Screw jacks are mechanical devices used to lift and position heavy loads. Operation is manual or fully automated with motor starters, programmable positioning systems, and linear actuator controllers. They are an efficient and versatile alternative to hydraulic jack systems.
Let's look at how screw jacks work, the ways they can be used, and important considerations for selecting the right screw jack system for your needs.
What is a Screw Jack?
A screw jack moves an object by converting rotary motion into linear motion. A horizontal screw is held next to either a gear or a series of ball bearings. As the screw rotates it engages the gear or bearings, causing a vertical lift shaft to move up or down. A platform on top of the shaft supports the load. The load remains static unless the screw is turned, which differs from hydraulic jacks that require constant liquid pressure to hold a load.
Screw jacks can be configured upright and perpendicular to the load or inverted and at an angle to the load. This configuration determines the type of movement achieved such as raising, lowering, pulling, pushing, tensioning, or angling the load.
This relatively simple operating principle means there are many uses of screw jacks in diverse applications including:
- Moving large tools and equipment in aerospace applications
- Vehicle lifts
- Lifting and/or leveling structures
- Raising and lowering safety barriers
- Moving and holding manufacturing machinery in place
- Positioning satellite antennas
- Moving solar panels to different angles
- Lifting and lowering platforms on stages
Considerations When Selecting a Screw Jack
The design of screw jack configurations is always based on the specific application and details of the load in question.
There is usually a choice between the two common types: the machine screw jack and the ball screw jack. Machine screw jacks work well for static loads, manual operation, and shorter duty cycles. Ball screw jacks are useful for moving loads longer distances, anticipated movement of loads, and longer duty cycles.
Here are additional considerations:
Jack Sizing Considerations
The appropriate jack size is determined by its load capacity, or how much force all components in the jack can bear without failing. Consider the total load, which includes dynamic, static, moving, decelerated, and accelerated loads as well as any shock loading (i.e., sudden, large changes in loading) that can be anticipated.
Ensure the jack is rated for overload without damage, including 30% for static loads and 10% for dynamic loads. Equipment selection is also impacted by how (or if) the load can be distributed between multiple jacks.
Duty Cycle
The duty cycle is the percentage of time the jack is moving during one full cycle. An ideal duty cycle for a ball screw jack is 35%, and 25% for a machine screw jack. Heat builds during the duty cycle and it must dissipate in order for the jack to function properly. Also, take into account the maximum operating temperature for the jack in question (200°F is common).
Column Strength
Column strength refers to how much compressive loading the lift shaft can support without buckling. As screw length increases, column strength decreases.
Horsepower Ratings
Consider the jack’s horsepower limit. Each jack has a value for the amount of torque it requires to raise a one-pound load. Its horsepower limit is the product of the jack’s required one-pound torque multiplied by the total number of pounds to be raised multiplied by the screw’s RPM.
Jack temperature, lubrication, duty cycle, environment, and mounting all impact horsepower.
Critical Speed
Critical speed is the speed at which the screw begins to vibrate. It can vary based on RPMs of the screw, end fixity, unsupported screw length, and screw diameter. It is an especially important factor when loads will travel longer distances.
Guidance Support
Screw jacks generate thrust but require external guidance systems to absorb non-thrust loads. Rails and guides may be added to provide support.
Brake Motor Sizing
A brake motor is a safety device used to stop and hold a load stationary to avoid injury or damage. Horsepower rating and torque requirements are factors in selecting an adequate brake motor.
Ball Screw Life
When selecting a ball screw jack, consider the estimated lifespan of the ball screw, as listed in product specifications.
Screw Jacks From Joyce/Dayton Corp.
Joyce/Dayton CORP. is an OEM specializing in lifting equipment, screw jacks, actuators, and controllers. We have been committed to excellence in manufacturing since our founding in 1873.
Contact us or request a quote to speak with a member of our team about your screw jack application.