Chapter 3: Additional System Details
34 M47-17028-EN
Limit switch assembly
A limit switch (2, Figure 14) is mounted to the underside of the base plate (1). The purpose of the limit
switch is to protect the hydraulic cylinder from overtravel. The limit switch is actuated by a guide rod (3)
that is mounted to the piston (8a). This allows the guide rod to move in conjunction with the piston. The
guide rod extends through the base plate into the base (see Figure 14). As the piston reaches the limit
of its travel, the stop collar (4) on the guide rod will trip the limit switch. When the limit switch is tripped,
an electrical signal is sent to the 59 Series control unit and the controls shut down the HPS. The limit
switch must be reset before the HPS can be restarted. To reset the limit switch, the frame must unload
and allow the piston to travel away from the limit switch. The frame can only unload by gravity. To
increase the unload rate, the dump valve can be manually opened by turning the system disconnect
switch OFF. Once the frame is unloaded, turn the disconnect switch back ON and restart the system.
600DX frames have an additional limit switch. The purpose of this limit switch is to prevent the screw
columns from running out of the backlash nuts while jogging the adjustable crosshead in either direction
(up or down). When the limit switch is tripped, the crosshead motor stops and can not be operated in that
direction until the limit switch is reset. The adjustable crosshead can only be operated in the opposite
direction. Jog the adjustable crosshead away from the limit that was tripped to reset the limit switch and
resume normal operation.
Load measurement
A strain gauged load cell is used to measure the force applied to the test specimen. The load cell (17,
Figure 9 on page 27) is mounted between the compression table and piston. This mounting
arrangement provides a load measuring system that is independent of the hydraulic system. This means
that the force measured by the load cell is free of interference due to piston friction and hysteresis; it is a
direct force measurement.
Because of the dual test space design of the frame, the load cell are always subjected to a compressive
load - even during a tensile test. The controlling software will report the load on the specimen in the
appropriate direction (tension or compression) according to the test setup.
Figure 14. Typical limit switch and position encoder assemblies (300DX shown).
1. Base plate
2. Limit switch
3. Guide rod
4. Stop collar
5. Encoder string
6. Position encoder
7. C o m p res sion table
8. Hydraulic cylinder
a. Piston
b. Cylinder body
9. Lower crosshead
Left side of frame. Cover and some components removed for clarity.
8a
8b
9
7
1
2
3
4
6
5
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