This blog will explain all eight and how they can affect the function and reliability of your screw jack system.
A jack’s load capacity is limited by the physical constraints of important components such as the drive sleeve, lift, shaft, and bearings. All anticipated loads must be calculated properly to ensure they meet the rated capacity of the jack. Loads experienced by jacks can include static, dynamic, moving, acceleration & deceleration, as well as shock loads, cutting, and other reactionary forces.
The duty cycle is the percentage of time that a screw jack is in use versus total time. The main factor in calculating duty cycle is the ability of the screw jack to dissipate heat that builds up during operation. As the generated heat increases or decreases, the duty cycle will be affected accordingly. Some screw jacks are limited by their maximum operating temperature rather than their duty cycle.
There are many application-specific variables that can influence horsepower values. These include mounting, environment, duty cycle, and lubrication. The best way to determine if performance is within horsepower limits is to measure the temperature of the jack. The horsepower limit of a jack is reflected by the ability to dissipate the heat generated from the inefficiencies of its components in intermittent operation.
Column strength is the ability of the jack’s lift shaft to hold a compressive load without buckling. With longer screw lengths, column strength can be much lower than jack capacity. If there is a possibility for the lift shaft to go into compression, the application should be sized accordingly. If the maximum column strength is exceeded, the operator should change the jack figuration and put the shaft in tension, use a larger screw jack, add a bearing mount for rotating jacks, and change the lift shaft mounting.
Critical speed refers to the speed that excites the natural frequency of the screw. It varies with the diameter, unsupported length, end fixity, and the rpm of the screw. Because of the nature of most screw jack applications, critical speed is often overlooked. Despite this, it is an important factor to consider.
Any system that uses linear motion requires both thrust and guidance. Screw jacks are designed to provide only thrust, so a guidance system must be designed to absorb all loads other than thrust. Preferred systems include harderend ground round shafting and square profile rail.
For screw jacks where the possibility of injury exists, a brakemotor is recommended. Horsepower requirements will determine the size of the motor and assure that the standard brake has enough torque to both stop and hold the load.
A benefit of screw jacks is the ability to predict the lifespan of the ball screw. Many manufacturers will provide charts with their products. Once you have this information, it makes selecting the right screw jack significantly easier.
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