Power Jacks C-Series - Manual (Page 19)

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5
Operation
5.2.21 Corrosion Resistant Properties
Screw Jacks can be supplied with alternative materials and/or paint specifications for high
corrosive areas. These options include stainless steel, chrome plating, Electro-nickel plating,
epoxy paint, etc. Check the unit specification is suitable before installation.
5.2.22 Using Screw Jacks within a Rigid Structure or Press
Power Jacks recommend that the screw jack selected has a greater capacity than the rated
capacity of the press or of the load capacity of the structure. We also recommend that a torque
clutch or similar device be used to prevent overloading of the screw jack unit. Unless these
precautions are taken, it is possible to overload the screw jack without realising it.
5.2.23 Screw Jack Drift after Drive Motor is Switched Off
The screw jack will drift after the motor drive is switched off unless a brake of sufficient capacity
is used to prevent it. The amount of drift will depend upon the load on the screw jack and the
inertia of the rotor in the motor.
For machine screw jacks with no load, the amount of drift will depend upon the size and speed
of the motor. For example, a 1500 RPM input directly connected to a screw jack without a load
will give on average 35mm to 60mm of drift; a 1000 RPM input will give about 1/2 as much drift.
Note that the drift varies as the square of the velocity (RPM). The drift of the screw jacks screw
can be controlled by using a magnetic brake on the motor. Variations of drift will also be seen if
the motor drives the screw jack via a reduction gearbox.
5.2.24 Screw Jacks Operation where Vibration is Present
Screw Jacks will operate in areas with vibration, however the vibration may cause the lead
screw to “creep” or “inch” under load. For applications involving slight vibration, select the
higher of the worm gear ratios. If considerable vibration is present, use a motor equipped with a
magnetic brake, which will prevent the screw jack from creep and/or back-driving.
5.2.25 Use of Screw Jacks Fitted With Emergency Stop Disc
To prevent over travel of the lead screw a stop disc or nut can be fitted to a screw jack that is
hand operated. It should not be used as a full power stop.
5.2.26 Use of Screw Jacks Fitted With Emergency Stop Nut
For motor driven units it is possible for the full capacity of the screw jack or even a greater force
(depending on the power of the motor) to be applied against the stop. These stops are called
"full power stop nuts". They must only be used as an emergency device and if such a condition
occurs an assessment made to discover why it happened in order to carry out preventative
action. If the full power stop nut is used at full load in an emergency it might be driven into the
unit jamming so tightly that it must be disassembled in order to free it.
It is recommended that external stops are fitted where possible, however they must only be
used as a last resort (Note - limit switches are one possible solution to constrain screw jack
movement safely - consult Power Jacks for system advice). Under ideal conditions where a slip
clutch or torque limiting device is used, a stop pin or stop nut may be used - but Power Jacks
should be consulted.
5.2.27 Screw Jack System Arrangements
Perhaps the greatest single advantage of Power Jacks screw jacks is that they can be linked
together mechanically, to lift, lower, move or position in unison. Typical mechanical system
arrangements link 2, 4, 6 or 8 screw jacks together and are driven by one motor. As an
alternative screw jacks can be individually driven by electric motors and with suitable feedback
devices such as encoders be synchronised electronically by a control system.