Screw Jack Working Principle
Very common throughout the world, screw jacks are found
wherever there is a need to lift, position, align and hold
load. Their high reliability and synchronization makes screw jacks suitable for a wide variety of uses that alternative
methods of handling cannot achieve. This spectrum of applications reaches across market sectors and includes Steel
Works 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 conventionally 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 devise used to convert rotational motion into linear motion. It utilises 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 in to 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. It could, however, come at a greater initial cost and does not cover as greater ratio range as worm drives.
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 utilise 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 minimise 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.
Types of Screw Jack
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.