Power Jacks have been manufacturing and developing screw jacks at their Fraserburgh factory since the 1930s. Screw jacks whether in single-face gearbox, cubic gearbox or special designs can be supplied to suit customer requirements. They are available in metric (mm/kN) and imperial (inch/tonne) designs with capacity ratings range from 5 kN to 3500 kN and ? tonne to 250 tonnes as standard. Some of the features of specific screw jack designs are discussed below.

Anti-Backlash Screw Jacks Position Accurately

Machine Screw Jacks are used for positioning loads in numerous applications, however when the load direction alternates between tension and compression within the operating cycle and a high position accuracy must be maintained then a specific type of screw jack is required. The Anti-Backlash Screw Jack is recommended for such applications, as it is able to reduce the axial backlash between the lead screw and the trapezoidal screw thread in the gear to a practical minimum. This design type is available in all 4 screw jack series of C, E, M and S.

Axial backlash exists in standard screw jacks as the "gap" or "play" between the flanks of the lead screw thread and the internal thread on the worm gear nut. This "gap" or "play" is due not only to normal manufacturing tolerances, but to the fact that there must be some clearances to prevent binding and galling when the screw jack is operating under load.

In most applications either the load direction is constant so axial backlash does not occur or the position error caused by the axial backlash is considered negligible for the application. However some applications require minimal or regulated axial backlash for accurate and repeatable positioning. Such an application is for positioning the rollers in steel straightening machines for tube, bar or sheet steel production.

Power Jacks anti-backlash mechanism on screw jacks can be used to reduce axial-backlash to a practical minimum (typically 0.013 mm) or a regulated value. Reduction in axial backlash below this minimum is not normally recommended as it can cause binding and excessive wear. As the screw jack is operated over time the axial backlash will increase as the lead screw thread on the gear wears due to normal operation. To compensate for this, Power Jacks anti-backlash device can be adjusted to re-set for the required amount of regulated backlash. To help with the identification of wear Power Jacks screw jacks can also be fitted with a wear monitoring device for visual use or with electronic sensing for integration into a motion control system.

Stainless Steel Screw Jacks for Linear Motion Without Corrosion

A complete stainless steel screw jack is available from Power Jacks for linear motion applications where there is a high demand for corrosion resistance, hygienic installations, and/or easy to clean designs.

There are two series of stainless steel screw jacks the E-Series Metric screw jacks and the M-Series Imperial (inch) screw jacks. The E-Series has 6 basic models in the range with capacities ranging from 10 kN up to 300 kN. The M-Series has 9 basic models in the range with capacities from 2 tonnes to 100 tonnes.

The screw jacks worm gearbox has a curved housing made from stainless steel and can be specified with a sealed gear cavity to keep water and other contaminants out. Alternatively, a standard gearbox construction can be specified.

The lubrication of the gearbox and the lifting screw can also be altered for the application for example food grade grease or nuclear grade grease. As the requirements for each application can vary greatly so can the specification of the stainless steel screw jacks. This can extend from a unit with just a stainless steel lifting screw and plated housing to a fully stainless steel screw jack. Each screw jack has the same high standard array of options as other screw jack designs available from Power Jacks, which results in thousands of available variants. This allows the specification of the screw jack to be tailored to meet the requirements of the application taking all factors and conditions into account.

Power Jacks stainless steel screw jacks have been successfully used in applications worldwide ranging from food processing lines to nuclear power stations to sub-sea oil wellheads.

Metric Ball Screw Jack with Integral Safety Device as Standard

Power Jacks E-Series range of Metric Ball Screw Jacks is designed with an integral safety device as standard. This provides extra safety for the customer in case of ball failure or excessive wear in the screw jack. Most ball screw jacks only meet the demands of customers to have a more efficient screw jack over machine screw types. The need to improve safety was identified during application work with customers who requested extra safety features on screw jack systems due to their working environments including operations with human cargo, fragile or high value industrial cargo or work in hazardous areas.

To provide the best solution for these requirements it was decided to review the screw jack design with a view to improving design rather than adding more equipment to the whole screw jack system, which increases costs and the working envelope. These ball screw jacks now accommodate a special ball nut that incorporates a safety device as well as integral wipers. The addition of the integral safety device to the E-Series screw jacks provides the customer with the two advantageous safety features of load sustaining and wear indication:

1. In the unlikely event of excessive wear in the ball screw drive, the safety device will contact the screw shaft, and act as an "ACME" Thread. This will provide early warning of any possible ball screw failure, and is capable of providing drive in the event of any such failure. It also allows a company's control system to alert an operator to wear of this kind by monitoring the increase in motor current required to drive the system due to the increased friction generated by the device.
2. It allows the ball nut on the screw jack to sustain a load in the event of catastrophic ball failure. The safety of industrial and human cargo is therefore improved. Ball screw systems without this device could collapse under load or drop the carried load.

The ball screw unit used in Power Jacks screw jacks and actuators is designed and manufactured to Power Jacks own specification from certified materials, in an ISO 9000 environment. The ball screw drive is custom manufactured with high precision, close conformity raceways. These have been tested and proven as having amongst the highest load carrying capacities. The E-Series ball screw jacks are available in capacity ratings from 10 kN up to 300 kN as standard with up to 1000 kN available on special request.

S-Series High Performance Machine Screw Jacks

To meet the increasing industrial demands on machine screw jacks Power Jacks team of experienced design engineers set out to provide customers with a new high performance cubic machine screw jack. It has typically 50% higher duty cycle capabilities than standard machine screw jacks, thus reducing the need for ball screw jacks or larger de-rated machine screw jacks in high duty applications.

In addition, the range is engineered to have long lasting service life, high durability, large selection of configurations, versatile mounting and an extensive accessory list. Ideal for applications that demand more from a machine screw jack, but do not warrant a ball screw jack. Installed singularly or as part of multi-unit jacking systems. Power Jacks high duty S-Series screw jacks are a range of metric cubic screw jacks designed to have a higher duty cycle and mounting flexibility over conventional worm gear screw jacks.

The higher duty rating has been attained by improving the design of the screw jacks worm gearbox for a higher thermal efficiency. Achieved by increasing surface area and mass to the optimum point for the available foot print for each screw jack size.

This balance of area and mass is critical as both can detrimentally affect the thermal performance. The gearboxes lubricant was also changed from grease to oil. In general, tests showed that the S-Series screw jacks had a 50% higher duty cycle than conventional worm gear screw jacks, irrespective of the lifting screw type in the unit. Other benefits built into the design are improved flexibility for different linear travel rates, mounting arrangements for the screw jack, and accessory attachment mounting. There is the choice of 3 standard gear ratios with two different lifting screw leads, covering a multitude of linear travel rates, in translating (ST) or rotating (SR) screw format.

C-Series Cubic Screw Jacks

The C-Series complements Power Jacks' existing high-performance S-Series cubic screw jack and has been designed to offer new application versatility with standard screw jack performance.

It represents the latest stage in the evolution of our product range, based as it is upon the principles of Power Jacks' classic E-Series screw jack that is long established and proven in the market. Power Jacks has adapted those principles into this new C-Series cubic design which we believe offers exciting new market opportunities. The C-Series is durable and adaptable and boasts a modern, sharp design. One of its key assets is the versatility it offers in terms of mounting methods, reflecting our commitment to offering customers the widest possible range of options.

The standard C-Series unit is complemented by anti-backlash, anti-rotation and safety nut versions, and comes in the four standard capacity sizes - 10 kN, 25 kN, 50 kN and 100 kN. The C-Series special features include two screw lead options and two gear ratio options for each screw jack size, aluminium bronze worm gear and optimum lubrication via two integrated systems. The gearbox housing is made of either highly durable SG iron or aluminium, providing a robust housing that firmly and accurately holds the gear set in a reservoir of lubricant suited to the most demanding industrial applications. The worm gear set is of a proven design commonly used across Power Jacks' screw jack portfolio, and there is forced grease lubrication of the lead screw. It is ideally placed to serve a host of industrial applications, operating individually or as part of a multi-unit jacking system.

(For further information on screw jacks or 3D CAD, catalogues and maintenance manuals are freely available from Power Jacks website www.power jacks.com or alternatively in India contact Power Jacks distributor Amcats for assistance.)