02-07-2012, 12:28 PM
Lifting Torque Required for a Screw Jack
Lifting Torque.doc (Size: 22.5 KB / Downloads: 36)
What is the Lifting 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 lifting 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.
Can Screw Jacks be Supplied with an Emergency Stop Discs, Pin or Nut?
To prevent over travel of the lifting screw a stop disc, pin or nut can be fitted to a screw jack that is hand operated.
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 conditions occurs an assessment made to discover why it happened in order to carry out preventative action. Should the full power stop nut be 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.