What is Jackscrew?- Types And How does it Work

screw jack

What is jackscrew?

A jackscrew, or screw jack, is a type of jack that is operated by turning a lead screw. It is commonly used to lift moderately heavyweights, such as vehicles; to raise and lower the horizontal stabilizers of aircraft; and as adjustable supports for heavy loads, such as the foundations of houses.

A screw jack consists of a heavy-duty vertical screw with a top-mounted load table that is screwed into a threaded hole in a stationary support frame with a wide base on the floor. A rotating collar on the head of the screw has holes into which the handle, a metal rod, fits.

When the handle is turned clockwise, the screw moves further out of the base and lifts the load resting on the load table. In order to withstand large loads, the screw is usually formed with Acme threads.

Related Article: What is a lead screw?

Related Article: What is Screw?

How a Screw Jack Works?

A screw jack is a gearbox assembly (either worm gear or bevel gear) and a transmission product (lead screw, ball screw, or roller screw) which through the use of a motor is used to convert rotary into linear motion. They can be used to push, pull, tension, lock, unlock, tilt, pivot, roll, slide and lift or lower loads, anything from a few kilos to thousands of tonnes.

Screw jacks are essential components of automated machines. Safety and legislative concerns encourage automation of the handling and lifting of heavy loads, especially in regions where workplace and health and safety legislation have been developed.

The trend to electromechanical actuation from hydraulic actuation.

Screw jacks typically work in heavy-duty applications. The competing technology at high loads is usually hydraulics. However, hydraulics are less energy-efficient than electromechanical actuation by screw jacks. Hydraulic systems waste energy when the fluid circulates at constant pressure, regardless of how much work is required by hydraulic positioning.

A hydraulic jack or ram requires constant pressure to hold its position when a load is held in place. In contrast, an electric motor used to drive a spindle lift truck consumes energy only when it is driving the load to a required position.

The advantages of electro-mechanical over hydraulic can be summarized by:

  • Demand for increased safety, in the event of power loss, screw jacks can be self-locking
  • Demand for machinery that operates with better energy efficiency
  • Demand for machinery that operates greater levels of precision
  • Machinery that requires less maintenance
  • Machinery that requires less manual intervention to set up processes
  • Increased range of actuation in terms of variable positioning
  • Accurate and smooth delivery of force
  • Cleaner machinery
A jackscrew, or screw jack, is a type of jack that is operated by turning a lead screw.

Types of Screw Jack

There are 3 main types of screw jacks: machine/worm gear screw jacks, ball screw jacks, and bevel gear jacks. Within these, there are 3 subcategories related to the mode of operation: Translating, Keyed and Rotating/Traveling nut.

1. Translating Screw Jack

The rotation of the worm wheel acts directly on the lead screw and the lead screw moves linearly. If the end of the lead screw is not fixed (depending on customer requirements), the lead screw will tend to rotate due to the friction between the screw threads.

2. Translating Keyed Screw Jack

As above, but the lead screw is wedged so that it cannot turn. This is important if the lead screw end is not to be attached to the load. Both encrypted and non-encrypted transmission screws are commonly used in applications where more than one screw jack is attached to a common load.

3. Rotating Screw Jack

The lead screw is attached to the worm wheel so that rotation of the worm wheel causes the lead screw to rotate, thereby sliding the nut along the lead screw when the nut is attached to the load.

4. 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 plate of the jack body to accommodate the translating screw cover.

5. Rotating Screw Jack System

4x screw jack. Translate the lifting nuts. Clearance under the lower mounting level of the jack body is not required. When designing the lifting plate, however, it must be taken into account that the plate can move over or past the lifting screw.

The translation speed of a screw jack is influenced by the number of times the screw spindle is started. With single start screw jacks, the system is normally self-locking because the friction angle of the threads is greater than the helix angle. i.e. an axial force exerted on the screw does not result in rotation. This is not the case with a multi-start screw because the angle of friction may be less than the angle of lead.

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