A Compared with some other types of jack,the Worm gear machine screw jack has an important advantage that is it is self-locking, which means when the rotational force on the screw is removed, it will remain motionless where it was left and will not rotate backwards, regardless of how much load it is supporting. This makes them safer than the hydraulic jack itself, for example, if the force on the hydraulic actuator is released accidentally, it will move backwards under load.

Self-locking is a term used to describe screw jacks that require power to move in either direction. They hold their position when power to the system is off. Exceptions include Ball screw jacks, machine screw jacks having double lead screws and electric cylinders that are more than 30% efficient and all ball screw jacks. A brake must be used on the input shaft of any jack that is not self-locking. A brake should also be included for applications that expose the jack or actuator to vibration.

Self-locking means that lead screw nuts and lead screws cannot be moved without external force application. It has to do with the pitch and coefficient of friction. Self-locking allows the user to eliminate a costly brake in many applications. Single-start trapezoidal lead screw drives are self-locking. This means that the flank angle and the sliding friction prevent the nut or lead screw from moving without outside forces being applied. As soon as the static friction is exceeded, the components are no longer self-locking. Multistart trapezoidal screw drives have a "residual self-locking" feature; high helix screw drives have no self-locking feature.

Large frictional forces cause most screws in practical use to be "self-locking", also called "non-reciprocal" or "non-overhauling". This means that applying a torque to the shaft will cause it to turn, but no amount of axial load force against the shaft will cause it to turn back the other way, even if the applied torque is zero. This is in contrast to some other simple machines which are "reciprocal" or "non locking" which means if the load force is great enough they will move backwards or "overhaul". Thus, the machine can be used in either direction. For example, in a lever, if the force on the load end is too large it will move backwards, doing work on the applied force. Most screws are designed to be self-locking, and in the absence of torque on the shaft will stay at whatever position they are left. However, some screw mechanisms with a large enough pitch and good lubrication are not self-locking and will overhaul, and a very few, such as a push drill, use the screw in this "backwards" sense, applying axial force to the shaft to turn the screw. A push drill, one of the very few mechanisms that use a screw in the "backwards" sense, to convert linear motion to rotational motion. It has helical screw threads with a very large pitch along the central shaft. When the handle is pushed down, the shaft slides into pawls in the tubular stem, turning the bit. Most screws are "self locking" and axial force on the shaft will not turn the screw.

Now come back to screw jacks. In the case of large shocks, the self-locking function of the lift may be invalidated. Worm gear screw lift self-locking function depends on the following parameters: The size of the lead. Different gear ratios. Lubrication. Friction coefficient. Environmental effects such as high or low temperature, vibration, etc. Installation location. Ball screw type and large pitch of the trapezoidal screw series without self-locking function. Therefore, in these cases must use the appropriate brake or brake motor. For smaller guides (Single line), providing a limited self-locking function. Self-locking function depends on the specific situation.

A Worm gear screw jack offers a low-cost solution to a wide variety of industrial lifting, lowering, pushing and pulling applications. Available in many sizes, these worm gear screw jacks feature a self-locking acme screw and can be ordered with a number of options and configurations, the most basic including Translating, Keyed or Rotating designs.

Translating Screw Jack: A translating jack has a lifting shaft that moves through the gear box. A nut is integrated with the worm gear such that the worm gear and nut rotate together. When the lift shaft is held to prevent rotation, the lift shaft will move linearly through the gear box to move the load.

Rotating Screw Jack: A rotating jack has a lift shaft that moves a nut as it turns. The lift shaft is fixed to the worm gear. This causes the load, which is attached to the travel nut, to move along the lift shaft.

Keyed Screw Jack: The lift shaft of a translating style screw jack must be attached to something which prevents the lift shaft from rotating. If it is not, the lift shaft (and the load) will turn and not translate. A feature can be added to a machine screw jack to prevent lift shaft rotation. This type of jack is referred to as a "keyed jack" and is available in upright and inverted models. A keyed jack has a keyway machined along the length of the lifting screw. A matching key is fastened to the cover of the jack which will eliminate lift shaft rotation. The keyway in the screw causes greater than normal wear on the internal drive sleeve threads, somewhat reducing jack life.
Ball screw jacks cannot be keyed for non-rotation the same way that machine screw jacks are keyed. Contact us for a design solution if you need a keyed ball screw jack. Ball screw jacks can also be supplied with a device that prevents rotation of the lift shaft. Anti-rotation is accomplished by a square guide attached to the screw translating inside a square stem cover attached to the jack. The square stem tube is supplied with lube fittings.

A The linear actuators are grease lubricated for the ram and gearbox assemblies.

A Linear Actuators are not recommended for use in applications where it they are run into dead stops or can be jammed. Examples of jamming include over-travelling the limit switches and jamming the nut and screw internally at the extreme ends of the stroke, or driving the actuator against an immovable object and thus overloading the actuator severely.

A Unless otherwise stated back-driving is possible in all electric Linear Actuators. Actuators that use a ball screw as the lead screw have an electrical brake (typically motor mounted) to prevent the load from back-driving the actuator. Note if a machine screw actuator is considered self-locking, it may still back-drive if significant vibration and cyclic temperature variations are present.

A The linear actuators generally have a mounting option at the end of the ram and at the gearbox end of the actuator to allow a pivoting movement.

A Linear actuators can be supplied with limit switches. Have the standard option of electro-mechanical limit switches. Other limit switch types such as inductive proximity, magnetic proximity, etc.. Are not pre-set on actuators. Limit switches allow you the flexibility to set the limits of travel on your actuator to fit your particular application. The customer is responsible for properly setting the limit switch in the unit. If the limit switches are not set, or are improperly set, the unit may be damaged during operation. In addition, limit switches may require resetting if the translating tube of your linear actuator is rotated manually, as this will change the limit switch setting.

A Side loading, or radial loading is a force applied perpendicular to the linear actuator centre line. Eccentric loading is any force whose centre of gravity does not act through the longitudinal axis of the actuator. Both side loading and eccentric loading should always be avoided as they can cause binding and shorten the life of the linear actuator.