A The standard screw jacks are grease (EP2) lubricated for the lifting screw and gearbox assemblies.

A Mount an electric motor to a screw jack with the extensive range of motor adaptors designed to be used in conjunction with a flexible jaw coupling that connects the motor drive shaft to the screw jacks worm shaft.

A The screw jack will drift after the motor drive is switched off unless a brake of sufficient capacity is used to prevent it. The amount of drift will depend upon the load on the screw jack and the inertia of the rotor in the motor. Due to different construction, the ball screw jack must be considered separately. Machine screw jacks require approximately one-half as much torque to lower the load as they do to raise the load. For machine screw jacks with no load, the amount of drift will depend upon the size and speed of the motor. For example, a 1500 RPM input directly connected to a screw jack without a load will give on average 35mm to 60mm of drift; a 1000 RPM input will give about 1/2 as much drift. Note that the drift varies as the square of the velocity (RPM). The drift of the screw jacks screw can be controlled by using a magnetic brake on the motor. Variations of drift will also be seen if the motor drives the screw jack via a reduction gearbox.

A We recommend that the screw jack selected has a greater capacity than the rated capacity of the press or of the load capacity of the structure. We also recommend that a torque clutch or similar device be used to prevent overloading of the screw jack unit. Unless these precautions are taken, it is possible to overload the screw jack without realizing it.

A 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). Under ideal conditions where a slip clutch or torque limiting device is used, a stop pin or stop nut may be used. Note that the standard stop disc used on the end of the ball screw on ball screw jacks prevents the ball screw from running out of the ball nut during shipping and handling, thereby preventing loss of the recirculating balls. It should not be used as a full power stop.

A Perhaps the greatest single advantage of screw jacks is that they can be linked together mechanically, to lift and lower in unison. Typical arrangements involving the screw jacks, bevel gear boxes, motors, reducers, shafting and couplings are shown in application section of the web site. Typical mechanical system arrangements link 2, 4, 6 or 8 screw jacks together and are driven by one motor. As an alternative screw jacks can be individually driven by electric motors and with suitable feedback devices such as encoders be synchronized electronically by a control system.

A In addition to the efficiencies of the screw jacks and the bevel gearboxes, the efficiency of the screw jack arrangement must be taken into consideration. The arrangement efficiency allows for misalignment due to slight deformation of the structure under load, for the losses in couplings and bearings, and for a normal amount of misalignment in positioning the screw jacks and gearboxes. Two screw jacks lift system, I and T configuration,, transmission efficiency 95%. Three screw jacks lift system, I and T configuration, transmission efficiency 90%. Four screw jacks lift system, H, U and T configuration, transmission efficiency 85%. Six screw jacks lift system, 2H and U configuration, transmission efficiency 80%. Eight screw jacks lift system, 2H and H configuration, transmission efficiency 80%. Notes: don't ignore bevel gearbox efficiency 98%.

A This will be limited by input torque requirements on the first worm shaft in the line. The torque on the worm shaft of the first screw jack should not exceed 300% of its rated full load torque on the machine screw jacks.