RACO International is pleased to announce their partnership with Kelston Actuation in North America. This relationship allows RACO to align its existing line of Electric Actuators with a complete line of High Quality Screw Jacks.
By definition, a screw jack is a mechanical lifting device operated by turning a lead screw. Very common throughout commercial and industrial applications around the world, screw jacks are found wherever there is a need to lift, position, or align and hold load. Their high reliability and synchronization make screw jacks suitable for a wide variety of uses that alternative methods of handling cannot achieve. These applications reach across market sectors which include Iron and Steel and Mill equipment, Water Processing, Pharmaceutical, Medical and Laboratory Equipment, Packaging Equipment, Nuclear and Aerospace and General Mechanical Handling. Additionally, screw jacks are increasingly finding uses as alternatives to conventional pneumatic and hydraulic systems.
A Screw Jack (also known as a Jack Screw, a Worm Screw Jack, a Machine Screw Jack or a Lead Screw Jack) is a device used to convert rotational motion into linear motion. It utilizes the property of a screw thread providing a mechanical advantage i.e. it can be used to amplify force.
Machine Screw Jack and Ball Screw Jack
A screw jack may incorporate a machine cut lead screw or a rolled or ground ball screw in order to transfer rotational energy in to linear energy. In both cases, the rotation of the worm screw acting on the lead screw via the worm wheel converts rotation into linear motion.
Machine screw jack: The worm wheel acts directly on the lead screw (lifting screw).
Ball screw jack: The worm wheel acts on the ball screw (via the ball nut) which actuates the lead screw. This system offers greater efficiency between the input and the useful output compared with a machine screw jack. In addition, it allows for greater actuation speeds and, due to the low friction, is very durable. However a ball screw jack is not inherently self-locking and, as a consequence of its enhanced precision components, the initial outlay is greater. The resulting improved efficiency however implies this can be offset against smaller drive train components and a significant reduction in the necessary power.
Many applications do not warrant the extra expenditure of a ball screw jack since they do not require continual drive. In configuring a screw jack a prediction is made of the frequency of actuation and this will point to the appropriate screw jack to be selected.
As opposed to the worm drive systems discussed here, a bevel gear system could be used to convert rotation to linear motion. This would offer greater efficiency to a machine screw jack due to it making a rolling contact as opposed to the sliding contact of worm drive components.
Lead Screws: Also known as power screws, lead screws come with several different types of thread profile which are suitable for different applications. Acme lead screws are defined by their trapezoidal thread profile and 29° flank angle and are commonly found in American Imperial machine screw jacks. An alternative to the Acme lead screw in a machine screw jack would be a square lead screw.
European or other international screw jacks utilize a trapezoidal lead screw with a 30° flank angle and complies to an ISO metric standard.
Ball screw jacks require the thread of the lead screw to have a profile that allows for the travel of the balls. To enhance load distribution and minimize wear, the ball screw track has a gothic arch profile.
Popularity of the trapezoidal screw thread comes from the fact that it is easier to machine and is therefore more economical than square and ball screw thread forms. Additionally, due to the large area of contact between the lead screw threads and the worm wheel, there is a large load carrying capacity. This results in high friction which is detrimental to efficiency but also means the system is more likely to be self-locking. This low efficiency implies that such screw jacks are more suited to non-continuous or intermittent operation.
Screw Jack Variations depending on specific applications
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 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 used in applications where more than one screw jack is attached to a shared 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.
RACO International is able to provide both Electric Actuator Solutions and Screw Jack Systems for many demanding applications. Our capabilities for Screw Jacks range from mini units designed for lifting 1 kN (224.8 lbs) in compact configurations and lifting up to 1000 kN (224,809 lbs) for heavy industrial applications. Our RACO Electric Actuators have maximum capabilities of 314,700 lbs of thrust and standard stroke lengths up to 78 inches.