There are a few variations of screw jack available depending on the specific application. These variations can be achieved with either a machine screw jack or ball screw jack and are largely chosen based upon the system architecture in to which they are to be fitted:
Translating Screw Jack
The rotation of the worm wheel acts directly on the lead screw and the lead screw translates linearly. Unless the end of the lead screw is fixed (dependent on customer requirements), the lead screw will tend to rotate due to friction between the screw threads.
Translating Keyed Screw Jack
As above but the lead screw is keyed such that it cannot rotate. This is important if the lead screw end is not to be fixed to the load. Both the keyed and un-keyed translating screws are commonly used in applications where more than one screw jack is attached to a common load.
Rotating Screw Jack
The lead screw is fixed to the worm wheel so rotation of the worm wheel causes rotation of the lead screw thus translating the nut along the lead screw when the nut is attached to the load.
Translating Screw Jack System
6x Screw Jack System. The lead screw ends are to be attached to a common load. Clearance is required under the bottom mounting plane of the jack body to accommodate the translating screw cover:
Rotating Screw Jack System
4x Screw Jack System. The lifting nuts translate. Clearance under the bottom mounting plane of the jack body is not required but allowance must be made in the lifting platen design to allow the platen to pass over or past the lifting screw:
Translation speed of a screw jack is affected by the number of starts of the lead screw. With single start screw jacks the system is usually self-locking due to the friction angle of the threads being greater than the lead angle. i.e. an axial force applied to the screw does not result in rotation. This is not the case with a multi-start screw as the friction angle may be lower than the lead angle.
If the system in to which the screw jack is to operate has a lot of vibration the screw jack is more likely not to self lock since there is dynamic loading on the threads which may overcome friction. In this case, and in the case of a multi-start screw, the screw jack should have a locking mechanism.
