Power Jacks C-Series - Manual (Page 16)

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5
Operation
5.2.3 Anti-Backlash Screw Jack – When To Use
For reduced axial backlash of the lead screw in the screw jack select a model with the "Anti-
Backlash" mechanism. This is typically used when the load direction changes from tension to
compression and minimal axial backlash is required. This design is only available for translating
screw jacks. It can be combined with Anti-Rotation mechanism as well.
5.2.4 Input Torque Required for a Screw Jack
The input torque for a single screw jack depends on the load, the worm gear ratio, type of screw
(machine screw, ball screw or roller screw) and the pitch of the lead screw. Torque values are
listed in the individual product specification charts based on capacity loads. For loads from 25%
to 100% of screw jack model capacity, torque requirements are approximately proportional to
the load.
Note: The input torque, as well as the efficiency and side load ratings, is the same for both
translating screw and rotating screw jacks.
5.2.5 Maximum Input Power & Speed for a Screw Jack
The input power to the screw jacks should not exceed the power rating shown in the
specifications table. Maximum input speed in rpm (revolutions per minute) to a screw jacks
worm shaft should not exceed 1800 rpm for C-Series screw jacks.
5.2.6 Efficiency of a Screw Jack
Screw Jack model efficiencies are listed in the individual product specification charts.
5.2.7 Expected Life of a Screw Jack
The life expectancy of a screw jacks lead screw, bearings, nut and worm gear set varies
considerably due to the extent of lubrication, abrasive or chemical action, overloading, excessive
heat, improper maintenance, etc. For detailed life calculations consult Power Jacks Ltd.
5.2.8 Screw Jack with Anti-Rotation (Keyed) Mechanism
This design is only available for translating screw jacks. If the structure/object connected to
the lead screw is not prevented from rotating or the lead screw is not always in contact with the
structure then a screw jack with an "Anti-Rotation" mechanism (keyed) should be used.
5.2.9 Standard Screw Jacks How To Prevent The Load from Rotating
For multiple screw jack systems, fix the lead screw end fittings (e.g. top plate or clevis) to the
common member being lifted by all the units. For single screw jack applications, bolt the lead
screw end fitting (e.g. top plate or clevis) to the load and ensure the load is guided to prevent
rotation.
A guided load is always recommended to ensure that the screw jack does not receive any side
load and so guidance can be scaled suitably for the load without altering the screw jack design
unnecessarily. Note that an external guidance system can provide a higher restraining "key"
torque than compared to an anti-rotation mechanism in a screw jack.
5.2.10 Self-Locking of Screw Jacks
Screw Jacks with 24:1 gear ratios are considered self-locking in most cases. Consult Power
Jacks for a recommendation specific to your application.
All screw jacks with double start lifting screws are considered not to be self-locking.
Screw Jacks considered not self-locking will require a brake or other holding device.