Power Jacks C-Series - Manual (Page 15)

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5
Operation
5.1 Operational Recommendations
5.1.1 Select a screw jack which has a rated capacity greater than the maximum load that may be
imposed on it.
5.1.2 The screw jacks should have a greater raise than is needed in the actual installation. Should
it be necessary to operate the screw jacks at the extreme limits of travel it should be done
cautiously.
5.1.3 It is important that the lead screws should not be closed below the specified closed height
dimension of the screw jacks, otherwise serious damage may result to the worm gear.
5.1.4 Lead screw end stops are to prevent over-travel or loss of screw. These are not load supporting
and should be treated as an emergency device only and must not be allowed to come into
contact with the worm gears during normal working cycles otherwise serious damage will
result to worm gears and bearings.
5.1.5 The maximum worm shaft speed for these screw jacks should not exceed 500 R.P.M. for heavy
loads. Refer to Power Jacks Limited for higher worm shaft speeds for lighter loads.
5.1.6 The lead screws should not be permitted to accumulate dust and grit on the threads. If possible,
lead screws should be returned to the closed position (retracted) when not in use.
5.1.7 If equipped with bellows boot the boot must not be compressed bellow its minimum height
(consult Power Jacks product literature or engineers).
5.2 Operational Features
5.2.1 How a Rotating Screw Jack Works
The rotation of the worm shaft causes the worm gear to rotate. For rotating screw jacks the lead
screw is fixed to the worm gear and they rotate at the same speed. As the worm gear turns, the
friction forces on the screw thread act to turn the nut also. The greater the load on the screw
jack unit, the greater the tendency of the nut to turn. If the nut turns with the screw, it will
not raise the load. Therefore the nut needs to be fixed to a structure to prevent rotation. The
restraining torque required for the structure, also known as the "lead screw key torque" can be
found in product literature or requested from Power Jacks.
5.2.2 How a Translating Screw Jack Works
The rotation of the worm shaft causes the worm gear to rotate. For translating screw jacks
the worm gear is threaded to accommodate the lead screw thread. As the worm gear turns,
the friction forces on the screw thread act to turn the screw also. The greater the load on the
screw jack unit, the greater the tendency of the screw to turn. If the screw turns with the nut
(worm gear), it will not raise the load. In those cases where a single unit is used, and where the
load cannot be restrained from turning, it is necessary to use a screw jack with an anti-rotation
mechanism (keyed screw jack). Lead screw key torque (refer to product literature or requested
from Power Jacks) must be checked as excessively heavy unguided loads could break the Anti-
rotation mechanism (key).