A For Compression Loads: The column strength of a screw is determined by the relationship between the length of the screw and its diameter. Column strength charts are included in the Screw Jacks catalogue for each screw jack size.

A Larger Screw Jacks have their maximum raise stroke available limited only by the available length of bar stock from suppliers (note - special steel production runs can be organised for special applications) and the practical ability to handle, machine and transport the lead screw and the complete screw jack. Practical lengths will also be affected by whether the screw is to be subjected to compression or tension loads. Depending on diameter the length can be limited due to deformation of material in the machining process or column strength of the screw when subjected to compression loads.

A Screw Jacks with 24:1, 28:1, 32:1, 36:1 and 40:1 gear ratios are considered self-locking in most cases. 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.

A We recommend the following methods for preventing the rotation of the lifting screw on standard screw jacks. 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. It should also be noted that an external guidance system can provide a higher restraining “key” torque than compared to an anti-rotation mechanism in a screw jack.

A For all machine screw jacks a keyed lifting screw is available. Note the keyway in the screw causes greater than normal wear on the internal threads of the worm gear. For ball screw jacks the lifting screw cannot be keyed, as the keyway would interrupt the ball track, permitting loss of the recirculating balls. Instead the option of a square anti-rotation tube can be fitted to ball screw jacks to prevent rotation.

A 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. For machine screw jacks the mechanism uses an internal key to prevent the lead screw from rotating. For ball screw jacks a guided element fixed to the ball screw is used to prevent the ball screw from rotating. What other design features are available? Other designs & features available include: * Bellows boot screw protection * Safety nut (translating or rotating screw types) * Motor adapter kit * Double Clevis - reinforced cover pipe with clevis * Anti-corrosion protection. As well as stainless steel various platings and paint finishes are available * Trunnion mounts for base of screw jack * Limit switch kits (mechanical, proximity types) * Thrust rings for screw jacks that need to withstand high shock loads * Upgraded rotating screw jack for improved column strength for compressive load

A 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.

A 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. We can not accept responsibility for the overheating and rapid wear that may occur should these limits be exceeded. Power increases in direct proportion to the speed, and the motor size will be out of proportion to the screw jack model design rating should the speed become excessively high. When selecting the maximum permissible speed for a screw jack arrangement, always check to see that the power rating of the screw jack model is not exceeded.