MTS 318 Load Unit - Manual (Page 19)

318 Load Unit Introduction 19
Load frame
The load frame is the basic structure which provides the reaction mass for the
force train. The base of the load frame is one end of the reaction mass and the
crosshead is the other end of the reaction mass. Installing a specimen and other
fixtures or components between the load unit base and the crosshead create a
force train.
The load frame and the other hydraulic components mounted to it collectively
create the load unit. The base houses the actuators, servovalves, and hydraulic
manifold. The crosshead is mounted above the base by two columns. A control
panel lets you operate the crosshead lifts, locks, and grips to assist in specimen
installation procedures.
Crosshead lifts
and locks
The crosshead can be positioned anywhere along the load frame columns. It is
moved along the column with hydraulic lifts. When the crosshead is in an
appropriate test position, it is hydraulically clamped to that position. This lets
you change the load unit to test specimens of different lengths.
Actuator manifold
The Series 298 Actuator Manifold (also called a hydraulic service manifold or
HSM) acts as the hydraulic interface between the HPU and the components
mounted to the manifold (actuator, servovalves, and accumulators) of the load
unit. It contains the required hydraulic porting and plumbing to accommodate the
hydraulic components. The manifold can also control the hydraulic pressure to
the load unit.
Actuators
The Series 244 Actuators can be located in the middle of the load unit base or
crosshead. It is a hydraulically powered piston that applies displacement of (or
force into) a specimen. It can apply equal power in tension and compression. One
end of the test specimen is installed into a fixture which is mounted to the end of
the actuator rod.
Servovalves
The Series 252 Servovalves regulates the direction and flow of the hydraulic
fluid to and from a hydraulic actuator. The servovalve responds to the polarity
and magnitude of the command signal generated by the controller.
Accumulators
The Series 111 Accumulators suppress line-pressure fluctuations. The load unit
includes a pressure-line accumulator to provide fluid storage so a constant line
pressure can be maintained at the servovalves for maximum performance. The
return-line accumulator minimizes return-line pressure fluctuations.
Pressure control
The load unit can be configured for several pressure configurations. The free low
configuration passes the hydraulic pressure from the HPU (or hydraulic service
manifold) through the manifold to the hydraulic components. The hydraulic
pressure options include on/off control, high/low/off control, and high/low/off
control with a proportional valve to ramp the pressure transitions.
Transducers The load unit includes a force transducer and an LVDT.
Force
The force transducer (also called load cell or force sensor) measures the amount
of tension or compression and rotational torque applied to it. It has four strain
gages that form a balanced Wheatstone bridge. When forces are applied to the
bridge, it becomes unbalanced and produces an electrical signal that is
proportional to the force applied to it. The force transducer is a resistive device
and requires a DC conditioner to process the axial signal from the Wheatstone
bridge.