearances and matched
orifices, a very slight displacement of the piston rod from center creates a high restoring
force, avoiding rod-to-end cap contact.
Piston rod seal One low-pressure seal assembly in each end cap contains a seal to guide excess fluid to
the drainback port and a wiper to prevent external contamination from entering the
actuator.
LVDT assembly The LVDT indicates actuator piston rod displacement. The LVDT core is secured to the
piston rod by a core mount. On pedestal base and open housing configurations, the core
mount is adjusted to establish a zero reference point for the piston rod. On the static
support configuration, the core mount is fixed to provide a midstroke zero reference
point.
Series 248 Component Identification (Continued)
C OMPONENT D ESCRIPTION