What is Jackscrew?- Types And How does it Work

What is a jackscrew?

A jackscrew, also known as a screw jack, is a mechanical device that relies on the rotation of a lead screw to operate. This tool finds frequent use in lifting moderately heavy objects—for instance, raising vehicles, adjusting the horizontal stabilizers on aircraft, or serving as adjustable supports beneath substantial loads like house foundations.

Structurally, a screw jack features a robust vertical screw topped with a load table. This screw is threaded into a fixed support frame that typically has a broad base for stability. At the head of the screw, there is a rotating collar fitted with holes, allowing a metal rod to be inserted and used as a handle.

Turning the handle clockwise causes the screw to extend further from the base, which in turn raises the load positioned on the load table. To ensure the device can safely bear significant weight, the screw is generally manufactured with Acme threads, which are well-suited to handling heavy loads.

Related Article: What is a lead screw?

Related Article: What is Screw?

How a Screw Jack Works?

A screw jack combines a gearbox assembly—such as a worm or bevel gear—with a transmission mechanism, like a lead screw, ball screw, or roller screw. Powered by a motor, this setup converts rotary motion into precise linear movement.

In practical terms, screw jacks are remarkably versatile. They can be used to push or pull, apply tension, lock or unlock, tilt, pivot, roll, slide, and, most commonly, lift or lower loads. Their capacity ranges from handling just a few kilograms to managing several thousand tonnes.

Within automated machinery, screw jacks have become indispensable. As safety standards and legislation evolve, particularly in areas with well-developed workplace health and safety regulations, the demand for automated solutions to lift and handle heavy items continues to grow.

A noticeable shift has taken place in recent years, moving from hydraulic systems to electromechanical actuation, with screw jacks often at the center of this change. While hydraulic setups have traditionally been the go-to technology for heavy-duty tasks, they lag behind in terms of energy efficiency. Hydraulics tend to waste energy, as the fluid is kept under constant pressure even when little or no movement is needed.

On the other hand, a hydraulic jack or ram must maintain steady pressure to hold a load in position, consuming energy regardless of activity. By contrast, an electric motor driving a spindle or screw jack uses power only when it’s actively moving the load. This fundamental difference gives electromechanical screw jacks a distinct advantage, particularly in applications where energy efficiency is a priority.

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

Types of Screw Jack

Screw jacks can generally be grouped into three primary types: machine or worm gear screw jacks, ball screw jacks, and bevel gear jacks. Each of these types can be further classified based on their mode of operation, which typically falls into one of three categories: translating, keyed, and rotating or traveling nut designs.

1. Translating Screw Jack

When the worm wheel rotates, it drives the lead screw, resulting in linear movement of the screw. If the end of the lead screw is left unfixed, which may be the case depending on specific customer requirements, the screw can begin to rotate as well. This rotation typically occurs because of friction present between the threads of the screw.

2. Translating Keyed Screw Jack

In this case, the lead screw is intentionally immobilized to prevent rotation. This detail is particularly significant when the end of the lead screw does not connect directly to the load. In practical applications, both keyed (encrypted) and plain (non-encrypted) transmission screws are frequently employed, especially in scenarios where multiple screw jacks are linked to a single load.

3. Rotating Screw Jack

The lead screw is connected to the worm wheel in such a way that, as the worm wheel turns, it drives the rotation of the lead screw. When the nut is fixed to a load, this rotation causes the nut to travel along the length of the lead screw, allowing the load to move accordingly.

4. Translating Screw Jack System

A six-unit screw jack system is planned, with each lead screw intended to connect to a shared load. When designing the setup, it is important to ensure there is sufficient clearance beneath the jack body’s bottom mounting plate. This space is necessary to properly fit the cover for the translating screw.

5. Rotating Screw Jack System

A system incorporating four screw jacks is used. The lifting nuts must be translated accordingly. Notably, there is no requirement for clearance beneath the lower mounting surface of the jack body. Nevertheless, when designing the lifting plate, one should ensure that it is capable of moving over or past the lifting screw as needed.

The translation speed in a screw jack system is largely determined by the number of thread starts on the screw spindle. Typically, single-start screw jacks exhibit self-locking behavior, owing to the fact that the friction angle of the threads exceeds the helix angle. As a result, applying an axial force to the screw does not induce rotation.

However, this self-locking property is not present in multi-start screws, where the friction angle can be lower than the lead angle.

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