Articles
Screw jacks offer benefits for various applications
by Derrick Corea Technosoft Innovations, IncScrew jacks are an old concept but as relevant today as
always. For the correct type of application, a screw jack, or a set of screw
jacks, can provide a smooth and controllable linear movement when operating a
motor, which is generally electric, although it can also be hydraulic or
pneumatic. Normally, a screw jack is self-locking and the load cannot recede,
although this is not always the case, especially if ball screws are used. The
self-locking feature is often seen as an attractive feature when hydraulic and
pneumatic actuators are replaced and it is necessary to prevent the load from
moving once it is in position. Normally, screw jacks are available with
capacities of a few kilograms up to 100 kN, depending on the manufacturer and
the model.
'Classic' screw jack designs have only one mounting face,
but most manufacturers today offer designs that are 'cubic' and can be mounted
on their other faces (Fig. 1). Similarly, manufacturers generally offer two
kinds of screw jacks: those in which the spindle moves (screw) and those in
which the spindle rotates and a moving nut moves. Screw jack manufacturers also
tend to develop modular designs that allow products to be quickly assembled
from standard components so that customer needs can be met without incurring
the costs associated with fully customized units (see page 50 of the panel).
The applications for screw jacks are extremely diverse, but
the following is an indicative list: mobile platforms in theater stages;
adjustments in woodworking machinery; alignment of radio telescope plates (Fig.
2); plastics processing machinery; height-adjustable platforms in manufacturing
industries; management systems in the nuclear industry; and adjustable
manufacturing equipment that allows to process different product lines. In most
cases, screw jacks are only used periodically, so care should be taken if high
work cycles are anticipated. For example, this is what Servomech says for four
of his models of
Options and accessories
To further increase the versatility of screw jacks, manufacturers offer a variety of options and accessories. Often, screw jacks are used for safety or mission-critical applications where it is essential that the load does not fall when power is removed from the drive motor. Although screw jacks are generally inherently self-locking and cannot be driven backwards, there is a risk that excessive wear or other factors may cause catastrophic nut failure. For such applications, manufacturers offer 'locknuts' that comprise the main nut that supports the load and a second adjacent nut that carries the load in case the main nut fails. A small space between the two nuts also allows measuring the wear of the main nut. Another option related to safety is the 'emergency stop nuts' or the 'total power stop nuts' that come into contact with the cat's body in case of over-displacement, although it is generally recommended to use external stops, instead of Relying on devices that could damage the screw the cat's body in case they have to stop the movement at full power.
Screw jacks are high precision engineering mechanisms, but
standard designs feature an exposed screw. This can be protected in several
different ways, including fixed protective tubes, elastomeric bellows and
spiral springs, depending on the nature of the likely contaminants and the
physical limitations of the equipment design. For applications in the food,
beverage, pharmaceutical or nuclear industry, screw jacks can be made of
stainless steel. Other options and accessories that are typically offered for
screw jacks include adjustable anti-reverse nuts, limit switches, wear
indicator switches, shaft encoders and handwheels. Often, the manufacturer can
also supply drive shafts, couplings and other components that allow synchronization
of movement from multiple screw jacks when a single motor is used (Fig. 3). An
interesting option of Marzorati is the multi-start ball screw spindles that
offer greater efficiency and, more importantly, a slower rotation of the screw
to produce the same linear speed.
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About Derrick Corea 
Technosoft Innovations, Inc
27 connections, 0 recommendations, 100 honor points.Joined APSense since, January 2nd, 2018, From Suite C Morrisville, United States.
Created on Dec 23rd 2019 04:52. Viewed 204 times.
